Instructables
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.
 
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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.
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arush22 months ago

hi friend

Currently I'm working with some dc motors and i need negative voltage with high current, but when I connect the GND of the "power supply 1" to the 12V of the "power supply 2", to use the GND of "power supply 2" as -12V, i have a short circuit, and therefore the "power supply 2" is switched off. Is there some way to get -12V with high current?

thanks

matthewbeckler (author)  arush22 months ago
Most likely your power supplies are connecting GND to the AC earth ground (ground plug on your AC outlet), which produces a short from 12V to earth ground on PS2.

Why do you need negative voltage? Most applications don't need negative voltage. If you are driving DC motors, you should be using an H-bridge circuit in order to drive the motors in either direction, without needing to use negative voltage. More details here: http://en.wikipedia.org/wiki/H-bridge

There are a lot of different H-bridge chips that are really easy to work with. Which chip to select depends primarily on how much current your motor needs, and what voltage (6v? 12v? 24v?).

thanks for your response :), the problem that i have is due to the driver that im using to control the motor, i have a analog output signal of -10v to 10v from the PC (data acquisition card), and the driver only converts this signal in "high current voltage" (the driver is similar to use the h-bridge but using two transistors), then i can control the movement speed and the rotation direction using one output, but for the driver i need -12V and 12V (with this voltage is enough because i don't require high speed) with high amperage, but when i use the 2 atx in parallel to get 12 and -12v volts one of them is turned off :S.... then i dont know if it is possible to connect two atx in parallel, or is necessary to do other thing

matthewbeckler (author)  arush22 months ago

I see, sounds like old PLC-style control systems :-) How much current do you need? If it's just a few amps, you could probably build your own from an AC-AC transformer (down to about 15 volts AC) and then a full-bridge rectifier to generate the positive and negative DC voltages, and finally a 7812 and 7912 linear regulators (with heatsinks and in/out capacitors). If you need more current, you're probably going to have to drop some cash into a real power supply, which may be a little more difficult to find such a bipolar power supply.

Alternatively, I have used some fancy power supplies in EE labs where the negative terminal of the power supply wasn't permanently connected to AC earth ground (it included a little wire that you could use to connect them if you wanted). If you had two such power supplies, or a power supply with two outputs, you could connect them together in parallel like you mentioned.

Zarix3 months ago

I know some people have been having trouble getting their PSU to stay on. And this was my solution. I could not find any information on the current rating of the 12v CPU plug in the newer supplies. But when I put my load there it works for me. Maybe that is where the load needs to be for some newer supplies to stay on.

stuffandwires3 months ago

hello, this is a nice project that i'll try to make in these next days. Regarding the fuse matter, i see only a fuse holder in the front. Is that the only first fuse that you had connected to ground? The rest of them are in a 3x fuse holder or individually?
thank in advance

matthewbeckler (author)  stuffandwires3 months ago

The way I built it, there was only one fuse, on the ground line, which isn't a great idea. What would be best is to put a fuse on each power rail (3.3v, 5v, 12v, etc). If you could find a 3x fuse holder that would really help to reduce the space needed for fuse holders. Good luck with your build!

mothball3 months ago

I have LCD Controller that need 12V4A. Could I connect through 12V PSU directly?

matthewbeckler (author)  mothball3 months ago
That should work, if your PSU can provide > 4 amps for the 12v line (check the label on the side of the PSU for the current rating).
harshesh2 years ago
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 (author)  harshesh2 years ago
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!

I always wondered why all tutorials use or suggest to use specifically a 10 Watt 10 Ohm resistor for dummy load. In high school I was taught: P=V*I => P=V*V/R
In this case: P= 5*5/10 = 2.5 Watts So I always wondered, why use a 10 Watt resistor instead of 5 Watt. Even considering possible spikes the resistor wouldn't be damaged. A constant load that produces 5 Watts of heat or more would be necessary to damage it.

matthewbeckler (author)  myouknowwho7 months ago
Yep, only 2.5W burned at 5v. I would guess that 10W 10Ohm resistors are more standard and easier to find, if you can find a power resistor at all. They are not used for many applications and can be difficult to find.

[head-bulb.jpg]

What about use a incandescent bulb instead? Maybe hader to find soon becouse of led lamps tough.

ionsight8 months ago
do you guys think a 160w unit would be practical for use testing my arduino and various small electronics. It could be a nice compact test supply
matthewbeckler (author)  ionsight8 months ago
Most power supplies' wattage rating is an overall power rating, for all the different output voltages (mostly 5v and 12v though). You should check the label on the side of the PSU to see how many amps of current it can provide on the 5v rail. This will directly indicate how many arduinos you could run. I think most USB devices will draw less than 500mA, unless they are a high-power device like some some cell phones and tablets, which can draw up to 2 amps sometimes, depending on how you connect them. FWIW, the USB and ATX specification for 5v rail is the same for both, 5v +/- 0.25v.
sdevi reddy11 months ago
sir small doubt regarding the power supply!
im done with my led cube project and im using our clg LAB RPS to power my cube.now im planing to use pc smps.can i dircetly connect the smps 5v rail same as rps? smps has very high current ratings,does it make any damaga to my cube?
thankyou
matthewbeckler (author)  sdevi reddy11 months ago
I am not sure what you mean by "RPS". I would expect that the SMPS 5v rail will work correctly with your LED cube project. Normally, a device such as this will only draw as much current as needed, regardless of how much current the power supply can provide.
tnq sir!
RPS- regulated power supply.
nwlaurie1 year ago
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
nwlaurie1 year ago
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
Crucial971 year ago
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
Crucial971 year ago
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 (author)  Crucial971 year ago
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!
hschmutz1 year ago
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 (author)  hschmutz1 year ago
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!
Skarz881 year ago
How do you use the -12v and -5v?
matthewbeckler (author)  Skarz881 year ago
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?
Zakinator1 year ago
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 (author)  Zakinator1 year ago
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!
Awesome, thanks so much! Just figured it would be good to check before electrocuting myself...
mcdabcar1 year ago
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 (author)  mcdabcar1 year ago
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!
jmunoz31 year ago
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 (author)  jmunoz31 year ago
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!
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-In1 year ago
is this stuff can Charge a Car Battery rating @ 12v, 50A? if yes, how long it would take to fully charge the battery?
matthewbeckler (author)  Zach-In1 year ago
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!
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 (author)  salexandridis2 years ago
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?
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