Solenoid valve driving device

A solenoid valve driving device includes a bypassing current path of which one end is connected to a point between a first switching device and a first solenoid valve and another end is connected to a point between a second switching device and a second solenoid valve, and an element disposed in the bypassing current path and having a characteristic that causes current to flow from the first switching device side to a second switching device side. By adopting the above described constitution, through current generated when the first switching device is switched from an OFF-state to an ON-state during duty control of the first switching device can be suppressed by the inductance of the second solenoid valve. In this way, one solenoid valve among a pair of the first and second solenoid valves, which is not operated, has a function of suppressing the through current.

CROSS REFERENCE TO RELATED APPLICATION 
This application is related to prior Japanese Patent Applications No. 
H.8-320080 filed on Nov. 29, 1996 and No. H.9-304239 filed on Nov. 6, 
1997, the contents of which are incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to a solenoid valve driving device and, in 
particular, relates to a driving device for driving a plurality of 
solenoid valves which are parallel-supplied with electric power by an 
electric power supply source in common. 
2. Related Art: 
A solenoid coil of a solenoid valve which has been used for an anti-skid 
control device and has been operated under duty control (PWM control) has 
been driven by a driving circuit as shown in FIG. 9. In the driving device 
shown in FIG. 9, a first solenoid valve 3 is operated under duty control. 
In duty control, the mean current flowing through the first solenoid valve 
3 is controlled by switching a first driving IC 4. 
The driving circuit shown in FIG. 9 is provided with the first and the 
second driving ICs 4, 6 which receive and are driven by respective driving 
signals from a central processing unit (hereinafter referred to as "CPU 
7"). The CPU 7 sends the electronic signals to the first and the second 
driving ICs 4, 6 via a buffer not shown or the like. When the driving ICs 
4, 6 are turned on, a current from an electric power supply source 1 is 
supplied to the first solenoid valve 3 and the second solenoid valve 5 
corresponding to the first driving IC 4 and the second driving IC 6, 
respectively. For example, when the first driving IC 4 is turned on, 
current flows from the electric power supply source 1 to the first 
solenoid valve 3 and the first driving IC 4 via a diode 2 and a choke coil 
9. Immediately after the first driving IC 4 is turned off, the 
recirculation of the current occurs through the first solenoid valve 3 and 
a recirculation diode 8 which is connected in parallel to the first 
solenoid valve 3. A recirculation diode may be connected in parallel to 
the second solenoid valve 5, however, since it produces the same effect as 
the recirculation diode 8, the representation in the figure and the 
description thereof will be omitted. If the second solenoid valve 5 is not 
operated under duty control, the recirculation diode is not required for 
the second solenoid valve 5. 
In this circuit structure, there is a problem that, when the first driving 
IC 4 is switched from an OFF-state (shut-down state) to an ON-state 
(conductive state) while the first solenoid valve 3 is operated under duty 
control, the recirculation diode 8 connected in parallel to the first 
solenoid valve 3 does not come to a state in which it prevents the current 
flowing in a reverse direction at the same time when the first driving IC 
4 is turned on. That is, the recirculation diode 8 comes to a reverse 
direction current prevention state (a one- way current passing state) in 
which current is prevented from flowing from the electric power supply 
source 1 to the first driving IC 4 through the recirculation diode after 
an instantaneous delay time has passed since the first driving IC 4 is 
turned on. During this delay time, bypassing current flows from the 
electric power supply source 1 to the first driving IC 4 through the 
recirculation diode 8 (hereinafter referred to as "through current"). 
Since the change of the through current is very rapid, surge voltage is 
produced by the parasitic inductance of the wire in the circuit. As a 
result, a problem is produced such that the recirculation diode 8 and the 
first driving IC 4 are required to have a large margin on withstand 
voltage. In addition, the surge voltage and the surge current cause 
radio-noises. 
Therefore, as shown in FIG. 9, a choke coil 9 has been provided in the 
upstream of the first solenoid valve 3 as a coil for preventing a rapid 
change of the through current. However, the choke coil provided in the 
circuit increases the number of parts and a production cost. In 
particular, if there are many solenoid valves driven by a solenoid valve 
driving device, for example, in the case of an anti-skid control device, 
many choke coils are required to prevent the surge voltage with the result 
that the production cost is further increased. If the choke coil 9 is not 
used, high-response recirculation diodes having a small delay in response 
need to be used, which also increases the production cost. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a solenoid valve 
driving device which can reduce its production cost by making use of one 
solenoid valve among a pair of solenoid valves as a choke coil in the 
conventional technique. 
