Patent Publication Number: US-6657844-B2

Title: Electromagnetic drive control device

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electromagnetic drive control device for controlling electromagnetic driving devices which are employed in various instruments as a drive source. 
     An electromagnetic driving device typically drives an actuator making use of an electromagnetic force generated between a permanent magnet and a coil in which an electrical current flows. The driving force can be controlled by controlling the current flowing through the coil. Since the electromagnetic driving device can be made relatively compact in size, it is widely employed, as a driving source, for an objective lens driving device of a camera, a scanning position compensating device for a laser scanning device, a driving device for a linear motor car, and the like. 
     As an example, the scanning position compensating device will be described. In the scanning position compensating device for a laser scanning device, a driving coil is swingably (rotatably) supported in a magnetic field generated by a magnet which is secured to the electromagnetic driving device. As the electrical current is fed to the driving coil, it swings due to the electromagnetic force. The driving coil typically supports a prism, which swings with respect to the optical axis as the driving coil swings, thereby deflecting a passage of the laser beam. In this type of drive control device for controlling the electromagnetic drive that controls the direction of the electrical current flowing through the coil, a circuit as shown in FIG. 9 is generally employed. 
     The circuit shown in FIG. 9 includes a drive circuit  41 , which includes an operational amplifier OP, a resistor R, and a current buffer circuit  411 . The current buffer circuit  411  is configured such that an NPN transistor TR 1  and a PNP transistor TR 2  are connected in accordance with a complimentary emitter follower connection. The drive circuit  41  is a so-called voltage-current conversion circuit, which outputs an electrical current in accordance with a voltage of an input drive control signal CS to a drive circuit  30 . 
     In such a voltage-current conversion type drive control circuit, the output current I is grounded through a drive coil  24  and the resistor R. The drive circuit  41  operates such that the voltage R*I equals the drive control signal CS. When the drive control signal CS is positive, a positive voltage +Vcc is applied to an terminal A of the drive coil  24 , and thus, the current flows from the terminal A to a terminal B. When the drive control signal CS is negative, a negative voltage −Vcc is applied to the terminal A, thereby the electrical current flowing from the terminal B to the terminal A. As the direction of the electrical current flowing through the drive coil  24  switches as described above, the direction of the electromagnetic force caused between the drive coil  24  and the magnet  223  switches. Thus, the drive coil  24  can be driven to operate as desired. Further, depending on the voltage of the drive control signal CS, the voltage output by the drive circuit  41  varies. Then, the current flowing through the drive coil  24  varies, and the electromagnetic force between the drive coil  24  and the magnet  223  varies. Accordingly, by controlling the voltage of the drive control signal CS, the amount of the swing movement of the drive coil  24  can be controlled. 
     In the conventional drive control circuit as described above, when power sources (i.e., +Vcc and −Vcc) are turned from ON to OFF and the voltages change from 0V to designated values (+Vcc and −Vcc), one of the power sources may reach the designated voltage earlier than the other. In such a case, the performance of the circuit may become unstable. In a particular case, the output of the operational amplifier OP is fixed, for example, to +Vcc or −Vcc. In such a case, a maximum (or minimum) drive current is output from the drive circuit  41  to the drive coil  24 . Then, an electrical damage and/or a mechanical damage of the electromagnetic drive device will be caused. 
     Further to the above, when the power sources are in OFF condition, the following problem may occur. When the power sources are in OFF condition, no electrical current flows through the coil  24 . Since the drive coil  24  is in an electrically open status, no electromagnetic force is generated between the drive coil  24  and the magnet  223  when the power sources are in the OFF condition. If an oscillation or a shock is applied from outside to the drive coil  24  under such a condition, the drive coil  24  may be swung greatly exceeding a limited movable range. In such a case, thin feed lines connected to the drive coil  24  may be cut, or a supporting mechanism for the drive coil  24  may be mechanically damaged. 
     As described above, the conventional drive control device provided with two power sources has defects. 
