Replacing wall warts with PC power supplies

I wrote a long, long time ago about my adventures trying to find a wall wart for my old 8-port Netgear dual-speed hub. The other day I stumbled across a novel idea for a replacement.
I won’t rehash how you determine whether a unit is a suitable replacement–read the above link if you’re curious–but suffice it to say a $5 universal adapter from Kmart is fine for my answering machine or my cordless phone and can probably provide the 5 volts my Netgear needs, but my Netgear also needs 3 amps and the universal adapter I keep around can only deliver 20% of that. The beefiest 5v unit I could find at Radio Shack could only deliver 1.5 amps.

A PC power supply delivers 5V and 12V on its hard drive connectors. And PC power supplies deliver plenty of amperage: one of mine will deliver 25 amps on its 5V line, and 10 amps on its 12V line.

In a pinch, I could just obtain a suitable plug barrel that fits my Netgear from Radio Shack, clip the power connector off a dead CPU fan, and solder the plug to the red wire (5 volts) and a black wire (ground), put it in a PC, and use that to run my Netgear hub. The increased power draw would be equivalent to putting three typical PCI cards in the system. Just be sure to wire things right–reverse polarity can kill some devices.

Rather than using one of the PCs I actually use, it would be better to obtain a cheap microATX case, short the green and one of the black wires on the 20-pin motherboard connector with a paper clip, insulate the paper clip with electrical tape, and then wire things up to the drive connectors. Or, for that matter, you could use some of the other leads available on the 20-pin connector if you have a device that needs 3.3 volts (pinout here.) You could also just use a bare ATX power supply with a paper clip connecting the green wire and one of the black wires on the 20-pin motherboard connector, if you’re into the ghetto look.

An AT power supply would also work and it offers the advantage of being really cheap and common (here’s a nice writeup about an AT power supply’s capabilities), but most AT boxes require you to hook up enough 5-volt devices to chew up about 20% of its rating on that power rail before they’ll power up. I have a 200-watt AT power supply that delivers 20 amps on its 5-volt rail, so my 3-watt Netgear hub probably wouldn’t be quite enough on its own. So it might be necessary to either connect an obsolete motherboard to the power supply or connect a 1-ohm resistor between a +5 lead and ground, if you don’t have a plethora of power-hungry 5-volt devices to plug in.

But PC power supplies provide a cheap and commonly available way to replace odd wall warts, or at the very least, to reduce the clutter around the computer room.

11 thoughts on “Replacing wall warts with PC power supplies

  • March 22, 2003 at 10:01 pm
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    I think this might be a white trash idea more so than Ghetto. I mean if Red Green knew a thing or two about computers, this is an idea that probalby would have popped in his head.

  • March 23, 2003 at 3:09 am
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    There are downsides to the project ๐Ÿ™‚ Getting a rather large box with a fan to drive a hub?

    Anyhow, I got a pointer on that 1-ohm resistor. Not a good idea. I know they mentioned this in the text you refered to but they don’t say anything about the consequences which is not good….

    If you stick a 1 ohm resistor between 5 volts and ground you will need a resistor capable of (at least) 25 Watts!!! Do you know what size of resistor you will need for this?

    Anyhow, if you decide to do this, you might consider drawing the resistor outside the box so you can use it in your kitchen to cook eggs and so on ๐Ÿ™‚

    Here is the explanation:

    Volts = 5 V.
    Resistance = 1 ohm
    Current = voltage / resistance = 5 amps

    Power = volts * amps = 25 W.

    Roughly speaking…

    Dave T.

  • March 23, 2003 at 2:03 pm
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    Alternatively, you could connect four 4-ohm, 10-watt resistors in parallel, yielding the equivalent of a 1-ohm, 40-watt resistor (and it would run cooler, too).

    Here is the explanation:

    Series & Parallel Resistance

    Therefore,

    4 ohms / 4 resistors = 1 ohm

    Also,

    10 watts * 4 resistors = 40 watts

    Roughly speaking… ๐Ÿ˜‰

    If push comes to shove, they’re available from DigiKey.

  • March 23, 2003 at 3:24 pm
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    Yup, Thats correct Mark. However, I still advise against building this thing in order to make a 200W powersupply act stable to power a hub that it was never meant to do it in the first place. I am against exposing that type of current in that type of manner outside of the powersupply unless someone with real experience in building such things does it. That was my only point.

    Also, roughly speaking :), your 40W calculation is a bit off. Connecting 4, 4-ohm resistors in parallel will, like you said, give you 1-ohm, giving you a total of 25W like before.

    5 volts/4 ohms = 1.25 amps.
    5 volts x 1.25 amps = 6.25 watts
    4 x 6.25 watts = 25 watts.

    10 Watts is the maximum rating for the resistor. If I was building a heat generator like that then I would make sure that its maximum rating was maybe double of what I really needed.

    /Dave T.

  • March 23, 2003 at 6:14 pm
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    However, I still advise against building this thing in order to make a 200W powersupply act stable to power a hub that it was never meant to do it in the first place.

