ATX 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.

Step 1: Background

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.

Comments

[nwlaurie]

Just built it over the Easter weekend - it works exactly as expcted. Many thanks for an excellent circuit diagram (I know it's not complicated but it's still really handy to have a decent diagram!).
HINT: if you have a choice of old power supplies, pick a QUIET one!!! And test repeatedly as you build.
I opted o remove some of the unwanted output cable bundles at the root, if you do this make sure you have a really powerful soldering iron or you'll end up doing more cooking than clearing (I used a gas-powered one for this bit).
Thanks again,
Nick

[nwlaurie]

Just built it over the Easter weekend - it works exactly ad expcted. Many thanks for an excellent circuit diagram (I know it's not complicated but it's still really handy to have a decentdiagram!).
Thanks again,
Nick

[Crucial97]

Hi sorry for getting back to you late, i was testing my power supply last night and a resistor popped. so i am going to remake another since we have 3 non-used power supplys

[Crucial97]

Hi,
I have been reading your instructable and I have built my own until i found the grey power-good wire. what did you do with your (probably grey) power-good wire?
Thanks,
Zach

[matthewbeckler]

Hi Crucial97, thanks for the comment. I connected it to an LED to act as a "power is good, we're ready to go!" indicator. Check out the diagram on step 2 (http://www.instructables.com/id/ATX--%3E-Lab-Bench-Power-Supply-Conversion/step2/Planning/) and you can see how I connected the LED+resistor to the grey wire. Good luck with your project!

[hschmutz]

Hy there
I know this is an older post but i will try to make this power supply for me.
What fuse do you take ?

[matthewbeckler]

Hi, thanks for the comment. Choose your fuse based on your application (what you're going to power with your power supply). The fuse is primarily there to protect your circuit from mistakes in your wiring, not to protect the power supply from your mistakes (it doesn't need protection). If you are running small circuits like an Arduino that will draw less than 500mA, then a 1 amp fuse should be fine. If you are charging RC batteries from your power supply, then you'll need a 10 or 15 amp fuse. Basically get a fuse that can handle just a bit more than the maximum amount of current than you expect to need for your application. Good luck!

[Skarz88]

How do you use the -12v and -5v?

[matthewbeckler]

Hi Skarz88, not really sure what exactly you are asking? Those wires produce voltages at -12 volts and -5 volts relative to the ground (black) wires. They are generally not very useful to the electronics hobbyist since they have very limited current capacity (not the tens of amps available on the +5v and +12v lines). Does that answer your question?

[Zakinator]

I'm having a bit of trouble seeing what you did with the wires that supply voltages that you didn't want. When you originally cut the supply wires, did you just remove the ones that you didn't want from the circuit board within the power supply? Or do you need to be more careful than that to make sure that they don't shock anything?

[matthewbeckler]

Yep, If you don't want to use a particular voltage. I would cut the wires off at the same height (maybe 1-2 inches from the circuit board) and tape them together so they don't accidentally touch against something else. For the voltages you do want to use, the number of wires to use probably depends on how much current you want to draw at each voltage. If you want to charge batteries or some other high-current load, you probably want to keep as many of the wires together as you can, and connect them to a binding post. Good luck!

[Zakinator]

Awesome, thanks so much! Just figured it would be good to check before electrocuting myself...

[mcdabcar]

mcdabcar
Hi this might sound crazy but what size of fuses should I use on my +5V an + 12V as they are the only one I am going to use

[matthewbeckler]

Not crazy at all, but a very good question. Unlike what I did (put a single fuse on ground) you definitely want to put fuses on each non-ground line you are using (+5v and +12v). How large of fuse depends on what you are doing with each voltage. If you need to draw a lot of current, like for running an RC car battery charger, then you will need a large fuse (10-20 amps?). If you are doing small things like arduino-style projects, then you want to have a smaller current fuse so it limits the maximum current that can flow through your circuit, maybe 0.5 amps?

Hope that helps! If you need more advise, let us know what you're planning to power with your power supply, and that will help us know how much current it might draw. Good luck!

[jmunoz3]

Matthew, great tutorial. I wanted to point out that on your schematic you have placed the 2, 330 ohm resistors on the cathode leg leads of the 2 LED lights. I do believe that they belong on the anode legs. Once again great tutorial.

[matthewbeckler]

Thanks, glad you like the tutorial. You can put the resistor on either side of the LED, either the anode or cathode side. I haven't really ever heard of a convention for where to put the resistor. Thanks for the comment!

