Teaching an old PSU new tricks

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 3×3 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.

Comments closed
    • SpotTheCat
    • 10 years ago

    ATX power supplies are used to charge lipoly batteries for RC planes etc. Secretsquirrel spent some time and money doing his. I didn’t spend the time or money, but mine can give me over 400 watts for my hotwire cutter. Nice.


    • holophrastic
    • 10 years ago

    Actually, I’m really interested in the whole using a PSU to power LCDs. Of course, I know very little except what I see when I tear apart a typical desktop LCD.

    Does anyone have any real working knowledge of doing this? If I could ditch the internal lcd psu and run things off of the computer’s modern psu, my all-in-one rig would become an entire 2 inches thinner.

      • zaeric19
      • 10 years ago

      I took an LCD panel out of a laptop for a case mod where I mounted the LCD in the side panel of my computer. I had to get a power inverter, LCD controller and 12v DC power supply (ran a 2nd power cord rather than connect it to the computer power supply) from ebay that worked with the display I had. Since a laptop has a DC source (battery) it uses an inverter to supply AC to the fluorescent lights that provide the back light while everything else runs on DC power. With a desktop monitor you have an AC input so I would guess that the AC goes directly to the fluorescent lights and there is a DC power supply powering everything else. It might just be easier to find a power inverter for a car’s cigarette lighter and to have the power supply supply 12v to that. Just make sure the power inverter can handle the amount of power your LCD would need and that your computer’s power supply can handle the added load.

    • rephlex
    • 10 years ago

    But what if the standard ATX voltages aren’t what you want? What if you need AC not DC? You could try this: [url<]http://www.gearslutz.com/board/geekslutz-forum/540317-ehx-12ay7-test-poor-mans-balanced-power-regenerator.html[/url<] BTW, to convert the output of the amplifier from AC to DC all you need are two diodes and a capacitor, one diode for each channel's hot wire with a capacitor connected across both to the cold wire, as the second diagram on [url<]http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectct.html[/url<] shows.

    • Bauxite
    • 10 years ago

    Spare PSUs can be good for powering a desktop card jerry rigged into a laptop, all it needs is the 12V and beating discrete laptop cards let alone integrated is easy with last gen desktop. (you know, leftover from an upgrade)

    The basic converter for the card only runs about $50, I would suggest grabbing the prefab power switch (or make one) and not using paper clips.

    Hard part is finding something to put it all into so its not a jumbled mess of wires on the desk.

    • bcronce
    • 10 years ago

    My friend took apart one of this PSUs because it was REALLY dusty. Even though it was unplugged for a good hour, one of the caps still had some decent juice and it instantly gave him a 1 inch burn that turned into blisters within seconds. He said it was quite painful. I didn’t see it happen, but I heard the pop+crackle and turned around.

    • wiak
    • 10 years ago

    stupid manufactors cant make a simple adapter so i can power a external usb hub with a molex connector 😛

    meybe this guide will work, pretty sure it will work, must resist, do NOT mess with expensive psu 😛

      • EtherealN
      • 10 years ago

      Buy a cheap one and try things out with that one first. 🙂

      Hell, if you have a case like me (HAF fulltower) you can mount two PSU’s in it for added fun! 😀

    • sluggo
    • 10 years ago

    Designing PSU’s was my job for a while, and I’ve a fair bit of hacking. Some really unspeakable, shameful things … it still hurts to talk about it. But if you want to know how to very quickly reheat a cup of coffee, I may be able to help.

    And if you need a bit of 24V, you can connect the ground lead of the load to the dark blue wire (-12V) from the ATX supply and the power lead to a yellow wire (+12V). Be careful, though, as the -12V supply is often a simple 7912, which may be good for 1.5A max. If you plan to use this on an extended basis, I’d recommend fishing around in the supply, finding the 7912 and putting a heatsink on it.

    • SomeOtherGeek
    • 10 years ago

    Yea, I helped out a friend testing car lights with a PSU as we didn’t have anything else around. It was a fun project and I learned a lot that day. And became a convert in HW hacking.

    Thanks for this tho cuz it is giving me some ideas: I have this observatory (mini) with a 11 inch telescope that my dad, who just passed away, gave me. It has no power adapters to run the motor on the scope, so I think I’ll use a computer to do that for me as I will need the computer to track things. Looking forward to it.

    • Waco
    • 10 years ago

    I use my old SinTek SLI500 (yes, it’s a POS riced out PSU, the 5v rails died somehow. Hell, the company isn’t even in business now…) to run my power tools. The batteries died long ago so I spliced all the 12v lines together and put connectors on the end that plug into where the batteries used to go.

    The tools are made to run on 14.4 volts but the 12.5 volts and ~30 amps from the PSU seems to do just fine.

    • blanchjd
    • 10 years ago

    Well, I am currently using an old junky ATX PS to power my R/C battery chargers out in the garage. I’ve modded it to include banana jacks on the case for easy plug-in of the chargers. Other than a high pitch squeal every time I turn it on ( I told you it was crappy) its been great.