To achieve the above described object, a solenoid valve driving device 
according to the present invention is provided with a bypassing current 
path of which one end is connected to a point between a first switching 
device and a first solenoid valve and another end is connected to a point 
between a second switching device and a second solenoid valve, and an 
element disposed in the bypassing current path and having a characteristic 
that causes current to flow through the bypass current in a direction 
which does not substantially prevent the recirculation current of the 
first solenoid valve. 
By adopting the above described constitution, the through current generated 
when the first switching device is switched from an OFF-state to an 
ON-state during duty control of the first switching device can be 
suppressed by the second solenoid valve. In this way, one solenoid valve 
among a pair of first and second solenoid valves, which is not operated, 
has a function of suppressing the through current. As a result, the number 
of parts in the solenoid driving device can be reduced. It is to be noted 
that a pair of first and second solenoid valves include a combination of 
at least one first solenoid valve which is operated under duty control and 
at least one second solenoid valve which is not supplied with a current 
while the first solenoid valve is operated under duty control. 
When the bypassing current path is formed in an electronic control device, 
noises can be reduced. 
When the present invention is applied to a brake system for an anti-skid 
control device or the like, the number of parts can be remarkably reduced 
because there are many sets of first and second solenoid valves used in 
the anti-skid control device. Further, the present invention is easily 
applied to the anti-skid control device, because current is not usually 
supplied to the second solenoid valve used as a pressure decreasing 
control valve when the first solenoid valve used as a pressure increasing 
control valve is operated under duty control in the anti-skid control 
device. 
A diode may be used as the element disposed in the bypassing current path. 
The solenoid valve driving circuit according to the present invention may 
be constituted as in the following. That is, the solenoid valve driving 
device comprises a control device for operating under duty control one of 
a first switching device and a second switching device and for shutting 
down the other switching device, a bypassing circuit path of which one end 
is connected to a point between the first switching device and the first 
solenoid valve and another end is connected to a point between the second 
switching device and the second solenoid valve, and a changing device for 
changing a direction of the current passing through the bypassing current 
path based on which of the first switching device and the second switching 
device is operated under duty control. 
According to the above described constitution, the through current is 
suppressed by an opposing solenoid valve which is not operated under duty 
control. In addition, both of the first and second solenoid valves can be 
operated under duty control. That is, for example, when a diode is used as 
described above, only one solenoid valve can be operated under duty 
control. However, according to the above described constitution, both 
solenoid valves can be operated under duty control.

PREFERRED EMBODIMENTS OF THE INVENTION 
A solenoid valve driving device according to the present invention will be 
hereinafter described based on the accompanying drawings. 
A first embodiment will be described with reference to FIG. 1. The parts 
having the same or corresponding functions as the parts described in FIG. 
9 has will be designated by the same reference numerals and the 
description of those parts will be omitted. As shown in FIG. 1, the first 
embodiment has a feature such that a recirculation diode 10 is disposed in 
a path which connects the point between a first solenoid valve 3 and a 
first driving IC 4 with the point between a second solenoid valve 5 and a 
second driving IC 6. The recirculation diode 10 is connected such that it 
substantially allows current to pass only from the first solenoid valve 3 
side to the second solenoid valve 5 side. A CPU 7 sends an electric signal 
to the first driving IC 4 and controls it so that the first solenoid valve 
3 is operated under duty control. While the first driving IC 4 is operated 
under duty control, the second driving IC 6 is maintained in a shut-off 
state (an OFF-state). 
The functions and advantages of the recirculation diode 10 connected in 
this way will be hereinafter described. While the first solenoid valve 3 
is operated under duty control by the control of the first driving IC 4, 
the first solenoid valve 3 is supplied with current by an electric power 
supply source 1 when the first driving IC 4 is being turned on. Then, when 
the first driving IC 4 is switched from an ON-state to an OFF-state, a 
recirculation path is formed by the recirculation diode 10 and the first 
and second solenoid valves 3, 5 during the OFF state of the first driving 
IC 4 and recirculation current flows therethrough. Next, when the first 
driving IC 4 is switched from the OFF-state to the ON-state, since the 
recirculation diode 10 has a delay in response, the above-described 
through current flows from the electric power supply source 1 to the first 
driving IC 4 via the recirculation diode 10. However, since there is the 
second solenoid valve 5 disposed between the electric power supply source 
1 and the recirculation diode 10, the through current is prevented from 
increasing by the inductance of the second solenoid valve 5. That is, 
since the recirculation diode 10 is disposed such that the second solenoid 
valve 5 which is one of a pair of first and second solenoid valves and is 
not operated under duty control has the function of a conventional choke 
coil, the conventional choke coil can be omitted. 