[dcopeland1]

You can use the resistors on the anode side as well. Just as long as you do have a resistor in series with the LED's. Remeber an LED is a diode that will light up when forward Biased. So you can have a load resistor on the cathode to ground if you want and it will work the same. If wanted to try something fun, build a clamper or limiter circuit with a Diode, two resistors, a 1.5V battery and a 5 power supply. Now just a quick warning to others that are wanting build one of these. I can attest to the Capacitors on the 110V side of the PS. These bad boys hurt if you touch the outer shell. Trust me. So be careful with what you do...

[Zach-In]

is this stuff can Charge a Car Battery rating @ 12v, 50A? if yes, how long it would take to fully charge the battery?

[matthewbeckler]

For charging batteries, you want to use a battery charger product, some electronics specially designed for charging lead acid batteries. The reason for this is that improper charging can cause a build-up of explosive hydrogen gas, so you want to do it right. Also, the chemical reactions involved with discharging and recharging need to proceed in the proper way, based on the charging voltage and charging current. For more details about the charging profile, check out this page http://batterytender.com/resources/battery-basics.htm

Good luck!

[salexandridis]

Here I am, with a new ATX Power Supply which is functional! The circuit presented on this Instructable works just perfectly! But.... when I try to get more current it switches off. For example, it can handle a 12V fan (which was salvaged from the same PC), but it can't handle driving a 12V motor from a hand vacuum. Any explanation...?

[matthewbeckler]

I know that some power supplies have short-circuit protection circuitry, which might shut-down the power supply if it detects a sudden large current draw. Perhaps this is what is happening to you? Does it work if you have the 12V motor connected when you turn it on, instead of connecting it after the PSU is already running?

[salexandridis]

I know that I said that I would answer soon, but I was busy. When the motor is already connected to the PSU, inothing changes. The motor spins for a fraction of a second and then the PSU shuts down. Having been desperate enough, I tried lots of different devices. Firstly I removed the resistors and tried the PC's fan. It worked smoothly! Then I connected a 10W LED, which would just flash and I started playing with the switch (the one which shorts the black and green cables). It didn't take long to realise that every time I turned the switch on and off, the period of time the LED was on was getting longer and longer, when suddenly I mannaged to make the PSU co-operate with the LED! It was a mirracle, but stil, every time I want to turn on the LED I have to play with the switch a little bit. Of course a LED is not a device that drains a lot of current. The next step was to connect a Flyback Transformer Driver circuit, which would demand all the current the +12V cable could supply (which in my case is 8 Amps). Guess what! This time I wasn't that successful.
Do you think that I can modify something in order to deactivate the PSU's short-circuit protection? An idea of mine, is to slowly increase the Amps by using a trimmer - potentiometer. I have no idea what kind of trimmer to connect or whether this would ever work. What's your opinion?

[matthewbeckler]

Hey there, thanks for the interesting reply. While most LEDs don't draw too much current (10-30 milliamps typically) a 10W LED will draw a lot of current. A typical 5mm red LED will have a voltage drop of about 1.5v and a current draw of about 20 mA, so that's 0.03 W. If you had a 10W LED that dropped 1.5v then that would be 6.67 A of current! So, certainly not insignificant.

It's really weird that you can get a large load to stay on by playing with the switch, I'll have to think more about why this might be.

I don't have any experience with disabling the short-circuit protection on a power supply, and wouldn't really know where to begin.

I don't think it would work to use a simple trimmer/potentiometer to limit the current draw. If you have a motor controller driving your motor, you could slowly increase the motor's speed after power-on to try and gradually increase the current draw?

Good luck!

[salexandridis]

Actually the LED is rated to 12V and at ±950mA:
http://www.ebay.com/itm/1pcs-10w-High-Power-Square-Warm-White-Led-800-900LM-/110867200712?pt=LH_DefaultDomain_0&hash=item19d0333ac8

I don;t know whether this makes any difference, but just to let you know.
Also, I managed to make the PSU work with the 10W LED attached without having to play with the switch. I didn't change anything. It seems that the PSU is just "used" to it!

But the motor still doesn't work...

[salexandridis]

I don't know... I'll try and answer soon!

[Vspec]

My psu needed the power resistor on the +12v instead of the +5v in order to turn on.

It's made by Delta electronics, manufacture date 2008.

[tomato1991]

Just wanted to say thank you for posting this.
Nice and easy to understand, and it was the exact thing I needed.

[seraine]

For some reason, my standy led takes about a minute to turn off when it loses power because it takes a while for the voltage to drain off. Is anyone else experiencing this, and is there a simple solution?

