Patent Publication Number: US-7903383-B2

Title: Solenoid valve driving circuit and solenoid valve

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solenoid valve driving circuit in which, after a first voltage is impressed on the solenoid coil of a solenoid valve for driving the solenoid valve, a second voltage is impressed on the solenoid coil and the driven state of the solenoid valve is maintained, as well as to a solenoid valve having such a solenoid valve driving circuit. 
     2. Description of the Related Art 
     Conventionally, it has been widely practiced to arrange a solenoid valve within a fluid passage, and by impressing a voltage on a solenoid coil of the solenoid valve from a solenoid valve driving circuit, the solenoid valve is energized to open and close the fluid passage. In this case, after the solenoid valve is driven by impressing a first voltage on the solenoid coil of the solenoid valve from the solenoid valve driving circuit, the driven state of the solenoid valve is maintained by impressing a second voltage on the solenoid coil from the solenoid valve driving circuit. 
     Recently, it has been desired that the driven state be maintained with low power consumption. In Japanese Patent No. 3777265 and Japanese Laid-Open Patent Publication No. 2006-308082, it has been proposed that, within a time period during which the driven state is maintained, and as a result of controlling conduction between the power source and the solenoid coil by means of a switch, energization and deenergization of the solenoid coil is carried out repeatedly, so that the driven state of the solenoid valve can be maintained with a lower level of power consumption. 
     Incidentally, the current flowing through the solenoid coil tends to vary over time as a result of various factors, such as changes in electrical resistance in the solenoid coil induced by temperature changes of the solenoid coil, timewise changes of the power source voltage (first voltage and second voltage) impressed on the solenoid coil from the DC power source through the solenoid valve driving circuit, and due to vibrations or shocks and the like, which are imparted to the solenoid valve from the exterior thereof. Owing thereto, within the time period during which the driven state of the solenoid valve is maintained, so as to prevent the above-mentioned various factors from occurring and causing stoppage of the solenoid valve, a current, which takes into consideration the aforementioned various factors, is superimposed on the minimal required current for maintaining the driven state. Accordingly, even when the above-mentioned various factors do not occur, the current taken in consideration of these factors still flows through the solenoid coil, and hence, electrical power savings of the solenoid valve driving circuit and the solenoid valve cannot be promoted. 
     Further, as a result of the current that flows through the solenoid coil being large, when driving of the solenoid valve is halted after maintaining the driven state, the solenoid valve cannot be stopped in a short time period. 
     Moreover, in the case that a plurality of DC power sources, having different power source voltages, are prepared and utilized on the side of users of the solenoid valves, on the manufacturer&#39;s side, even if there are solenoid valve driving circuits and solenoid valves having roughly the same capability with respect to opening/closing the same fluid passage, because it is necessary to separately manufacture the solenoid valve driving circuits and solenoid valves corresponding to differences of the various power source voltages, manufacturing costs tend to rise. 
     Still further, because the electrical power consumption of a solenoid valve driving circuit and a solenoid valve corresponding to the case of a relatively high power source voltage (e.g., 24V) is larger than the electrical power consumption of a solenoid valve driving circuit and a solenoid valve corresponding to the case of a relatively low power source voltage (e.g., 12V), on the side of a user equipped with a DC power source having a relatively high power source voltage, electrical power savings of the solenoid valve driving circuit and the solenoid valve cannot be achieved. 
     SUMMARY OF THE INVENTION 
     The present invention has the object of providing a solenoid valve driving circuit and a solenoid valve, which are capable of realizing, in one sweep, a reduction in electrical power consumption, a rapidly responsive drive control for the solenoid valve, and a reduction in costs. 
     In accordance with the present invention, a solenoid valve driving circuit is provided, in which, after a first voltage is impressed on the solenoid coil of a solenoid valve for driving the solenoid valve, a second voltage is impressed on the solenoid coil and a driven state of the solenoid valve is maintained, 
     the solenoid valve driving circuit being electrically connected, respectively, to a direct current power source and to the solenoid coil, and further including a switch controller, a switch, and a current detector, wherein the current detector detects a current flowing through the solenoid coil, and outputs a detection result, as a current detection value, to the switch controller, 
     wherein the switch controller generates a first pulse signal based on a comparison between a predetermined activation current value and the current detection value, and a second pulse signal based on a comparison between a predetermined holding current value and the current detection value, and supplies the first pulse signal and the second pulse signal to the switch, and 
     wherein the switch applies a power source voltage of the direct current power source as the first voltage to the solenoid coil during a time period when the first pulse signal is supplied thereto, and applies the power source voltage as the second voltage to the solenoid coil during a time period when the second pulse signal is supplied thereto. 
     Herein, within the time period that the solenoid valve is driven, the necessary excitation force (activation force) for driving a movable core (plunger) that makes up the solenoid valve and for driving a valve plug installed onto the end of the plunger, and the necessary excitation force (holding force) needed to maintain (hold) the plunger and the valve plug at a predetermined position during a time period in which the driven state of the solenoid valve is maintained, are values resulting from multiplying the number of windings (turns) of the solenoid coil and the current that flows through the solenoid coil (respective excitation forces=number of windings×current). Therefore, assuming that the activation force needed to drive the solenoid valve, the minimum necessary holding force for maintaining the driven state, and the number of windings, respectively, are known ahead of time, an optimal current (activation current value) corresponding to the activation force, as well as an optimal current value (holding current) corresponding to the holding force, can easily be calculated. 
     Further, at the time of supplying the first pulse signal or the second pulse signal to the switch from the switch controller, the power source voltage is applied to the solenoid coil as a first voltage or a second voltage, whereby the supply of electrical power to the solenoid coil is carried out from the DC power source, and thus, the current flowing through the solenoid coil increases. On the other hand, at times when supply of the first pulse signal or the second pulse signal to the switch from the switch controller is halted, the supply of electrical power is stopped, and thus, the current flowing through the solenoid coil is reduced. Accordingly, by timewise controlling the supply of the first pulse signal and the second pulse signal with respect to the switch, the current flowing through the solenoid coil can be maintained at desired current values (i.e., an activation current value optimal for the activation force, and a holding current value optimal for the holding force). 
     In the present invention, the current detector detects the current flowing through the solenoid coil, and the current detection value is fed back to the switch controller. In the switch controller, the first pulse signal is generated based on a comparison between the activation current value, as an optimal current corresponding to the activation force, and the fed back current detection value, whereas the second pulse signal is generated based on a comparison between the holding current value, as an optimal current corresponding to the holding force, and the fed back current detection value. The switch applies the first voltage to the solenoid coil only at times corresponding to a pulse width of the first pulse signal, or applies the second voltage to the solenoid coil only at times corresponding to a pulse width of the second pulse signal. 
     That is, during the time period when the solenoid valve is driven, the switch controller generates the first pulse signal so that the current detection value becomes the activation current value corresponding to the activation force, and supplies the first pulse signal to the switch, whereby the switch, based on the pulse width of the first pulse signal, controls the application time of the first voltage to the solenoid coil. Owing thereto, the current flowing through the solenoid coil is maintained at the activation current value corresponding to the activation force, and the activation force induced by such a current is impressed to energize the plunger and the valve plug. 
     More specifically, on the side of the user of the solenoid valve, in the case that a DC power source has been prepared beforehand having a relatively high power source voltage (e.g., 24V), and a solenoid valve that uses a relatively low power source voltage (e.g., 12V) is applied with respect to such a DC power source, the activation current value is set in the switch controller at or below a rated value (rated current) of the current flowing through the solenoid coil. Then, if the pulse width of the first pulse signal is adjusted such that the current detection value becomes the thus set activation current value, the current flowing through the solenoid coil during the time period that the solenoid valve is driven is maintained at the activation current value, and thus, even for a user for whom a DC power source having a relatively high power source voltage has been prepared, a power savings can be achieved for the solenoid valve driving circuit and the solenoid valve. In this case, since the relatively high power source voltage is applied as the first voltage to the solenoid coil, it is possible for the solenoid valve to be driven in a shorter time. 
     As described above, by adjusting the pulse width of the first pulse signal in the switch controller, the current that flows through the solenoid coil can be maintained at the activation current value, which is at or below the rated current. Therefore, on the side of the manufacturer, without concern to any difference in the power source voltage supplied to the solenoid coil from the DC power source provided on the user&#39;s side, the solenoid valve driving circuit and the solenoid can be made commonly usable in accordance with a relatively low power source voltage, wherein by providing such a commonly usable solenoid valve driving circuit and solenoid valve to the user, costs can be reduced. 
     Accordingly, with the present invention, by generating the first pulse signal based on a comparison between the current detection value that is fed back to the switch controller from the current detector and the activation current value during a time period in which the solenoid valve is driven, power savings of the solenoid valve driving circuit and the solenoid valve, common usage and cost reduction, and a rapidly-responsive drive control for the solenoid valve, are all capable of being realized. 
     On the other hand, during a time period in which the driven state of the solenoid valve is maintained, the switch controller generates a second pulse signal so that the current detection value becomes the holding current value corresponding to the holding force, whereupon the second pulse signal is supplied to the switch, and the switch thereby controls, based on the pulse width of the second pulse signal, the application time at which the second voltage is applied to the solenoid coil. Owing thereto, the current flowing through the solenoid coil is maintained at the holding current value corresponding to the holding force, and the holding force induced by the current is impressed to energize the plunger and the valve plug. 
     Accordingly, with the present invention, by generating the second pulse signal based on a comparison between the current detection value that is fed back to the switch controller from the current detector during a time period in which the driven state of the solenoid valve is maintained and the holding current value, the driven state of the solenoid valve can be maintained with smaller power consumption, and further, the solenoid valve can be stopped in a short time. 
     Further, by feeding back the current detection value to the switch controller, even if the current tends to vary over time due to changes in electrical resistance inside the solenoid coil or due to changes in the power source voltage as a result of temperature changes in the solenoid coil, the second pulse signal is generated responsive to such changes, whereby a solenoid valve driving circuit and a solenoid valve, which are capable of responding to changes in the use environment, such as changes in electrical resistance and power source voltage or the like, can be realized. 
     In this manner, with the present invention, a reduction in electrical power consumption of the solenoid valve driving circuit and the solenoid valve, rapidly responsive drive control for the solenoid valve, and a reduction in costs for the solenoid valve driving circuit and the solenoid valve, can all be realized together in one sweep. 
     Herein, the switch controller preferably includes: 
     a single pulse generating circuit for generating a single pulse; 
     a short pulse generating circuit, which, during a time period in which the solenoid valve is driven, generates a first short pulse having a pulse width shorter than a pulse width of the single pulse based on a comparison between the activation current value and the current detection value, whilst, during a time period in which a driven state of the solenoid valve is maintained, generates a second short pulse having a pulse width shorter than the pulse width of the first short pulse based on a comparison between the holding current value and the current detection value; and 
     a pulse supplying unit, which, during the time period in which the solenoid valve is driven, supplies the first short pulse to the switch as the first pulse signal after the single pulse has been supplied to the switch as the first pulse signal, whilst, during the time period in which the driven state of the solenoid valve is maintained, supplies the second short pulse to the switch as the second pulse signal. 