Moreover, as compared with the conventional technique described with 
reference to FIG. 9, a diode 2 for restricting the direction of the 
current from the electric power supply source can be omitted in the 
present embodiment. That is, for example, when a well-known MOS-FET (Metal 
Oxide Semiconductor-Field Effect Transistor) is used as the first driving 
IC 4, the MOS-FET has a parasitic diode (a diode which allows current to 
flow only in one direction from the grounding side to the first solenoid 
valve 3 side and is connected in parallel with the first driving IC 4). 
Therefore, if the electric power source is reversely connected by a 
malfunction or by a misoperation in FIG. 9 (if the current flows as 
follows; the grounding side.fwdarw.the first driving IC 4.fwdarw.the first 
solenoid valve 3 and the recirculation diode 8.fwdarw.the choke coil 9), 
there is a possibility that excessively large current passes through the 
parasitic diode and the recirculation diode 8, whereby the first driving 
IC 4 and the recirculation diode 8 are broken by such an overcurrent. 
Furthermore, the resistance of the choke coil 9 in FIG. 9 is usually set 
much smaller than the resistance of the first solenoid valve 3 in order 
not to prevent the power supply from the electric power supply source 1. 
Therefore, the choke coil 9 will not sufficiently suppress the current 
from passing through the first driving IC 4 and the recirculation diode 8. 
Hence, an overcurrent passes through the parasitic diode and the 
recirculation diode 8. Accordingly, the diode 2 is disposed to prevent the 
overcurrent from passing through the first driving IC 4 and the 
recirculation diode 8. In the present embodiment, however, since the 
second solenoid valve 5 which is used instead of the choke coil 9 has the 
sufficient resistance corresponding to the solenoid valve 3, even if 
current passes from the parasitic diode to the recirculation diode 10, it 
is expected that the resistance of the second solenoid valve 5 fully 
suppresses the current. Therefore, the diode 2 shown in FIG. 9 can be 
omitted, which also produces an advantage that production costs are 
expected to be reduced. 
Next, the second embodiment will be described with reference to FIG. 2. The 
parts having the same functions and advantages as those of the above 
described embodiment has will be designated by the same reference numerals 
and the description of those parts will be omitted. 
In the second embodiment, the first and the second solenoid valves 3, 5 are 
disposed in an actuator 20 and the CPU 7, the first and second driving ICs 
4, 6 and the recirculation diode 10 are built in an electronic control 
unit 21 (hereinafter referred to as "ECU 21"). The actuator 20 is 
connected to the ECU 21 with wire harnesses 24, 25. Capacitors 22, 23 are 
connected in parallel with the first and the second driving ICs 4, 6, 
respectively. The capacitors 22, 23 are provided to prevent the first and 
the second driving ICs 4, 6 from being broken by static electricity 
applied to the ECU 21. 
If the capacitors 22, 23 are disposed as shown in the present embodiment, 
the following advantages can be produced by forming the recirculation 
diode 10 in the ECU 21 in addition to the functions and advantages in the 
above-described first embodiment. That is, the noises caused by the 
inductance of the wire harnesses 24, 25 can be suppressed. When the first 
driving IC 4 is switched from the OFF-state to the ON-state, since the 
first driving IC 4 is grounded, the voltage at a point A lowers. As a 
result, current flows through the recirculation diode 10 in a reverse 
direction due to the electric charge accumulated in the capacitor 23, 
which is added to the through current described in the first embodiment, 
that is, the through current passing through the second solenoid valve 5 
and the recirculation diode 10. Although the through current passing the 
second solenoid valve 5 is suppressed by the inductance of the second 
solenoid valve 5, there may be a case where the through current from the 
capacitor 23 is very large because it is not affected by the second 
solenoid valve 5. For example, as shown in FIG. 3, if the recirculation 
diode 10 is disposed in the actuator 20, the through current from the 
capacitor 23 passes through the wire harness 25, the recirculation diode 
10, and the wire harness 24 and reaches the capacitor 22 connected across 
the first driving IC 4. When the through current passes through the wire 
harnesses 25, 24, it is predicted that the noises such as magnetic fields 
and the like are generated by the inductance of the wire harnesses 24, 25. 
