JPH10252930A - Solenoid valve drive device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably perform the switching from the over drive to the holding drive without any complicated timing control. SOLUTION: A holding drive distribution switch circuit 62 is turned on in response to the condition that the level of high voltage to be applied to a solenoid SL1 becomes smaller than the battery voltage VB due to the over-drive, a capacitor 5 is charged by the back electromotive force to be generated in the solenoid SL1 through the holding drive distribution switching circuit 62 when the level of the first over-drive control signal S11 rises, a constant current circuit 4 is operated when the charge to the capacitor 5 is advanced, and the charged voltage reaches the prescribed level, and the prescribed current for the holding drive is supplied to the solenoid SL1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic valve driving device which applies a high voltage for a predetermined time when an electromagnetic valve is opened to supply an overdrive current to a solenoid coil to shorten the valve opening time. Things.

[0002]

2. Description of the Related Art For example, in order to shorten the valve opening time and valve closing time of a fuel injection solenoid valve, a high voltage is applied to a solenoid of the solenoid valve when the valve is opened, and an overdrive current flows. Japanese Patent Application Laid-Open No. Hei 8-306528 discloses a driving method in which a voltage having the opposite polarity (the same polarity as the induced electromotive force) is applied to the device to flow a demagnetizing current. To drive the solenoid valve as described above, (1) an overdrive drive power supply and switching means for distributing and switching the power supply,
(2) Holding drive power supply for holding the opening of the injection valve and switching means for distributing and switching the same, (3) Demagnetizing (reverse excitation) driving power supply for accelerating the closing of the injection valve and distributive switching It is necessary to provide switching means, and to prepare individual timing signals for each switching means.

[0003] As described above, the overdrive current is supplied to the solenoid of the solenoid valve to shorten the valve opening time.
If a configuration is used in which a demagnetizing current flows by applying a voltage of the opposite polarity when the valve is closed, the size of the control signal generation circuit tends to increase, and the amount of hardware tends to increase.

[0004]

In particular, when the overdrive current state at the time of opening the valve is shifted to the holding current state, the current is sharply dropped to generate an induced voltage, which is used for degaussing drive power supply or failure diagnosis. When adopting the configuration used,
Since the current flowing through the solenoid rapidly decreases in a short period of time, the timing signal at this time is not accurate, especially if the time difference between the overdrive end timing and the timing to enter the holding current mode is inappropriate, the time difference may be increased. If the amount is small, sufficient degaussing driving energy is not accumulated. On the other hand, if the time difference is too long, the solenoid driving current falls below the rating, and in the worst case, inconveniences such as valve closing during driving occur.

Accordingly, the timing of the control signal must be accurate, a more accurate timer is required, and it is necessary to use high-precision parts. In addition to the problem that the circuit scale and the amount of hardware increase, there is also a problem that the temperature dependency of the timer circuit makes it difficult to operate in a wide temperature range.

[0006] It is therefore an object of the present invention to provide a solenoid valve drive which can solve the above-mentioned problems in the prior art.

[0007]

The features of the present invention for solving the above-mentioned problems include a high-voltage power supply for supplying a high voltage for overdrive, a constant current circuit for holding and driving, and a high-voltage power supply for the high-voltage power supply. A high voltage on / off switch provided on the output side for controlling application of the high voltage to the solenoid of the solenoid valve in response to an external overdrive control signal having a level lower than the high voltage; on/
A capacitor for storing electrical energy due to back electromotive force generated in the solenoid in response to the off switch being turned on from off, and the overdrive of the solenoid valve due to application of the high voltage. In an electromagnetic valve driving device configured to cause a holding drive current to flow from the constant current circuit to the solenoid at a predetermined timing after the termination, the overdrive control signal provided between the high voltage switch and the high side of the solenoid Self-holding first switch means that is controlled to open and close in response;
The voltage level of the high side of the solenoid is higher than a predetermined level in response to the overdrive control signal, which is provided between the high side of the solenoid and the constant current circuit, with a delay more than the first switch means. A self-holding type second switch which is turned on when the voltage becomes smaller; and a current controller which is responsive to a charge voltage of the capacitor and switches a return loop when the charge voltage reaches a predetermined level. It is in the point.

