Voltage compensating drive circuit for a thermal printer

In an exothermic printer including a power supply for producing a supply voltage of varying magnitude, a plurality of exothermic printing elements disposed on a substrate for recording print characters on a thermally sensitive medium in response to a current driving pulse being applied thereto, the printing density of said print characters being responsive to variations in the duration of the current drive pulses, the improvement comprising compensating drive circuit means coupled to said power supply for detecting changes in the magnitude of said supply voltage, said compensating drive circuit means including oscillator means adapted to control the duration of said current driving pulses in response to changes in magnitude of said supply voltage to thereby stabilize the printing density of said print characters recorded on a thermally sensitive medium and including exothermic drive means adapted to select said exothermic elements to be energized and apply thereto current drive pulses having a voltage sufficient to record print characters on a thermally sensitive medium and feedback means coupled intermediate said exothermic drive means and said compensating drive circuit means for detecting changes in the voltage of the current drive pulses applied to the exothermic elements and for producing a compensating signal representative thereof, said compensating drive circuit means being adapted in response to said compensating signal to control the duration of said current driving pulses applied to said exothermic elements.