    +5 volts DC is +5 volts DC; the 200W PC power supply doesn’t care what it powers, and the hub doesn’t care that you’re not using its factory-original power brick.

    I am against exposing that type of current in that type of manner outside of the powersupply unless someone with real experience in building such things does it. That was my only point.

    On which I feel certain all three of us concur. ๐Ÿ™‚

    Also, roughly speaking :), your 40W calculation is a bit off.

    Au contraire, mon frere; since each of the four resistors can individually dissipate a maximum of 10 watts, I was describing the maximum power dissipation of the resistor bank. Sorry, I should have clarified that.

    10 Watts is the maximum rating for the resistor.

    Yes, for each resistor. So:

    10 watts * 4 resistors = 40 watts

    If I was building a heat generator like that then I would make sure that its maximum rating was maybe double of what I really needed.

    IMPO a 160% overdesign (40/25 ratio) instead of 200% (50/25 ratio) is perfectly acceptable for this application; but there’s nothing to prevent you from going higher if you prefer (and again, your resistors would run cooler).

  • March 24, 2003 at 12:37 am
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    Thanks for the clarification Mark, although I am a bit sceptical about one thing:

    “+5 volts DC is +5 volts DC; the 200W PC power supply doesn’t care what it powers, and the hub doesn’t care that you’re not using its factory-original power brick.

    I know that the hub itself doesn’t really care where it is getting it’s current from as long as it is a stable supply. However, the power supply is built to give high current on two voltages. I don’t have much experience using just the 5 volts output from a PC power supply and leaving the 12 volt supply unused. I know that the 5 volt supply is doing most of the work but it makes me wonder if it is a good thing to leave the +12v supply without any load whatsoever? What do you think?

    Cheers,

    Dave T.

  • March 24, 2003 at 10:13 am
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    Points taken, that an AT power supply is more trouble than it’s worth if you’re just going to power one device, and an ATX power supply, though easier, is overkill.

    But when an el cheapo microATX power supply and case can be had for $20–the same cost as the shoddy wall wart that came with my hub–the overkill makes sense. And if you can use the same big box to power another device or three (modem, scanner, cable/DSL router…) then you’ve saved some electrical outlets. You’ve still got a shoddy power supply, but then it’s a severely underloaded shoddy power supply.

    And you can always run some cables and make that case do double-duty as a SCSI enclosure to justify the space it’s taking up.

  • March 24, 2003 at 12:14 pm
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    I shudder when I hear the words “short”, “paper clip”, and “electrical tape” used in close proximity. I expect “chewing gum” and “twine” to follow. ๐Ÿ™‚

    Seriously, I’m all for experimentation and well-placed frugality. But I also believe that we have things like wire and solder for a reason. For anything beyond a temporary setup, these are probably better choices than office supplies.

    If it were my choice, I wouldn’t sacrifice piece of mind (or possibly safety) for “alternative” setups (I also believe that we have dedicated power bricks for a reason). And I disagree with Dave’s point about cost in the previous comment. If you place value on your effort – total hardware costs held constant – to rig up something like this, I’d argue that it’s more expensive than just buying a new brick.

    I’m sure Dave’s real purpose here is to tally a tricked-out case/PS like this in our computer arms race. ๐Ÿ™‚

  • March 24, 2003 at 9:18 pm
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    Dave T.: I know that the 5 volt supply is doing most of the work but it makes me wonder if it is a good thing to leave the +12v supply without any load whatsoever? What do you think?

    It depends on the power supply you have; one power supply I worked on would vary its +12 volts (with no load attached) way out of tolerance if you varied the load on the +5 volts, but when you attached a small load to the +12 volts, it was stable. Then there was the supply that wouldn’t come up at all unless both the +5 volts and the -12 volts had small loads on them.

    I’d say try loading only the +5 volts and see, and if you find your particular supply will run without a load on +12 volts, then don’t worry about it. Otherwise, put a load on +12 volts.

    Dave F.: You’ve still got a shoddy power supply, but then it’s a severely underloaded shoddy power supply.

    I had thought that your readers who happened to have 145W eMachines power supplies left over from an upgrade could use those, since the problem with those is being overloaded anyway.

  • December 14, 2003 at 10:10 am
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    I’ve done this for assorted bench projects. I’ve yet to find a PC power supply that won’t run with half an amp load. Radio Shack sells 5 watt 8 ohm resistors (little ceramic brick thingies)that work great, I’m running a project like that right now ’cause I’m building the circuit in stages and it won’t draw enough current to keep the power supply on itself until most of the system is online (Running a bunch of stall-motors for animation on my model railroad).

    The only thing your formerly wall-wart powered thingie might notice is that the power is a heck of a lot smoother than coming off that cheap plug-brick.

    But you could do away with the resistor by moving more things in your office over to that switching supply. If you run out of +5 and +12 there’s generally a peripheral or two left going -5 to ground and -12 to ground. Modern ATX supplies have a lot of 3.5V around too, but I haven’t come up with a reasonable use for that yet.

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