4.5 V, 6 V, 9 V , 12 V Power Supply Circuit

THIS little power supply supplies a range of switch selectable output controlled voltages from 4.5 to 12 volts, selectable by a switch.

The supply provides up to 400 mA and the output can stand up to a short circuit with no destruction. Hence , it is well suited for the experimenter or for use with high drain appliances.

How it works

The 120V line voltage is decreased to 15 volts by transformer T1, and this secondary voltage can now be fullwave rectified by rectifier bridge D1 D4.

The bridge rectifier output is filtered by C1 to deliver somewhere around 20 volts dc. The series combination, of Zener diode ZD1 fed by resistor R1, supplies a stabilized voltage of about 13 volts which can be used along the voltage divider R2, R3, R4 and R5.

Hence a series of reference voltages 4.5 V, 6 V, 9 V , 12 V are created for the regulator, in which the positive rail is set and the negative rail is the one which is adjusted.

Transistor Q3 is an emitter follower in which the output (emitter) is around 0.6 V higher (more positive) compared to base. The base voltage is determined by SW2 from one of the tappings on the reference voltage divider.

Due to the fact Q3 are not able to manage the specified output current, it makes Q2, a power transistor, which could handle the necessary load.

Once the load surpasses 400 mA (around), the voltage decrease around R6 forward biases Q1 which usually activates and shunts current away from the base of Q2.

Therefore the regulator seems to lose control and the output voltage drops, restricting the current to 400 mA. As the power dissipated in Q2 in case of short circuit conditions is about 10 watts, Q2 needs to be fitted to a heatsink.

Furthermore, resistor R7 limits the current provided by Q3 to a safe value (for Q3) under short circuit conditions. If a fully variable supply is needed, a 10 k potentiometer needs to be employed instead of the voltage divider. The potentiometer wiper is subsequently fed straight to the Q3 base.