Bench Power Supply circuit

Posted on Jul 16, 2012

This power supply is able to deliver adjustable center-tapped DC from 0 to about 30 volts, as well as AC directly from the Variac and from the main transformer before the rectifier. What's primitive by today's standard is that it is not voltage regulated; i.e. if you adjust it to 12V for a heavy load, and then disconnect the load, the voltage will go up quite a bit. But for motors and very low power electronics it didn't matter. I had to buy the electrolytic capacitors, binding posts and bridge rectifier at Radio Shack. Pretty much everything else was scrounged from somewhere. The voltmeter originally had a hand-drawn paper scale inserted into it, but over the years it warped and blocked the meter needle so I just removed it. Nowadays I have better voltmeters anyway.

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Back before we had computers to occupy all our time, my brother and I were always tinkering with motors and electronic junk and such. This required electrical power, and good power supplies were hard to come by, so we each ended up building our own. We had few parts and lots of time, so we each ended up taking apart and rewinding transformers at one time or another - I rewound the primary of a 220V transformer (from Germany) for 110V, Matthias rewound, I think, both windings of a transformer to convert it to 110V and put many taps in the secondary for various voltages. This project got started when I was given a defective Variac (a variable autotransformer - the shiny copper coloured thing at the bottom right of the inside front panel) and an old rusty car battery charger with a big beefy 6 ampere transformer inside. I was able to fix the Variac by reconnecting the break in the winding.

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