    • Usacomp2k3
    • 10 years ago

    I remember the voltages in the easiest way possible: They follow alphabetical arrangement:
    Orange, Red, Yellow = 3.3V, 5V, 12V

      • flip-mode
      • 10 years ago

      Hey, that’s nifty and near impossible to forget.

        • Dashak
        • 10 years ago

        Challenge accepted.

    • Lianna
    • 10 years ago

    I modded quite a few PSUs for regular use by taking off upper part of their cases, cutting the fan wire (or inserting fanmate device), and putting it on the case (or screwing it behind the case) to keep it cool enough. Silent computing FTW! Now I just use NoiseBlockers.

    I did use a hacked PSU to power small car race track, because we found two different sets of tracks for close to nothing, but it turned out that with regular batteries it was no fun – you could push cars to the max, and they were just driving steady through the corners. With the PSU, it became an art of driving. It was nothing like EVGA and XFX PSU ads with starting the real car or powering… power?… toys 😉

    I still prefer regular wall warts for everyday use, though.

    • Thorburn
    • 10 years ago

    When at university I managed to pick up a pair of Gallatin-2M Xeon’s (basically the same as the original Pentium 4 Extreme Edition, but two of them for less than half the price of one P4EE) on eBay, along with an ASUS PC-DL motherboard.

    Unfortunately my PSU was too weedy to power those, my GeForce 6800 Ultra, and my hard drives, and I’d spent all my money on said Xeon’s, and therefore couldn’t afford a better one. I did however have a cheap old PSU sitting around and my case had empty space above the PSU…..

    So using the same green to black trick as above I chopped up a ATX extension lead and used it to splice together the wiring so that both PSU’s powered up together. One powering the motherboard, the other the hard drives and optical drives.

    • spiked_mistborn
    • 10 years ago

    I’ve hooked up a car stereo a few times, and also used one to power an old 802.11b router and signal booster that I found at a thrift shop, but my favorite was the 4D cell air mattress pump. I don’t usually keep a stock of D batteries sitting around, and I figured that the 5v line on the PSU would be close enough to the 6v that is provided by the batteries that it wouldn’t matter. Apparently the motor in the pump was able to utilize the extra current that the PSU could provide because it filled a queen size air mattress in like 20 seconds, but I think it let out a small amount of the magic smoke that all electronic components run on. I put batteries in the pump and it still works ok, so no harm done.

    • thesmileman
    • 10 years ago

    The big issue is they are really inefficient especially when there is little to no load. Running two of these and you power bill will probably go up by about $8-$10 a week.

      • pragma
      • 10 years ago

      True, typical PSU aren’t efficient at low loads, wasting some 5 or maybe 10 watts.
      Your calculation is way off though. It’s $8-$10 per [b<]year[/b<] more likely.

        • OneArmedScissor
        • 10 years ago

        That’s assuming a load from a desktop type of computer, though, which, even at the most minimal, is still more than any number of other electronic doodads. It’s likely just going to get worse from there, especially considering that most peoples’ unused PSUs are going to be either old or cheapos with very poor efficiency.

        If it’s not for something you leave on constantly, it probably won’t really add up over time, but that’s something you’d have to measure for yourself with your own toys, and unfortunately, most people don’t even seem to know they can do that.

      • bcronce
      • 10 years ago

      $8/week is about a 450watt draw. So unless most PSU pull more power while idle than under load….

      • ew
      • 10 years ago

      [quote<]Running two of these and you power bill will probably go up by about $8-$10 a week.[/quote<] Not intended to be a factual statement.

    • Bombadil
    • 10 years ago

    I am thinking of trying to charge a car battery with a 350W Antec supply I have sitting in storage.

      • cass
      • 10 years ago

      Charging batteries requires more than 12v. You really need the ability to push 15v and lots of amps. Battery chargers need lots of sensing and ability to raise/lower the charge voltage. Cheap chargers suck.

        • bcronce
        • 10 years ago

        I second this. I had a bad battery a month ago and did a lot of reading on testing it because it was only half-way bad and hard to figure out. A working battery should have about 13v which is higher than what a PSU puts out and the battery would actually push power into the PSU and probably ruin it. Also, ~14.5v(typical alternator voltage), a battery can pull over 100 amps.

    • bthylafh
    • 10 years ago

    Not much of a hack at all, but at my old job we had an ancient Full-AT power supply with a Big Red Switch we used as an auxiliary p/s to power extra disk drives when a customer’s p/s didn’t pack the gear. I always liked flipping the BRS; it felt like it meant business.

    • Corrado
    • 10 years ago

    I made a ‘Stereo’ once using an old 230w ATX power supply and a 2x CDROM with a headphone jack on it that I plugged speakers into. Worked pretty well. I soldered a switch into the green -> ground to turn it on. It was one of the old CD drives with the play/pause and Next Track buttons as well as the volume switch.

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