However, as shown in the second embodiment, if the recirculation diode 10 
is mounted in the ECU 21, it can prevent the noises such as magnetic field 
from being generated because the through current from the capacitor 23 
does not pass through the wire harnesses 24, 25. 
Even in the constitution shown in FIG. 3 as a third embodiment, i.e., in 
the case where the recirculation diode 10 is disposed in the actuator 20, 
the same functions and advantages as those in the first embodiment can be 
obtained. 
The present invention is not limited to the above described embodiments but 
it can be modified to various ways as shown below. For example, as shown 
in FIG. 4 as a fourth embodiment, instead of the recirculation diode 10 
disposed in the above described embodiment, a recirculation transistor 40 
may be used. In this case, the recirculation transistor 40 is switched by 
the CPU 7. That is, the recirculation transistor 40 is switched such that, 
when the first driving IC 4 is switched from the ON-state to the 
OFF-state, the current passes from the first solenoid valve 3 side to the 
second solenoid valve 5 side through the recirculation transistor 40. 
Moreover, as shown in FIG. 5, instead of the recirculation transistor 40, a 
MOS-FET 50 for recirculation use may be used. When the MOS-FET 50 is used, 
a diode 51 may be disposed because there may be the case where the MOS-FET 
has a parasitic diode not shown. 
When the constitutions shown in FIG. 4, FIG. 5 are used, they can produce 
the better effects than the above described embodiments. 
Furthermore, in FIG. 4, an another recirculation transistor may be 
additionally connected in parallel to the recirculation transistor 40 
between a connecting point of the first solenoid valves 3 and the first 
driving IC 4 and a connecting point of the second solenoid valves 5 and 
the second driving IC 6 so that it allows current to pass in the direction 
opposite to the recirculation transistor 40. The CPU 7 controls the 
above-described recirculation transistor 40 so that the (recirculation) 
current passes through the recirculation transistor 40 when the first 
driving IC 4 is turned off while the first solenoid valve 3 is operated 
under duty control and the second solenoid valve 5 is not operated. On the 
contrary, the CPU 7 controls the recirculation transistor not shown so 
that the recirculation current passes through the recirculation transistor 
not shown when the second driving IC 6 is turned off while the second 
solenoid valve 5 is operated under duty control and the first solenoid 
valve 3 is not operated. 
Although the above described embodiments are limited by conditions that the 
first solenoid valve 3 can be operated under duty control and that the 
second solenoid valve 5 can not be operated under duty control, if the 
another recirculation transistor is provided as described above, these 
solenoid valves are not subjected to the above described constraints of 
the operations thereof. It is to be noted that, as shown in FIG. 6, the 
order of electric connection between the electric power supply source 1 
and a ground terminal may be as follows; the electric power supply source 
1.fwdarw.the first and the second driving ICs 4, 6.fwdarw.the first and 
the second solenoid valves 3, 5.fwdarw.the ground terminal. Furthermore, 
the solenoid valve driving devices in the above described embodiments may 
be applied to an anti-skid control device or a traction control device of 
a brake system for a vehicle. That is, the above described first solenoid 
valve 3 is utilized as a pressure increasing control valve 50 and the 
second solenoid valve 5 may be utilized as a pressure decreasing control 
valve 51 in the brake system for a vehicle shown in FIG. 7. The pressure 
increasing control valve 50 is used as a valve for allowing or prohibiting 
the flow of brake fluid from a master cylinder 54 to a wheel cylinder 52 
which produces wheel braking force. The pressure decreasing control valve 
51 is used as a valve for controlling the flow of the brake fluid from the 
wheel cylinder 52 to a reservoir 55 when the pressure of the brake fluid 
applied to the wheel cylinder 52 is reduced. The pressure increasing 
control valve 50 is a normally-open valve whose valve body is in a 
communicating state when the power supply is shut off. The pressure 
decreasing control valve 51 is a normally-closed valve whose valve body is 
in a closed state when the power supply is shut off. In ordinary anti-skid 
control, the pressure decreasing control valve 51 is not operated under 
duty control. Only the pressure increasing control valve 51 is operated 
under duty control when the pressure of the wheel cylinder is gradually 