According to this configuration, when the high voltage on / off switch is turned on in response to the overdrive control signal, the first switch is turned on, and the on state is maintained. As a result, a high voltage is applied to the solenoid from the high voltage power supply, and the high side is at substantially the same level as the high voltage.

On the other hand, the overdrive control signal is simultaneously applied to the second switch means, but since the second switch means has a slower response speed than the first switch means, the solenoid is turned on when the first switch means is turned on. Will respond to the overdrive control signal after the high side becomes high level. Therefore, the second switch means is not turned on at this time and maintains the off state.

Thereafter, as time passes, the level of the high voltage applied to the high side of the solenoid gradually decreases due to the release of electric charge from the high voltage power supply, and when the level becomes lower than the level of the overdrive control signal, the second switch means is turned on. And the on state is maintained. Next, when the application of the overdrive control signal is stopped and the high voltage on / off switch is turned off, a counter electromotive force is generated in the solenoid, and the capacitor starts to be charged. When the charging voltage of the capacitor exceeds a predetermined level, the current control means is operated to switch the return loop, and the holding drive current from the constant current source is supplied to the solenoid via the second switch means.

As a result, fine timing adjustment is required without separately preparing a timing signal corresponding to a predetermined period to be held and driven with respect to the constant current circuit and the second switch means. Therefore, the switching from the overdrive to the holding drive can be performed stably and reliably.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing an example of an embodiment of a solenoid valve driving device according to the present invention. The electromagnetic valve driving device 1 shown in FIG. 1 includes two fuel injection valves FV1 and FV2.
Are alternately driven, so that the fuel injection valves FV1, FV
The current for driving is alternately supplied to each of the solenoids SL1 and SL2 at a predetermined timing.
In these solenoids SL1 and SL2, a high voltage is applied for a predetermined time immediately after the start of driving, and overdrive driving is performed to shorten the valve opening time of the fuel injection valve, and thereafter maintain the valve open state. Current is supplied to the fuel injection valve, and finally the fuel injection valve is driven so as to shorten the valve closing time by performing reverse excitation.

For this reason, the solenoid valve driving device 1 turns on the high voltage obtained from the overdrive voltage boosting circuit 2 and the high voltage obtained from the overdrive voltage boosting circuit 2 for obtaining the high voltage for overdrive from the battery voltage VB. A high-voltage switch 3 for off-control, a constant current circuit 4 for supplying a holding drive current, a capacitor 5 for storing energy for back excitation by back electromotive force generated in the solenoids SL1 and SL2, And a current detecting resistor 42 for detecting the holding drive current and the charging current of the capacitor 5. The high voltage from the high-voltage switch 3, the holding drive current from the constant current circuit 4, and the charging voltage of the capacitor 5 correspond to the first switch unit 6 and the second switch unit 6 provided corresponding to the fuel injection valves FV 1 and FV 2. The fuel is distributed by the switch unit 7 and supplied to the solenoids SL1 and SL2 of the fuel injection valves FV1 and FV2 at a predetermined timing. Since the first switch unit 6 and the second switch unit 7 have the same configuration, the detailed configuration of the second switch unit 7 is omitted in FIG.

The overdrive booster circuit 2 and the high-voltage switch 3 are a booster circuit and a switching circuit having a known configuration, and their detailed circuit diagrams are shown in FIG. In FIG. 2, the overdrive booster circuit 2 includes a booster coil 21, a diode 22, and a high-voltage storage capacitor 2.
3. The switching transistor 24 is connected as shown in the figure, and has a known circuit configuration in which the boost control circuit 25 controls the switching transistor 24 to turn on and off in response to the terminal voltage of the capacitor 23. The high-voltage switch 3 is composed of a switching transistor 31 and is turned on and off in response to an overdrive control signal S1 supplied from a control signal generation unit, which will be described later, to a gate electrode 31A of the high-voltage switch via a drive circuit 32. The output of the high voltage stored in 23 is controlled.

Returning to FIG. 1, the constant current circuit 4 has a switching transistor 41 for turning on and off the application of the battery voltage VB, and a constant current control circuit 44. The voltage generated at both ends is supplied to the constant current control circuit 44 as a feedback control signal as a detection voltage signal VD indicating the level of the current flowing therethrough.

