Variable Power Supply Unit. DIY.

This little circuit is a copy of a design which was built around twenty years ago. The heart of the unit is based around a 723 regulator chip. It has variable voltage and current output and regulation is more than good for such a simple circuit.

The two off-board 10,000uF smoothing capacitors are mounted on the chassis with the 10n capacitor and 2K7 bleeder resistor soldered directly across each of their terminals. These are then connected to the circuit proper.

As a matter of point, I accidentally shoved 85volts d.c up the two smoothing capacitors and since these were only rated at 63volts d.c, the resultant explosion caused my YL to run for cover! The mess took an age to clean from the bench too!

Note also the two 10n capacitors in the main supply rail MUST be rated at the proper voltage potential.

The pre-regulator centered around the 2N3053 and zener diode protects the input of the 723 from D.C overload (it’s maximum input is stated @ 40v). In this case the 723 is zener protected at 33 volts.

The 10K trimpot is used to adjust the regulated d.c output to a minumim value. Once set it can be left to any prefered minimum voltage. The main output voltage is set by the 10K panel-mount linear pot.

For those of you who wish to use the unit without the variable current control, simply omit any connection to pins 2 and 3 of the regulator chip. Total current output will depend on the amount of 2N3055 pass transistors used.

For the voltage meter I used the three-digit meter (DVM) seen elsewhere on these pages. My completed unit was housed in a wooden case approx. 6″ x 7″ x 9″.
After a few coats of English Oak wood stain the unit, along with its digital voltage meter, looks pretty neat. And a worthwhile addition to the bench.