increased, that is, when a duty pressure increasing operation is carried 
out. The solenoid valve driving device described in the above described 
first embodiment or second embodiment can be easily applied to a brake 
system for performing anti-skid control like this. In addition, when the 
second driving IC 6 is turned on in a driving circuit using the 
recirculation diode 10, the current passes through not only the second 
solenoid valve 5 but also the first solenoid valve 3. However, when the 
pressure decreasing control valve 51 is turned on (in a communicating 
state) in the brake device shown in the figure, that is, when the pressure 
of the wheel cylinder 52 is reduced, the pressure increasing control valve 
50 is also surely turned on (in a closed state). Therefore, even when a 
solenoid valve driving device provided with the recirculation diode 10 is 
applied to the brake system shown in FIG. 7, it is possible that the first 
solenoid valve 3 is used as the pressure increasing control valve and that 
the second solenoid valve 5 is used as the pressure decreasing control 
valve. Moreover, a solenoid valve driving device according to the present 
invention may be applied also to control the pressure increasing control 
valve 56 and the pressure decreasing control valve 57 for the wheel 
cylinder 53. By applying the present invention to the brake device for a 
vehicle having more than four wheels, the number of the choke coils 
necessitated in the conventional device which can be eliminated by the 
above-described embodiments increases in the entire brake system, 
resulting in realizing a great cost reduction. 
Further, the solenoid valves 3, 5 of the solenoid valve driving device 
described in FIG. 4 or FIG. 5 may be applied to each valve in the brake 
system. In the constitution provided with the recirculation transistor 40 
shown in FIG. 4 and the recirculation transistor not shown, both of the 
pressure increasing control valve 50 and the pressure decreasing control 
valve 51 can be operated under duty control. 
FIG. 8 is a flow chart showing an example of a control method for 
controlling the respective parts of the anti-skid control device shown in 
FIG. 7 (pressure increasing control valves 50, 56, pressure decreasing 
valves 51, 57 and a pump). The flow chart shown in FIG. 8 can be applied 
to the first embodiment, the second embodiment, and the like, if 
necessary. 
A flow chart shown in FIG. 8 which is periodically carried out for each 
wheel will be described. In step 100, when the ignition switch of a 
vehicle is turned on, an initial check of each flag or the like is 
performed. In step 110, each wheel speed VW of a front-right wheel, a 
front-left wheel, a rear-right wheel and a rear-left wheel is calculated 
based on the output of wheel speed sensors not shown. In step 120, a 
vehicle body speed VB is calculated based on each wheel speed VW. In step 
130, a wheel acceleration dVW of each wheel is calculated. In step 140, a 
slip ratio SW of each wheel is calculated. 
In step 150, it is determined whether the slip ratio SW of a wheel which is 
an control object at present is larger than the first reference slip ratio 
KSW or not. If the negative determination is made, since it is determined 
that the wheel is not likely to be locked, the processing advances to step 
160. In step 160, an ABS flag F is set at F=0, whereby a brake system is 
set in an ordinary braking state. If the affirmative determination is made 
in step 150, the processing advances to step 170. In step 170, the ABS 
flag F which shows that the brake system is under anti- skid control is 
set at F=ABS. Moreover, in step 170, electric power is supplied to a motor 
(not shown) for driving a pump at the same time. 
In step 180, it is determined whether the slip ratio SW of the wheel which 
is a control object at present is larger than a second reference slip 
ratio MSW (SW&gt;MSW) or not. When a negative determination is made, the 
processing advances to step 190. In step 190, pressure increasing duty 
control is performed to the wheel which is the control object. For 
example, if a wheel provided with a wheel cylinder 52 is a wheel to be 
controlled, in pressure increasing duty control, a pulse-like current is 
provided to the pressure increasing control valve 50. At this time, the 
pressure decreasing control valve 51 is maintained in a closed valve 
position as in the case of the normal braking state. In many cases, step 
190 is carried out in two or more control cycles later after the anti-skid 
control is started and decreasing control of the wheel cylinder pressure 
of a control object wheel is once performed. 
