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A Cheap High-Current Bench Power Supply

If you've ever had to buy a high-current bench supply, you'll know they don't come cheap. But you may well have such a supply sitting unloved and (until now!) unwanted in the back of a cupboard. It's the power supply in that old computer you never quite got around to throwing away!

by Nicholas Vinen

DANGER!

Internally, computer power supplies carry the full 230V AC mains voltage and should be regarded as potentially lethal.

Much of the exposed internal circuitry, heatsinks, etc floats at the mains voltage.

NEVER open a computer power supply case or work on the supply with the IEC mains cable plugged in (turning it off is NOT good enough!).

Heed the warnings in the text!

Click for larger image

Here at SILICON CHIP we are big fans of re-using and recycling old electronics. We’re loathe to throw away anything which is still operational, even if it’s obsolete. Manufacturing these devices involves much effort, so just throwing them away when they still work would be a shame.

This means that, among other things, we have a number of computer power supplies lying around, gathering dust. Some of these are still inside old computers which are too slow to be useful while others are left over from upgrades (where the old supply wasn’t up to the task of powering a new motherboard or CPU). Others were rescued from machines that were recycled or thrown away.

Even if you don’t have a spare computer power supply, these days they are cheaper to buy than an equivalent bench supply. They don’t have particularly good voltage regulation, either in terms of absolute output voltage or ripple but they do have multiple voltage rails, in some cases capable of delivering upwards of 30A. If all you need is a high current fixed voltage supply (12V, 5V and possibly 3.3V), using a computer supply is a cheap and efficient option.

Note that we are not modifying the supply to provide different output voltages than those offered. Of course that can be done (see the articles in SILICON CHIP, December 1998 and October 2003) but here we are just making it much easier to use the existing rails for a bench supply.

Choosing a supply

Our first task was to decide which supply to adapt. We have some of the old “AT” supplies as well as the newer “ATX” supplies. The latter are far more common these days and safer to work with since there is no external mains power switch. As ATX supplies are now pretty much universal (and also more powerful), that is what most constructors would use.

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