The constant current circuit 4 is further provided with a series circuit of a flywheel diode 45 and a switching circuit 46 between the source of the switching transistor 41 and the ground as shown in FIG. A flywheel diode control circuit 47 is provided for performing control. The flywheel diode control circuit 47 responds to the charging voltage of the capacitor 5 and the injection control signal D1 indicating the fuel injection period, and when the injection control signal D1 indicates that the injection period is in progress, the charge voltage of the capacitor 5 is reduced. When the signal has reached the predetermined level, a signal for turning on the switching circuit 46 is output.

The constant current control circuit 44 has an injection control signal D1
When the operation is permitted, the switching transistor 41 is turned on and off so that the detection result of the current detection resistor 42 indicates a predetermined constant current value, thereby controlling the solenoid SL1 or SL2 to a predetermined level. Can be supplied.

The first switch unit 6 includes an overdrive distribution switch circuit 61 for controlling application of a high voltage from the high voltage switch 3 to the solenoid SL1;
A holding drive distribution switch circuit 62 for controlling the flow of the holding drive current supplied from the constant current circuit 4 to the solenoid SL1, and a control for flowing the energy stored in the capacitor 5 for reverse excitation of the solenoid SL1. And a demagnetizing drive distribution switch circuit 63 for performing the operation. First
Each of these circuits provided in the switch unit 6 includes a first overdrive control signal S11 and a first reverse excitation control signal R1 supplied from a control signal generation unit described later.
Operate in response to

FIG. 3 shows a detailed circuit diagram of the first switch unit 6. In the overdrive distribution switch circuit 61, a thyristor 611 having an anode connected to the high-voltage switch 3 and a cathode connected to the high side of the solenoid SL1 is provided as a switching element, and a thyristor 611 is provided between the cathode and the gate. A parallel circuit of a resistor 612 and a capacitor 613 is connected. The ignition control circuit 61 is provided at the gate of the thyristor 611.
4 is provided, and the first overdrive control signal S
11 is applied to the input terminal of the ignition control circuit 614. The ignition control circuit 614 includes transistors 615 and 61
6, the resistors 617 to 621 and the diode 622 are connected as shown in the figure, and when the first overdrive control signal S11 changes from a low level to a high level, the potential of the gate of the thyristor 611 is substantially changed to the battery voltage VB. Potential. As a result, the potential of the gate becomes higher than the potential of the cathode, and the thyristor 611 is turned on. Here, since the thyristor 611 is a self-holding switching element, once turned on in this way, the thyristor 611 can maintain the on state even if the gate potential returns to the ground level again thereafter.

The holding drive distribution switch circuit 62 also includes a thyristor 62 having an anode connected to the high voltage switch 3 and a cathode connected to the high side of the solenoid SL1.
3 is provided as a switching element, and a parallel circuit of a resistor 624 and a capacitor 625 is connected between the cathode and the gate of the thyristor 623. A firing control circuit 626 is provided at the gate of the thyristor 623. The ignition control circuit 626 includes a transistor 627, resistors 628 to 630, and a diode 631 connected as shown in FIG.
It operates in response to the potential of 15 collectors. Therefore, the ignition control circuit 626 also sets the potential of the gate of the thyristor 623 to a potential substantially equal to the battery voltage VB when the first overdrive control signal S11 changes from the low level to the high level.

However, in the holding drive distribution switch circuit 62, the value of the capacitor 625 is selected to be larger than the value of the capacitor 613, so that the resistor 624
The time constant of the thyristor 611
Are set to be larger than the time constants of the resistor 612 and the capacitor 613 provided in the first stage. Therefore,
When the level of the first overdrive control signal S11 changes from a low level to a high level, the gate voltage of the thyristor 611 rises rapidly, but the gate voltage of the thyristor 623 rises slowly. That is,
The response speed of the overdrive distribution switch circuit 61 to the first overdrive control signal S11 is fast, and the response speed of the holding drive distribution switch circuit 62 is slow.

