| 摘要 |
PURPOSE:To prevent a power transistor from damaging by detecting an abnormal voltage between the collector and the emitter of the transistor only when a drive signal is ON. CONSTITUTION:A load 2 such as a motor from a DC power source 1 is controlled by a power transistor (Tr) 4. The Tr 4 is controlled by a drive Tr 8 through a PWM signal insulating photocoupler 7. In this case, a protecting circuit of Tr 4 is composed of a photocoupler 15, signal Tr 16 for deciding the upper limit of the abnormal detecting voltage, a signal Tr 19 for turning ON, OFF Tr 16, a signal TR 23 for taking the logic product of the abnormal voltage and an ON drive signal, and a logic IC 26. Thus, when the base current of the Tr 4 becomes insufficient, Tr 16 becomes ON, and TR 23 becomes ON while Tr 4 is turned ON through the photocoupler 15. As a result, IC 26 outputs abnormal signal of the Tr 4 being at H level.
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