Abstract: A detection circuit for determining whether heating members within a thermal head are broken is disclosed. The circuit comprises a voltage source connected in series with a resistor, wherein the voltage source and resistor are connected in parallel with a diode. The diode allows current to flow from a thermal head driving power supply to a thermal head. invention also includes a comparator for comparing the voltage The present level at the output terminal of the diode with a reference voltage. The parallel connection of the diode and the voltage source/resistor allows the output voltage of the diode to maintain an expected voltage level while the heating members are operating normally (i.e. the members are not broken). The parallel connection also allows the voltage level at the output terminal of the diode to increase as the members operate abnormally (i.e. a worn or broken condition). The comparator monitors this voltage level, and detects undesirable fluctuations therein.