Solenoid drive circuit

A solenoid drive circuit, when providing a holding current to a solenoid by means of periodically turning a drive transistor on and off, by-passes a Zener diode until immediately before the end of a holding period as a result of an on state of a first switching transistor, and interrupts the by-pass of the Zener diode as a result of an off state of the first switching transistor during a time period immediately before the end of the holding period, as above-mentioned, until the lapse of a predetermined time after the end of the holding period. Thereby, current due to a counter-electromotive force is utilized in order to shorten the on time of the drive transistor to thereby save the supply of electric power. Moreover, by interrupting the by-pass of the Zener diode immediately before the end of the holding period, an off-responsibility of current flowing through the solenoid is not disturbed at the end of the holding period.

BACKGROUND OF THE INVENTION 
The present invention relates to a solenoid drive circuit for driving a 
solenoid valve and the like. 
FIG. 1 is a prior art solenoid drive circuit for driving a solenoid valve. 
A solenoid 1 of the solenoid valve is connected to a power source at a 
first end, and is grounded via a collecter-emitter circuit of a drive 
transistor 2 at a second end. A serial circuit of a diode 3 and a Zener 
diode 4 is inserted in parallel with the solenoid I. A base of the drive 
transistor 2 is connected via a resistor to an input terminal IN wherein a 
drive signal for driving the solenoid valve is applied to the input 
terminal IN. 
According to the drive circuit of FIG. 1, an off-responsibility of current 
flowing through the solenoid 1 rises by existence of the Zener diode 4. 
However, in the drive circuit of FIG. 1, due to existence of the Zener 
diode 4, current due to a counter-electromotive force cannot be used as a 
holding current when a holding current lower than a starting current is to 
be applied to the solenoid 1 via lowering an average current flowing 
through the solenoid 1 by turning the drive transistor 2 on and off 
periodically. Because of this, in the configuration of the circuit of FIG. 
1, it is necessary to increase the supply of electric power in order to 
counteract the shortage due to the inability to utilize the current by 
counter-electromotive force as the holding current, by extending the on 
time of the drive transistor 2. That is, as shown in the drawing of FIG. 
2, which is explanatory of the operation of the circuit of FIG. 1, current 
which flows through the solenoid 1, when the drive transistor 2 is changed 
from on to off, cannot change instantaneously as shown by the full line 
because of the use of the Zener diode. On the other hand, if the Zener 
diode 4 does not exist, current by counter-electromotive force flows 
through the solenoid 1 when the drive transistor 2 is changed from on to 
off, as shown by the broken line. Therefore, if it is possible to utilize 
the current by counter-electromotive force as the holding current, it is 
possible to conserve the supply of electric power during a holding period. 
SUMMARY OF THE INVENTION 
It is an object, therefore, of the present invention to overcome the 
disadvantages and limitations of the prior art solenoid drive circuit by 
providing a new and improved solenoid drive circuit. 
Another object of the present invention is to provide a solenoid drive 
circuit for conserving the supply of electric power without disturbing the 
off-responsibility of current flowing through a solenoid. 
The above objects are attained by a solenoid drive circuit comprising: a 
drive transistor which is inserted in series with a solenoid and which 
repeats an on state and an off state in response to a drive signal which 
is at first level state during a starting period and which repeats 
periodically the first level state and a second level state during a 
holding period, said drive transistor being conductive to apply a starting 
current to said solenoid when said drive signal is in the first level 
state, and repeating the on state and the off state to provide a holding 
current, lower than said starting current, to said solenoid when said 
drive signal repeats the first level state and second level state 
periodically; a serial circuit of a diode and a Zener diode which is 
inserted in parallel with said solenoid; and a first switching transistor, 
which is inserted in parallel with said Zener diode and which repeats an 
on state and an off state in response to a control signal which is a first 
level state immediately prior to an end of said holding period and which 
becomes a second level state after a lapse of a predetermined time after 
an end of said holding period, said first switching transistor being in 
the off state to interrupt a by-pass of said Zener diode when said control 
signal is at the first level state, and being in the on state to by-pass 
said Zener diode when said control signal is the second level state.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 3, a reference numeral 10 shows a solenoid in a solenoid valve. The 
solenoid 10 is connected to a power source VB at a first end and is 
grounded via a collector-emitter circuit of a drive transistor 11 at a 
second end. A base of the drive transistor 11 is connected via a resistor 
12 to a drive signal input terminal 13. A drive signal applied to the 
drive signal input terminal 13, as shown by a waveform (A) of FIG. 4, is 
at a high level during a starting period t.sub.1 -t.sub.2 of the solenoid 
valve, and repeats the high level and a low level with cycle T during a 
holding period t.sub.2 -t.sub.3 of the solenoid valve. The drive signal is 
at the high level during the initial portion of the holding period, a 
high-level portion (a) of the waveform (A) of FIG. 4, and at the last 
portion, a high-level portion (z) of the waveform (A) of FIG. 4. The drive 
transistor 11, following the drive signal applied to the input terminal 
13, is in an, on state during the starting period t.sub.1 -t.sub.2 of the 
solenoid valve and applies a starting current to the solenoid 10, and is 
turned on and off periodically during the holding period t.sub.2 -t.sub.3 
of the solenoid valve and therefore applies a holding current, lower than 
the starting current, to the solenoid 10. 