As a result, when the level of the first overdrive control signal S11 changes from a low level to a high level, first, the thyristor 611 is turned on within a short time,
A high voltage is applied to the high side of the solenoid SL1.
Thereafter, the potential of the gate of the thyristor 611 gradually rises. However, since the potential of the cathode of the thyristor 611 has already risen to the high voltage level, even if the potential of the gate reaches the battery voltage VB, the thyristor 611 also rises. 623
Are not turned on and remain off. After a lapse of a predetermined time, as described later, the high-voltage switch 3 is turned off, the high-side potential of the solenoid SL1 decreases, and the thyristor 623 turns on and self-holds only when the potential of the solenoid SL1 becomes lower than the level of the battery voltage VB. Enter the state. As a result, switching from overdrive to holding drive can be performed stably and reliably without fine timing adjustment. This will be described in more detail later.

The demagnetizing drive distribution switch 63 is configured as a switching circuit in which a thyristor 632, a resistor 633, and a capacitor 634 are connected as shown. A first signal supplied from a control signal generation unit described later
When the reverse excitation control signal R1 is applied to the gate of the thyristor 632 via the drive circuit 635, the thyristor 632 is turned on, and the high side of the solenoid SL1 and one end of the capacitor 5 are conducted by the thyristor 632 and stored in the capacitor 5. The applied high-pressure energy flows in the reverse direction to the solenoid SL1, and the solenoid SL1 is reversely excited. That is, an operation for shortening the valve closing time of the fuel injection valve FV1 is performed.

FIG. 4 is a circuit diagram of a control signal generating unit for generating various control signals to be supplied to each part of the solenoid valve driving device 1 shown in FIG. The control signal generation unit 9 includes inverters 91 and 92, an OR gate 93,
First timer 94, second timer 95, and AND gate 9
6 to 99 are connected as shown in FIG.
The first drive signal N1 indicating the injection period DT1 of the fuel injection valve FV1 is applied to the
The second drive signal N2 indicating the injection period DT2 of V2 is applied.

Next, the operation of the control signal generation unit 9 will be described with reference to FIG. Both the first drive signal N1 and the second drive signal N2 correspond to the respective injection periods DT1, D
This signal is a low level signal when T2 is indicated, and the injection periods DT1 and DT2 are determined so as not to overlap.

The first drive signal N 1 and the second drive signal N 2 are inverted in level by the corresponding inverters 91 and 92, taken out through the OR gate 93, and output by the first timer 9.
4 so that the overdrive control signal S
1 is taken out. As shown in FIG. 5 (H), the overdrive control signal S1 is a signal which is at a high level during a predetermined period TP which is an overdrive period from the fall timing of each of the first drive signal N1 and the second drive signal N2. , A signal indicating the overdrive period of the fuel injection valve FV1 and the fuel injection valve FV2. A first overdrive control signal S11 indicating only the overdrive period of the fuel injection valve FV1 is output from the AND gate 96 (FIG. 5B), and the fuel injection valve FV2 is output from the AND gate 97.
A second overdrive control signal S12 indicating only the second overdrive control signal is output (FIG. 5E). In addition, from the OR gate 93,
An injection control signal D1 indicating the fuel injection periods DT1 and DT2 of the fuel injection valve FV1 and the fuel injection valve FV2 is output (FIG. 5 (G)).

The output from the OR gate 93 is the second timer 9
5, an output is obtained which is at a high level for a predetermined period TR which is a reverse excitation period from each rising timing of the first drive signal N1 and the second drive signal N2. Second
The output from the timer 95 is processed by the AND gates 98 and 99, and a first signal indicating a reverse excitation period for the fuel injection valve FV1.
The reverse excitation control signal R1 (FIG. 5C) and the fuel injection valve FV
2. A second reverse excitation control signal R2 indicating a reverse excitation period for
(FIG. 5F) is output.

Next, the operation of the solenoid valve driving device 1 shown in FIG. 1 will be described with reference to FIG. FIG. 6 (A)
Is a level inversion signal of the first drive signal N1 shown in FIG. 5A and is a signal indicating the injection period of the fuel injection valve FV1. The signals shown in (B) to (E) of FIG.
(B), (G), (H), and (C). In the following, only the drive control of the solenoid SL1 for fuel injection by the fuel injection valve FV1 will be described.

First, at time t1, the first drive signal N1
Rises, the levels of the first overdrive control signal S11, the injection control signal D1, and the overdrive control signal S1 also rise at the same time. As a result, the high-voltage switch 3 is turned on, and at the same time, the overdrive distribution switch circuit 61 is turned on.
A high voltage is applied to the solenoid SL1. On the other hand, since the value of the capacitor 625 of the holding drive distribution switch circuit 62 is larger than the value of the capacitor 613, the holding drive distribution switch circuit 62 does not immediately turn on at the time t1 and remains off for a while. As described above.

