DIY Power Supply

Obtaining a free power supply is incredibly quick and easy with a few basic tools and an old desktop computer. Most desktop computers are perfectly fine when they are exchanged for a newer, top-of-the-line system. If you have no other use for a working desktop, you can harvest the power supply to provide a source of 12V, 5V, and 3.3V DC to test your electrical creations.

Where can you find an old desktop computer? Check w/ local offices, colleges, schools, etc. Nearly every place will have a storage room full of old systems, most will be happy to donate one to your quest. Be sure that any existing data on the computer is wiped or inaccessible.

Step 1: Materials

-- Desktop computer

Step 2: Tools

-- Screwdriver set

-- Multimeter

-- Hot Glue Gun

-- Wire
Breadboard wires are perfect, but regular solid core or stranded wire works fine.

-- Optional: Breadboard

Step 3: Remove Outer Casing.

All desktop computers are unique. Examine the case for screws or another opening mechanism. The one I used had a pull-tab that pops open one side.

Preferable to remove the outer casing without damaging it (this should be totally doable).

Step 4: Remove Power Supply

1. Unplug all connected cords.

2. Locate screws for power supply and unscrew.

3. Wiggle it out of the computer.

If it resists, check for hidden screws.

Step 5: Plug in Power Supply & Test Connections

1. Plug power supply in using desktop cable.

A small light will go on when the power supply is on. You may need to "MacGuyver" a switch to keep it on (see above photo w/ screw + rubber bands.)

2. Using a multimeter, measure the voltage across the pins of the largest connector.

The color of the wires provides information about each pins' function: black is ground, and red is positive VDC.

There is likely only one ground pin.

3. Find where the voltage is 12 VDC, 5 VDC, and 3.3 VDC. Mark these pins and the ground pin on the connector.

There is one pin that supplies +12 VDC, and a few that supply 5 VDC and 3.3 VDC.

Step 6: Connect Wires & Test.

1. Plug in a connector or individual wires to the pins marked ground, 12 VDC, 5 VDC and 3.3 VDC.

Recommended to attach wires with some adhesive like hot glue.

2. Check connections w/ a simple circuit like an LED (+ resistor).

Check that the power supply lights up an LED w/ the 3.3 VDC (or 5VDC) wires.

3. Use for testing all the circuits!

Or at least the ones that work w/ 3.3, 5, and 12 VDC.. which are about what you'll need for hobbyist electronic projects so it's perfect!

Comments

[LesB]

Good to re-purpose these PSU's rather than sending them to
e-waste or the landfill!

I have seen other instructions for re-purposing PCU's.Usually they instruct to add a preload
resistor to prevent the supply from over-voltaging, since in its natural habitat this type of supply
always has a load attached.

Your experiment proves that this at least some of the time
is not necessary. And it is certainly
better design for a supply to be able to work normally unloaded. However, with some PSU's it may be
necessary to provide a preload.

So if this works without a preload, cool. If the voltages are too high or if the unit trips off (from
over-voltaging), just add a preload in the form of a 5 ohm, 10 watt resistor permanently connected to the 5V output.That should stabilize the main regulator by
giving it a 5 watt preload.

[jenfoxbot]

Thanks for the helpful info, Les! Haven't had any problems w/ this power supply (yet), but it's probably best to add a preload resistor just in case. I'll add that in and update the tutorial when I get a chance.

[blodghram]

The button you have on the back is used for testing purposes and is mostly found on Dell power supplies, most other power supplies don't have this feature. However, if you take the big 24pin connector and connect the green and gray wires togehter (you can build a switch in to connect them) you will switch on the power supply

[jenfoxbot]

Aha! That makes a lot more sense. I've seen a few power supplies w/ flip switches, so I had assumed the button was similar. When I get a chance I'll follow your advice and make a better, more permanent switch without rubber bands :)

[blodghram]

the flip switches are there to cut the power from the wall. the power supply gets the signal from your motherboard so they don't really have a switch to turn the actual psu on on it's own, except for the Dell ones and a few others that has that diagnostics button which will power the system as long as it's pressed down. but yeah, bridging the green/gray wires is an old trick for us pc techs to see if the psu powers up in a dead computer

[russ_hensel]

Just a note to let you know I have added you to the collection: Encyclopedia of ATX to Bench Power Supply Conversion ! >> https://www.instructables.com/id/Encyclopedia-of-ATX-to-Bench-Power-Supply-Conversi/ Take a look at about 70 approaches to this project.

[jenfoxbot]

Awesome! Thank you v. much for adding mine to the list! Will definitely check out all the projects :)