     It should be noted that a drive control device employing a single power source also has a similar problem, if a relatively long time is required till the voltage of the power source reaches the designated value after turning ON the power source. In such a case, the maximum current may flow through the drive coil and the electromagnetic drive device may be electrically damaged when the power source is turned ON. Further, since the coil is in the unstable condition when the power source is in the OFF condition, the electromagnetic drive device may be mechanically damaged due to the external oscillation or shock. 
     As explained above, in the conventional electromagnetic drive control device, the operation of the electromagnetic driving device may be unstable, and the life thereof may be relatively short. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, it is an object of the present invention to provide an improved electromagnetic drive control device for an electromagnetic driving device, in which the above-described problems when the power sources are turned ON and/or when the power sources are in the OFF condition are resolved. 
     For the above object, according to the invention there is provided a drive control circuit for an electromagnetic driving device including a magnet and a drive coil that moves due to an electromagnetic force, when an electrical current flows therethrough. The drive control circuit may include a drive circuit that feeds an electrical current to the drive coil, the drive circuit including at least one voltage source, a short-circuit system that short-circuits the drive coil, the short-circuit system releasing the short-circuited condition of the drive coil in accordance with an output voltage of the at least one voltage source. 
     With this configuration, when the voltage source is in the OFF condition, since the drive coil is short-circuited, a counter electromotive force is generated when the external shock or oscillation is applied, which prevents the excessive movement of the drive coil. Further, when the voltage source is turned ON, the output current of the drive control circuit will be or will not be fed to the drive coil depending on the output voltage of the voltage source. Thus, the above-described problem of the overcurrent across the drive coil can be prevented. 
     Optionally, the short-circuit system may include a voltage detection circuit that detects the output voltage of the at least one voltage source. 
     Still optionally, the short-circuit system may include an electromagnetic relay system, which is provided with a magnet coil actuated in accordance with an output of the voltage detection circuit, a contact switch provided between both end terminals of the drive coil, the contact switch neutrally connecting the both end terminals of the drive coil, the contact switch disconnecting the both end terminals of the drive coil when the magnet coil is actuated. 
     Further optionally, the voltage detection circuit may include a switching circuit connected between the at least one voltage source and the magnet coil, the switching circuit being turned ON to connect the at least one voltage source and the magnetic coil when the output of the voltage source has satisfied a predetermined condition. 
     Still optionally, the drive circuit may have an input terminal to which a control signal is input, the drive circuit outputting an electrical current to the drive coil through the short-circuit system. 
     In a particular case, the at least one voltage source includes a positive voltage source and a negative voltage source. In this case, the short-circuit system may include a first voltage detection circuit that detects the output voltage of the positive voltage source and a second voltage detection circuit that detects the output voltage of the negative voltage source, and the short-circuit system may maintain or release the short-circuited condition of the drive coil in accordance with the output voltages of the positive and negative voltage sources. 
     According to one embodiment, the short-circuit system releases the short-circuited condition of the drive coil when the absolute values of the output voltages of the positive and negative voltage sources exceed predetermined values, respectively. 
     In this case, the short-circuit system may include an electromagnetic relay system which is provided with a magnet coil, a contact switch provided between both end terminals of the drive coil, the contact switch neutrally connecting the both end terminals of the drive coil, the contact switch disconnecting the both end terminals of the drive coil when the magnet coil is actuated. The voltage detection circuit may include a first switching circuit connected between the positive voltage source and the one end of the magnet coil and a second switching circuit connected between the negative voltage source and the other end of the magnet coil, the first and second switching circuits being turned ON when the absolute values of the output voltages of the positive and negative voltage sources exceed the predetermined values, respectively. 
     According to another embodiment, the short-circuit system releases the short-circuited condition of the drive coil when a difference between the output voltages of the positive and negative voltage sources exceeds a predetermined value. 
