Magnetic print hammer actuator protection circuit

Abstract

In a printer in which drive transistors coupled to hammer drive coils control energization of the coils within each of a plurality of magnetic print hammer actuators to provide firing of hammer springs associated with the print hammer actuators, leakage of one or more of the drive transistors when they are switched off is detected by a testing circuit which flags a fault condition to turn off the driver circuits if the current at a junction common to all of the drive transistors exceeds a predetermined threshold value. This provides an early warning system in which faulty transistors are routinely detected well before they have a chance to fail and thereafter burn out the associated coil included within a hammerbank of the printer. The testing circuit which comprises a resistor and an operational amplifier coupled in parallel between the common junction and a power supply terminal is activated by a transistor switch which responds to a test control microprocessor to repeatedly close and thereby test for transistor leakage as well as possible shorted coils when the printer is not printing. The microprocessor also repeatedly sequences through the drive transistors to momentarily switch each of them on so that the testing circuit can determine that they and the associated hammer drive coils conduct properly by insuring that a current of predetermined minimum value is present at the common junction. A power transistor coupled to provide rapid current rise when the drive transistors are switched on is also tested for leakage using a second testing circuit, when the printer is not printing.