     In this case, in the time period during which the solenoid valve is driven, after the power source voltage has been impressed as the first voltage on the solenoid coil only during a time corresponding to the pulse width of the single pulse, the switch then impresses the first voltage on the solenoid coil only during a time corresponding to the pulse width of the first short pulse. As a result, in the time period during which the solenoid valve is driven, after the current flowing through the solenoid coil has risen up to the activation current value within a time corresponding to the pulse width of the single pulse, the activation current value is maintained by a switching operation of the switch based on the first short pulse. Owing thereto, the solenoid valve driving circuit and the solenoid valve can be made commonly usable, and costs can be reduced easily. In particular, in the case that a DC power source having a relatively high power source voltage is electrically connected to the solenoid coil through the solenoid valve driving circuit and the solenoid valve is driven thereby, the solenoid valve is capable of being driven in a short time. Further, by maintaining the current flowing through the solenoid coil at the activation current value, unintended or mistaken operations of the solenoid valve driving circuit and the solenoid valve caused by the input of excessive voltage (surge energy) can be reliably prevented. 
     On the other hand, during a time period at which the driven state of the solenoid valve is maintained, by supplying the second short pulse as the second pulse signal to the switch, the driven state of the solenoid valve can be maintained with lower power consumption, and further, the solenoid valve can be stopped in a short time. 
     Herein, in place of the aforementioned structure, the switch controller may preferably include: 
     a single pulse generating circuit for generating a single pulse; 
     a repeating pulse generating circuit, which, during a time period in which the solenoid valve is driven, generates a first repeating pulse having a pulse width shorter than a pulse width of the single pulse based on a comparison between the activation current value and the current detection value, whilst, during a time period in which a driven state of the solenoid valve is maintained, generates a second repeating pulse having a pulse width shorter than the pulse width of the first repeating pulse based on a comparison between the holding current value and the current detection value; and 
     a pulse supplying unit, which, during the time period in which the solenoid valve is driven, supplies the first repeating pulse to the switch as the first pulse signal after the single pulse has been supplied to the switch as the first pulse signal, whilst, during the time period in which the driven state of the solenoid valve is maintained, supplies the second repeating pulse to the switch as the second pulse signal. 
     In this case, in the time period during which the solenoid valve is driven, after the power source voltage has been impressed as the first voltage on the solenoid coil only during a time corresponding to the pulse width of the single pulse, the switch then impresses the first voltage on the solenoid coil only during a time corresponding to the pulse width of the first repeating pulse. As a result, in the time period during which the solenoid valve is driven, after the current flowing through the solenoid coil has risen up to the activation current value within a time corresponding to the pulse width of the single pulse, the activation current value is maintained by a switching operation of the switch based on the first repeating pulse. In this case as well, the solenoid valve driving circuit and the solenoid valve can be made commonly usable, and costs can be reduced easily, and moreover, in the case that a DC power source having a relatively high power source voltage is electrically connected to the solenoid coil through the solenoid valve driving circuit and the solenoid valve is driven thereby, the solenoid valve is capable of being driven in a short time. Further, by maintaining the current flowing through the solenoid coil at the activation current value, unintended or mistaken operations of the solenoid valve driving circuit and the solenoid valve caused by the input of excessive voltage (surge energy) can be reliably prevented. 
     On the other hand, during a time period at which the driven state of the solenoid valve is maintained, by supplying the second repeating pulse as the second pulse signal to the switch, the driven state of the solenoid valve can be maintained with lower power consumption, and further, the solenoid valve can be stopped in a short time. 
     Accordingly, by providing each of the above-described structures for the switch controller, common usage and cost reduction of the solenoid valve driving circuit and the solenoid valve, driving of the solenoid valve in a short time, power savings of the solenoid valve driving circuit and the solenoid valve, and the ability to stop the solenoid valve in a short time, can easily be realized. 
     With the above-described invention, during a time period in which the solenoid valve is driven, supply of the first pulse signal is timewise controlled based on a comparison between the activation current value and the current detection value, whilst, during a time period in which the solenoid valve is maintained in the driven state, supply of the second pulse signal is timewise controlled based on a comparison between the holding current value and the current detection value. 
     With such a timewise control based on the current detection value, the control can be carried out only during the time period in which the solenoid valve is driven, or alternatively, only during the time period in which the solenoid valve is maintained in the driven state. 
     More specifically, in order to carry out a timewise control based on the current detection value only during the time period in which the solenoid valve is driven, the structure of the solenoid valve driving circuit is as follows. 
     Namely, a solenoid valve driving circuit is provided in which, after a first voltage is impressed on a solenoid coil of a solenoid valve for driving the solenoid valve, a second voltage is impressed on the solenoid coil and a driven state of the solenoid valve is maintained, 
     the solenoid valve driving circuit being electrically connected, respectively, to a direct current power source and to the solenoid coil, and further comprising a switch controller, a switch, and a current detector, 
     wherein the current detector detects a current flowing through the solenoid coil, and outputs a detection result, as a current detection value, to the switch controller, 
     wherein the switch controller generates a first pulse signal based on a comparison between a predetermined activation current value and the current detection value, and a predetermined second pulse signal, and supplies the first pulse signal and the second pulse signal to the switch, and 
     wherein the switch applies a power source voltage of the direct current power source as the first voltage to the solenoid coil during a time period when the first pulse signal is supplied thereto, and applies the power source voltage as the second voltage to the solenoid coil during a time period when the second pulse signal is supplied thereto. 
     In this case, preferably, the switch controller includes: 
     a single pulse generating circuit for generating a single pulse; 
     a short pulse generating circuit, which, during a time period in which the solenoid valve is driven, generates a first short pulse having a pulse width shorter than a pulse width of the single pulse based on a comparison between the activation current value and the current detection value, whilst, during a time period in which a driven state of the solenoid valve is maintained, generates a predetermined second short pulse having a pulse width shorter than the pulse width of the first short pulse; and 
     a pulse supplying unit, which, during the time period in which the solenoid valve is driven, supplies the first short pulse to the switch as the first pulse signal after the single pulse has been supplied to the switch as the first pulse signal, whilst, during the time period in which the driven state of the solenoid valve is maintained, supplies the second short pulse to the switch as the second pulse signal. 
     Further, in place of the aforementioned structure, the switch controller may preferably include: 
     a single pulse generating circuit for generating a single pulse; 
     a repeating pulse generating circuit, which, during a time period in which the solenoid valve is driven, generates a first repeating pulse having a pulse width shorter than a pulse width of the single pulse based on a comparison between the activation current value and the current detection value, whilst, during a time period in which a driven state of the solenoid valve is maintained, generates a predetermined second repeating pulse having a pulse width shorter than the pulse width of the first repeating pulse; and 
     a pulse supplying unit, which, during the time period in which the solenoid valve is driven, supplies the first repeating pulse to the switch as the first pulse signal after the single pulse has been supplied to the switch as the first pulse signal, whilst, during the time period in which the driven state of the solenoid valve is maintained, supplies the second repeating pulse to the switch as the second pulse signal. 
     In this manner, in the case that a timewise control is carried out based on the current detection value only during a time period in which the solenoid valve is driven, the aforementioned advantageous effects can easily be obtained with respect to the timewise control. 
     On the other hand, in order to carry out a timewise control based on the current detection value only during the time period in which the solenoid valve is maintained in the driven state, the structure of the solenoid valve driving circuit is as follows. 
     Namely, a solenoid valve driving circuit is provided in which, after a first voltage is impressed on a solenoid coil of a solenoid valve for driving the solenoid valve, a second voltage is impressed on the solenoid coil and a driven state of the solenoid valve is maintained, 
     the solenoid valve driving circuit being electrically connected, respectively, to a direct current power source and to the solenoid coil, and further including a switch controller, a switch, and a current detector, 
     wherein the current detector detects a current flowing through the solenoid coil, and outputs a detection result, as a current detection value, to the switch controller, 
     wherein the switch controller generates a predetermined first pulse signal, and a second pulse signal based on a comparison between a predetermined holding current value and the current detection value, and supplies the first pulse signal and the second pulse signal to the switch, and 
     wherein the switch applies a power source voltage of the direct current power source as the first voltage to the solenoid coil during a time period when the first pulse signal is supplied thereto, and applies the power source voltage as the second voltage to the solenoid coil during a time period when the second pulse signal is supplied thereto. 
     In this case, preferably, the switch controller includes: 
     a single pulse generating circuit for generating a single pulse; 
     a short pulse generating circuit, which generates a short pulse having a pulse width shorter than a pulse width of the single pulse based on a comparison between the holding current value and the current detection value; and 
     a pulse supplying unit, which, during the time period in which the solenoid valve is driven, supplies the single pulse to the switch as the first pulse signal, whilst, during the time period in which the driven state of the solenoid valve is maintained, supplies the short pulse to the switch as the second pulse signal. 
     Further, in place of the aforementioned structure, the switch controller may preferably include: 
     a single pulse generating circuit for generating a single pulse; 
     a repeating pulse generating circuit, which generates a repeating pulse having a pulse width shorter than a pulse width of the single pulse based on a comparison between the holding current value and the current detection value; and 
     a pulse supplying unit, which, during the time period in which the solenoid valve is driven, supplies the single pulse to the switch as the first pulse signal, whilst, during the time period in which the driven state of the solenoid valve is maintained, supplies the repeating pulse to the switch as the second pulse signal. 
     In this manner, in the case that a timewise control is carried out based on the current detection value only during a time period in which the driven state of the solenoid valve is maintained, the aforementioned advantageous effects can easily be obtained with respect to the timewise control. 
     Further, in each of the foregoing inventions, preferably, the switch controller adjusts the pulse width of the second pulse signal based on a vibration detection value from a vibration detector, which detects vibration of the solenoid valve. 
     When the holding force is reduced for the purpose of saving power, it may be envisaged that vibrations of the solenoid valve could be caused which might lead to stoppage of the solenoid valve. However, by providing the switch controller with the above-noted structure, even if the current flowing through the solenoid coil varies over time due to vibrations, by adjusting the pulse width responsive to such variations, a solenoid valve driving circuit and a solenoid valve, which are capable of responding to vibration-induced changes, can be realized. 
     Specifically, in the case that there are concerns over the solenoid valve coming into a stopped condition due to vibrations inside the solenoid valve caused by vibrations or shocks and the like, which are imparted to the solenoid valve from the exterior during a time period in which the driven state of the solenoid valve is maintained, by lengthening the pulse width and increasing the current (the holding current value) that flows through the solenoid coil, the holding force on the plunger and the valve plug in the solenoid valve is made to increase, whereby the solenoid valve coming into a stopped state can reliably be prevented. 
     In this manner, with the present invention, since the pulse width can be set longer to increase the current (holding current value) only in cases where a high holding force is needed, power savings of the solenoid valve driving circuit and the solenoid valve can be carried out with good efficiency. 
     Moreover, preferably, the solenoid valve driving circuit further includes: 
     an energization time calculator for calculating an energization time of the solenoid coil inside of a one-time operating period of the solenoid valve based on the current detection value; 
     an energization time memory for storing the energization time; and 
     an energization time determining unit for calculating a total energization time of the solenoid coil from each of respective energization times stored in the energization time memory, and determining whether or not the total energization time is longer than a predetermined first energization time, 
     wherein the energization time determining unit outputs a pulse width change signal to the switch controller instructing that the pulse width of the first pulse signal be changed, when it is determined that the total energization time is longer than the first energization time, and 
     wherein the switch controller lengthens the pulse width of the first pulse signal based on the pulse width change signal. 
