Everything for Electronics

Tech Forum

2020 Issue-5

Bench Power Supply

I would like to build an inexpensive AC power supply for my workbench. I want something much smaller and lighter than a variac, 0-30 VAC, and maybe one or two amps would be fine. Can anyone point to a good schematic or even a well-written circuit description?

Jeff Bowles
Columbus, OH

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You asked for help on an AC power supply, but without more details it can only be quite general.

Understand that a ‘Powerstat’, ‘Variac’ or similar device DOES NOT provide power line isolation since it is not a true transformer, with isolated windings. A non-isolated power supply on a workbench is as hazardous as the incoming line so if you need that isolation, get a unit that offers isolation, or, add an additional transformer for safety.

Should you be looking for a less expensive or smaller alternative to a ‘Variac’ then consider using a common lamp dimmer in the incoming power line to the transformer! Yes, I know, there’s going to be a slew of nay-sayers about this but, it works! If your load does not depend on the power line waveform for timing accuracy or speed; If your powered device can accommodate the chopped waveform from the triac lamp dimmer; if you don’t exceed the dimmer’s rating with a big starting load, like a motor. Yes, it’s a lot of if’s, but again, it works. I have repaired dozens of quality but old microscope stage illuminators that used a huge rheostat in the 120 volt primary of a 12 volt transformer to supply a12 volt halogen lamp. The rheostat never failed but the mechanical belt/wheel system that turned it always broke. I replaced the rheostat and all the bizarre mechanics with a simple compact lamp dimmer, and some minor mechanical adaptations. On occasion the transformer would buzz, because of the non-sine wave input, but tightening the lamination stopped the noise. Often overlooked is that transformers can generally be reversed, to step up, or down the voltage.

Since a transformer provides isolation, here’s a trick. For line voltage isolation, use two identical transformers, let’s say 120 / 24 volt. Power the first one normally, 120 volts on the primary. Connect the second one backwards, by connecting the two 24 volt windings together. and the output is from the second transformer! so it’s step down 120VAC in/24 VAC out....then step up 24 VAC /120VAC. It really provides excellent isolation since both transformers are isolated. Be mindful of the transformer’s current capacity and don’t overload the arrangement. Other arrangements of transformers can give buck or boost voltages...raid the scrap box!

Len Powell
Finksburg, MD

My interpretation of your request is that you don’t want an AC power supply as large as most of the Variacs listed as 500 VA to 3000 VA on eBay. However, there exists a Variac as small as 300VA.

Better yet search for Powerstat 10B which is rated at about 200VA. This smaller unit puts out 0-130 VAC with 120 VAC in at about 2 Amps. This is a bare unit which needs to be mounted in an enclosure. Don’t like the greater than 30 V output? Then wire a 120:36 V step down transformer to the output. I have used this technique to drive a foam cutting wire.

Dennis Crunkilton
Abilene, TX