Various type of power supply circuit from basic regulated power supply to advanced digital control power supply unit is already posted in bestengineeringprojects.com. Now here one more power supply circuit is posted and can be used to provide up to 300mA and voltage between 6V to 13V.
Circuit Description of Variable Switching Power Supply
The input AC is stepped down using a step down transformer T1 to 18V AC which is further rectified by a bridge rectifier. The rectified output is filtered by 1000 capacitor C1. The zener diode ZD1 is used to form the reference voltage and output voltage is selected by potentiometer VR1.
Transistor T1 forms emitter follower, and voltage at emitter of T1 is equal to the voltage at the varying point of VR1. Transistor T2 is used to sense the output voltage.
Timer IC (IC1) is here configured as monostable multivibrator (timer circuit) for the time period of 0.2ms. The collector of PNP transistor T3 is connected to pin 2 of IC1 and is used to trigger. When the circuit is switch on, the voltage at T1 is equal to voltage at centre point of VR1. If voltage at centre point of VR1 is 8 volt, the emitter of T2 is also at 8 Volt and base is at zero. When transistor T2 is in off state no voltage is developed across R4 and transistor T3 continues in the off state.