     Owing thereto, even in cases where the driving performance of the solenoid valve is decreased through use of the solenoid valve over a prolonged period, by setting the pulse width of the first pulse signal to be longer at times when the total energization time of the solenoid valve becomes longer than the first energization time, since the current (activation current value) flowing through the solenoid coil becomes larger, and the activation force can be increased, driving control of the solenoid valve can be carried out efficiently. 
     In this case, preferably, the energization time determining unit may externally output a usage limit notification signal notifying that the solenoid valve has reached a usage limit, when it is determined that the total energization time is longer than a second energization time, which is set to be longer than the first energization time. 
     Owing thereto, it becomes possible to quickly exchange the solenoid valve whenever the usage limit thereof is reached, so that reliability with respect to the usage limit (life) of the solenoid valve is improved. 
     Further, in place of the above-noted structure, the solenoid valve driving circuit preferably further includes: 
     a solenoid valve operation detector for detecting that the solenoid valve is under operation based on the current detection value; 
     a detection result memory for storing a detection result of the solenoid valve operation detector; and 
     an accumulated number of operation times determining unit for calculating an accumulated number of operation times of the solenoid valve from each of respective detection results stored in the detection result memory, and determining whether or not the accumulated number of operation times exceeds a predetermined first number of operation times, 
     wherein the accumulated number of operation times determining unit outputs a pulse width change signal to the switch controller instructing that the pulse width of the first pulse signal be changed, when it is determined that the accumulated number of operation times exceeds the first number of operation times, and 
     wherein the switch controller lengthens the pulse width of the first pulse signal based on the pulse width change signal. 
     If the pulse width of the first pulse signal is made longer at times when the accumulated number of operation times of the solenoid valve exceeds the first number of operation times, since the current (activation current value) flowing through the solenoid coil becomes larger, and the activation force can be increased, driving control of the solenoid valve can be carried out efficiently. 
     In this case, it is preferable for the accumulated number of operation times determining unit to externally output a usage limit notification signal notifying that the solenoid valve has reached a usage limit, when it is determined that the accumulated number of operation times exceeds a second number of operation times, which is set to be greater than the first number of operation times. 
     Owing thereto, it becomes possible to quickly exchange the solenoid valve whenever the usage limit thereof is reached, so that reliability with respect to the usage limit (life) of the solenoid valve is improved. 
     Further, the solenoid valve driving circuit further includes: 
     a current detection value monitoring unit for monitoring a decrease in the current detection value during a time period in which the solenoid valve is driven, 
     wherein the current detection value monitoring unit externally outputs a time delay notification signal for notifying that a time delay was generated in a time period from a drive start time of the solenoid valve to a time at which the current detection value decreases, when it is determined that the time period is longer than a predetermined set time period. 
     Owing thereto, it becomes possible to quickly exchange a solenoid valve for which the time required for the current detection value to decrease has become longer and thus the driving performance thereof has been degraded. That is, by providing the solenoid valve driving circuit having the aforementioned structure, detection of the usage limit (life) of the solenoid valve can be carried out efficiently, based on the responsiveness of the solenoid valve during the time period in which the solenoid valve is driven. 
     Further, preferably, the solenoid valve driving circuit further includes a light-emitting diode capable of emitting light when the current flows through the solenoid coil, wherein a series circuit made up of the light-emitting diode and the switch controller, and the solenoid coil, are electrically connected in parallel with respect to the direct current power source. 
     Although, conventionally, a series circuit made up of a light-emitting diode and a current limiting resistor for causing light to be emitted from the light-emitting diode have been connected electrically in parallel with respect to the DC power source and the solenoid coil. In the present invention, in place of the current limiting resistor, the series circuit made up of the switch controller and the light-emitting diode is connected electrically in parallel with respect to the DC power source and the solenoid coil, whereby, since the electrical energy consumed originally by the current limiting resistor is used for operating the switch controller, a solenoid valve driving circuit that exhibits high energy use efficiency can be realized. 
     Further, preferably, the solenoid valve driving circuit further includes a resistor, which is capable of adjusting an inrush current that flows to the switch controller at a drive start time of the solenoid valve, so as to remain below a maximum value of current flowing through the solenoid coil, wherein a series circuit made up of the resistor and the switch controller, and the solenoid coil, are electrically connected in parallel with respect to the direct current power source. 
     Owing thereto, it becomes possible for the switch controller to be reliably protected from an inrush current, and the solenoid valve can easily be applied as well with respect to a DC power source having a relatively high power source voltage. Further, by carrying out such a countermeasure with respect to the inrush current, unintended or mistaken operations of the solenoid valve driving circuit and the solenoid valve caused by a surge voltage, which is generated momentarily inside the solenoid valve driving circuit at starting and stopping times of the solenoid valve, can reliably be prevented. 
     Furthermore, the same respective advantageous effects concerning the aforementioned solenoid valve driving circuits can easily be obtained in a solenoid valve as well, to which the above-mentioned various solenoid valve driving circuits have been applied. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following descriptions when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram for a solenoid valve according to a first embodiment; 
         FIG. 2A  is a time chart of a relatively low power source voltage in the solenoid valve of  FIG. 1 ; 
         FIG. 2B  is a time chart of a single pulse signal supplied to a pulse supplying unit from a single pulse generating circuit; 
         FIG. 2C  is a time chart of a pulse signal supplied to the pulse supplying unit from a PWM circuit; 
         FIG. 2D  is a time chart of a control signal supplied to a gate terminal of a MOSFET from the pulse supplying unit; 
         FIG. 2E  is a time chart of a voltage impressed on a solenoid coil; 
         FIG. 2F  is a time chart of a current that flows through the solenoid coil; 
         FIG. 3A  is a time chart of a relatively high power source voltage in the solenoid valve of  FIG. 1 ; 
         FIG. 3B  is a time chart of a single pulse signal supplied to a pulse supplying unit from a single pulse generating circuit; 
         FIG. 3C  is a time chart of a pulse signal supplied to the pulse supplying unit from a PWM circuit; 
         FIG. 3D  is a time chart of a control signal supplied to a gate terminal of a MOSFET from the pulse supplying unit; 
         FIG. 3E  is a time chart of a voltage impressed on a solenoid coil; 
         FIG. 3F  is a time chart of a current that flows through the solenoid coil; 
         FIG. 4  is a circuit diagram for a solenoid valve according to a second embodiment; 
         FIG. 5  is a circuit diagram for a solenoid valve according to a third embodiment; and 
         FIG. 6  is a circuit diagram for a solenoid valve according to a fourth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the circuit diagram of  FIG. 1 , the solenoid valve  10 A according to a first embodiment is equipped with a solenoid valve driving circuit  14  connected electrically with respect to a DC power source  16 , and a solenoid coil  12  connected electrically with respect to the solenoid valve driving circuit  14 . In this case, the positive side of the DC power source  16  is connected electrically to the solenoid coil  12  through a switch  18  and a diode  32  inside of the solenoid valve driving circuit  14 , whereas the negative side of the DC power source  16  is connected to ground (earth). 
     The solenoid valve driving circuit  14  includes a surge absorber  30 , diodes  32 ,  34 ,  36 ,  39 , a MOSFET (metal oxide semiconductor field effect transistor)  38  serving as a switch, a switch controller  40 , resistors  42 ,  50 ,  52 ,  66 ,  70 ,  76 , condensers  44 ,  48 ,  56 , a light-emitting diode (LED)  54 , and a current detection circuit (current detector)  72 . 
     In this case, the solenoid valve driving circuit  14  may be arranged internally in the solenoid valve  10 A together with the solenoid coil  12 , or alternatively, may be arranged externally of a non-illustrated solenoid valve main body, which accommodates the solenoid coil  12  therein. Accordingly, the solenoid valve  10 A may be adopted as a structure in which the solenoid valve driving circuit  14  is connected electrically through a non-illustrated cable to the solenoid coil  12  inside of a commercially available solenoid valve, a structure in which the solenoid valve driving circuit  14  is unitized and attached externally to such a commercially available solenoid valve, or a structure in which the unitized solenoid valve driving circuit  14  is attached externally to a commercially available solenoid valve manifold. 
     Further, the switch controller  40  includes a constant voltage circuit  58 , a low voltage detection circuit  59 , a PWM circuit (short pulse generating circuit, repeating pulse generating circuit)  60 , an oscillator  61 , a single pulse generating circuit  62 , and a pulse supplying unit  64 . The switch controller  40 , the MOSFET  38 , the diode  39 , and the current detection circuit  72 , as mentioned above, can be configured, for example, as a customized IC (integrated circuit). 
     The surge absorber  30  is connected electrically in parallel with respect to a series circuit made up of the DC power source  16  and the switch  18 . Further, a series circuit made up of the diode  34 , the LED  54 , the resistor  42 , the switch controller  40  and the resistors  50 ,  52 ,  76 , is connected electrically in parallel with respect to the surge absorber  30 . Further, a series circuit made up of the diode  32 , the solenoid coil  12 , the MOSFET  38  and the resistor  70  is connected electrically in parallel with respect to another series circuit made up of the diode  34 , the LED  54 , the resistor  42 , the switch controller  40  and the resistors  50 ,  52 ,  76 . Still further, the condenser  56  is connected electrically in parallel with the LED  54 , and the condenser  44  is connected electrically in parallel with respect to a series circuit made up of the switch controller  40  and the resistors  50 ,  52 ,  76 . Further, the condenser  48  is connected electrically in parallel with respect to a series circuit made up of the resistors  50 ,  52 ,  76 , the diode  36  is connected electrically in parallel with the solenoid coil  12 , and the diode  39  is connected electrically between the drain terminal D and the source terminal S of the MOSFET  38 . 
     The aforementioned surge absorber  30  acts as a circuit protective voltage-dependent resistor, for causing the surge current that flows in the solenoid valve driving circuit  14  due to the surge voltage to be rapidly channeled to ground, at activation or stoppage times (times T 0  and T 1  shown in  FIGS. 2F and 3F ) of the solenoid valve  10 A when the switch  18  is opened and closed, as a result of the resistance value of the surge absorber  30  momentarily decreasing responsive to the surge voltage, which is momentarily generated inside the solenoid valve driving circuit  14 . The surge voltage is defined as a voltage which is larger than the power source voltage V 0 , V 0 ′ of the DC power source  16  (V 0 &lt;V 0 ′). 
     The diode  32  is a circuit protective diode for the purpose of preventing current from flowing in the direction of the positive electrode of the DC power source  16  through the diode  32  from the solenoid coil  12 , and the diode  34  is a circuit protective diode for the purpose of preventing current from flowing in the direction of the positive electrode of the DC power source  16  through the diode  34  from the LED  54 . Further, the diode  36  is a diode that refluxes (channels back) a current caused by a back electromotive force generated in the solenoid coil  12  at the stop time (time T 1 ) of the solenoid valve  10 A, in a closed circuit of the solenoid coil  12  and the diode  36 , for the purpose of rapidly attenuating the current. Concerning the diode  32 , this diode may be replaced by a non-polarized diode bridge (not shown) if desired. 
     The MOSFET  38  is a semiconductor switching element, which is placed in an ON state between the drain terminal D and the source terminal S at a time when the control signal Sc (first pulse signal S 1  or second pulse signal S 2 ) is supplied to the gate terminal G from the switch controller  40 , thereby electrically connecting the solenoid coil  12  on the drain terminal side D and the resistor  70  on the source terminal side S. On the other hand, the MOSFET  38  is placed in an OFF state between the drain terminal D and the source terminal S at a time when supply of the control signal Sc is halted with respect to the gate terminal G, whereby the electrical connection between the solenoid coil  12  and the resistor  70  is interrupted. 
