PURPOSE: To prevent a main transistor from being damaged when an overcurrent flows to the main transistor by a method wherein a resistance which is connected to an emitter for the main transistor and a capacitor which is connected to an emitter for a photocoupler are connected to a base for a control transistor.
CONSTITUTION: By the collector current, of a photocoupler P, which is changed according to an output voltage, the charging time of a capacitor C is changed, the ON-time of a control transistor Tr2 is controlled on the basis of the voltage of the capacitor C, the base current of a main transistor Tr1 is controlled, and the output voltage is changed into a constant voltage. On the other hand, when an output current becomes excessive, the voltage drop of a resistance R become large, the control transistor Tr2 is turned on, the base current of the main transistor Tr1 is limited, and the generation of an overcurrent is prevented. Thereby, the number of components is reduced, and the circuit constitution can be made simple.
YAMASHITA MASAJI
OSAKI MASAYASU
| JPS612781B2 | 1986-01-28 |