Design Note 27
Issue 1 February 1996
12V Solenoid Driver/Latch
DN27 - 1
Design Note 27
Issue1 February 1996
DN 27 - 2
This circuit has been designed to control
a solenoid by the operation of a single
push-button switch. It will supply loads
of over 1A and can be operated up to a
maximum speed of once every 0.6
seconds. When power is first applied to
the circuit, the solenoid will always start
in it’s off position. Other features of the
circuit are it’s automatic turn-off if the
load is shorted, and virtually zero power
consumption when off.
When the supply is connected, the 470Ω
and 270Ω base-emitter resistors ensure
all three transistors remain off. The 1µF
capacitor charges up to a value
approaching that of the supply rail. If the
push-button switch is then closed, the
charge is transferred to the bias network
of the ZTX300, turning it on. This,
through the 100Ω and 270Ω bias
resistors of the ZTX550s turn these
devices on which energises the load.
This action also holds the ZTX300 on (via
the two 560Ω resistors), irrespective of
the decaying charge on C1.
Since the ZTX300 is now on, when the
push-button is released the 1µF
capacitor will be discharged through the
transistor via the bleed resistor.
If the push-button is operated again it
will connect this discharged capacitor to
the bias network of the ZTX300 turning
it and thus the output off. Any excess
energy stored in the solenoid will be
dissipated in the 1N4000 protection
diode.
When the push-button switch is
released, the 1µF capacitor will charge
up ready to trigger the latch on again
when the switch is operated. The 0.1µF
capacitor inhibits false triggering due to
transient voltages.
This circuit was originally designed for
an Automotive application, but similar
circuits should also find use in industrial
control systems. Similar circuits for
higher current applications are possible
with a change to the output device. For
example, the ZTX751 2A rated PNP, or
the high gain ZTX790A could be used for
load currents of up to 2A, while the
ZTX951 allows operation to over 3A.
+12V
ZTX550
1A
SOLENOID
1N4000
ZTX550
560
560
470
0.1
ZTX300
27K
1µF
100
2W
270
Figure 1
12V Latch Circuit.
Single Switch Operated, High Current High Side Switch
Design Note 27
Issue 1 February 1996
12V Solenoid Driver/Latch
DN27 - 1
Design Note 27
Issue1 February 1996
DN 27 - 2
This circuit has been designed to control
a solenoid by the operation of a single
push-button switch. It will supply loads
of over 1A and can be operated up to a
maximum speed of once every 0.6
seconds. When power is first applied to
the circuit, the solenoid will always start
in it’s off position. Other features of the
circuit are it’s automatic turn-off if the
load is shorted, and virtually zero power
consumption when off.
When the supply is connected, the 470Ω
and 270Ω base-emitter resistors ensure
all three transistors remain off. The 1µF
capacitor charges up to a value
approaching that of the supply rail. If the
push-button switch is then closed, the
charge is transferred to the bias network
of the ZTX300, turning it on. This,
through the 100Ω and 270Ω bias
resistors of the ZTX550s turn these
devices on which energises the load.
This action also holds the ZTX300 on (via
the two 560Ω resistors), irrespective of
the decaying charge on C1.
Since the ZTX300 is now on, when the
push-button is released the 1µF
capacitor will be discharged through the
transistor via the bleed resistor.
If the push-button is operated again it
will connect this discharged capacitor to
the bias network of the ZTX300 turning
it and thus the output off. Any excess
energy stored in the solenoid will be
dissipated in the 1N4000 protection
diode.
When the push-button switch is
released, the 1µF capacitor will charge
up ready to trigger the latch on again
when the switch is operated. The 0.1µF
capacitor inhibits false triggering due to
transient voltages.
This circuit was originally designed for
an Automotive application, but similar
circuits should also find use in industrial
control systems. Similar circuits for
higher current applications are possible
with a change to the output device. For
example, the ZTX751 2A rated PNP, or
the high gain ZTX790A could be used for
load currents of up to 2A, while the
ZTX951 allows operation to over 3A.
+12V
ZTX550
1A
SOLENOID
1N4000
ZTX550
560
560
470
0.1
ZTX300
27K
1µF
100
2W
270
Figure 1
12V Latch Circuit.
Single Switch Operated, High Current High Side Switch