     In this case, the short-circuit system includes an electromagnetic relay system which is provided with a magnet coil, a contact switch provided between both end terminals of the drive coil, the contact switch neutrally connecting the both end terminals of the drive coil, the contact switch disconnecting the both end terminals of the drive coil when the magnet coil is actuated. The voltage detection circuit may include a first switching circuit connected between the positive voltage source and the one end of the magnet coil and a second switching circuit connected between the negative voltage source and the other end of the magnet coil, the first and second switching circuits being turned ON when the difference between the output voltages of the positive and negative voltage sources exceeds the predetermined value. 
     According to a further embodiment, the at least one voltage source includes a single voltage source, the short-circuit system includes a single voltage detection circuit that detects the output of the single voltage source, and the short-circuit system maintains or releases the short-circuited condition of the drive coil in accordance with the output voltages of the single voltage sources. 
     In this case, the short-circuit system releases the short-circuited condition of the drive coil when the output voltages of the single voltage sources exceed a predetermined value. 
     Further, the short-circuit system includes an electromagnetic relay system is provided with a magnet coil, a contact switch provided between both end terminals of the drive coil, the contact switch neutrally connecting the both end terminals of the drive coil, the contact switch disconnecting the both end terminals of the drive coil when the magnet coil is actuated, and the voltage detection circuit includes a single switching circuit connected between the single voltage source and one end of the magnet coil, the switching circuit being turned ON when the output voltages of the single voltage sources exceeds the predetermined value. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     FIG. 1 schematically shows a configuration of a laser beam printer, which employs an electromagnetic driving device according to an embodiment of the invention; 
     FIG. 2 is an exploded perspective view of an optical axis adjusting device provided with the electromagnetic driving device; 
     FIG. 3 is a cross sectional view of the optical axis adjusting device taken along the optical axis; 
     FIG. 4 is a circuit of a control device for the electromagnetic driving device according to an embodiment of the present invention; 
     FIGS. 5A-5E show timing charts illustrating an operation of the circuit shown in FIG. 4; 
     FIG. 6 is a modified circuit of the control device for the electromagnetic driving device; 
     FIGS. 7A-7E show timing charts illustrating an operation of the modified circuit shown in FIG. 6; 
     FIG. 8 is another modified circuit of the control device for the electromagnetic driving device; and 
     FIG. 9 shows a conventional circuit of the control device of the electromagnetic driving device. 
    
    
     DESCRIPTION OF THE EMBODIMENT 
     Hereinafter, an embodiment and its modifications will be described with reference to the accompanying drawings. 
     FIG. 1 schematically shows a configuration of a laser beam printer  1 , which employs an electromagnetic driving device according to an embodiment of the invention. The laser beam printer  1  includes a laser diode  101 , a collimating lens  102 , an optical axis adjusting device  103 , a cylindrical lens  104 , a polygonal mirror  105 , and fθ lens  106  and a photoconductive drum  107 . 
     A laser beam LB emitted by the laser diode  101  is collimated by the collimating lens  102 . The collimated laser beam LB is incident on the optical axis adjusting device  103 , by which the position of the optical axis is adjusted (i.e., the passage of the laser beam is adjusted). The beam LB passed through the optical axis adjusting device  103  is incident on the cylindrical lens  104 , and the cross sectional shape of the beam is changed to a circular shape. Then, the thus shaped laser beam LB is incident on the polygonal mirror  105 , which rotates at a high speed. The incident laser beam LB is reflected by the reflective side surfaces of the polygonal mirror  105  so that the laser beam LB scans within a predetermined angular range in a main scanning direction. The scanning beam LB passes through the fθ lens  106 , and is incident on the photoconductive surface  107   a  of the photoconductive drum  107 . The photoconductive drum  107  is arranged such that a beam spot formed by the incident laser beam LB moves in a direction parallel to the rotational axis of the photoconductive drum  107  (a main scanning is performed). While the main scanning movement of the beam spot occurs, the photoconductive drum  107  rotates about the rotational axis (i.e., an auxiliary scanning is performed), so that the circumferential surface of the photoconductive drum  107  is scanned with the laser beam LB. 