A serial circuit of a diode 14 and a Zener diode 15 is inserted in parallel 
with the solenoid 10. A collector-emitter circuit of a first switching 
transistor 16 is inserted in parallel with the Zener diode 15. A base of 
the first switching transistor 16 is grounded via a protection diode 17 
and collector-emitter circuit of a second switching transistor 18. The 
base of the first switching transistor 16 is also connected via the 
protection diode 17 and resistor 19 to a collector of the first switching 
transistor 16. A base of the second switching transistor 18 is connected 
via a resistor 20 to a control signal input terminal 21. A control signal 
applied to the control signal input terminal 2-, as shown by a waveform 
(B) of FIG. 4, inverts from a low level to a high level by synchronizing 
with the last rising to the high level of the drive signal, a rising edge 
of a high-level portion z of the waveform (A) of FIG. 4, and returns to 
the low level after a lapse of a predetermined time after the end of the 
holding time t.sub.2 -t.sub.3. The second switching transistor 18 is 
conductive in a high-level section of the control signal, and is cut off 
in a low-level section of the control signal. The first switching 
transistor 16, when the second switching transistor 18 is in off state, is 
conductive by a counter-electromotive force of the solenoid 10 and 
by-passes the Zener diode 15, and is cut off when the second switching 
transistor 18 is in on, state 
FIG. 4 is a waveform chart for explaining the operation of the circuit of 
FIG. 3. In FIG. 4, the waveform (A) shows the drive signal applied to the 
drive signal input terminal 13, the waveform (B) shows the control signal 
applied to the control signal input terminal 21, a waveform (C) shows a 
potential at a point P in the circuit of FIG. 3, and a waveform (D) shows 
a current which flows through the solenoid 10. 
When the drive signal assumes the high level at time t.sub.1, the drive 
transistor 11 is conductive, and the starting current flows through the 
solenoid 10. When the starting period has come to an end at time t.sub.2 
and the holding period starts, the drive signal starts repeating the high 
level and low level with cycle T. Because of this, the 
counter-electromotive force is generated in the solenoid 10. By the 
counter-electromotive force, the first switching transistor 16 is 
conductive, and thereby the Zener diode 15 is by-passed. The drive 
transistor 11 is conductive/cut off in response to the repetition of the 
high level and low level of the drive signal, and thereby current flowing 
through the solenoid 10 is lowered. Since the Zener diode 15 is by-passed, 
current due to the counter-electromotive force flows through the solenoid 
10 when the drive transistor 11 is in an off state, and, as shown in the 
waveform (D), the current by the counter-electromotive force works as the 
holding current. When the drive signal rises at the last high-level 
portion (z) of the holding period, the control signal inverts from the low 
level to the high level synchronously with a rising edge of the last 
high-level portion (z). As a result of the high level state of the control 
signal, the second switching transistor 18 is conductive. As a result, the 
first switching transistor 16 is cut off, and the by-pass of the Zener 
diode 15 is interrupted. When the holding period comes to an end at time 
t.sub.3, current flowing through the solenoid 10 falls immediately due to 
generation of a counter-electromotive voltage V.sub.z corresponding to the 
Zener voltage, as shown by the waveform (C), by existence of the Zener 
diode 15. Therefore, an off-responsibility of current at the end of the 
holding period is never disturbed. When the predetermined time passes 
after the end of the holding period, the control signal returns to the low 
level in order to be ready for the next operation. 
Though the above-mentioned embodiment uses a solenoid valve in the 
explanation, the present invention is of course applicable extensively to 
a drive circuit of a solenoid of a relay and other similar devices. 
As described above in detail, the solenoid drive circuit according to the 
present invention, when providing the solenoid the holding current by 
means of periodic on/off of the drive transistor, by-passes the Zener 
diode, by means of the on state of the first switching transistor until 
immediately before the end of the holding period, and interrupts the 
by-pass of the Zener diode by means of the off state of the first 
switching transistor, during a period from immediately before the end 
until the lapse of the predetermined time after the end of the holding 
period. Because of this, current due to the counter-electromotive force 
can be utilized as the holding current, and therefore the on time of the 
drive transistor during the holding period can be shortened to conserve 
the supply of electric power. Moreover, as the by-pass of the Zener diode 
is interrupted immediately before the end of the holding period, the 
off-responsibility of current flowing through the solenoid is not 
disturbed at the end of the holding time. 
From the foregoing, it will now be apparent that a new and improved 
solenoid drive circuit has been implemented. It should be understood of 
course that the embodiment is merely illustrative and is not intended to 
limit the scope of the invention. Reference should be made to the appended 
claims, therefore, rather than the specification, to determine the scope 
of the invention.