Inductive driver circuit effecting slow and fast current decay

A circuit for controlling current from a power supply to the coil of an electromagnetic solenoid whose armature is activated 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 to cause the armature to move from a first position to a second position and in said holding portion decreasing to a range of holding current 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 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 said solenoid coil for comparing a representation of the current through the solenoid coil with said lower and higher reference values and for 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 having a low impedance value coupled across said solenoid coil and being controlled by said command pulse to establish a low impedance current path through said solenoid coil during the presence command pulse, and said high impedance means coupled across said solenoid coil having an impedance value higher than said low impedance value to establish a high impedance current path through said solenoid coil at the termination of said command pulse;
• said low and high impedance values cooperating with the inductance of said solenoid coil to establish respective slow and fast L/R decay rates for the current through the coil such 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.

A method for controlling the current through a power switch to the coil of an electromagnetic, solenoid so as to move the armature thereof from a first position to a second position, holding the armature in the second position while minimizing the dissipation of power, and thereafter allowing a rapid return of the armature to its first position in response to the end of the command pulse, said method comprising

• (a) generating a command pulse having a beginning and an end,
• (b) establishing a series current path through said power switch to said coil in response to the beginning of said command pulse to build up current in said coil,
• (c) generating a first signal indicating that the current has built up to a peak level,
• (d) disconnecting said series current path in response to said first signal while still allowing current to continue to flow through said solenoid coil at a slow rate of decay,
• (e) generating a second signal indicating when the current has decayed to a lower holding current level,
• (f) establishing said series current path in response to said second signal to increase the current through said solenoid coil,
• (g) generating a third signal when the current exceeds a higher holding current level which is lower than said peak level,
• (h) disconnecting said series current path in response to this third signal while allowing current to flow through said solenoid coil at said slow decay rate,
• (i) alternately establishing and disconnecting said series current path until the end of said command pulse in response to said second and third signal, and
• (j) disconnecting said series current path in response to said end of said command pulse while allowing current to continue to flow through said coil at a fast decay rate in excess of said low decay rate.

A circuit for controlling the current supplied to an inductive load in response to the presence and absence of a command signal comprising:

• (a) current sensing means, electrically coupled to said load, for sensing the amount of current supplied to said inductive load and for generating a load signal representing the magnitude of at least first and second current levels supplied to said inductive load;
• (b) switch means, electrically coupled to said current sensing means and also to said inductive load, for completing and interrupting a current supply path to said load;
• said switch means enabled by the presence of said command signal to respond to said first and second current levels by respectively completing and interrupting said current supply path to said inductive load;
• (c) first decay means electrically coupled to said inductive load for completing and interrupting a first decay path, said first decay means enabled by the presence of said command signal to respond to said completion and interruption of said current supply path by respectively interrupting and completing said first decay path, said first decay path comprising a first impedance causing said inductive load current to decay at a first decay rate;
• (d) second decay means, electrically coupled to said inductive load and also to said first decay means, for completing a second decay path to said inductive load, said second decay means enabled by the absence of said command signal to respond to the disablement of said switch means and said first decay means by completing the second decay path, said second decay path comprising a second impedance causing said inductive load current to decay at a second decay rate which is faster than said first decay rate;
• said switch means and said first decay means cooperating to maintain the load current between said first and second levels while said command signal is present and to allow said inductive load current to increase from said first level to said second level through said current supply path when completed and to allow the inductive load current to decay through said first current decay path to said first level when said supply path is interrupted and thereafter to decay below said first level at said second decay rate when the absence of said command signal interrupts said supply and first current decay paths.