     In this embodiment, the optical axis adjusting device  103  includes an electromagnetic driving device. FIG. 2 is an exploded perspective view of the optical axis adjusting device  103 . FIG. 3 is a cross sectional view of the optical axis adjusting device  103  taken along the optical axis. 
     The optical axis adjusting device  103  has a casing  20 , which includes a base  21  formed with a circular opening  211 , a yoke  22  to be secured on the rear side of the base  21 , and a cover  23  to be secured on the front side of the base  21 . On the yoke  22 , a pair of semicylindrical protrusions  222  are formed opposed to each other. The circumferential surfaces of the pair of semicylindrical protrusions  222  are formed to fit in the circular opening  211  of the base  21 . On inner surfaces of the semicylindrical protrusions  222 , semicylindrical magnets  223  are provided. Further, on the yoke  22 , a circular window  221  is defined. Between the magnets  223 , a cylindrical drive coil  24  is arranged such that the central axis of the drive coil  24  coincides with the central axis of the circular window  221 . 
     The drive coil  24  is provided with a pair of horizontally extending swing arms  25 , which are held by the base  21 . With this configuration, the drive coil  24  is swingable, within a predetermined angular range, about the swing arms  25 . The drive coil  24  is formed by winding a line  242  around a cylindrical bobbin  241 , both ends of the line  242  being connected to flexible feed lines  243 , respectively. 
     The bobbin  241  holds a movable prism  26  having an inclined surface such that the inclined surface is perpendicular to the optical axis. 
     The cover  23  is formed with a circular window  231  the central axis of which coinciding with that of the circular opening  211 . In the circular window  231 , a prism  27  having an inclined surface, which is similar to the prism  26 , is fixed such that the inclined surface is oriented in an opposite direction with respect to the movable prism  26  (see FIG.  3 ). 
     The magnets  223  and the drive coil  24  constitute an electromagnetic device  30 . As an electrical current flows through the coil  24  via the feed lines  243 , due to interaction between magnetic fields generated by the magnets  223  and generated by the coil  24 , the coil  24  rotates about the swing arms  25 . The swinging amount (i.e., the swinging angle with respect to its neutral position) is determined by the amount of the electrical current flowing through the coil  24 . 
     As shown in FIG. 3, when the coil  24  swings, the movable prism  26  inclines with respect to the optical axis LO as indicated by dotted lines. Then, the position of the laser beam LB output from the optical axis adjusting device  103  is shifted in an up-down direction in FIG.  3 . The shifting amounts of the output laser beam LB is determined by the swinging angle of the coil  24 . Therefore, by controlling the swinging angle of the drive coil  24 , the optical axis adjustment of the laser beam LB can be performed. 
     FIG. 4 is a circuit diagram of a drive control circuit  40  for the electromagnetic driving device  30  according to an embodiment of the present invention. The drive control circuit  40  employs two power sources. 
     A drive control signal CS is transmitted from an external device. The drive control circuit  40  includes a drive circuit  41 , a bypass unit  42  and a voltage detection circuit  43 . The drive circuit  41  outputs an electrical current to the drive coil  24  in accordance with the drive control signal CS. The bypass unit  42  allows the electrical current output by the drive circuit  41  to flow through the coil  24 , or bypasses the electrical current so as not to flow through the coil  24 . The voltage detection circuit  43  detects voltages of a positive voltage source +Vcc and a negative voltage source −Vcc, which are the power sources of the drive circuit  41 , and controls the bypass unit  42  in accordance with the detected voltages. 
