Stepping motor driving circuit having failsafe function

A driving circuit for driving a stepping motor having a plurality of excitation coils having different phases, comprising:

• a plurality of excitation transistors for driving said excitation coils of said stepping motor, respectively;
• at least one chopping transistor for effecting chopping control of current flowing to said excitation coils;
• a serial circuit comprised of a fault diagnosis transistor and current limiting means;
• said serial circuit being connected in parallel with said chopping transistor;
• switching control means for selectively turning on and off said fault diagnosis transistor;
• output state detecting means for detecting output states of said chopping transistor assumed respectively when said fault diagnosis transistor is turned on and turned off;
• and
• fault detecting means for comparing the detected output levels of said chopping transistor with respective predetermined logic levels and detecting faults in said driving circuit from results of said comparisons.