How to convert a ATX Power Supply to a Bench Supply

Back when I went back into playing with electronics I turned an old Computer Power Supply into a Bench Supply. Over time I had found out if used incorrectly you can screw stuff up and maybe even yourself.

This guide we will go step by step from beginning to end on converting one of these ATX Power Supplies for safe electronics tinkering.

Lets start with the Basics
A Computer Power Supply is a Switch Mode power supply. Wikipedia states

A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from AC source to DC loads, such as a personal computer, while converting voltage and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy.

In layman’s terms they’re designed to run for long periods with low power dissipation.

On the power supply there should be a label that has the specs listed for the power supply. For my build I am using a DELL branded dual 12 volt rail power supply.
Max Output: 300 Watts
+12 Volt A: 17 Amps
+12 Vvolt B: 9 Amps
+3.3 Volt: 10 Amps
+5 Volt: 13 Amps
-12 Volt: 0.3Amps
+5 Volt Standby: 2 Amps

Now this does not mean you can get 300 Watts on a single 12 volt rail. It’s the Amps times Volts. So for the 12 Volt A rail 204 Watts. Honestly this is total over kill and you would probably melt wires after hitting 8 amps of continuous power.
Some times it will state +5v and +3.3V will not exceed 90W.

ATX Power Supplies use a special standard. However some OEM companies like DELL and HP attend to stray away and change a few things. For an example the DELL Power Supply I am using doesn’t use the same pinout for the Motherboard power plug. It uses the same color scheme for the wires so it helps. If you attend to use an odd ball power supply then do some research on it. Find out what pin is what.

Here is a typical pinout of the mother board connector of the power supply.
atx-pinout
Just remember the following.

Orange = +3.3 Volts
Red = +5 Volts
Yellow = +12 Volts
Blue = -12 Volts
Black = Ground or 0 Volts

These also carry over to the drive connectors as well.
There is also special pins on the motherboard connector such as Power-OK, +5v StandBy and Power-ON. Here is a little tip for the 5V StandBy, as long as the power supply is plugged into the wall that rail is live. It is used for powering USB devices, Realtime Clock, Wake on LAN and other functions. This could be used to power items around the bench so keep it in mind.
The -12Volt rail is kinda useless but could be handy for Audio circuits and powering Operational Amps. For the power supply it’s rated 300mA. If you really need a -12volt rail you can use a LM7912 regulator and get 1amp to 1.5amps.

Now since we got the basics down for the exterior of the power supply lets take a look on the inside.

Before committing on the supply of choice it needs to be inspected of the interior. After removing the top half of the case look at the tops of the Electrolytic Capacitors. If this supply had a lot of hours or is fairly old the caps could be on their way out. If there are any that look like this then find a different supply or if you want to you could recap it.
IMG_6810
Left side has Vented and the middle one has bulged. They should all be flat for signs of good Capacitors.
Check the fan to see if it spins freely by hand. If not then replace it.
Remove the main circuit board and inspect the bottom of it.
nutone+im3303+overheated+traces
If you see signs like shown above then go find a different power supply. Brown marks such as this means it has dangerously over heated. Granted if it still works but time will only tell for how long.

If all passes or you found a different supply that passes then give it a good cleaning and come back soon for the next installment.

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