ATX Based Lab Power Supply

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Computer power supplies cost around US$15,but lab power supplies can run you $100 or more! By converting the cheap (free) ATX power supplies that can be found in any discarded computer, you can get a phenomenal lab power supply with huge current outputs, short circuit protection, and very tight voltage regulation.

In this instructable I will show you how to quickly convert one of those many computer power supplies into something that you can use to power your electronics projects, for electroplating, for electroetching, for heating wires for foam cutting, etc.

The voltages that can be output by this unit are 24v (+12, -12), 17v (+5, -12), 12v (+12, 0), 10v (+5, -5), 7v (+12, +5), 5v (+5, 0) which should be sufficient for most electrical testing. Many ATX power supplies with a 24-pin connector for motherboards will not supply the -5V lead. Look for ATX power supplies with a 20-pin connector, a 20+4-pin connector, or an AT power supply if you need -5V.

PLEASE, YOU ARE WORKING WITH AC VOLTAGE!!! IF YOU ARE NOT SURE OF WHAT YOU ARE DOING - DO NOT ATTEMPT THIS.

These instructions were originally posted by me on http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

I finally had to package the PSU in a nicer box. The wooden book was from a local craft chain called Michaels which I lined with foil and then packaged the electronics in. I also added back the 3.3V terminal as this was useful and I missed not having that in my previous version.

Step 1: Harvesting the ATX Power Supply

1) Unplug the power cord from the back of the computer. "Harvest" a power supply from a computer by opening up the case of the computer, locating the gray box that is the power supply unit, tracing the wires from the power supply to the boards and devices and disconnecting all the cables by unplugging them.
2) Remove the screws (typically 4) that attach the power supply to the computer case and remove the power supply.
3) Discharge the power supply by either letting it sit unconnected for a few days, or by attaching a 10 ohm resistor between a black and red wire (from the power cables on the output side). Using a resistor will only take a few seconds to fully discharge the power supply.
4) Gather the parts you need: binding posts (terminals), a LED with a current-limiting resistor, a switch (optional), a power resistor (10 ohm, 10W or greater wattage, see Tips), and heat shrink tubing.

Step 2: Preparing the ATX Power Supply

5) Cut off the connectors (leave a few inches of wire on the connectors so that you can use them later on for other projects).
6) Open up the power supply unit by removing the screws connecting the top and the bottom of the PSU case.
7) Bundle wires of the same colors together. The color code for the wires is: Red = +5V, Black = Ground (0V), White = -5V, Yellow = +12V, Blue = -12V, Orange = +3.3V, Purple = +5V Standby, Gray = power is on (output), and Green = Turn DC on (input).

Step 3: Drill Holes for Attaching Binding Posts

8) Drill holes in a free area of the power supply case by marking the center of the holes with a nail and a tap from the hammer. Use a Dremel to drill the starting holes followed by a hand reamer to enlarge the holes until they are the right size by test fitting the binding posts. Also, drill holes for the power ON LED and a Power switch.

Step 4: Attach Binding Posts and Start Wiring

9) Screw the binding posts into their corresponding holes and attach the nut on the back.

Step 5: Connecting the Wires

10) Connect one of the red wires to the power resistor.
11) All the remaining red wires to the red binding posts.
12) Connect one of the black wires to the other end of the power resistor.
13) One black wire to a resistor (330 ohm) attached to the anode of the LED(see the next image)
14) One black wire to the DC-On switch
15) All the remaining black wires to the black binding post.
16) Connect the white to the -5V binding post, yellow to the +12V binding post, the blue to the -12V binding post, the gray to the cathode of the LED.
  • Note that most power supplies have either a mauve or brown wire to represent "power good"/"power ok". Check the ATX plug (the plug with many connections) to see if there is a small mauve or bown wire plugged into the same hole as an orange wire (+3.3V) or a red wire (+5V). If the small wire is connected to the orange in the ATX plug then do the same, hook these two together. If it is connected to the red, then hook it to the red wire. This wire must be connected to either an orange wire (+3.3V) or a red wire (+5V) for the power supply to function. When in doubt, try the lower voltage first (+3.3V).

Step 6: Wrapping Up the Connections

17) Connect the green wire to the other terminal on the switch.
18) Make sure that the soldered ends are insulated in heatshrink tubing.
19) Organize the wires with a electrical tape or zip-ties.
20) Check for loose connections by gently tugging on the wires.
21) Inspect for bare wire, and cover it to prevent a short circuit.
22) Put a drop of super-glue to stick the LED to its hole.
23) Put the power supply cover back on.

Step 7: Testing the Power Supply

24) Plug the power cord into the back and into an AC socket.
25) Flip the main switch on the PSU (on the back). The fan will come on.
26) Check to see if the LED light comes on. If it has not, then power up by flipping the switch you placed on the front.
27) Plug in a 12V bulb into the different sockets to see if the PSU works, also check with a digital voltmeter. It should look good and work like a charm!

Updated the packaging a bit so it looks sexier and wife acceptance factor increases ... used a ready made unfinished wooden box from Michaels.

