PURPOSE:To prevent erroneous printing and the burnout of a heat generating resistor, by connecting a protective circuit between a driving power source and a driver logic circuit and cutting off the supply of power to a heat generating resistor when driver voltage is lower than constant voltage. CONSTITUTION:The protective circuit II connected between a driver logic circuit and a power source VDD contains a voltage detection circuit II' and a logical circuit II''. When divided voltage due to voltage dividing resistors RD1, RD2 is set to V1 and collector voltage to V2 and V1=x is formed, a current I1 is flowed between a base and an emitter and V2 comes to a LOW level. Whereas, in the case of V1x, no current I1 is flowed and V2 comes to a HIGH level. This voltage level V2 is inputted to the logical circuit II''. An inputted voltage signal V2 and a STROBE input signal SS are respectively reversed in the circuit II'' and logical operation is performed according to a real value table shown by a NAND circuit NA. Only when STROBE input SS and voltage V2 are LOW and STROBE input SS' and voltage V1 are HIGH, STROBE OUTPUT SS'' comes to LOW.
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