     The operation of the drive circuit  41  will be described in detail hereinafter. The drive circuit  41  has an operational amplifier OP, a resistor R, and a current buffer circuit  411  including an NPN transistor TR 1  and a PNP transistor TR 2 . The resistor R is serially connected between the inverted input terminal of the operational amplifier OP and the ground. The control signal CS is input to the non-inverted input terminal of the operational amplifier OP. A point, where the resistor R and the inverted input terminal of the operational amplifier OP are connected, is connected, via the bypass unit  42 , to a terminal (terminal B) of the drive coil  24 . The current buffer circuit  411  is configured such that the bases of the transistors TR 1  and TR 2  are directly connected, the emitters of the transistors TR 1  and TR 2  are directly connected, the output OPout of the operational amplifier OP is input to the bases of the transistors TR 1  and TR 2 , and the emitters of the transistors TR 1  and TR 2  are connected, via the bypass unit  42 , to the other terminal (terminal A) of the drive coil  24 . The collector of the NPN transistor TR 1  is connected to the positive voltage source +Vcc, and the collector of the PNP transistor TR 2  is connected to the negative voltage source −Vcc. 
     The bypass unit  42  includes an electromagnetic relay which normally turned ON. Specifically, the bypass unit  42  includes a contact switch  421  provided between the terminals A and B of the drive coil  24 , and a magnet coil  422  for turning ON/OFF the contact switch  421 . When the electrical current does not flow through the magnet coil  422  (i.e., in a neutral state), the contact switch  421  is closed (i.e., connects the terminals A and B). When the electrical current flows through the magnet coil  422 , the contact switch  421  is opens, i.e., the contact switch  421  does not connect the terminals A and B. 
     The voltage detection circuit  43  includes a positive side transistor TR 11  which is a PNP transistor connected between one terminal of the magnet coil  422  and the positive power source +Vcc, and a negative side transistor TR 12  which is an NPN transistor connected between the other terminal of the magnet coil  422  and the negative power source −Vcc. 
     The emitter of the positive side transistor TR 11  is connected to the positive power source +Vcc, the collector is connected to one terminal of the magnet coil  422 , and the base is connected to a connection point of dividing resistors R 11  and R 12 , which divide a voltage difference between the positive power source +Vcc and the ground level. With this configuration, when the positive power source +Vcc reaches a predetermined voltage or more, and the base voltage reaches a threshold voltage of the positive side transistor TR 11  or more, the positive side transistor TR 11  is turned ON. 
     The emitter of the negative side transistor TR 12  is connected to the negative power source −Vcc, the collector is connected to the other terminal of the magnet coil  422 , and the base is connected to a connection point of dividing resistors R 13  and R 14 , which divide a voltage difference between the negative power source −Vcc and the ground level. With this configuration, when the absolute value of the negative power source −Vcc reaches a predetermined voltage or more, and the absolute value of the base voltage reaches a threshold voltage of the negative side transistor TR 12  or more, the negative side transistor TR 12  is turned ON. 
     FIGS. 5A-5E show a timing chart illustrating an operation of the circuit shown in FIG.  4 . According to the embodiment, the positive and negative voltages sources are turned ON/OFF in response to the operation of a power switch. It is assumed that the initial voltages of the positive and negative voltage sources are turned OFF, i.e., the voltages +Vcc and −Vcc are both 0V, and that the voltage of the control signal CS is also 0V. In this condition, the transistors TR 11  and TR 12  are both in OFF condition, and no electrical current flows through the magnet coil  422 . Accordingly, the contact switch  421  is closed and the terminals A and B are short-circuited. 
     When the positive and negative voltage sources +Vcc and −Vcc are turned ON, the positive voltage +Vcc increases and the negative voltage −Vcc decreases as shown in FIG.  5 A. It is assumed that the increasing ratio of the positive voltage +Vcc and the decreasing ratio of the negative voltage −Vcc are different as shown in FIG.  5 A. In the example shown in FIG. 5A, the decreasing ratio of the negative voltage −Vcc is smaller. 
     If the control signal CS or the output of the operational amplifier OP is not controlled, an abnormal voltage is output as the output voltage OPout of the operational amplifier OP. Then, as shown in FIG. 5B, a relatively large current is output from the positive voltage source +Vcc to the NPN transistor TR 1 , which is output from the buffer circuit  411 . 