     In the circuit diagram of  FIG. 1 , as an example of the semiconductor switching element, a case in which an N-channel depression mode MOSFET  38  is adopted is shown. However, the solenoid valve  10 A according to the first embodiment is not limited to this arrangement, and any type of semiconductor switching element may be used, which is capable of rapidly switching the electrical connection between the solenoid coil  12  and the resistor  70 , corresponding to whether the control signal Sc is being supplied or not. Specifically, in place of the aforementioned MOSFET  38 , for example, an N-channel enhancement mode, a P-channel depression mode, or a P-channel enhancement mode MOSFET, a bipolar transistor, or a field effect transistor, may also be adopted as a matter of course. 
     Further, the diode  39  is a protective diode for the MOSFET  38 , which serves to pass the current that flows in the direction of the solenoid coil  12  from the resistor  70 . 
     Furthermore, the aforementioned first pulse signal S 1  is defined as a control signal Sc, which is supplied to the gate terminal G of the MOSFET  38  during the time period in which the solenoid valve  10 A is driven (i.e., the time periods T 3 , T 3 ′ from time T 0  times T 2 , T 2 ′ in  FIGS. 2F and 3F ). On the other hand, the second pulse signal S 2  is defined as a control signal Sc, which is supplied to the gate terminal G of the MOSFET  38  during the time period in which the driven state of the solenoid valve  10 A is maintained (i.e., the time periods T 4 , T 4 ′ from times T 2 , T 2 ′ to time T 1  in  FIGS. 2F and 3F ). 
     The LED  54 , during a time period when the switch  18  is in an ON state (i.e., the time period from time T 0  to T 1  shown in  FIGS. 2F and 3F ), due to the LED  54  becoming illuminated in response to a current flowing in the direction from the diode  34  to the resistor  42 , provides a notification to the exterior that the solenoid valve  10 A is in operation. 
     The condenser  56  is a bypass condenser for passing high frequency components included within the current that flows in the direction from the diode  34  to the resistor  42 , whereas the condenser  48  is a bypass condenser for passing high frequency components included within the current that flows in the direction from the constant voltage circuit  58  to the resistors  50 ,  52 ,  76 . Further, the condenser  44  is a condenser capable of adjusting the momentary interruption time of the solenoid valve driving circuit  14  including the switch controller  40  by causing a change in the capacitance thereof, as well as serving as a bypass condenser for draining to ground the high frequency components included within the current that flows in the direction of the constant voltage circuit  58  and the low voltage detection circuit  59  from the resistor  42 . 
     The resistor  42  operates as an inrush current limiting resistor, for the purpose of suppressing an inrush current, which flows in the switch controller  40  when the switch  18  is in an ON state, so as to remain below a rated value (rated current) of the current I flowing through the solenoid coil  12 . Accordingly, the resistor  42 , by carrying out a countermeasure against the inrush current, functions as a resistor for preventing mistaken operations of the solenoid valve driving circuit  14  and the solenoid valve  10 A, caused by the surge voltage generated in the solenoid valve driving circuit  14  at start and stop times of the solenoid valve  10 A. 
     When the current I flows to the resistor  70  from the solenoid coil  12  through the MOSFET  38 , a voltage Vd corresponding to the current I is generated at the resistor  70 . 
     Herein, within a time period (refer to  FIGS. 2F and 3F ) from the time T 0  when the switch  18  is placed in an ON state until the time T 1  when the switch assumes an OFF state, a DC voltage V is impressed on the constant voltage circuit  58  from the DC power source  16  through the switch  18 , the diode  34 , the LED  54  and the resistor  42 . The constant voltage circuit  58  converts the DC voltage V to a voltage V′ having a predetermined level, and then supplies the voltage V′ to the resistors  50 ,  52 ,  76 . The DC voltage V represents a DC voltage, which has been reduced from the power source voltage V 0 , V 0 ′, by respective voltage drops of the diode  34 , the LED  54 , and the resistor  42 . 
     The oscillator  61  outputs a pulse signal Sp having a predetermined repeating frequency (i.e., a repeating frequency corresponding to the period of the time period T 5  of  FIGS. 2C and 3C ) to the PWM circuit  60 , the single pulse generating circuit  62  and the current detection circuit  72 , during a time when the DC voltage V is supplied to the switch controller  40 , and more specifically, during a time period in which the aforementioned switch  18  is in an ON state. 
     The low voltage detection circuit  59  monitors whether or not the DC voltage V impressed on the constant voltage circuit  58  is at or below a predetermined voltage level. In the case that the DC voltage has been detected to be at or below the voltage level, a low voltage detection signal Sv indicating that the DC voltage V, which is a drive voltage for operating the switch controller  40 , is a relatively low voltage, is output to the single pulse generating circuit  62  and the pulse supplying unit  64 . 
     The single pulse generating circuit  62  generates a single pulse signal Ss having a predetermined pulse width based on the pulse signal Sp from the oscillator  61  and supplies the single pulse signal Ss to the pulse supplying unit  64 . In this case, the single pulse generating circuit  62  essentially is preset to count the number of pulses of the pulse signal Sp input from the oscillator  61 , and to generate a single pulse signal Ss (see  FIG. 2B ) having a pulse width (i.e., the pulse width of the time period T 3  shown in  FIG. 2F ) corresponding to a predetermined count number. However, it is also possible for a single pulse signal Ss (see  FIG. 3B ) to be generated, which has a predetermined pulse width (i.e., the pulse width of the time period T 9  shown in  FIG. 3F ) corresponding to the resistance value of the resistor  66 . 
     That is, the single pulse generating circuit  62  is a pulse generating circuit that is capable of adjusting the pulse width of the single pulse signal Ss corresponding to the resistance value of the resistor  66 . Further, the single pulse generating circuit  62  outputs a notification signal St to the PWM circuit  60 , for notifying passage of the time periods T 3 , T 3 ′. 
     The notification signal St is defined as a signal for notifying the PWM circuit  60  that a shift has occurred from the time period during which the solenoid valve  10 A is driven (the time periods T 3 , T 3 ′ shown in  FIGS. 2F and 3F ) to a time period in which the driven state is maintained (the time periods T 4 , T 4 ′ shown in  FIGS. 2F and 3F ), which is output to the PWM circuit  60  from the single pulse generating circuit  62  at times T 2  and T 2 ′. In this case, times T 2 , T 2 ′ are set in the single pulse generating circuit  62  corresponding to an operation of the solenoid valve  10 A (first operation or second operation), which shall be described subsequently. Further, in the case that the low voltage detection signal Sv is input from the low voltage detection circuit  59 , the single pulse generating circuit  62  halts generation of the single pulse signal Ss and output of the notification signal St. 
     The current detection circuit  72  samples the voltage Vd of the resistor  70  at the timing of the pulse signal Sp input from the oscillator  61 , and the sampled voltage Vd is output as a pulse signal Sd to the PWM circuit  60 . As described above, because the voltage Vd represents a voltage that corresponds to the current I flowing through the solenoid coil  12 , the amplitude (voltage Vd) of the pulse signal Sd represents a voltage value (current detection value), which is indicative of the current I flowing through the solenoid coil  12 . 
     The PWM circuit  60  generates a pulse signal Sr (first short pulse, first repeating pulse, second short pulse, or second repeating pulse) having a repeating period (i.e., the time period T 5  shown in  FIGS. 2C and 3C ) corresponding to the repeating frequency of the pulse signal Sp from the oscillator  61 , and a predetermined duty ratio (i.e., the ratios T 6 /T 5 , T 7 /T 5  of the time periods T 6 , T 7  to the time period T 5 ) corresponding to the voltage value, and supplies the pulse signal Sr to the pulse supplying unit  64 , based on a comparison between a voltage value corresponding to a desired current value (i.e., the first current value (activation current value) I 1  and the second current value (holding current value) I 2  shown in  FIGS. 2F and 3F ) with respect to the current I flowing through the solenoid coil  12  and the amplitude (voltage Vd) of the pulse signal Sd from the current detection circuit  72 . 
     In the solenoid valve  10 A, within the time periods T 3 , T 3 ′ (refer to  FIGS. 2F and 3F ), an excitation force (activation force), which is caused by the current I flowing through the solenoid coil  12 , is exerted on an unillustrated movable core (plunger) constituting the solenoid valve  10 A, as well as on the valve plug that is installed onto an end of the plunger, thereby driving the solenoid valve  10 A. On the other hand, during time periods T 4  and T 4 ′, another excitation force (holding force), which is caused by the current I flowing through the solenoid coil  12 , is exerted on the plunger and the valve plug, so that the plunger and the valve plug are held in a predetermined position, whereby the driven state of the solenoid valve  10 A is maintained. 
     In this case, the excitation force (activation force) required for driving the plunger and the valve plug within the time periods T 3 , T 3 ′ which define time periods during which the solenoid valve  10 A is driven, or the minimum necessary excitation force (holding force) for holding the plunger and the valve plug in a predetermined position within the time periods T 4 , T 4 ′ which define time periods during which the solenoid valve  10 A is maintained in the driven state, are values obtained by multiplying the number of windings (turns) of the solenoid coil  12  and the current I that flows through the solenoid coil  12  (respective excitation forces=number of windings×current I). Therefore, assuming that the activation force needed to drive the solenoid valve  10 A, the minimum necessary holding force for maintaining the driven state, and the number of windings, respectively, are known ahead of time, an optimal current value (first current value I 1  as the activation current value) corresponding to the activation force, as well as an optimal current value (second current value I 2  as the holding current value) corresponding to the holding force, can easily be calculated. 
     Further, during the time periods in which the first pulse signal S 1  and the second pulse signal S 2  are supplied from the switch controller  40  to the gate terminal G of the MOSFET  38 , because the power source voltages V 0 , V 0 ′ are impressed on the solenoid coil  12  as the first or second voltage, and the supply of electrical power to the solenoid coil  12  from the DC power source  16  is carried out through the switch  18  and the diode  32 , the current I flowing through the solenoid coil  12  increases. On the other hand, during time periods in which supply of the first pulse signal S 1  and the second pulse signal S 2  from the switch controller  40  to the gate terminal G of the MOSFET  38  is halted, because the supply of electrical power is halted, the current I flowing through the solenoid coil  12  is reduced. 
     Accordingly, by timewise controlling the supply of the first pulse signal S 1  and the second pulse signal S 2  with respect to the gate terminal G, the current I flowing through the solenoid coil  12  can be maintained at the desired current value (the first current value I 1  and the second current value I 2 ). 
     Consequently, in the solenoid valve driving circuit  14 , the voltage Vd corresponding to the current I flowing through the solenoid coil  12  is output from the resistor  70  to the current detection circuit  72 , and a pulse signal Sd having the amplitude of the voltage Vd indicated by the current detection value is fed back to the PWM circuit  60  of the switch controller  40  from the current detection circuit  72 . 
