PURPOSE: To provide the titled power source circuit in an impact-type printer free of variation in a driving current with the temperature of a hammer coil, comprising a detecting part for the temperature of a hammer coil, and a driving voltage controlling part for varying a printing hammer driving voltage according to the detected temperature.

CONSTITUTION: In the impact-type printer, a device comprising a sensor 7 for detecting the temperature of the printing hammer 8 and the driving voltage controlling part for varying the printing hammer driving voltage in accordance with the detected temperature is used as a driving power source circuit for the printing hammer. In the controlling part, when a detected temperature signal from the sensor 7 is impressed on an output voltage detecting terminal 6, a switching regulator controller (CTL) 5 varies the width of a switching pulse, whereby a DC voltage outputted from the hammer coil 9 through rectifying circuits 1, 2 can be varied according to the temperature.