If the affirmative determination is made in step 180, step 200 is 
performed. In step 200, it is determined whether the sign of the wheel 
acceleration dvw of the control object wheel is negative or not. That is, 
it is determined whether the wheel speed of the control object wheel is 
decelerating or accelerating. If the negative determination is made, step 
210 is performed. In step 210, because it can be considered that the wheel 
speed tends to recover toward the vehicle body speed VB and that the wheel 
cylinder pressure has been suitably adjusted, pressure holding control is 
performed to hold the brake fluid pressure applied to the wheel cylinder 
of the control object wheel. For example, in this pressure holding 
control, current is continuously provided to the pressure increasing 
control valve 50 to set the valve position in a closed state and current 
is not provided to the pressure decreasing control valve 51 to set the 
valve position in a closed state. 
If the determination is affirmative in step 200, step 220 is performed. It 
is determined in the step 220 whether the wheel acceleration dvw of the 
control object wheel is smaller than a reference wheel acceleration KdVW 
(KdVW&lt;0) or not. If the determination is affirmative, step 230 is 
performed. In step 230, the wheel cylinder pressure of the control object 
wheel is continuously decreased. For example, current is continuously 
provided to both the pressure increasing control valve 50 and the pressure 
decreasing control valve 51 for a specified time, whereby the wheel 
cylinder pressure is rapidly decreased when it is estimated that the wheel 
speed is being decelerated by a large deceleration and the tendency for 
the wheel to be locked is strong. If the determination is negative in step 
220, step 240 is performed. In step 240, a pressure decreasing duty 
control is performed for the wheel to be controlled. The pressure 
decreasing duty control is performed when the wheel cylinder pressure of 
the control object need not much rapidly be decreased. For example, in 
this pressure decreasing duty control, current is not provided to the 
pressure increasing control valve 50 and therefore its valve position is 
held in a communicating state, and the pressure decreasing control valve 
is operated under duty control, whereby the wheel cylinder pressure is 
decreased or increased in response to duty control of the pressure 
decreasing control valve. As a result, smooth decrease in the wheel 
cylinder pressure can be realized. 
If control according to the flow chart shown in FIG. 8 is executed, in step 
190, only the pressure increasing control valve among a pair of pressure 
increasing control valve and pressure decreasing control valve which are 
provided for one wheel cylinder is controlled under duty control. While 
the pressure increasing control valve is operated under duty control, the 
current is not provided to the pressure decreasing control valve. That is, 
the pressure decreasing control valve is maintained in an OFF-state. On 
the contrary, in a step 240, only the pressure decreasing control valve 
among the pair of the pressure increasing control valve and the pressure 
decreasing control valve is operated under duty control. While the 
pressure decreasing control valve is operated under duty control, the 
current is not provided to the pressure increasing control valve. That is, 
the pressure increasing control valve is in an OFF-state. Therefore, if, 
in the anti- skid control device in which control shown in FIG. 8 is 
performed, a recirculation transistor not shown is additionally provided 
in parallel to the recirculation transistor 40 between the connecting 
point of the first solenoid valves 3 and the first driving IC 4 and the 
connecting point of the first solenoid valve 5 and the second driving IC 6 
in FIG. 4 such that it allows current to pass in the direction opposite to 
the recirculation transistor 40 and these recirculation transistors are 
turned on in response to duty control of each of the pressure increasing 
control valve and the pressure decreasing control valve, the above 
described functions and advantages can be obtained. Specifically, the 
recirculation transistor 40 is turned on in step 190 and the recirculation 
transistor connected in parallel to the recirculation transistor 40 is 
turned on step 240. 
Moreover, if step 220 and step 240 are omitted in the flow chart shown in 
FIG. 8 and step 230 is performed when the determination in step 200 is 
affirmative, only the pressure increasing control valve among the pair of 
the pressure increasing control valve and the pressure decreasing control 
valve which are constituted for one wheel cylinder is operated under duty 
control in step 190. While the pressure increasing control valve is 
operated under duty control, the current is not provided to the pressure 
decreasing control valve. Therefore, the constitution of the first 
embodiment, the second embodiment or the like (constitutions shown in FIG. 
1 to FIG. 6) in which the solenoid of the pressure decreasing control 
valve acts as a choke coil can be adopted for the control circuit of the 
brake system. Furthermore, even if the solenoid valve driving devices in 
the above described embodiments are applied to drive an air valve used in 
an air conditioner, it can obtain the same advantages as described above.