     In the PWM circuit  60 , based on a comparison between the voltage value corresponding to the current value (first current value I 1 ) optimal for the activation force and the amplitude (voltage Vd) of the fed back pulse signal Sd, a pulse signal Sr (first repeating pulse or first short pulse) is generated having a repeating period of time T 5  and a duty ratio of T 6 /T 5 . On the other hand, based on a comparison between the voltage value corresponding to the current value (second current value I 2 ) optimal for the holding force and the amplitude of the fed back pulse signal Sd, a pulse signal Sr (second repeating pulse or second short pulse) is generated having a repeating period of time T 5  and a duty ratio of T 7 /T 5 . 
     As stated above, the duty ratios T 6 /T 5  and T 7 /T 5  represent duty ratios corresponding to optimal current values (i.e., the first current value I 1  and the second current value I 2 ), and such duty ratios are set based on the resistance values of the resistors  50 ,  52 ,  76 . More specifically, the duty ratio T 6 /T 5  is a duty ratio corresponding to a predetermined voltage, which is generated by dividing the DC voltage V′ supplied from the constant voltage circuit  58  by each of the resistance values of the resistors  52 ,  76 , whereas the duty ratio T 7 /T 5  is a duty ratio corresponding to a predetermined voltage, which is generated by dividing the DC voltage V′ supplied from the constant voltage circuit  58  by each of the resistance values of the resistors  50 ,  52 ,  76 . Accordingly, in the PWM circuit  60 , the duty ratios T 6 /T 5  and T 7 /T 5  of the pulse signal Sr are adjustable by appropriately changing the resistance values of the resistors  50 ,  52 ,  76  corresponding to the sizes of the first current value I 1  and the second current value I 2 . 
     In this case, in the PWM circuit  60 , the second repeating pulse or the second short pulse having the duty ratio of T 7 /T 5  is generated as the pulse signal Sr (see  FIG. 2C ). Alternatively, until the notification signal St is received from the single pulse generating circuit  62 , the first repeating pulse or the first short pulse having the duty ratio of T 6 /T 5  is generated as the pulse signal Sr, whereas, after the notification signal St is received, the second repeating pulse or the second short pulse is generated as the pulse signal Sr (see  FIG. 3C ). 
     The first repeating pulse and the first short pulse are pulses having a pulse width (time period T 6 ) shorter than the pulse width of the single pulse signal Ss (see  FIG. 3C ). That is, the first repeating pulse is a pulse having a pulse width of the time period T 6 , which is generated to repeat at a period of time T 5 , whereas the first short pulse is a pulse having a pulse width of the time period T 6 . 
     Further, the second repeating pulse and the second short pulse are pulses having a pulse width (time period T 7 ) shorter than the pulse widths of the first repeating pulse and the first short pulse (see  FIGS. 2C and 3C ). That is, the second repeating pulse is a pulse having a pulse width of the time period T 7 , which is generated to repeat at a period of time T 5 , whereas the second short pulse is a pulse having a pulse width of the time period T 7 . 
     The pulse supplying unit  64  is constructed to include an OR circuit, for example, and serves to supply, as a control signal Sc, the single pulse signal Ss from the single pulse generating circuit  62 , or alternatively the pulse signal Sr from the PWM circuit  60 , to the gate terminal G of the MOSFET  38 . More specifically, the pulse supplying unit  64 , at the aforementioned time periods T 3 , T 3 ′, supplies the single pulse signal Ss or the pulse signal Sr (the first repeating pulse or the first short pulse) as the first pulse signal S 1  to the gate terminal G, whereas, at time periods T 4 , T 4 ′, supplies the pulse signal Sr made up of the second repeating pulse or the second short pulse signal as the second pulse signal S 2  to the gate terminal G. Further, in the case that the low voltage detection signal Sv is input from the low voltage detection circuit  59 , the pulse supplying unit  64  suspends supply of the first pulse signal S 1  or the second pulse signal S 2  to the gate terminal G. 
     The solenoid valve  10 A according to the first embodiment is constructed basically as described above. Now, with reference to  FIG. 1  through  FIG. 3F , operations of the solenoid valve  10 A shall be explained. 
     (1) An operation of the solenoid valve  10 A in the case that the first pulse signal S 1  having the pulse width of time period T 3  and the second pulse signal S 2  (second repeating pulse) having a duty ratio of T 7 /T 5  are supplied from the switch controller  40  to the gate terminal G of the MOSFET  38  (hereinafter, first operation), and (2) an operation of the solenoid valve  10 A in the case that the single pulse signal Ss having a pulse width of time period T 9  and the pulse signal Sr (first repeating pulse) having a duty ratio of T 6 /T 5  are supplied as a first pulse signal S 1  from the switch controller  40  to the gate terminal G, and thereafter, a pulse signal Sr (second repeating pulse) having a duty ratio of T 7 /T 5  is supplied as a second pulse signal S 2  from the switch controller  40  to the gate terminal G (hereinafter, second operation), shall be described below with reference to the circuit diagram of  FIG. 1  and the time charts of  FIGS. 2A through 3F . 
     Explanations shall be given assuming that, during the first operation, the power source voltage of the DC power source is set at V 0 , whereas during the second operation, the power source voltage of the DC power source is set at V 0 ′. More specifically, the first operation is an operation of the solenoid valve  10 A for a case in which, at the side of the user of the solenoid valve  10 A, a DC power source  16  having a relatively low power source voltage (e.g., V 0 =12V) is prepared. On the other hand, the second operation is an operation of the solenoid valve  10 A for a case in which, at the side of the user of the solenoid valve  10 A, a DC power source  16  having a relatively high power source voltage (e.g., V 0 ′=24V) is prepared. Further, explanations shall be made, assuming that, during the first operation and the second operation, the amplitude of the single pulse Ss supplied to the pulse supplying unit  64  from the single pulse generating circuit  62  and the amplitude of the pulse signal Sr supplied to the pulse supplying unit  64  from the PWM circuit  60  are substantially at the same level. 
     First, explanations concerning the first operation shall be given with reference to the circuit diagram of  FIG. 1  and the time charts of  FIGS. 2A through 2F . 
     At time T 0 , when the switch  18  is closed and the device is placed in an ON state (see  FIG. 2A ), a DC voltage V is applied by the constant voltage circuit  58 , which is reduced from the voltage V 0  of the DC power source  16  by voltage drops across each of the diode  34 , the LED  54  and the resistor  42 . At this time, the LED  54  emits light in response to current flowing in the direction of the resistor  42  from the diode  34 , thereby notifying externally of the solenoid valve  10 A that the solenoid valve  10 A is under operation. 
     The constant voltage circuit  58  converts the DC voltage V to a predetermined DC voltage V′, and supplies the DC voltage V′ to a series circuit made up of the resistors  50 ,  52 ,  76 . Further, the low voltage detection circuit  59  monitors whether or not the DC voltage V is at or below a predetermined voltage level. The oscillator  61  generates a pulse signal Sp having a frequency that is repeated at a period corresponding to the period of the time T 5 , and supplies the pulse signal Sp to the PWM circuit  60 , the single pulse generating circuit  62  and the current detection circuit  72 . 
     Based on the supply of the pulse signal Sp, the single pulse generating circuit  62  generates a single pulse signal Ss having a pulse width of the time period T 3  (see  FIG. 2B ) and outputs the generated single pulse signal Ss to the pulse supplying unit  64 . 
     The current detection circuit  72  carries out sampling, at the timing of the pulse signal Sp, with respect to the voltage Vd that corresponds to the current I in the resistor  70 , and the sampled voltage Vd is output as a pulse signal Sd to the PWM circuit  60 . 
     The PWM circuit  60 , based on a comparison between the voltage corresponding to the second current value I 2  and the amplitude (voltage Vd) of the pulse signal Sd, generates a pulse signal Sr of the second repeating pulse, having a duty ratio of T 7 /T 5  corresponding to the respective resistances of the resistors  50 ,  52 ,  76 , and further having a repeating period of the time period T 5 , and supplies the pulse signal Sr to the pulse supplying unit  64  (see  FIG. 2C ). 
     Within the time period T 3  from time T 0  time T 2 , a single pulse signal Ss from the single pulse generating circuit  62  is input to the pulse supplying unit  64 , and together therewith, the pulse signal Sr is input from the PWM circuit  60 . However, as described previously, because the pulse supplying unit  64  is constructed with an OR circuit therein, and since the respective amplitudes of the single pulse signal Ss and the pulse signal Sr are substantially the same amplitude, the pulse supplying unit  64  supplies the single pulse signal Ss as the first pulse signal S 1  to the gate terminal G of the MOSFET  38  (see  FIG. 2D ). 
     Owing thereto, based on the first pulse signal S 1  supplied to the base terminal G, an ON state is formed between the drain terminal D and the source terminal S, whereby the MOSFET  38  is connected electrically to the solenoid coil  12  and the resistor  70 . Therefore, the power source voltage V 0  is applied to the solenoid coil  12  as the first voltage from the DC power source  16  and through the switch  18  and the diode  32  (see  FIG. 2E ). On the other hand, the current I that flows in the direction of the resistor  70  from the solenoid coil  12  through the MOSFET  38  rapidly increases with the passage of time (see  FIG. 2F ). As a result, the plunger and valve plug are energized quickly by the excitation force (activation force) caused by the current I, and the solenoid valve  10 A shifts from a closed state into an open state. 
     Further, at time T 10 , the current I, which has increased rapidly over time, decreases slightly (see  FIG. 2F ). This is caused by the plunger being attracted to a non-illustrated fixed iron core, in accordance with the activation force. 
     Next, at time T 2 , when the current I flowing through the solenoid coil  12  reaches the predetermined first current I 1 , the single pulse generating circuit  62  stops generating the single pulse signal Ss, and supply thereof to the pulse supplying unit  64  is suspended (see  FIG. 2B ). In addition, a notification signal St is output to the PWM circuit  60  notifying that the time period T 3  has passed (i.e., that the single pulse signal Ss has been terminated). 
     On the other hand, the PWM circuit  60 , also during the time period T 4  from time T 2  to time T 1 , by the same circuit operation noted previously at the time period T 3 , generates the second repeating pulse as the pulse signal Sr, and supplies the same to the pulse supplying unit  64  (see  FIG. 2C ). In this case, because only the pulse signal Sr is input to the pulse supplying unit  64  from the PWM circuit  60 , the pulse supplying unit  64  supplies the pulse signal Sr as the second pulse signal S 2  to the gate terminal G of the MOSFET  38  (see  FIG. 2D ). 
     Owing thereto, based on the second pulse signal S 2  supplied to the gate terminal G, an ON state is formed between the drain terminal D and the source terminal S, whereby the MOSFET  38  is connected electrically to the solenoid coil  12  and the resistor  70 . Therefore, the power source voltage V 0  is applied to the solenoid coil  12  as the second voltage from the DC power source  16  and through the switch  18  and the diode  32  (see  FIG. 2E ). On the other hand, the current I that flows in the direction of the resistor  70  from the solenoid coil  12  through the MOSFET  38 , decreases rapidly, in a short time period from time T 2 , from the first current I 1  to a predetermined second current I 2 , and thereafter, the second current I 2  is maintained during the time period until time T 1  (see  FIG. 2F ). As a result, the plunger and valve plug are held at a predetermined position by the excitation force (holding force) caused by the second current I 2 , whereby the driven state (valve open state) of the solenoid valve  10 A is maintained. 