Step 8: Repackaging the Power Supply in Nicer Enclosure

I still use that supply quite a lot but Wife Acceptance Factor was low as it looked - well -ugly. So I used one of the cool looking wooden boxes we had bought from Michaels as a new skin for this power supply (above). Essentially installed the circuit board into the box (after lining the insides of the box with foil and then with insulating tape), reconnected all the wires to the terminals, installed a smaller fan, and added an old car amp meter that I had bought from Harbor Freight and viola a new sexier looking power supply.

Stained the box, carved some cool lettering with a Dremel ... still have to add labels for the different terminals ... but it now looks good when placed in the bookshelf with no hint of what it is till you flip it around!

Step 9: Troubleshooting

# If you are not sure of the power supply, test it in the computer before you harvest. Does the computer power on? Does the PSU fan come on? You can place your voltmeter leads into an extra plug (for disk drives). It should read close to 5V (between red and black wires). A supply that you have pulled may look dead because it does not have a load on its outputs and the enable output may not be grounded (green wire).
# If the LED light does not come on, check to see if the fan has come on. If the fan in the power supply is on, then the LED may have been wired wrong (the positive and negative leads of the LED may have been switched). Open the power supply case and flip the purple or gray wires on the LED around (make sure that you do not bypass the LED resistor).
# ATX power supplies are "switch-mode supplies"; they must always have some load to operate properly. The power resistor is there to "waste" energy, which will give off heat; therefore it should be mounted on the metal wall for proper cooling (you can also pick up a heatsink to mount on your resistor, just make sure the heatsink dosn't short anything out). If you will always have something connected to the supply when it is on, you may leave out the power resistor.
# You can add a 3.3 volt output to the supply by hooking the orange wires to a post (making sure the brown wire remains connected to an orange wire) but beware that they share the same power output as the 5 volt, and thus you must not exceed the total power output of these two outputs.
# If you don't feel like soldering nine wires together to a binding post (as is the case with the ground wires) you can snip them at the PCB. 1-3 wires should be fine. This includes cutting any wires that you don't ever plan on using.
# The +5VSB line is +5V standby (so the motherboard's power buttons, Wake on LAN, etc. work). This typically provides 500-1000 mA of current, even when the main DC outputs are "off". It might be useful to drive an LED from this as an indication that the mains are on.






Options: You don't need the switch in the front, just connect the green and a black wire together. The PSU will be controlled by the rear switch, if there is one. You also don't need the LED, just ignore the gray wire. Cut it short and insulate it from the rest.

Step 10: Almost 10 Years Later ...!

Am still using the original Power Supply though now it is packaged in a more professional case. I also added an ammeter in the 0 V line so that I could measure the Amps being delivered. I also added a USB socket to charge USB devices. The last diagram shows a simplified wiring diagram of all the parts together.

The fact that this PSU is short circuit proof has turned out to be a big advantage. It has withstood a lot of abuse.

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    165 Discussions

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    HD DigitalArtJ

    2 years ago

    Thank you for this great and clear explanation. I'll try it. Here are 4 old PSU around. I want to provide a digital camera and flash units with it.
    I do not understand that with the ammeter and the connection to 0V?

    If you are 12V with 0V and 5V with 0V connects simultaneously, the total current is then displayed as?
    Maybe you can post a circuit of your update version, would by nice.
    Could I install a voltmeter and a switch, to switch to the different outputs?
    Sorry, my English is just as bad as my knowledge in electronics.
    Here's perhaps someone who has experience or ideas with the PSU power supply of a digital camera and one or two flash units, each Note and advice would be great.
    Thanks again, your update looks just as good

    3 replies
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    abizarHD DigitalArtJ

    Reply 2 years ago

    Forgot to add to my previous reply. If you connect your device between +12V and -12V then the current will not flow through the 0V line and in such cases you will see no reading in the ammeter. The ammeter will only work when current is flowing into or out of the 0V line.

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    abizarHD DigitalArtJ

    Reply 2 years ago

    Hi HD DigitalArtJ, have added a circuit diagram showing where the ammeter was placed. (The circuit diagram does not show the power leads for the ammeter but most will require connection to +12V and 0V). As you know current flows from + voltage to 0V. If we place the ammeter in the zero volt line, all the current returning through the zero volt line will be measured. For example, if you connect a bulb between the 0V and 12V, the current passing through the bulb will be displayed. If at the same time you add another bulb to the 5V line and the 0V line, this new current will be added to the reading. That is, let us say the 2 A for the bulb connected to the 12-0V and 1 A for the 5-0V would result in a reading of 2+1 = 3A on the Ammeter.

    VPSU Ammeter.jpg
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    abizar

    2 years ago

    An update. Have further modified this power supply by putting it in a better case and adding an ammeter. The digital ammeter (ebay) was added in the 0 V (Gnd line) so any current returning through ground can be measured. Also added a USB power port that connects to the +5 V and 0 V.

    Picture1.jpg
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    BrenBren

    5 years ago on Introduction

    Thanks Abizar! Although I'm using my supply for a specific application rather than benchtop, it was nice to have a solid reference to speed things up. Very well written!

