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[h=1]How to Convert a Computer ATX Power Supply to a Lab Power Supply[/h]

Computer power supplies cost around US$30, 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 reasonably tight voltage regulation on the 5V line.​

On most power supply units (PSUs), the other lines are unregulated.
1
Look online or at your local computer store for an ATX computer power supply, or dismantle an old computer and remove the power supply from the

2
Unplug the power cable from the power supply and turn off the switch on the back (if there is one). Also, be sure you are not grounded so that remaining voltage doesn't flow through you to ground.

3
Remove the screws that attach the power supply to the computer case and remove the power supply.

4
Cut off the connectors (leave a few inches of wire on the connectors so that you can use them later on for other projects).

5
Discharge the power supply by letting it sit unconnected for a few days. Some people suggest attaching a 10 ohm resistor between a black and red wire (from the power cables on the output side), however this is only guaranteed to drain the low voltage capacitors on the output - which aren't dangerous to begin with! It could leave the high-voltage capacitors charged, resulting in a potentially dangerous - or even lethal - situation.

6


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.

7
Open up the power supply unit by removing the screws connecting the top and the bottom of the PSU case.

8
Bundle wires of the same colors together. If you have wires not listed here (brown, etc), see the Tips.

The color code for the wires is: Red = +5V, Black = Ground (0V), White = -5V, Yellow = +12V, Blue = -12V, Orange = +3.3V, Purple = +5V Standby (not used), Gray = power is on (output), and Green = PS_ON# (turn DC on by shorting to ground).

9


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 (optional).

10


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

11
Connect all the pieces together.

• Connect one of the red wires to the power resistor, all the remaining red wires to the red binding posts;
• Connect one of the black wires to the other end of the power resistor, one black wire to the cathode (shorter lead) of the LED, one black wire to the DC-On switch, all the remaining black wires to the black binding post;
• Connect the white to the -5V binding post, yellow to the +12V binding post, the blue to the -12V binding post, the gray to a resistor (330 ohm) and attach it to the anode (longer lead) of the LED;
• 

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  [*=left]Note that some power supplies may have either a gray or brown wire to represent "power good"/"power ok". (Most PSU's have a smaller orange wire that is used for sensing-- 3.3V- and this wire is usually paired at the connector to another orange wire. Make sure this wire is connected to the other orange wires, otherwise your lab power supply won't stay on.) This wire should 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). If a power supply is non ATX or AT compliant, it may have its own color scheme. If yours looks different that the pictures shown here, make sure you reference the position of the wires attached to the AT/ATX connector rather than the colors.
  [*=left]Connect the green wire to the other terminal on the switch.
  [*=left]Make sure that the soldered ends are insulated in heat shrink tubing.
  [*=left]Organize the wires with a electrical tape or zip-ties.
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  12​
  
  
  Check for loose connections by gently tugging on them. Inspect for bare wire, and cover it to prevent a short circuit. Put a drop of super-glue to stick the LED to its hole. Put the cover back on.
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  13​
  
  
  Plug the power cable into the back of the power supply and into an AC socket. Flip the main cutoff switch on the PSU if there is one. 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. Plug in a 12V bulb into the different sockets to see if the PSU works, also check with a digital voltmeter. Make sure you do not short any wires out. It should look good and work like a charm!
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    [*=left]Options: You don't need an additional switch, 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 an LED, just ignore the gray wire. Cut it short and insulate it from the rest.
    [*=left]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.
    [*=left]Feel free to add some pizazz to the dull grey box.
    [*=left]You can use your power supply 12V output as a car battery charger! Be careful, though: if your battery is too discharged, the power supply short circuit protection will trigger. In that case, it's better to put a 10 Ohm, 10/20 Watts resistor in series with the 12 V output, in order to not overload the power supply. Once the battery is near 12V charge(you can use a tester to verify that), you can remove the resistor, in order to charge the remaining of the battery. This can save you if your car has an old battery, if it's winter and your car does not want to turn on, or if you accidentally left the lights or the radio on for hours and hours.
    [*=left]You can also convert this to a variable voltage power supply - but that is another article (hint: Uses a 317 IC with power transistor).
    [*=left]You can add a 3.3 volt output (such as to power 3V battery-powered devices) 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.
    [*=left]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.
    [*=left]The voltages that can be output by this unit are 24v (+12, -12), 17v (+5, -12), 12v (+12, GND), 10v (+5, -5), 7v (+12, +5), 5v (+5, GND) 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.
    [*=left]ATX power supplies are switched-mode power supplies (info at Switched-mode power supply - Wikipedia, the free encyclopedia); 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 heat sink to mount on your resistor, just make sure the heat sink doesn't short circuit anything). If you will always have something connected to the supply when it is on, you may leave out the power resistor. You can also consider using a lighted 12v switch, which will act as the load necessary to turn on the power supply.
    [*=left]To get more room you can mount the fan on the outside of the PSU case
    [*=left]Also some power supplies need the grey and green to be connected together in order to run.
    [*=left]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).
    [*=left]You can take advantage of the hole left by the power supply cabling, to install a cigarette lighter connector. That way, you can connect car appliances to your power supply.
    [*=left]If you DO have a sense wire for the 3.3v. , connecting the the 3.3 v. part of the supply, using the 3.3v. voltage as a buck voltage against, say the 12v. to get 8.7v. will not work. You will see 8.7 v. with a volt meter but when you load that 8.7v. circuit the power supply may go into protective mode and shut the whole supply down.
    [*=left]If you aren't afraid of some soldering, you can replace the 10w power resistor with the cooling fan that was originally inside the PSU, be careful with the polarity though - match the red and black wires to each other.
    [*=left]The -5v rail was removed from the ATX specification and does not exist on all ATX power supplies.
    [*=left]You may have to drill the hole a little bit bigger.
    [*=left]If the power supply does not work, that is, no LED light, 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).
    [*=left]Some newer power supplies will have "voltage sense" wires that need to be connected to the actual voltage wires for proper operation. In the main power bundle (the one with 20 wires), you should have four red wires and three orange wires. If you only have two or less orange wires, you should also have a brown wire which must be connected with the orange. If you only have three red wires, another wire (sometimes pink) must be connected to them.
    [*=left]The fan on a PS can be quite loud; it's designed to cool a relatively heavily loaded PS as well as the computer. There is the possibility of just clipping the fan but is not a good idea. A work around is to cut the red wire going to the fan (12V) and connect it to a red wire going out of the PS (5V). Your fan will now be spinning significantly slower and thus quieter, but still provide some cooling. If you plan to draw a lot of current from the PS this might be a bad idea, be your own judge and see how hot the thing gets. You can also remove the stock fan and replace it with a quieter model (there will be soldering to do though.)
    [*=left]For use with items with a high starting load such as 12v fridge with a capacitor connect a suitable 12v battery to stop the PSU tripping
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    [*=left]http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply.