Driver circuit for printer electromagnet

A driver circuit for controlling the current delivered to the winding of an electromagnet by a voltage source, comprising in combination:

• a controllable switch;
• means coupling said switch to said winding, whereby when said switch is actuated current is enabled to flow through said winding from said source;
• a current sensor coupled to sense the current flowing through said winding and to deliver a first output signal having a level of voltage related to the quantity of current flowing through said winding;
• a reference circuit coupled to said source and responsive to the level of voltage supplied by said source for delivering a second output signal having a level of voltage related to the level of voltage supplied by said source;
• a bistable circuit operable in either of first and second states and transferrable from the one of said states in which it is operating to the other of said states upon receipt thereby of a respective trigger signal, said bistable circuit being responsive to the application of an input signal to said driver circuit for transferring to said first state, said bistable circuit being responsive to the relative levels of said first and second output signals for transferring to said second state when the level of voltage of said first output signal exceeds the level of voltage of said second output signal, said bistable circuit delivering a third output signal when said bistable circuit is in said first state;
• and
• means for transferring said third output signal to said switch for actuating said switch during the duration of occurrence of said third output signal.

A driver circuit for supplying the winding of an electromagnet with a controlled current provided by an unregulated voltage source, comprising:

• a switch connected in series with said winding, said switch being operable in a current passing state and a current inhibiting state;
• a current detector coupled to detect the current flowing in said winding and produce a winding signal whose voltage is related to the magnitude of the current flowing in said winding;
• a reference circuit connected to sid unregulated voltage source for generating a reference signal whose voltage varies as a predetermined function of the voltage variations of said source;
• and,
• a controllable bistable device having first and second input terminals and an output terminal, said first input terminal being coupled to said reference circuit to receive said reference signal and said second input terminal being coupled to said current detector to receive said winding signal, said first input terminal also being coupled to receive a trigger signal, said output terminal being coupled to said switch for controlling the state of said switch, said bistable device having first and second output states represented by corresponding signals delivered at said output terminal, said first output state causing said switch to be controlled to operate in its current passing state and said second output state causing said switch to be controlled to operate in its current inhibiting state, said first and second output states being controlled by the polarity of the voltage difference existing between said two input terminals, the polarity of said voltage difference when a trigger signal of an appropriate level is applied to said first input terminal causing said bistable device to have said first output state, said first output state continuing until the polarity of the difference between the voltage of said winding signal and said reference signal causes said bistable device to have said second output state.