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    weatherjack

    8 years ago on Step 7

    I have been thinking about building one of these. I am an amateur electronic hobbiest. I still cannot figure out he +/- o equal certain voltages eg. +5v and -12v makes 17v, guess I am thinking +5-12=7? Anyone can shine some light? I feel pretty silly for asking but.. Knowledge is King!

    6 replies
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    Synckweatherjack

    Reply 8 years ago on Step 7

    Voltage is the electric potential difference between two points. So if you have the points +5v and +12v the potential difference between them is 12-5=7v. If you have the points +5v and -12v the potential difference is 5-(-12)=5+12=17v.

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    zackerSynck

    Reply 5 years ago on Step 7

    so when you have two plus's you subtract the smaller from the larger to get the volts? and if you have one plus and one minus, you add both together to get the voltage? what about two minus's like -12v and -5v? also, if i wanted say, 17v, i use one lead in the +5 and the other lead in the -12 jacks? does it matter what lead? red or black? what about if Im connecting them to something? would it matter what voltage went to neg and what went to pos? also, no ground in those scenarios?

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    abizarzacker

    Reply 5 years ago on Step 7

    Zacker, what are you trying to power? In general stuff like light bulbs, LEDs, etc. a ground is not required as long as the polarity of the device is respected. The + terminal of device needs to be connected to the more positive (or less negative) voltage. So, if I was using the -12V and the -5V terminals. I would attach the -5V to the positive terminal of the stuff I was powering and the -12V to the negative terminal. The device would think it is being powered by +7V! If I was using the +12V and +5V, then I would connect the +12V to the positive terminal (as it is more positive than +5V) and the +5V to the negative terminal. The device would still think it is being powered by +7V.
    This will not work if the device really needs 0V (ground). For example, if it is connected to something else which is at 0V from another power supply. Hope this is not too confusing!

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    zackerabizar

    Reply 5 years ago on Step 7

    Its a little confusing, ill just have to read it a couple times to get it to sink in...lol thanks for explaining it.
    I am looking for something to power an electrolysis tank for rust removal. I basically need like, 12V with about 2 Amps. A lot of people will try and go with as many Amps as possible but the more seasoned guys say 2Amps works well, it just takes a bit longer. I used me 12v car battery charger set to the 30 Amp "Quick" Charge setting but after 3 Hrs. it shut its self down. Most people leave these going for 8 hrs. or more depending on size of items being cleaned or amount of rust... in the three hrs. I ran mine on the 30 amp setting, its got almost all the rust off and old vise. So Im thinking if I can run this at 12v with anywhere in the 2 amps and above, it would be good. But then Im seeing all these PSU's being turned into bench power supply's and now, not only do I want to make this for the Rust Removal, but I may as well use it for a bench power supply too.. lol its just confusing, all these Volts and numbers and - / + and stuff... Like the pin outs on this PSU, some wires are +12v or +12VC DC or +12 VB DC or VA DC or worse, +12 VD DC / SE... lol

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    abizarzacker

    Reply 5 years ago on Introduction

    Hi Zacker. I do use the powersupply for electrolysis too, like you, mostly for converting rust back to iron. For electrolysis or for electroplating I typically use the 0V to +5V as it has the most current capacity. If you use 12V your powersupply may shut down if the current draw is too high. The current draws are high if the salt in your tank is too high.

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    zackerabizar

    Reply 5 years ago on Introduction

    Hmmm.... What are the Amps on the +5 side? Ill be doing mostly small parts, the Vice I did was an 8" Cardinal Milling Machine vice and probably the biggest thing id have to de rust. Mostly it would be smaller stuff. I like to buy old tools and clean them up to look as good as new (or as close to it as I can get... lol) I recently re did a 1950 Craftsman Drill Press and some of the Accessories that were available for it at the time. Well, I gotta get this puppy wired up if it works well, I have a Dell 750W PSU to use next. But that ones a whole new set of issues, its got both a 24 and a 20 pin Molex on it...

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    Machine

    5 years ago on Step 8

    The wood box looks fancy!

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    Jeffrey G C

    5 years ago on Introduction

    in step 5 you said, "most power supplies have a brown or mauve wire." My PSU doesn't fit in there because I have neither of these wires, I assume I need some kind of current consumption device but I'm not sure where else to put it. There aren't any wires doubled in the 20 pin connector and the PSU was manufactured 8/01 by Allied, model AL-A300ATX

    1 reply
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    Never mind, I didn't read carefully enough

    I read mauve and it says somewhere else purple too, I got things mixed around

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    RadGlobe

    5 years ago on Introduction

    thanks Abizar!!! I have favorited 2 of your 3 instructables that I plan to do myself...great work!!!

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    samumar

    6 years ago on Step 5

    Hello and nice instructable!
    I liv e in Europe and using 220V AC input.
    Do I have to change the 10 ohm 10 W resistor with some different resistor or is it good using the same?
    I need 6 amp out in the 12V line!

    Thanks a lot!


    Sam

    1 reply
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    abizarsamumar

    Reply 5 years ago on Introduction

    10 ohm should be good.
    Most of the PSU are compatible with 100-240V input so you could use it in most countries.