Inductive load driver circuit effecting slow hold current delay and fast turn off current decay

A circuit for controlling current from a power supply to the solenoid coil of an electromagnetically activated armature in response to a command pulse having a beginning and an end bounding an actuating portion and a holding portion, the current in said actuating portion increasing to an actuating current level sufficient to overcome a bias causing the armature to move from a first position to a second position and the current in said holding portion decreasing to a range sufficient to maintain the armature in said second position, said circuit comprising:

• (a) power switch means coupled to said power supply and adapted to be coupled to a one of first and second sides of said solenoid coil, said power switch means responsive to first and second inputs thereto to alternately complete and interrupt a charging current path between said power supply and said solenoid coil;
• (b) reference means for generating a lower and higher reference value representing respectively a lower level and a higher level of said holding current;
• (c) comparator means coupled to said reference means, said power switch means, and a one of said first and second sides of said solenoid coil for comparing a representation of the current through the solenoid coil with said lower and higher reference values and generating said first input to said first switch means when said current through said solenoid is less than said lower holding level and generating said second input when said current through the solenoid coil is greater than said higher holding level;
• (d) solenoid current decay means comprising low impedance means and high impedance means, said low impedance means coupled across both sides of said solenoid coil and being enabled by the presence of said command pulse to establish a low impedance current path through said solenoid coil when the charging current path is interrupted, said low impedance means comprising silicon controlled rectifier means, and said high impedance means coupled across both sides of said solenoid coil having an impedance value higher than said low impedance means to establish a high impedance current path through said solenoid coil;
• (e) SCR turn off means coupled to said power switch means responsive to the termination of said command pulse to momentarily cause said power switch means to complete said charging current path so as to rapidly turn off said silicon controlled rectifier means by diverting current therefrom;
• and
• (f) said low and high impedance means cooperating with the inductance of said solenoid coil to establish respective slow and fast L/R decay rates for the current through the coil so that said slow L/R decay rate allows said holding current to flow through said solenoid coil prior to the end of said command pulse even though said power switch interrupts said path between said power supply and said solenoid coil and so that said fast L/R decay allows said armature to be rapidly biased back to its first position after the end of said command pulse.