     When the voltage of the positive voltage source +Vcc reaches the threshold value VT, as shown in FIGS. 5A and 5C, the positive side transistor TR 11  is turned ON. At this stage, however, the voltage of the negative voltage source −Vcc has not reached the threshold value −VT, and the negative side transistor TR 12  remains in OFF condition. Accordingly, an electrical current does not flow through the magnet coil  422 , and the contact switch  421  remains closed. Thus, the terminals A and B are shorted, and the drive coil  24  is protected from an overcurrent output from the drive circuit  41 . 
     Thereafter, when the negative voltage −Vcc has reached the threshold value −VT as shown in FIG. 5A, the negative side transistor TR 12  is turned ON as shown in FIG.  5 D. Then, through the positive side transistor TR 11  and the negative side transistor TR 12 , the electrical current flows from the positive voltage source +Vcc to the negative voltage source −Vcc. The electrical current flows through the magnet coil  422  and actuate the same. Then, the contact switch  421  of the bypass unit  42  opens, and accordingly, the electrical current output by the drive circuit  41  flows from the terminal A to the terminal B through the drive coil  24 , and is grounded through the resistor R. At this stage, the control signal CS and the output of the operational amplifier OP have been stabilized, and the overcurrent condition has been resolved. Therefore, the overcurrent does not flow through the drive coil  24 . 
     It should be noted that, when the decreasing ratio of the negative voltage source −Vcc is greater than the increasing ratio of the positive voltage source +Vcc, the similar control is performed, and the coil  24  is protected from the overcurrent. 
     Thus, in the optical axis adjusting device  103  described above, a magnetic force is generated by the drive coil  24 , which swings about the swing axis  25 , thereby inclining the prism  26  to shift the optical axis LO, or the chief ray of the laser beam LB. 
     The shifting amount of the optical axis LO depends on the electrical current flowing through the drive coil  24 , which depends on the output of the drive circuit  41 . The output of the drive circuit  41  depends on the control signal CS. Accordingly, by adjusting the voltage of the control signal CS, the position of the chief ray of the laser beam LB can be adjusted. 
     As described above, when the voltage sources are turned ON, even if one of the positive voltage source and the negative voltage source reaches a predetermined voltage value earlier than the other and the drive circuit outputs the overcurrent, the voltage detection circuit  43  and the bypass unit  42  protects the drive coil from the overcurrent unit until both of the positive and negative voltage sources reach respective threshold values. Therefore, the drive coil  24  is protected from being electrically and/or mechanically damaged. 
     Further, when the voltages sources are turned OFF, the terminals A and B are short-circuited, and the drive coil  24  forms a closed circuit. In this condition, if an oscillation or shock is applied and the drive coil  24  is moved, a counter electromotive force is generated since the coil  24  moves within the magnetic field of the magnets  223 . As well-known in the art, the counter electromotive force generated as above functions to prevent the movement of the drive coil  24  due to the oscillation of the shock externally applied. Accordingly, with the above-described configuration, the drive coil  24  is protected from the external oscillation or the like when the voltage sources are turned OFF. 
     FIG. 6 is a drive control circuit  40 A according to a modification of the embodiment. In this modification, the voltage detection circuit  43  of the above-described embodiment has been changed to a modified voltage detection circuit  43 A. The same reference numerals are given to elements having the similar function to those employed in the above-identified embodiment, and description thereof will not be repeated. 
     As understood from FIG. 6, the resistors R 12  and R 14  shown in FIG. 4 are omitted, and instead, a Zener diode ZD is connected, in a forward direction, between the bases of the transistors TR 11  and TR 12 . Further, a resistor R 15  is provided between the emitter and the positive voltage source +Vcc, and a resistor R 16  is provided between the emitter and the negative voltage source −Vcc. It should be noted that the Zener diode ZD has a characteristic such that the breakdown voltage ZV thereof is substantially the same as a difference ΔVT of the positive voltage +Vcc and the negative voltage −Vcc when both of them reach predetermined values, respectively (i.e., the sum of the absolute values of the positive and negative voltages). 
