A very convenient small power supply circuit to share

A very handy little power supply circuit with a fully adjustable output voltage from the lowest possible voltage.

The transformer steps down the input power AC to the required low voltage AC, which is then rectified to equivalent DC by the bridge rectifier.

Zener diode ZD1 provides the required regulation for the output. The biasing of this Zener diode is obtained through D5 and related components. C3 and C4 are used to filter out ripple.

VR1 works like a potential divider, enabling the user to apply a desired potential to the bottom of the TR2 transistor. Since TR1 and TR2 are connected as emitter followers, any voltage that appears at the base of TR2 is replicated at the collector of TR1.

This means that when VR1 is adjusted, the TR1 output also adjusts the equivalent voltage at the output terminals. However, since the minimum emitter drop of a Darlington transistor is about 1.2V, the emitter output will always lag behind the value of 1.2V and a 1.2V drop will appear at the output.

C1 and C2 act like an electronic smoothing network and help eliminate various interference and hum in the circuit.

As a purely linear design, the TR1 will likely exhibit significant heating as the difference between input and output increases.

This means that if VR1 is adjusted to have 3 V at the output, and the input from the transformer is 24V, then TR1 may dissipate a lot of power to compensate for the input/output difference.

Switch S1 was introduced to prevent this and help control dissipation to a great extent. Therefore, when making lower output adjustments, it is recommended to switch S1 to the center tap to reduce the input/output differential by 50%, thereby reducing TR1 dissipation by 50%.