     In addition, at time T 1 , when the switch  18  is opened and the device is placed in an OFF state (see  FIG. 2A ), since the supply of the DC voltage V to the switch controller  40  is suspended, the low voltage detection circuit  59  outputs a low voltage detection signal Sv to the single pulse generating circuit  62  and to the pulse supplying unit  64 , whereby, based on input of the low voltage detection signal Sv thereto, the pulse supplying unit  64  stops supplying the second pulse signal S 2  to the gate terminal G of the MOSFET  38 . Owing thereto, because the MOSFET  38  is rapidly switched from an ON state to an OFF state between the drain terminal D and the source terminal S thereof, a condition is reached in which application of the voltage V 0  to the solenoid coil  12  from the DC power source  16  is halted. In this case, although a back electromotive force is generated in the solenoid coil  12 , a current caused by the back electromotive force is refluxed (i.e., flows backward) inside of a closed circuit made up of the solenoid coil  12  and the diode  36 , so that the current is quickly attenuated. 
     Next, explanations concerning the second operation shall be given with reference to the circuit diagram of  FIG. 1  and the time charts of  FIGS. 3A through 3F . 
     At time T 0 , when the switch  18  is closed and the device is placed in an ON state (see  FIG. 3A ), a DC voltage V is applied by the constant voltage circuit  58 , which is reduced from the voltage V 0 ′ of the DC power source  16  by voltage drops across each of the diode  34 , the LED  54  and the resistor  42 . At this time, the LED  54  emits light in response to the current flowing in the direction of the resistor  42  from the diode  34 , thereby notifying externally of the solenoid valve  10 A that the solenoid valve  10 A is under operation. 
     The constant voltage circuit  58  converts the DC voltage V to a predetermined DC voltage V′, and supplies the DC voltage V′ to a series circuit made up of the resistors  50 ,  52 ,  76 . Further, the low voltage detection circuit  59  monitors whether or not the DC voltage V is at or below a predetermined voltage level. The oscillator  61  generates a pulse signal Sp having a frequency that is repeated at a period corresponding to the period of the time T 5 , and supplies the pulse signal Sp to the PWM circuit  60 , the single pulse generating circuit  62 , and the current detection circuit  72 . 
     Based on supply of the pulse signal Sp and the resistance value of the resistor  66 , the single pulse generating circuit  62  generates and outputs to the pulse supplying unit  64  a single pulse signal Ss having a pulse width of the time period T 9  (see  FIG. 3B ). 
     The current detection circuit  72  carries out sampling, at the timing of the pulse signal Sp, with respect to the voltage Vd that corresponds to the current I in the resistor  70 , and the sampled voltage Vd is output as a pulse signal Sd to the PWM circuit  60 . 
     Based on a comparison between a voltage value corresponding to the first current value I 1  and the amplitude (voltage Vd) of the pulse signal Sd, during a time period T 3 ′ until the time T 2 ′ at which the notification signal St from the single pulse generating circuit  62  is input, the PWM circuit  60  generates a pulse signal Sr of the first repeating pulse, having a duty ratio of T 6 /T 5  corresponding to the respective resistances of the resistors  50  and  52 , and further having a repeating period of the time period T 5 , and supplies the pulse signal Sr to the pulse supplying unit  64  (see  FIG. 3C ). 
     Within the time period T 9  from time T 0  time T 8 , a single pulse signal Ss from the single pulse generating circuit  62  is input to the pulse supplying unit  64 , and together therewith, the pulse signal Sr is input from the PWM circuit  60 . However, as described previously, because the pulse supplying unit  64  is constructed with an OR circuit therein, and since the respective amplitudes of the single pulse signal Ss and the pulse signal Sr are substantially the same amplitude, the pulse supplying unit  64  supplies the single pulse Ss as the first pulse signal S 1  to the gate terminal G of the MOSFET  38  (see  FIG. 3D ). 
     Owing thereto, based on the first pulse signal S 1  supplied to the gate terminal G, an ON state is formed between the drain terminal D and the source terminal S, whereby the MOSFET  38  connects electrically the solenoid coil  12  and the resistor  70 . Therefore, the power source voltage V 0 ′ is applied to the solenoid coil  12  as the first voltage from the DC power source  16  and through the switch  18  and the diode  32  (see  FIG. 3E ). On the other hand, the current I that flows in the direction of the resistor  70  from the solenoid coil  12  through the MOSFET  38  rapidly increases over time within the time period T 9  until reaching the first current value I 1  (see  FIG. 3F ), and the plunger and valve plug are energized quickly by the excitation force (activation force) caused by the current I, whereby the solenoid valve  10 A shifts from a closed state into an open state. 
     Subsequently, at time T 8 , just after elapse of the time period T 9 , the single pulse generating circuit  62  stops generating the single pulse Ss and supply thereof to the pulse supplying unit  64  is suspended (see  FIG. 3B ). 
     On the other hand, the PWM circuit  60 , also during the time period from time T 8  to time T 2 ′, by the same circuit operations noted previously at the time period T 9 , generates the first repeating pulse as the pulse signal Sr, and supplies the same to the pulse supplying unit  64  (see  FIG. 3C ). In this case, because only the pulse signal Sr is input to the pulse supplying unit  64  from the PWM circuit  60 , the pulse supplying unit  64  supplies the pulse signal Sr as the first pulse signal S 1  to the gate terminal G of the MOSFET  38  (see  FIG. 3D ). 
     Owing thereto, based on the first pulse signal S 1  supplied to the gate terminal G, an ON state is formed between the drain terminal D and the source terminal S, whereby the MOSFET  38  connects electrically the solenoid coil  12  and the resistor  70 . Therefore, the power source voltage V 0 ′ is applied to the solenoid coil  12  as a first voltage from the DC power source  16  and through the switch  18  and the diode  32  (see  FIG. 3E ). On the other hand, the current I that flows in the direction of the resistor  70  from the solenoid coil  12  through the MOSFET  38  is maintained at the first current I 1  during the time period from time T 8  until time T 2 ′ (see  FIG. 3F ). 
     In  FIG. 3F , the waveform shown by the dashed line represents a situation in which feedback control of the current I is not carried out by the solenoid valve driving circuit  14 , and shows a timewise change of the current I in the case that application of the power source voltage V 0 ′ continues until time T 2 . On the other hand, the two-dot-dashed line waveform shows a timewise change of the current I during the time period T 3  (i.e., the time period from time T 0  to time T 2 ) of  FIG. 2F  (i.e., a timewise change of the current I at the relatively low power source voltage V 0 ). 
     Herein, an integration over time of the current I flowing through the solenoid coil  12 , that is, the partial area (current I×time) surrounded by the time waveform of the current I, the current values at two times, and the zero level (i.e., the dashed line extending in the horizontal direction in  FIGS. 2F and 3F ), indicates the amount of energy that is supplied to the solenoid coil  12  from the DC power source  16 . Accordingly, the energy amounts (current I×time periods T 3 , T 3 ′) supplied to the solenoid coil  12  from the DC power source  16  during the time periods T 3  and T 3 ′ from time T 0  times T 2  and T 2 ′ represents the energy amounts required to drive the solenoid valve  10 A. 
     Because the same solenoid valve  10 A is used for both of the above-noted first operation and second operation, the energy amount required to drive the solenoid valve  10 A is the same, irrespective of differences in operation. As a result, the timewise integration of the current I during the first operation (the area of the current I×the time period T 3 ) is the same as the timewise integration of the current I during the second operation (the area of the current I×the time period T 3 ′). 
     Accordingly, assuming that the timewise integrations of the current I (the area of the current I×the time periods T 3 , T 3 ′) during the first operation and the second operation are adjusted identically, during the second operation (the solid line in  FIG. 3F ), the current I flowing through the solenoid coil  12  rises to the current level I 1  over a shorter time period than in the first operation (the two-dot-dashed line in  FIG. 3F ). Additionally, by supplying the energy amount from the DC power source  16  to the solenoid coil  12  within the time period T 3 ′, which is shorter than the time period T 3  (refer to  FIG. 2F ), the solenoid valve  10 A can be driven in a short time. 
     Next, at time T 2 ′, the single pulse generating circuit  62  (see  FIG. 1 ) outputs a notification signal St to the PWM circuit  60 , for notifying passage of the time period T 3 ′. Accordingly, based on the notification signal St, during the time period T 4 ′ from time T 2 ′ to time T 1 , in place of the aforementioned pulse signal Sr having the duty ratio of T 6 /T 5 , the PWM circuit  60  generates a pulse signal Sr of the second repeating pulse, having a duty ratio of T 7 /T 5 , based on the respective resistances of the resistors  50  and  52 , and further, having a repeating period of the time period T 5 , and supplies the pulse signal Sr to the pulse supplying unit  64  (see  FIG. 3C ). In this case, because only the pulse signal Sr is input to the pulse supplying unit  64  from the PWM circuit  60 , the pulse supplying unit  64  supplies the pulse signal Sr as the second pulse signal S 2  to the gate terminal G of the MOSFET  38  (see  FIG. 3D ). 
     Owing thereto, based on the second pulse signal S 2  supplied to the gate terminal G, an ON state is formed between the drain terminal D and the source terminal S, whereby the MOSFET  38  connects electrically the solenoid coil  12  and the resistor  70 . Therefore, the power source voltage V 0 ′ is applied to the solenoid coil  12  as a second voltage from the DC power source  16  and through the switch  18  and the diode  32  (see  FIG. 3E ). On the other hand, concerning the current I that flows in the direction of the resistor  70  from the solenoid coil  12 , after being reduced rapidly in a short time period from time T 2 ′, from the first current value I 1  to the second current value I 2 , the current I is maintained at the second current value I 2  during the time period until time T 1  is reached (see  FIG. 3F ). As a result, the plunger and valve plug are held at a predetermined position by the excitation force (holding force) caused by the second current I 2 , whereby the driven state (valve open state) of the solenoid valve  10 A is maintained. 
     In addition, at time T 1 , when the switch  18  is opened and the device is placed in an OFF state (see  FIG. 3A ), since the supply of the DC voltage V to the switch controller  40  is suspended, the low voltage detection circuit  59  outputs a low voltage detection signal Sv to the single pulse generating circuit  62  and to the pulse supplying unit  64 , whereby, based on input of the low voltage detection signal Sv thereto, the pulse supplying unit  64  stops supplying the second pulse signal S 2  to the gate terminal G of the MOSFET  38 . Owing thereto, because the MOSFET  38  is rapidly switched from an ON state to an OFF state between the drain terminal D and the source terminal S thereof, a condition is reached in which application of the voltage V 0 ′ to the solenoid coil  12  from the DC power source  16  is halted. In this case, although a back electromotive force is generated by the solenoid coil  12 , a current caused by the back electromotive force refluxes (i.e., flows backward) inside of a closed circuit made up of the solenoid coil  12  and the diode  36 , so that the current is quickly attenuated. 
     In this manner, in the solenoid valve  10 A according to the first embodiment, a voltage Vd corresponding to the current I flowing through the solenoid coil  12  is output from the resistor  70  to the current detection circuit  72 , and in the current detection circuit  72 , a pulse signal Sd having an amplitude of the voltage Vd serving as a current detection value is fed back to the PWM circuit  60  of the switch controller  40 . 
     In the PWM circuit  60 , based on a comparison between the voltage value corresponding to the current value of either the first current value I 1  (activation current value) or the second current value I 2  (holding current value) and the amplitude (voltage Vd) of the fed back pulse signal Sd, a pulse signal Sr (first repeating pulse, first short pulse, second repeating pulse, or second short pulse) is generated having a pulse width of the time period T 5  and a predetermined duty ratio of T 6 /T 5  or T 7 /T 5 , and the pulse signal Sr is supplied to the pulse supplying unit  64 . 
