Electronic Corner

A variable power supply it's a basic tool for each lab. This project shows how to build a cheap one easilly that can go from +/-0.6 V to +/- 23.

Transformer:

You will need a center tapped transformer with a primary coil of 240/220 volts for europe, or 120 volts for North America and a secondary coil of +/- 24 V center tapped at 2 A or more. You can use another transformer if you need a different voltage range but pay attention not to exceed the 35V of the capacitors

Output Current:

This power source can supply safely up till 1.5A because it's limited by the Linear ICs

Box:

Parts of this circuit are subject to lethal potential, so it's mandatory to close it in a box. I used the container of an old switching power supply for pc, because i bought it for a few dollars an it had already a switch and an ATX connector for a cable; moreover i could easilly add a fan if needed (but i didn't).

Heat:

The two ICs must be mounted on heatsink, but a fan it's usually not needed.

Diode:

D1 and D2 are used to reduce the output voltage of 0.6 V because the ICs alone (LM317 AND LM337) can't go under +/-1.25V . If this minimum output voltage is sufficient you can replace the two diodes with two pieces of wire (increasing also the maximum output voltage by 0.6V)

Schematic:

Bill Of Material:

Wire connections:

AC1-AC2-GND are connected to the secondary coil of the transformer

TP1-TP2 are connected to an external 5KOhm potentiometer that regulates the positive voltage

TN1-TN2 are connected to an external 5KOhm potentiometer that regulates the negative voltage

V+ / V- / GndOUT are connected to three banana sockets mounted on the panel.

PCB:

Components Placement: