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introATX to Lab Bench Power Supply Conversion
In my sophomore year of college at the University of Minnesota, I
started into my main electronics classes, and needed a good power supply
for working on lab projects at home. My roommate Adam
told me about somebody online who had converted an ATX computer power
supply into a lab bench power supply, so I decided to do the same thing.
You can also check out this link
for a very similar guide by their user Abizarl
. I have also documented this project on my website at http://www.mbeckler.org/powersupply/
if you are interested.
Warning! There are several large capacitors in ATX power supplies, that will store a dangerous charge for a long time. Please let your power supply discharge, completely unplugged from the wall outlet, for a few days before opening it up. You can probably be seriously hurt, so please be very careful. I am not a lawyer, but I hereby release myself from as much liability as I can, for any sort of injury you sustain, or any trouble you get into.
Warning! There are several large capacitors in ATX power supplies, that will store a dangerous charge for a long time. Please let your power supply discharge, completely unplugged from the wall outlet, for a few days before opening it up. You can probably be seriously hurt, so please be very careful. I am not a lawyer, but I hereby release myself from as much liability as I can, for any sort of injury you sustain, or any trouble you get into.
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step 1Background
First, a bit of background on a typical ATX power supply:
Computer power supplies are Switch Mode Power Supplies (SMPS) , which use high-frequency switching circuit elements to provide a high-quality output voltage, with good energy efficiency. One side effect of this technology is the minimum load requirement that each power supply has. In order to function properly, the power supply needs at least a very small electrical load connected to it. In other words, ATX power supplies will only work if you have something connected to it. We will be using a power resistor to provide this minimum load.
Also, modern power supplies do not simply have an OFF/ON switch, they have what is known as a "soft" power switch. This normally makes no difference to the user, as the computer behaves the same, but when you shutdown your computer, the motherboard can turn off the power supply when it has finished shutting down. This requires us to add our own power switch to the power supply chassis.
To protect our circuit from accidental (and careless!) short circuits, we will install some fuse-holders and fuses, which will disconnect the circuit supply lines if too much current flows. The size of the fuses are up to you, but a 1 amp fuse will work just fine for most circuits. You really should put fuses on all supply lines.
Update: While the diagrams show fuses on all voltage rails and no fuse on the ground line, when I actually built my power supply, I was young and foolish and only put a fuse on the ground wire. It's much safer and a better idea to put fuses on all signal lines and not the ground line. Thanks to many emails and messages on Instructables about this oversight.
Computer power supplies are Switch Mode Power Supplies (SMPS) , which use high-frequency switching circuit elements to provide a high-quality output voltage, with good energy efficiency. One side effect of this technology is the minimum load requirement that each power supply has. In order to function properly, the power supply needs at least a very small electrical load connected to it. In other words, ATX power supplies will only work if you have something connected to it. We will be using a power resistor to provide this minimum load.
Also, modern power supplies do not simply have an OFF/ON switch, they have what is known as a "soft" power switch. This normally makes no difference to the user, as the computer behaves the same, but when you shutdown your computer, the motherboard can turn off the power supply when it has finished shutting down. This requires us to add our own power switch to the power supply chassis.
To protect our circuit from accidental (and careless!) short circuits, we will install some fuse-holders and fuses, which will disconnect the circuit supply lines if too much current flows. The size of the fuses are up to you, but a 1 amp fuse will work just fine for most circuits. You really should put fuses on all supply lines.
Update: While the diagrams show fuses on all voltage rails and no fuse on the ground line, when I actually built my power supply, I was young and foolish and only put a fuse on the ground wire. It's much safer and a better idea to put fuses on all signal lines and not the ground line. Thanks to many emails and messages on Instructables about this oversight.
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I understand the earler AT box did not do this but am unable to get the full 12v.
Tim
I think that computer power supplies have a short-circuit-prevention where any large spikes in the load after they have turned on are treated as a short circuit and the PSU shuts down to avoid damage. I'm not sure how you could get around this issue, hopefully someone else has run into this problem and has a solution.
Also, I have two PSUs and want to keep one as a backup for my pc. One of them is 250W and came from my old pc, so I know it works perfectly. The other is 300W (PS-PR+P4-300X, PREMIER, ATX POWER SUPPLY 300 WATT), but I got it from dumpster diving and there's some red stuff coming out of some of the capacitors (I assume that's bad). As far as I know, they both work though. I connected the greedn to ground and they both powered a small motor) Which would be better (and safer) to keep as a backup and which should I use for this project?
None of the plans I see online use the 3.3v, is there a reason for this, or can I use it without problems if I want?
Also, some show one wire of each color connected to the post, and some say connect them all. After installing the resistor I have 11 ground wires and half a dozen of each of the voltage wires. Should all really be connected?
Thanks for this project,
Semper Cogitant
I can think of no reason not to use the 3.3v line. My power supply is very old (I bought that computer with paper route money in 7th grade, so about 1997-1998?) and didn't come with 3.3v output. I remember reading that in addition to requiring a small load on the 5v line, you might also need a small load on the 3.3v line, so that the regulation circuitry doesn't get confused by an absence of a load.
More wires = more current capacity. For my projects, I have the voltage rails fused to 1 amp of current, which is easily carried by just one of the power supply wires. If you were building this power supply for a larger current situation, such as a battery charger or ham radio project, where you were actually going to use most of the power supply's current capacity at a given voltage, then you'd probably want to use more of the wires, lest you risk overloading the maximum current through one of the wires.
Good luck, let us know how it turns out!
I used a lighted switch instead of a separate LED, and I put the connectors and fuses in a little project box on top of the unit because it's a pretty cramped space inside.
Thanks for the great article.
As far as I know, most ATX power supplies are a "switch mode" power supply, meaning that they first do the full-wave rectification of the AC signal and then use inductors and capacitors with some control logic to produce the proper output voltage. I'm not sure what you mean by "pulsed DC", but you can find more information here:
http://en.wikipedia.org/wiki/Smps