Current limiting solenoid driver

An apparatus for controlling, in response to a solenoid control signal and with respect to a predetermined current reference signal, electric current from a source thereof to actuate and hold actuated an electric solenoid in a manner to prevent damage form a short circuit of the solenoid, the apparatus comprising:

• switching means for rapidly switching current applied to the solenoid on and off to generate a pulsed current, the switching means including at least one switching transistor having a collector, an emitter and a base, the solenoid being connected between the source of electric current and the collector of the switching transistor;
• detecting means for detecting current flow through the solenoid and generating in response thereto a peak signal representative of the amount of current flowing through the solenoid, the detecting means including a current sensing resistor connected between the emitter of the switching transistor and ground,
• a differential amplifier having an inverting input, a noninverting input and an output, the noninverting input thereof being connected to the emitter of the switching transistor, and
• peak detector having an input and an output, the input thereof being connected to the output of the differential amplifier, the peak detector having an impedance sufficiently high to enable the differential amplifier to respond quickly and being further constructed such that a voltage appearing at the peak detector output has a predetermined value when current through the solenoid is sufficient to hold the solenoid actuated, the voltage appearing at the peak detector output when a short circuit is present across the solenoid increasing rapidly to a sufficiently high value that the time required during a short circuit condition to allow the voltage preparing at the peak detector output to decay to a value allowing the switching transistor to switch on again is longer than the time the switching transistor is switched on, thus providing a duty cycle sufficiently low to prevent damage to the switching transistor;
• comparing means for comparing the peak signal with the predetermined current reference signal and generating a deviation signal representative of the difference therebetween;
• and
• logical gating means for gating the solenoid control signal with the deviation signal and generating a solenoid current control signal to switch the switching transistor off and on respectively when the peak signal reaches predetermined maximum and minimum levels.