     The pulse supplying unit  64  supplies the single pulse signal Ss from the single pulse generating circuit  62  as the first pulse signal S 1  to the gate terminal G of the MOSFET  38 , and thereafter, supplies the pulse signal Sr from the PWM circuit  60  as the second pulse signal S 2  to the gate terminal G of the MOSFET  38 . Alternatively, the pulse supplying unit  64  supplies the single pulse signal Ss and the pulse signal Sr as the first pulse signal S 1  to the gate terminal G of the MOSFET  38 , and thereafter, supplies the pulse signal Sr as the second pulse signal S 2  to the gate terminal G of the MOSFET  38 . 
     More specifically, in the time period (time period T 3 , T 3 ′) during which the solenoid valve  10 A is driven, the PWM circuit  60  of the switch controller  40  generates the pulse signal Sr, made up of the first repeating pulse or the first short pulse, and supplies the same to the pulse supplying unit  64 , so that the current detection value corresponding to the amplitude (voltage Vd) of the pulse signal Sd becomes the first current value I 1  corresponding to the activation force of the solenoid valve  10 A, and the pulse supplying unit  64  supplies the pulse signal Sr as the first pulse signal S to the gate terminal G of the MOSFET  38 . Owing thereto, the MOSFET  38  controls the application time of the first voltage (power source voltage V 0 , V 0 ′) to the solenoid coil  12  based on the pulse width of the first pulse signal S 1 . As a result, the current I that flows through the solenoid coil  12  is maintained at the first current value I 1  corresponding to the activation force, while the activation force caused by the current I (first current value I 1 ) is applied for energizing the plunger and the valve plug. 
     In greater detail, for a case in which, at the side of the user of the solenoid valve  10 A, a DC power source  16  having a relatively high power source voltage V 0 ′ (e.g., V 0 ′=24V) is prepared beforehand, whereas with respect to such a DC power source  16 , a solenoid valve  10 A is applied that is intended for use with a relatively low power source voltage V 0  (e.g. V 0 =12V), in such a case, in the PWM circuit  60  of the switch controller  40 , the first current value I 1  is set to be at or below a rated value (rated current) of the current I that flows through the solenoid coil  12 . Assuming the pulse width (time period T 6 ) of the pulse signal Sr is adjusted such that the current detection value becomes the thus set first current value I 1 , then since the current I flowing through the solenoid coil  12  during the time period (time period T 3 , T 3 ′) in which the solenoid valve  10 A is driven is maintained at the first current value I 1 , even on the side of a user who has prepared the DC power source  16  having the relatively high power source voltage V 0 ′, electric power savings of the solenoid valve  10 A and the solenoid valve driving circuit  14  can be achieved. In this case, because the relatively high power source voltage V 0 ′ is applied as the first voltage to the solenoid coil  12 , the solenoid valve  10 A can be driven in a shorter time. 
     As described above, since, by adjusting the pulse width (time period T 6 ) of the pulse signal Sr in the PWM circuit  60  of the switch controller  40 , the current I flowing through the solenoid coil  12  can be maintained at the first current value I 1  at or below the rated current, on the side of the manufacturer, without concern to differences in the power source voltages V 0 , V 0 ′ supplied to the solenoid coil  12  from the DC power source  16  prepared on the side of the user, the solenoid valve  10 A and the solenoid valve driving circuit  14  can be made commonly usable in conformity with a relatively low power source voltage, and by providing such a commonly usable solenoid valve  10 A and solenoid valve driving circuit  14  to the user, costs can be reduced. 
     Accordingly, with the solenoid valve  10 A according to the first embodiment, by generating the pulse signal Sr of the first repeating pulse or the first short pulse based on a comparison between the pulse signal Sd having the voltage Vd corresponding to the current detection value that is fed back to the switch controller  40  from the current detection circuit  72  and the voltage value corresponding to the first current value I 1  during a time period (time period T 3 , T 3 ′) in which the solenoid valve  10 A is driven, power savings of the solenoid valve  10 A and the solenoid valve driving circuit  14 , common usage and cost reduction, and a rapidly-responsive drive control for the solenoid valve  10 A, are all capable of being realized. 
     On the other hand, during a time period (time period T 4 , T 4 ′) in which the driven state of the solenoid valve  10 A is maintained, the PWM circuit  60  of the switch controller  40  generates a pulse signal Sr of the second repeating pulse or the second short pulse, so that the current detection value corresponding to the amplitude (voltage Vd) of the pulse signal Sd becomes the second current value I 2  corresponding to the holding force for the solenoid valve  10 A, whereupon the pulse signal Sr is supplied to the pulse supplying unit  64 , and the pulse supplying unit  64  supplies the pulse signal Sr as the second pulse signal S 2  to the gate terminal G of the MOSFET  38 . Owing thereto, the MOSFET  38  controls the application time during which the second voltage (power source voltage V 0 , V 0 ′) is applied to the solenoid coil  12 , based on the pulse width of the second pulse signal S 2 . As a result, the current I flowing through the solenoid coil  12  is maintained at the second current value I 2  corresponding to the holding force, and the holding force induced by the current I (second current value I 2 ) is applied to energize the plunger and the valve plug. 
     Accordingly, with the solenoid valve  10 A according to the first embodiment, by generating the pulse signal Sr of the second repeating pulse or the second short pulse based on a comparison between the pulse signal Sd having the voltage Vd corresponding to the current detection value that is fed back to the switch controller  40  from the current detection circuit  72  and the voltage value corresponding to the second current value I 2  during a time period (time period T 4 , T 4 ′) in which the driven state of the solenoid valve  10 A is maintained, the driven state of the solenoid valve  10 A can be maintained with smaller power consumption, and further, the solenoid valve  10 A can be stopped in a short time. 
     Further, by feeding back the pulse signal Sd having the voltage Vd corresponding to the current detection value to the PWM circuit  60  of the switch controller  40 , even if the current I tends to vary over time due to changes of the electrical resistance inside the solenoid coil  12  or changes in the power source voltage V 0 , V 0 ′ as a result of temperature changes in the solenoid coil  12 , the pulse signal Sr is generated responsive to such changes, whereby the solenoid valve  10 A and the solenoid valve driving circuit  14 , which are capable of responding to changes in the use environment, such as changes in electrical resistance and power source voltage V 0 , V 0 ′ or the like, can be realized. 
     In this manner, with the solenoid valve  10 A according to the first embodiment, a reduction in electrical power consumption of the solenoid valve  10 A and the solenoid valve driving circuit  14 , rapidly responsive drive control for the solenoid valve  10 A, and a reduction in costs for the solenoid valve  10 A and the solenoid valve driving circuit  14 , can all be realized together in one sweep. 
     Further, at the time period (time period T 3 , T 3 ′) during which the solenoid valve  10 A is driven, after the power source voltage V 0 ′ has been impressed as the first voltage on the solenoid coil  12  only at a time period T 9  corresponding to the pulse width of the single pulse Ss, the first voltage is impressed on the solenoid coil  12  only at the time period corresponding to the pulse width (time period T 6 ) of the pulse signal Sr of the first repeating pulse or the first short pulse. As a result, within the time period during which the solenoid valve  10 A is driven, after the current I flowing through the solenoid coil  12  has risen up to the first current value I 1  within the time period T 9  corresponding to the pulse width of the single pulse signal Ss, the first current value I 1  is maintained by a switching operation of the MOSFET  38  based on the first repeating pulse or the first short pulse. Owing thereto, the solenoid valve  10 A and the solenoid valve driving circuit  14  can be made commonly usable, and costs can be reduced easily. In particular, in the case that a DC power source  16 , for which the power source voltage V 0 ′ thereof is relatively high, is electrically connected to the solenoid coil  12  through the solenoid valve driving circuit  14  and the solenoid valve  10 A is driven thereby, the solenoid valve  10 A is capable of being driven in a shorter time. Furthermore, by maintaining the current I flowing through the solenoid coil  12  at the first current value I 1 , unintended or mistaken operations of the solenoid valve  10 A and the solenoid valve driving circuit  14  caused by the input of excessive voltage (surge energy) thereto can be reliably prevented. 
     On the other hand, during a time period (time period T 4 , T 4 ′) at which the driven state of the solenoid valve  10 A is maintained, by supplying the pulse signal Sr of the second repeating pulse or the second short pulse as the second pulse signal S 2  to the MOSFET  38 , the driven state of the solenoid valve  10 A can be maintained with lower power consumption, and further, the solenoid valve  10 A can be stopped in a short time. 
     Accordingly, by providing a structure, including the PWM circuit  60 , the single pulse generating circuit  62  and the pulse supplying unit  64 , for the switch controller  40 , common usage and cost reduction of the solenoid valve  10 A and the solenoid valve driving circuit  14 , driving of the solenoid valve  10 A in a short time, power savings of the solenoid valve  10 A and the solenoid valve driving circuit  14 , and the ability to stop the solenoid valve  10 A in a short time, can easily be realized. 
     Further, in the solenoid valve driving circuit  14 , a series circuit made up of the diode  34 , the LED  54 , the resistor  42 , the switch controller  40  and the resistors  50 ,  52 ,  76 , and a series circuit made up of the diode  32 , the solenoid coil  12 , the MOSFET  38  and the resistor  70 , are electrically connected in parallel with respect to a series circuit made up of the DC power source  16  and the switch  18 . Although, conventionally, a series circuit made up of the LED  54  and a current limiting resistor for causing light to be emitted from the LED  54  have been connected electrically in parallel with respect to the DC power source  16  and the solenoid coil  12 , in the present invention, in place of the current limiting resistor, the series circuit including the switch controller  40  and the LED  54  is connected electrically in parallel with respect to the DC power source  16  and the solenoid coil  12 , whereby, since the electrical energy consumed originally by the current limiting resistor is utilized for operating the switch controller  40 , a solenoid valve driving circuit  14  exhibiting high energy use efficiency can be realized. 
     Further, owing to the arrangement of the resistor  42 , it becomes possible for the switch controller  40  to be reliably protected from an inrush current, and in addition, the solenoid valve  10 A can easily be applied as well with respect to a DC power source  16  having a relatively high power source voltage V 0 ′. Further, by carrying out such a countermeasure with respect to the inrush current, unintended or mistaken operations of the solenoid valve  10 A and the solenoid valve driving circuit  14  caused by a surge voltage, which is generated momentarily inside the solenoid valve driving circuit  14  at starting and stopping times of the solenoid valve  10 A, can reliably be prevented. 
     Further, in the PWM circuit  60 , the duty ratios T 6 /T 5  and T 7 /T 5  of the pulse signal Sr are adjustable by changing the resistance values of the resistors  50 ,  52 ,  76 , whereas in the single pulse generating circuit  62 , the pulse width of the single pulse signal Ss is adjustable by changing the resistance value of the resistor  66 . Owing thereto, irrespective of changes in the power source voltage V 0 , V 0 ′, the switch controller  40  and the MOSFET  38  can be operated stably, and the voltage range (i.e., the range of the power source voltage V 0 , V 0 ′) usable with the solenoid valve driving circuit  14  is capable of being widely set. 