Due to the discharge of the capacitor 23 of the booster circuit 2 for overdrive, the level of the voltage E applied to the solenoid SL1 decreases over time (FIG. 6).
(F)), time t at which the overdrive period TP ends
At time t2, which is slightly earlier than the third voltage, the voltage E becomes lower than the battery voltage VB.
Turns on.

At time t3, when the level of the first overdrive control signal S11 falls and the high pressure switch 3 is turned off, a back electromotive force is generated in the fuel injection valve FV1,
The terminal voltage of the solenoid SL1 of the fuel injection valve FV1 becomes negative. Thereby, the circulating current indicated by the symbol LP1 in FIG. 1 flows through the holding drive distribution switch circuit 62 which is already turned on at the time t2. That is, from the solenoid SL1 of the fuel injection valve FV1, the capacitor 5,
A current that returns to the solenoid SL1 flows through the diode 8, the current detection resistor 42, and the holding drive distribution switch circuit 62, whereby electric charge is accumulated in the capacitor 5. At this time, since the switching circuit 46 is off, the high side of the solenoid SL1 is set to 0 by the diode 45.
It is not clamped to V.

Therefore, the capacitor 5 is charged to the polarity shown in FIG. 1, and when the flywheel diode control circuit 47 detects that the charged voltage has reached a predetermined value, the switching circuit 46 is turned on. This is time t4.

When the switching circuit 46 is turned on, first, a current due to the back electromotive force of the solenoid SL1 flows through the flywheel diode 45. This is the circulating current indicated by the symbol LP2. The constant current control circuit 44 is activated by the control signal D1. Since the circulating current LP2 passes through the current detecting resistor 42, when the level of the circulating current LP2 becomes smaller than a predetermined value, the switching transistor 41 is turned on, and current is supplied from a battery (not shown) to the solenoid SL1. The constant current control circuit 44 controls on / off of the switching transistor 41 so that the drive current to the solenoid SL1 is maintained at a predetermined level. The waveform of the voltage E at this time is a rectangular wave (see FIG. 6F). This constant current supply operation is continued while the injection control signal D1 is in the high level state. When the injection control signal D1 becomes low level at time t5, the holding drive of the fuel injection valve FV1 by the constant current supply ends.

Next, at time t5, the degaussing drive distribution switch circuit 63 is turned on in response to the first reverse excitation control signal R1 (FIG. 6 (E)), and the negative voltage stored in the capacitor 5 is degaussed. The current is applied to the solenoid SL1 via the distribution switch circuit 63, and a demagnetizing current flows in the solenoid SL1 in a direction opposite to the current driving current, and the solenoid SL1 is reversely excited. The level of this demagnetizing current decreases with the passage of time due to the discharge of the capacitor 5, and at time t6
At the moment, the level of the first reverse excitation control signal R1 returns to the low level state. As a result, the closing time of the fuel injection valve FV1 can be reduced.

If the driving current drops to the holding driving level before the charging of the capacitor 5 is completed, the return loop remains at LP1. In this case, the constant current control circuit 44 turns on the switch 41 to start constant current driving because the current detection resistor 42 can detect that the drive current has decreased. Since the return loop remains at LP1, the charging voltage of the capacitor 5 gradually increases and reaches a predetermined level while the constant current drive (switching method) is performed. Thereafter, the fly-hole diode is turned on and the return loop is changed to LP2. Therefore, even in such a case, the drive current does not fall below the holding drive level.

As can be seen from the above description, the switching from the overdrive to the holding drive is performed in response to the fact that the level of the high voltage applied to the solenoid SL1 for the overdrive has become smaller than the battery voltage VB. The drive distribution switch circuit 62 is configured to be turned on, and the capacitor 5 is charged by the back electromotive force generated in the solenoid SL1 when the level of the first overdrive control signal S11 falls. 62 through the capacitor 5
The constant current circuit 4 is activated when the charging of the battery SL proceeds and the charging voltage reaches a predetermined level, so that a constant current for holding and driving can be supplied to the solenoid SL1. Therefore, it is not necessary to prepare a special control signal indicating a timing for starting the operation of the constant current circuit 4, and the switching from the overdrive to the holding drive can always be appropriately performed based on the operation state of the circuit after the end of the overdrive. Done. That is, the transition from the overdrive drive to the hold drive is performed stably, and it is possible to cope with more severe drive characteristics.

