Tuesday, 13 September 2011

Electricity is really just organized lightning.

I have recently been working on a project that requires a 12V supply. Ordinarily this is no problem my selection of bench supplies are generally more than a match for anything I throw at them.

My TS3022S Bench SupplyThis project however needed a little more "oomph" than usual, specifically 200W more. Funnily enough a precision variable output bench supply capable of supplying 20A are rare and *very* expensive beasties.

So we turn to a fixed output supply, after all I will want to run my project without hogging my bench supplies anyway. These can be bought from various electronics suppliers like Farnell from around the £50 mark and Chinese imports from Ebay sellers start around the £20 mark.

All very well and good but that is money I was not planning on spending and possibly a month of waiting for an already badly delayed project. So I decided to Convert an old ATX PSU into a 12V source. This is not a new idea and a quick search revealed many suitable guides online. I had a quick skim, decided I did understand the general idea and ploughed ahead.

Wikipedia has a very useful page on the ATX standard complete with pinout diagrams and colour codes. The pile of grey box ATX supplies available on my shelf was examined and one was helpfully labelled with a sticker proclaiming 22A@12V and we had a winner.

Opening the case of the donor 450W CIT branded supply revealed a mostly empty enclosure with the usual basic switching arrangement. I removed most of the wire loom aside from two of each output voltage (3.3V, 5V and 12V i figured the other voltages might be useful in future) and three commons, the 3.3V and 5V sense lines were also kept. Each of these pairs were cut to length and leads were wired to 4mm sockets.

The "PWR_EN" line was wired via a toggle switch to ground so the output can be switched on and off easily. The 5V standby and a 5V output line were wired to a green/red bi-colour LED (via 270Ω current limit resistors) to give indication that mains is present and when the output is on.

Holes were drilled for four 4mm sockets an indicator LED and a switch. The connectors and switches were all mounted in the PSU casework. I plugged it all in, put an 8.2Ω load resistor on the 5V line with an ammeter in line and a voltmeter across the 12V rail.

ATX bench power supply I turned the mains on and the LED lit up green (5V standby worked) and when I flicked the output switch the LED turned orange, the 12V line went to 12V and the expected 0.6A flowed through the load resistor.

Basically, Success!

I have since loaded the supply up to the 200W operating load and nothing unexpected has happened so I am happy. Seems converting an ATX PSU is a perfectly good way of getting a 200W 12V supply and I can recommend it for anyone as cheap as me willing to put an hour or so into such a project.