Teaching an old PSU new tricks

— 7:48 PM on April 14, 2011

Years back, while still an angsty teenager, I had a bug in my head telling me that A+ certification sounded like good times. With trepidation, I proceeded to check out related books from the library and download studying apps onto my Palm IIIe (yes we had apps in the prehistoric times before iDevices). Countless practice quizzes and several sets of AAA batteries later, one question continued to give me problems: in a standard Molex-style connection, what are the respective colors of the wires carrying +12V and +5V currents? For some reason, and despite various mnemonic devices and my PDA’s digital taunting, I could never reliably remember the correct color coding. As an active learner, it wasn’t until I began snipping wires and frying various electronics that everything finally clicked. For the record, yellow is +12V and red carries +5V.

Armed with this knowledge, I've put the oft-forgotten PSU to good use in a variety of ways befitting the T-shirt slogan "I void warranties." My first hack was innocent enough. After sweating out several sticky summer days in a used computer warehouse, I assembled a 3x3 block of 80-mm server fans joined together with packaging tape. The nine red and black wires stemming from the fans were grafted to a Molex tip ripped from a junked power supply, and the whole thing was given freshly squeezed 12V juice. The result was somewhat loud and hazardous, having no grills to guard the spinning rotors, but it ultimately kept me cool.

As it happened, the fans were powered by an old AT-style unit with a push-button power switch. Modern ATX power supplies require an extra step if you want to use them for your own devious purposes. Looking at the 20/24 pin motherboard connector, you will spot a lonely green wire in the bundle. This is your power button; touching the green lead to any of the black (ground) leads will bring the PSU to life. You can use anything from a paper clip to a wire or other electrically conductive material to complete the circuit. If the power supply has a built-in on/off switch, the green wire can be left connected to a ground line, with the switch used to toggle power.

While rigging up a noisy, finger-chopping desk fan is all well and good, it's a rather simplistic example of what can be done with a discarded PSU. In my experience, the most useful application of old power supplies has been powering or testing obscure electronics devices that have been separated from their wall-warts. Most small devices like routers, switches, modems, external drives, and even some LCD monitors require a DC input of either 5V or 12V. By MacGyvering PSU wiring with assorted connector tips from a universal adapter, you can power just about any of those gadgets directly from a standard power supply. Unless you enjoy the smell of smoke that accompanies electronic assassination, be sure to double check your wiring and the device's required input voltage first.

Oftentimes, the device you are powering will call for much less juice than a PSU can provide. Typical consumer adapters are limited to between 500 and 1500 milliamps, while even the crummiest of PSUs should be able to pump out at least 10 amps. The additional current capacity shouldn't damage less-demanding devices, and it'll give you the option of powering much bigger toys.

Back in the day, I built a LAN party rig with two 15" desktop LCD screens mounted in the case's side panel. The screens hung on a large piano hinge, sat side by side when in use, and folded flat for transport. Each screen required 4A of 12V power, and I was able to feed both from the system's PSU using a wiring harness that attached with just a single Molex connector. That harness was built using a couple of universal adapter tips, some custom wiring, and a male Molex plug taken from an old case fan. After replacing the original, cheap PSU with a quality Antec unit to fix some video distortion issues, the rig ran like a champ for several years before it was decommissioned.

PSU hacking can be useful beyond the realm of computer peripherals, as well. I recently constructed a 5V wiring harness for a local auto mechanic's handheld diagnostic scanner. With the charger MIA and no time to order a replacement online, an old power supply stepped up to save the day. The electronics inside automobiles typically runs on 12V power, too. I've used old PSUs to test head units and car-mounted LCDs from the comfort of my own home. If you're feeling ambitious, and your PSU can sustain 18 amps or more on the 12V rail, it's entirely possible to build a custom home stereo using car audio components. You could even mount a head unit in a spare 5.25" drive bay and connect your sound card to an auxiliary input for integrated amplification.

Old PSUs can be great for projects involving motors, LEDs, displays, or other devices requiring DC current. If I had been handed a power supply to use in my science fair projects as a kid, things might have turned out differently. At the very least, my baking-soda volcano would have had some rad wires coming out of the top. One of my grown-up dream projects is someday to build a custom slot car track (and cars) from scratch using a computer's power supply to make it all run.

With a little creativity and some duct tape, an old PSU can be dusted off and put to any number of clever uses. As always, be careful when playing with electricity. Even if you know what you're doing, it's still possible to damage yourself or your electronics using an unofficial power source. Old and/or low-quality power supplies should be approached with caution, as they can produce "dirty" power that could also prove harmful to your devices.

If you’ve hacked a PSU and lived to tell the tale, we'd love hear about it in the comments section below.

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