How to build a 24V Power Supply from 2 ATX PSU

Picture of How to build a 24V Power Supply from 2 ATX PSU
Two ATX PSU = One juiced DIY 24 Volt DC Power Supply

Before we get going, i have to make it clear that this is by no means the safest solution, and that might even be and under estimation. I am no expert, and copying my steps is at your own cost. If you are in doubt don't try it.


This is my first instrucable since i couldn't find this project anywhere i thought i might aswell document it with pictures.
Also English is my 2nd language so bear with me.

Well, what you get when done is a 24 Direct Current (DC) Power Supply (PSU).

Parts list:

2* XXXX W ATX power supply.


A good length of mains cable.

Scrap wood or what you think is suitable.

Common tools (screw driver/hammer, saw, nails/screws and so forth)

Soldering iron with accessories or alot of male/female connectors.

Nice to have list:

A clue gun.

Multimeter. I nice to test things out before frying electronics.

Common sens.

Everything was sourced for free except for tools. This made this a perfect fit for my needs.


If i missed something please point it out, but if you have the above tools i have no doubt in my mind that you can make it.

Look all syncronyms and words up on wikipedia before asking.
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john52471 year ago
Hi Zilver. All very excellent! But you do too much!
You can leave PSU A complete in its own case. Only open PSU B to disconnect the case ground from the circuit board. This leaves the mains ground still working on the metal case. The two metal cases then still have a safety earth ground and can touch each other - or even be bolted together.
PSU A only needs a link between green and black to start it. PSU B needs an on-off switch. Now take about 8 black wires from PSU A to be your negative output for CNC
and about 8 yellow wires to the positive output for 24V CNC.
Then join 8 yellow wires from PSU A to 8 black wires on PSU B as you have done.
Some say that the thin red and thin orange "sense" wires should connect to their thicker red and orange supply wires. Some people also put 12V car bulbs on the 5V rail because some very old ATX need a load on 5V to regulate properly.
To be honest most newer ATX don't need anything.
My 24V ATX is wired all inside the 2 metal cases which are bolted together and all the external computer wiring loom is gone. I have one mains socket with on-off, one start button for PSU B and two 24V output terminals on 4mm banana sockets.
Yours is an excellent Indestructible - my only worry was to leave as many safety grounds on as possible and just "float" the black OV wiring of PSU B.
Considering that English is not your first language you make really good use of many English idioms ......
Zilver (author)  john52474 months ago

Hi John.

I know i did, but i had no use for the cabinets, and wanted to be on the safe side, since the PSU had screws to ground i wanted to circumvent it completely, and show others the same method.

Sure you can do it KISS, and if i need 24V for another project with different PSU i might do it. but these PSU arent' bolt on friendly in the first place.

Nice short guide you wrote i liked it, i still see some are confused, and for those i would recommend doing like i did.

true78 john524711 months ago

So John, are you saying you float both 12v dc case grounds and then just have the cases ground the110vac? The ac grounded cases can then still touch each other and the red and black 12 leads are just treated as standard 12v wiring such as those found on a car battery? I ask because I'd like to take two 12v 10a computer power supplies and use them to run an all in one gecko driver/controller. (g540 model) It will be running three 3.5a NEMA 23's. If I recall the gecko asks for 15-40v so I think this combination would be a good middle of the line match.

john5247 true7811 months ago
Yes, find the circuit board screw that connects black negative to case ground and insulate underneath the PCB where the screw used to be. The screw will be near all the black, red, yellow output wires. Don't remove the screws at the other (high voltage end) of the circuit board.
Test the black wire to ground continuity with an ohm meter as you take the screw out so you can see the connection break as you do it. Check it again after you fit the insulation under the board.

Now you can stand the two ATX supplies next to each other - or even bolt them together and they will have a power safety ground.

The max you can get out of 2 ATX supplies is 2 x 12 volts. The 12 volts are on the yellow wires. Join yellow to black in the middle and tape it. (Like two car batteries) The other yellow and black across both supplies should give you 24 volts, but only 10 amps. You can add the volts up, but the max amperage is that of one supply.

Some guys on some older supplies swear by loading the 5 volt red wires with a small lightbulb. loading the 5 Volt wire can help the regulation of the whole supply (all the outputs). If you use a 12 volt car bulb like a stop light it will only glow on 5 volts. You may see a slight increase in your 24 volts if you do the lightbulb thing.

Also, some guys say that you can short black and green in one supply, but you might need a switch on the other one. As your load is potentially 10.5 Amps you might want to put switches on both green wire start circuits.

I know nothing about Gecko. You should check it's current at 24 volts if you can. They might be talking about drawing 3.5A at 15 volts.
A better indicator would be how many Watts it needs. Then you can work out the current at 24 volts.

Don't forget there are lots of Chinese switch mode supplies on ebay.
The ATX gig is only good if you have access to cheap or free ones.

good luck
Manchester UK

Thanks for the detailed explanation. I ended up doing it exactly as you explained. However the powersupplies I found had negative to case going through two of the screws so had to isolate both of them. Just a heads up!

Were both screws near the end of the circuit board with all the red, yellow and orange wires leave the board?

The screws at the high voltage end of the board should stay in place as a saftey ground.