     Concerning adjustment of the duty ratios T 6 /T 5  and T 7 /T 5  and the pulse width of the single pulse signal Ss, instead of the aforementioned resistors  50 ,  52 ,  66 ,  76 , a non-illustrated memory may be used to store the duty ratios T 6 /T 5  and T 7 /T 5  and the pulse width of the single pulse signal Ss, and then, as necessary, the duty ratios T 6 /T 5  and T 7 /T 5  and the pulse width may be read out from the memory to the PWM circuit  60  and the single pulse generating circuit  62 . Accordingly, by changing the data stored in the memory, the duty ratios T 6 /T 5  and T 7 /T 5  and the pulse width can be set appropriately to desired values, corresponding to the specifications of the solenoid valve  10 A. 
     In the above explanations of the solenoid valve  10 A according to the first embodiment, within the time period at which the solenoid valve  10 A is driven, supply of the first pulse signal S 1  is timewise controlled based on a comparison between the voltage value that corresponds to the first current value I 1  and the amplitude (the voltage Vd corresponding to the current detection value) of the pulse signal Sd. On the other hand, within the time period at which the driven state of the solenoid valve  10 A is maintained, supply of the second pulse signal S 2  is timewise controlled based on a comparison between the current value that corresponds to the second current value I 2  and the amplitude of the pulse signal Sd. 
     In the solenoid valve  10 A according to the first embodiment, it is a matter of course that such a timewise control based on the current detection value can be carried out solely during a time period in which the solenoid valve  10 A is driven, or alternatively, during a time period in which the driven state of the solenoid valve  10 A is maintained. 
     More specifically, in order to carry out the timewise control based on the current detection value only during the time period in which the solenoid valve  10 A is driven, in the time period (time period T 3 ′) when the solenoid valve  10 A is driven, the solenoid valve  10 A is driven based on the aforementioned second operation, whereas, in the time period (time period T 4 ′) when the driven state of the solenoid valve  10 A is maintained, the PWM circuit  60  generates either a predetermined second repeating pulse having a duty ratio of T 7 /T 5  and a repeating period of the time period T 5 , or a predetermined second short pulse having a pulse width of the time period T 7 , and outputs such pulses to the pulse supplying unit  64 . 
     Even in this case, during the time period in which the solenoid valve  10 A is driven, the above-mentioned effects of the timewise control based on the current detection value can easily be obtained. 
     On the other hand, only during the time period in which the driven state of the solenoid valve  10 A is maintained, in order to carry out the timewise control based on the current detection value, the aforementioned first operation is performed. Even in this case, during the time period in which the driven state of the solenoid valve  10 A is maintained, the above-mentioned effects of the timewise control based on the current detection value can easily be obtained. 
     Further, in the solenoid valve  10 A according to the first embodiment, although the solenoid valve driving circuit  14  is constructed to include an LED  54  therein, even if the LED  54  is omitted, the aforementioned effects can still be obtained as a matter of course. 
     Next, with reference to  FIG. 4 , explanations shall be given concerning a solenoid valve  10 B in accordance with a second embodiment of the present invention. In the following descriptions, constituent elements, which are the same as those in the solenoid valve  10 A (see  FIGS. 1 to 3F ) are designated by the same reference numerals, and detailed explanations of such features shall be omitted. 
     The solenoid valve  10 B according to the second embodiment differs from the solenoid valve  10 A according to the first embodiment, in that it includes a vibration sensor  98 . 
     The vibration sensor  98  detects vibrations generated inside the solenoid valve  10 B as a result of vibrations and/or shocks imparted to the solenoid valve  10 B from the exterior. Detection results are output as a vibration detection signal So (vibration detection value) to the PWM circuit  60  of the switch controller  40 . Based on the vibration detection signal So from the vibration sensor  98 , the PWM circuit  60  increases the duty ratio T 7 /T 5  (i.e., the pulse width of the time period T 7 ) of the pulse signal Sr that is supplied to the pulse supplying unit  64  during the time period T 4 , T 4 ′ (refer to  FIGS. 2F and 3F ). Owing thereto, even if there are concerns that the current I (second current value I 2 ) flowing through the solenoid coil  12  might change over time due to vibrations inside the solenoid valve  10 B, causing stoppage of the solenoid valve  10 B during the time period (time period T 4 , T 4 ′) in which the driven state of the solenoid valve  10 B is maintained, by increasing the duty ratio T 7 /T 5 , the current I can be raised. 
     When the holding force is reduced in order to conserve power, it may be envisaged that vibrations inside the solenoid valve  10 B could be caused which might lead to stoppage of the solenoid valve  10 B. However, according to the solenoid valve  10 B of the second embodiment, by providing the switch controller  40  with the above-noted structure, even if the current I (second current value I 2 ) flowing through the solenoid coil  12  changes over time due to vibrations inside the solenoid valve  10 B, by adjusting the pulse width of the pulse signal Sr (second pulse signal S 2 ) corresponding to such changes, a solenoid valve  10 B and solenoid valve driving circuit  14 , which are capable of responding to such vibration-induced changes, can be realized. 
     That is, during the time period (time period T 4 , T 4 ′) in which the driven state of the solenoid valve  10 B is maintained, in the event it is feared that the solenoid valve  10 B may reach a stopped state due to vibrations, the pulse width (time period T 7 ) of the pulse signal Sr (second pulse signal S 2 ) is lengthened and the current I (second current value I 2 ) flowing through the solenoid coil  12  is increased, whereby the holding force on the plunger and the valve plug inside the solenoid valve  10 B is made to increase, so that the solenoid valve  10 B can be prevented from coming into a stopped state. 
     Accordingly, in the solenoid valve  10 B according to the second embodiment, because the pulse width of the second pulse signal S 2  can be set longer so that the level of the current I becomes greater only in those cases when a high holding force is necessary, power savings of the solenoid valve  10 B and the solenoid valve driving circuit  14  can be carried out efficiently. 
     In existing solenoid valves, although it is known to detect valve-open and valve-closed states of the solenoid valve by detection of the pressure inside the solenoid valve utilizing an internal pressure sensor, wherein restarting of the solenoid valve is carried out based on such a detection result, by applying the features of the above-described solenoid valve  10 B to the existing solenoid valve, stoppage of the solenoid valve during a time period (time period T 4 ) in which the driven state of the existing solenoid valve is maintained can reliably be prevented. 
     Next, with reference to  FIG. 5 , explanations shall be given concerning a solenoid valve  10 C in accordance with a third embodiment of the present invention. 
     The solenoid valve  10 C according to the third embodiment differs from the solenoid valve  10 B according to the second embodiment (see  FIG. 4 ), in that the solenoid valve driving circuit  14  further includes an operation detector (energization time calculator and solenoid valve operation detector)  100 , a flash memory (energization time memory and detection result memory)  102 , and a determining unit (energization time determining unit and accumulated number of operation times determining unit)  106 . 
     The operation detector  100  includes a counter, which calculates the energization time of the solenoid coil  12  (total time during which the power source voltage V 0 , V 0 ′ is impressed on the solenoid coil  12 ) in one operational period (the time period from time T 0  time T 1  in  FIGS. 2F and 3F ) of the solenoid valve  10 C based on the pulse signal Sd, and the detection result is stored in the flash memory  102 . Alternatively, the operation detector  100  detects that the solenoid valve  10 C is in operation based on the pulse signal Sd, and stores the detection result thereof in the flash memory  102 . 
     The determining unit  106  calculates the total energization time of the solenoid coil  12  based on the totality of the energization time that has been stored in the flash memory  102  after the end of each operation of the solenoid valve  10 C, and determines whether or not the total energization time is longer than a predetermined first energization time. Alternatively, the determining unit  106  calculates an accumulated number of operation times of the solenoid valve  10 C from each of respective detection results stored in the flash memory  102 , and determines whether or not the accumulated number of operation times exceeds a predetermined first number of operation times. 
     In this case, when the determining unit  106  determines that the total energization time is longer than the predetermined first energization time, or alternatively, that the accumulated number of operation times has exceeded the predetermined first number of operation times, the determining unit  106  outputs a pulse width change signal Sm to the single pulse generating circuit  62  and the PWM circuit  60  of the switch controller  40 , instructing that the pulse width (time period T 3 , T 9 ) of the single pulse signal Ss and the pulse width (time period T 6 ) of the pulse signal Sr should be changed. Based on the pulse width change signal Sm, the single pulse generating circuit  62  sets the pulse width of the single pulse signal Ss to be longer than the currently set pulse width. On the other hand, based on the pulse width change signal Sm, the PWM circuit  60  sets the pulse width of the pulse signal Sr to be longer than the currently set pulse width. 
     Further, when the determining unit  106  determines that the total energization time has become longer than a predetermined second energization time, which is set to be longer than the predetermined first energization time, or alternatively, when the determining unit  106  determines that the accumulated number of operation times exceeds a predetermined second number of operation times, which is set to be greater than the first predetermined number of operation times, the determining unit  106  externally outputs a usage limit notification signal Sf, notifying that the solenoid valve  10 C has reached a usage limit. 
     In this manner, by means of the solenoid valve  10 C according to the third embodiment, even in cases where the driving performance of the solenoid valve  10 C is decreased through use of the solenoid valve over a prolonged period, by setting the pulse widths of each of the single pulse signal Ss and the pulse signal Sr to be longer at times when the total energization time of the solenoid valve  10 C becomes longer than the first energization time, or when the accumulated number of operation times exceeds the first number of operation times, the current I (first current value I 1 ) flowing through the solenoid coil  12  becomes larger, and the activation force can be increased. Thus, driving control of the solenoid valve  10 C can be carried out efficiently. 
     Further, because the determining unit  106  outputs the usage limit notification signal Sf to the exterior when the total energization time of the solenoid valve  10 C becomes longer than the second energization time, or when the accumulated number of operation times exceeds the second number of operation times, it becomes possible to quickly exchange the solenoid valve  10 C whenever the usage limit thereof is reached, so that reliability with respect to the usage limit (life) of the solenoid valve  10 C is improved. 
     Next, with reference to  FIG. 6 , explanations shall be given concerning a solenoid valve  10 D in accordance with a fourth embodiment of the present invention. 
     The solenoid valve  10 D according to the fourth embodiment differs from the solenoid valve  10 C according to the third embodiment (see  FIG. 5 ), in that the solenoid valve driving circuit  14  further includes an activation current monitoring unit (current detection value monitoring unit)  104 . 
     The current detection value monitoring unit  104  monitors a time period T 11 , from time T 0  time T 10 , at which the current I (and the voltage Vd corresponding thereto) slightly decreases during a time period (time period T 3 , T 3 ′) at which the solenoid valve  10 D is driven. When it is determined that the time period T 11  has become longer than a predetermined set time, a time delay notification signal Se is output to the exterior, for notifying that a time delay was generated in the time period T 11 . 
     In this manner, by means of the solenoid valve  10 D according to the fourth embodiment, it becomes possible to quickly exchange the solenoid valve  10 D for which the time period T 11  has become long, and thus the driving performance thereof has degraded. That is, by providing the solenoid valve driving circuit  14  having the aforementioned structure, detection of the usage limit (life) of the solenoid valve  10 D can be carried out efficiently, based on the responsiveness of the solenoid valve  10 D during the time period at which the solenoid valve is driven. 
     The solenoid valve driving circuit and solenoid valve according to the present invention are not limited to the aforementioned embodiments. Various other structures and configurations may be adopted as a matter of course without deviating from the essence and gist of the present invention.