| 摘要 |
PURPOSE:To protect a recording element, by stopping the supply of a current when a current is supplied to the recording element for an abnormally long time. CONSTITUTION:A protective circuit 11 is interposed between an enable signal 10 and an AND gate 3 and, when the above-mentioned enable signal 10 is a L-level, the output of a comparator 13 is a L-level and a point E comes to a L-level and no enable is applied. Next, when the signal is a H-level, the output of an inverter 14 comes to a L-level and Q1 is turned OFF and, at the same time, a current is flowed to a condenser CX through an external resistor RX to raise potential at a point B. The rising in potential is determined by the time constants of the above-mentioned resistor RX and the condenser CX. When the potential at the point B is set to VB, the comparator 13 is reversed when the potential VB became equal to the reference voltage V-comp of the comparator 13. When a time constant at this time is set to tCR, the point E comes to a L-level when the pulse width of the signal is longer than the above- mentioned time constant and an output power transistor 4 is turned OFF and the damage of a heat generating resistor element 5 is prevented. |