Due to the above structural features, the amount of hardware of the control unit of the electromagnetic drive device 1 can be reduced, the circuit can be downsized, and fine timing adjustment of the circuit is not required. No parts are required, and the cost of the apparatus can be reduced. In addition, since there is no need for fine timing adjustment, manufacturing variations can be reduced, and
The usable temperature range of the device can be extended. Further, even if the characteristics and specifications of the fuel injection valve are changed, it is not necessary to readjust the timing, and the applicable fuel injection valve can be expanded.

Although the operation of the first switch unit 6 has mainly been described above, the operation of the second switch unit 7 for switching the drive current for the fuel injection valve FV2 is exactly the same. Further, in the present embodiment, the example in which the first switch unit 6 and the second switch unit 7 operate alternately and the current for overdrive and the constant current circuit are shared has been described. It is not limited to the embodiment.

[0040]

According to the present invention, the following effects can be obtained. (1) The amount of hardware of the control unit of the solenoid valve driving device can be reduced, and the circuit becomes compact. (2) Since fine timing adjustment is not required, high precision components are not required. (3) For the above reasons, the cost of the apparatus can be reduced. (4) Fine timing adjustment is not required, and manufacturing variations can be reduced. (5) Since delicate timing adjustment is not required, the specifiable temperature range of the device can be expanded. (6) Even if the characteristics / specifications of the solenoid valve are changed, it is not necessary to readjust the timing, and the applicable solenoid valve can be expanded. (7) The transition from the overdrive drive to the hold drive is performed stably, and it is possible to cope with more severe drive characteristics.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of an embodiment of a solenoid valve driving device according to the present invention.

FIG. 2 is a detailed circuit diagram of the overdrive booster circuit and the high-voltage switch shown in FIG. 1;

FIG. 3 is a detailed circuit diagram of a first switch unit shown in FIG. 1;

4 is a circuit diagram of a control signal generation unit for generating various control signals for the solenoid valve driving device shown in FIG.

FIG. 5 is a waveform diagram showing waveforms of input / output signals of the control signal generation unit shown in FIG.

FIG. 6 is a waveform chart of signals of respective parts for explaining the operation of the solenoid valve driving device shown in FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solenoid valve drive device 2 Overdrive booster circuit 3 High voltage switch 4 Constant current circuit 4A Current path 5 Capacitor 6 First switch unit 7 Second switch unit 42 Current detection resistor 44 Constant current control circuit 45 Flywheel diode 46 Switching Circuit 47 Flywheel diode control circuit 61 Overdrive distribution switch circuit 62 Holding drive distribution switch circuit 63 Demagnetization drive distribution switch circuit D1 Injection control signal FV1 Fuel injection valve FV2 Fuel injection valve R1 First reverse excitation control signal S1 Overdrive control signal S11 First overdrive control signal SL1 solenoid SL2 solenoid VB Battery voltage VD Detection voltage signal

Claims (1)

[Claims] 1. A high-voltage power supply for supplying a high voltage for overdrive, a constant current circuit for holding and driving, and an external power supply provided at an output side of the high-voltage power supply and having a lower level than the high voltage. A high-voltage on / off switch for controlling the application of the high voltage to the solenoid of the solenoid valve in response to the overdrive control signal; and A capacitor for storing electrical energy due to back electromotive force generated in the solenoid, and a holding drive current from the constant current circuit at a predetermined timing after the overdrive of the solenoid valve by application of the high voltage is completed. In the solenoid valve drive device, wherein the solenoid valve drive device is provided between the high pressure switch and a high side of the solenoid. Self-holding first switch means that is controlled to open and close in response to an overdrive control signal; and the first switch means provided between the high side of the solenoid and the constant current circuit in response to the overdrive control signal. A self-holding type second switch means that is turned on when the voltage level of the high side of the solenoid becomes smaller than a predetermined level in response to the charging voltage, and the charging voltage in response to the charging voltage of the capacitor. And a current control means for switching the return loop when the pressure reaches a predetermined level.