OVERCURRENT PROTECTIVE CIRCUIT OF CONSTANT VOLTAGE POWER SUPPLY CIRCUIT

PURPOSE:To stabilize operation to improve reliability by using an overcurrent protecting transistor TR in the saturation area. CONSTITUTION:The base bias of an overcurrent protecting TRQ2 is obtained from between the emitter of an output control TRQ1 and the output of a detecting circuit OP which detects variance of voltage and consists of an operational amplifier, and an overcurrent is detected by a current detecting resistance R5 connected between the output of this detecting circut OP and the base of the output control TRQ1; and if the overcurrent is generated in the load side, the circuit between the base and the emitter of the output control TRQ1 is short- circuited between the collector and the emitter of the TRQ2. In this case, the overcurrent protecting TRQ2 is so set that it is saturated when the excessive current is flowed. Thus, the overcurrent protective TRQ2 is turned on to cut off the output control TRQ1 when the excessive current is flowed because of short- circuit of the load or the like.