     FIGS. 7A-7E show timing charts illustrating an operation of the modified circuit shown in FIG.  6 . 
     As shown in FIG. 7A, if one of the positive voltage source +Vcc and the negative voltage source −Vcc reaches the predetermined voltage earlier than the other, only when both of the voltage sources reach the predetermined values, respectively, and the difference ΔVT between the voltage sources exceeds the breakdown voltage ZV of the Zener diode ZD, will the electrical current flow through the magnetic coil, and the contact switch  421  is opened. 
     As shown in FIG. 7B, the overcurrent is output by the drive circuit  41  when the voltages sources are turned ON occurs when the rising operation of one of the transistors of the current buffer circuit  411  is excessively delayed with respect to the rising operation of the other transistor. If rising condition of the both transistors has proceeded in some extent, the overcurrent has disappeared. Further, in an actual circuit, the rising condition of the ON operation of the transistors are substantially the same, a n d the difference will not be so large. Therefore, even when the contact switch  421  is opened based on the difference ΔVT of the positive voltage source +Vcc and the negative voltage source −Vcc as shown in FIGS. 7C and 7D, the overcurrent will not flow through the drive coil  24 . Furthermore, according to the above-described modification, it is not necessary to keep the drive coil  24  short-circuited until the slower voltage source reaches the predetermined voltage. Therefore, the electromagnetic driving device  30  can be controlled rapidly. 
     In the above-described modification, until the difference between the positive voltage +Vcc and the negative voltage −Vcc reaches the predetermined value, the drive coil  24  is short-circuited by the bypass unit  42 . Therefore, the drive coil  24  is in the short-break condition, and the mechanical damage can be prevented. 
     It should be noted that the transistors TR 1 , TR 2 , TR 11 , and TR 12  are not limited to bi-polar transistors, but can be field effect transistors. The bypass circuit  42  is not limited to one employing an electromagnetic relay, but can also be an electronic switch whose ON/OFF condition can be switched by an electrical current flowing therethrough, for example. 
     In the above-described embodiment and modification, two voltage sources are employed. However, the present invention is not limited to the application of such systems, but can be applicable to an electromagnetic driving device employing a single voltage source. FIG. 8 shows a circuit diagram illustrating an example of such a circuit employing a single voltage source. 
     As shown in FIG. 8, the circuit includes a voltage detection circuit  43 B, which includes a transistor TR 11 , resistors R 11  and R 12  connected in series. The base of the transistor TR 11  is connected to a point where the transistor TR 11  and TR 12  are connected. The resistor R 12  is grounded, and the resistor R 11  is connected to the voltage source +Vcc, to which the collector of the transistor TR 11  is also connected. 
     When the voltage source +Vcc is turned ON, until the voltage reaches a predetermined voltage +VT, the transistor TR 11  remains in an OFF condition. Accordingly, an electrical current does not flow through the magnet coil  422 , and the contact switch  421  is closed to short-circuit the drive coil  24 . During this period, an overcurrent may be generated due to a surge voltage or the like. Since the drive coil  24  is short-circuited, it is protected from an electrical damage. 
     When the voltage +Vcc increases and has reached the predetermined voltage +VT, the transistor TR 11  is turned ON, thereby the magnetic coil  422  is actuated to open the contact switch  421 . Then, the output of the drive circuit  41  is applied to the drive coil  24  to drive the electromagnetic driving device  30 . Similarly to the above-described embodiment and modification, when the voltage source is turned OFF, the contact switch  421  is closed to connect the both ends of the drive coil  24 , it is protected from a mechanical damage even if the electromagnetic device  30  is oscillated by an external force. 
     As described above, according to the invention, the drive coil is protected from the overcurrent when a voltage source or voltage sources are turned ON and the voltage thereof is increasing. Further, when the voltage sources are turned OFF, a short-break is applied and the electromagnetic driving device is prevented from mechanical damages. 
     The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2000-379803, filed on Dec. 14, 2000, which is expressly incorporated herein by reference in its entirety.