[cardboardwizard]

This is likely because of the inductors and similar components found within the psu. When on they build up a large magnetic field so when turned off the field collapses and produces current which will keep the LED on for a short while.

[matthewbeckler]

I see the same exact behavior when I unplug my PSU. I don't know if there's a way to change this behavior, but I also can't think of a reason why it would be a problem.

[arzenpunk]

Great, you did a great job, just a short question... how many amps this kind of power supplies can give?

[matthewbeckler]

Excellent, an easy question! Most (all?) power supplies come with a nice label on the side that should list how many amps of current are available at each voltage rail. Some newer power supplies that I've seen have split 12 volt lines for some reason, but I'm not really sure how that works.

[harshesh]

Will a 10W 100Ohm Resistor work as the power resistor ?
When I use the 10W100Ohm Resistor and connect the green line with the black line, Some noise comes for a second and then stops .. Sometimes the noise comes for 5 seconds and then the Fan starts but then immediately stops ! .. Is there something wrong with the Power Supply I am using or is it the resistor ?

[matthewbeckler]

Hi! Somewhat counter-intuitively, the larger the resistance you have, the less current it was draw from the power supply. A 10 ohm resistor will draw 500 milliamps (mA), a 100 ohm resistor will draw 50 mA, and a 1000 ohm resistor will draw 5 mA. Presumably there's some minimum current draw that your power supply requires on the 5v line (and maybe the 3.3v line too?) and if you don't draw enough current (resistor is too large) then it won't stay on.

It really sounds like the behavior you describe is due to insufficient load current, or perhaps some other problem. I would try a power resistor with a smaller resistance value to try and draw more current. You could try connecting two of your 10W 100 ohm resistors in parallel, producing an equivalent resistor of 20W 50 ohms that will draw 100 mA, and dissipate P = V * I = 5 * 0.1 = 0.5 watts shared across both power resistors. Good luck!

[salexandridis]

Hello!
I also tried to convert my 350W ATX, but there are aome problems.
First of all I'd like to mention that I didn't a 10Ω 10W resistor, so I replaced it with two 22Ω 5W resistors in parallel. Is this fine?
When I turn on the switch of the ATX, the Stand By LED lights up. But when I short the green wire to ground, the fan spins for a second (or less) and then.... nothing. The Power On LED doesn't even light up. Firstly, I thought that my PSU is "clever". I thought that the fan was heat controlled. So, in order to test that, I connected an LED to the 5V output, using also a 330Ω resistor. And guess what... The same exactly thing happened. It just blinked.
Any ideas of what's going on here?
P.S. The PSU certainly works, because I just unplugged it from a working PC!

[bikerboy_id]

Hey slalexandiris,

Did you find a solutions to this problem of power source not staying on?
I have a 300W ATX wich does the same thing, although I use the reccomended 10Ω 10W resistor. (tried the load on 3.3V, 5V, 12V)

Hope you solved it by now :)

[salexandridis]

Unfortunately not. It seems that my ATX is pretty much useless. I don't know why, but it doesn't work. I didn't through it away though because I will salvage it's parts. They might come in handy in the future. I will probably buy an Switching Power Supply on eBay:

http://www.ebay.com/itm/Led-Strip-Universal-Regulated-12V-Power-Supply-Transformer-2A-3A-5A-10A-20A-30A-/110840547570?pt=US_Lighting_Parts_and_Accessories&var=&hash=item19ce9c88f2

[bikerboy_id]

Yeah, I have come to the same conclusion, it must be an internal short. I've put it aside for now.
Anyways, by chance I put my hands on a 450W source that works perfect, it only took a couple of hours to finish this project.

Thank you and also Matthew for a very good written project!

[matthewbeckler]

Hey there, sorry to see that you're having a bit of trouble.

1. The pair of 22 ohm 5W resistors in parallel will have an equivalent resistance of 11 ohms. This will draw I = V / R = 5v / 11 ohm = 0.45 amps, making for TOTAL power dissipation of P = V * I = 5v * 0.45A = 2.2727 Watts, which is split between the two 5W resistors, so you should be just fine.

2. That's a good idea about the PSU being too clever, and I've heard from other people that that happens. It sounds like the PSU thinks it has no load (and shuts off), but if you have the 0.45A load on the 5v line that should work ok I would think.

Perhaps you can try moving that load to 12V or 3.3V, or adding extra loads to those lines?

Good luck, let us know if you solve it!

[salexandridis]

Thank you! I'll try!