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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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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 used a 350W ATX power supply to power a CD car radio and an automated aerial over 12V and here's the results:
- the 3.3V line had to be loaded with almost 2 Ohms of resistance!
The reason is this: the 12V are supplied from the same transformer as the 3.3V and are not regulated as opposed to the 3.3V line. So when I cranked up the volume the PS would shut down since the load on the 12V caused the 3.3V to dip. But since the 3.3V line had no current going out the PS decided to shut down. So in order to get any good power out of the 12V line the 3.3V line should be loaded with at least 1 Amp.
Now I can draw almost 3 Amps on the 12V line without a problem.
PS: your mileage might wary with the load resistor :)
So my brother-in-law is an R/C plane hobbiest, saw my little project last night, and really wanted one of these to charge his batteries with. so with another PSU i had laying around, i first tested it by jumping "PS_ON" and ground, and it worked like a champ.
after all internal mods were done, i went to test the PSU before giving it to my brother-in-law and nothing......my +5vSB LED comes on but thats it. i did notice that this PSU had a +3.3vS line so i connected it to the +3.3v wires in the PSU, no worky worky. it puzzles me that this PSU would turn on before taking the PSU "apart" but not now. im going to take the switch out of the loop tonight and solder the "PS_ON" to ground, but other than that any ideas? maybe i have to connect the 3.3vS wire to the +5.5v line?
just a little background on my self, 9.5 year aircraft avionics tech with basic Electronic Principals knowledge. I can use an "O" scope and DMM like nobody business, and know OHM's Law very well. i appologize for not doing anymore troubleshooting before posting this, but im at work and would like as many ideas as possible to go home with to troubleshoot. thank you everyone in advance!
Yeah, that's a strange case for your second PSU, that it would originally work just with shorting PS_ON to ground, but then when you did the mods that same thing doesn't work. Odd.
Perhaps you created a short by accident when you did the modifications? Did you cut down the extra wires near the PCB? Could a stray strand of wire shorted against the case or another wire? I'd look for that.
I've heard that some newer power supplies might need loads on the 3.3v line so that might be something else to try?
PS. You did plenty of troubleshooting, definitely more troubleshooting than most people do before posting. Good luck, let us know how you get it working!
P.S. do you have any experience with server PSU's? im about to buy one i found online for $15 that has a 32amp rating on the 12v rail. for R/C hobbyists that charge batteries with these its perfect. wish me luck :)
I haven't played with server PSUs before, but I'd hope they operate in a similar fashion, but might have a different pin out, you know? Probably would still have a pin for "ground to turn it on" but it might be in a different place in the main connector. Let us know what you find out!
I'm also building a reprap prusa, which needs a lot of current at 12v for heating coils and the like, so I picked up this 12v 30A dedicated PSU from Amazon for about $40 including shipping. If you can get your server PSU working that'd definitely be a better deal. Good luck! http://www.amazon.com/Universal-Regulated-Switching-Computer-Project/dp/B004CIW8LA/ref=sr_1_1?ie=UTF8&qid=1325513725&sr=8-1
http://www.newegg.com/Product/Product.aspx?Item=N82E16817101053
recently been made to maintain the equipment that has ATX supplies, maintenance consists of cleaning the equipment with dry air blower.
These devices are connected 24 hours during the last 3 years.
When you reconnect the equipment one source gave no sign of life, and broke in two others the next day.
What is the reason for this situation?
Just to confirm, you are still connecting the green wire to ground to turn it on, right? Even with the LED in your rocker switch, the green wire needs to be connected to ground to signal the PSU to turn on.
Connect the 5vS wire to one of the 5v wires, and then connect one of the 10 ohm 10 watt resistors between 5v and ground.
Connect the 3.3vS wire to one of the 3.3v wires, and then connect another 10 ohm 10 watt resistors between 3.3v and ground.
Use your rocker switch to either leave the green wire disconnected, or connect it to ground. If your switch has an LED you can power it from the 5vSB (5v standby, which is always on) wire if you want it to be lit up whenenever it's plugged in, or power the rocker switch LED from the regular 5v line if you only want it on when the switch is set to "on".
Does that make sense? Good luck, hope you get things working!
you're working on a PhD in electrical engineering. if your not an expert, who is? :)
You probably need to have a 10 ohm 10W power "sandbar" resistor between the regular red wire 5v line and ground. You might also need to do the same on the 12v wire or the 3.3v wire. Separate resistors for all lines you want to add a constant load. Different PSU's have different loading needs, you'll have to experiment a little bit. I've even had some people report not needing any load at all!
Some power supplies have "sense" wires which need to be connected to their corresponding voltage wires, but these are uncommon. I think you said you have one, so connect the 3.3v sense wire to the 3.3v supply line.
If the power supply doesn't come on (meaning you can measure proper voltages at the 5v and 12v lines, as well as the PG wire goes high (5v I think), then your power supply might be broken. Good luck!
(also, there are lots and lots of different areas in EE, and my area of digital electronics and IC chip test are pretty far from analog circuits and power electronics :- )
Brilliant instructable I am new in electronic engineering and wanted a lab power supply will you please tell me that how Can we add a potetiometer(voltage regulator),rheostat, voltmeter and ammeter to calculate and vary voltage and current?
Thanks
You should be able to either modify an existing volt/ammeter to do what you need, or you could always whip up some sort of microcontroller + tiny current sampling resistor + 7-segment displays for your volt or ammeter. Good luck!
What do you mean by "data wire"? Are you talking about the green wire that you normally connect to ground to turn the supply on? What do you mean by "crossed"? Did you permanently connect the wire to 3.3v or just briefly touch them together?
Does your power supply have a rocker switch on the back, near the AC plug? If so, that might be an option for you. Normally, the green ("data") wire should be left unconnected, and connected to ground when you want to turn it on.
Good luck, let us know what you figure out!
Yeah by data wire i mean the green wire that needs to be grounded for the psu to turn on. And i accidentally touched it to the 3.3 wire for maybe a second or two and now the data wire is useless for a switch to turn it on/off... There is not a rocker switch in the back sadly. Lol so the problem i have now is that when i plug this in it turn right on even with the data wire not on ground, do you think there is a way to fix the green wire so i can use it and ground for the swittch shown in your schematic or should i find another place to put a switch?
Thanks for taking the time to give me advice, i appreciate it!
You could go get some more, and put them in series, to increase the resistance and power capacity. If you had 5 1 ohm, 10W resistors in series, the equivalence resistance would be 5 ohms, 50 W power capacity. This equivalent resistor would draw I =V/R = 5 / 5 = 1 Amp of current, and burn P = V * I = 5 * 1 = 5 Watts of power, spread across all five resistors, so each resistor would burn 1 Watt of power. That sounds much better.
Keep in mind that some people here have mentioned that their power supplies worked just fine without any load resistors at all, so you might want to try that first to see if your PSU requires a load. Good luck, let us know what you come up with!
Everything seems fine but should I still have the power resistor? I just want to make sure nothing is going wrong :P
One thing I'd suggest to to attach a small load to each rail in turn and measure the voltage again, under load. You know how when a AA/AAA battery is running out, it will still measure 1.5 volts without a load, but once you attach a load, the voltage will drop really fast? Maybe the same thing could be happening here. I doubt it, but that might be one more test to try. Congrats on finding an awesome PSU to convert!
If you get a chance please respond.
Thanks again.
-Nick