The circuit board has two completely independant "halves" The only link between them is magnetically in the transformer and optically in the opto-isolaters that carry the feedback signals.

Also try and leave the green ground wire from the mains power connector pin 3 attached to the metal case - that's the main safety ground.

good luck with all your doings


Hi John5247 and Zilver

Question? Is there a way to use the additional dc power levels ie +5, +3.3? I ask cause I saw this

and thought it would be useful. If any thing it could supply power for the BOB or some other device. I'm also asking cause I'm wondering what the power would be if I where to float the 5v and 3.3v. Would it double?

Yes. with the Dc negative black wire floated (disconnected) from the metal case and mains power ground you can get a range of voltages from the two ATX supplies.

Assuming the yellow 12 volt output of the first supply is connected to the black negative of the second supply, you can now get the following voltages. 3.3, 5, 12 as usual from the first supply. Then 15.3, 17 and 24 volts. THERE IS ONLY ONE REAL NEGATIVE NOW. The black wire of the second supply is at +12 volts from the first supply.

The original plan to give 24 volts from 2 supplies can be modified further. Instead of using 12v yellow to black negstive on 2nd supply, you could use + 5volts Red from 1st supply to black on supply 2. Now you can have 8.3, 10 and 15 volts. You cabn still use +12V yellow on supply one, but you lose the 24 volts.

Finally you could use oragnge +3.3V connected to black on PSU 2 and get 6.6, 8.3 again, and 15.3 Volts.

The easiest way to do this is to make black, orange , red and yellow connectors for each supply. Just remenber black from PSU one is your only negative. The black from PSU 2 should be clearly labelled as your voltage selector wire back to PSU one.

These supplies are really clever and well protected. Any mistake or short and the supply will shut down. Reset just means turning off and on again.

Ok you said I will lose 24vdc so there is no way to have all said dc volts and 24.

I was thinking there would be 7 maybe 8 voltage ranges including the 24. The reason i asked cause the BOB wants +5 and -5 to the rail the has the relay for the spindle and limit switchs, but then I need 24 thou 12 would work but wouldn't be as strong of course to power the drivers to steppers. While the BOB has a USB port for power I don't want to be tied to it and the PC. The BOB also has a manual controller/gcode recorder so I don't need a PC if I wanted to run the CNC manually.

I just thought if I ran a red +5 and white -5 from PSU 1/A to BOB that would power it. I'd start wiring and testing but I need a couple switches. Don't really need the other volts but it could be useful maybe.

This if my first CNC and has taken some time to build by hand and save for the electronics. But it has taken this long and I'd rather take my time and get things right.

Thx for your advise and everyone's else as well.

Yes you can use PSU one black and red for +5V, then black and white -5V for the BOB - just check the current drawn by the BOB. Then as before PSU one yellow goes to PSU two black and you will get +24 volts at PSU two yellow.

Let's make this really clear for any TL:DR guys out there reading this.

Open up two ATX power supplies. Isolate the black zero volts wiring from the metal case by removing a PCB screw. Link the green "power on" wire to an 0 volts black to switch the supplies on seperately. Mark the black from PSU 2 with pvc tape to distinguish it from the real black zero volts. THE TAPE MARKED BLACK IS NOT ZERO VOLTS anymore - it is the link wire to get higher volts out of PSU two.

Now every voltage on all the coloured wires from both PSUs are available, but beware that some are quite low current. They all reference to black zero volts on PSU one - even the -12 and -5V outputs.

Good luck with all your doings

Oh one other thing. The black ground from PSU1 and the yellow from PSU2, do they have to be bundled together or are they essentially 24vdc each, so if I have 6black and 6yellow I could run 1n1 to one of the driver boards and so on then have a couple free 24v leads hanging there?

I wonder cause if I bundle them I then have to run them to a distribution block or just run a pigtail from one board to the other. Maybe a heavier guage wire thou it can't be to much heavier.

Yes - you can use the original PSU wiring for distribution just like the PC does. All the yellows form one PSU are internally connected. Only some expensive high end supplies have "dual rail" outputs where the 12V and 5V rails have two independant outputs isolated for motherboard and graphics.

true78 john524711 months ago

Ok, so I got a pile of these two IBM think station power supplies for free, the whole computers, they are 200w max. I may have already shaved off too many contacts on the first one I opened up, as it had three grounded screw mount tabs in total inside the power box, I'll have to open it up and have another look at it. The blacks are all non grounded to the case now, and the middle plug-in tab is still grounded to the case What I did was take a chisel and shave the contact off down to the bare board about a cm around the mounting post so no conductor was left, I then added a 15mm plastic washer in between just to make sure nothing would touch if it were to shift after being dropped or something. My biggest question are: maybe repetitive....

1. If I float each 12v dc board in both supplies can I leave *both* the 115v ac boxes grounded with their original cords and still allow them to touch.

2. To protect against any ac/dc short making its way out of the power supply case through one of the 12vdc wires, could an inline 10a fuse be put in the inside of the case. My thought is anything shorting inside to the wall of the case will ground out on the 115ac cord ground, but in the event that a wire contact broke free and hit a 12vdc output feed it would at least pop the fuse before exceeding 10a.

3. The 3 steppers are 3.5a max each, and for the size of table top cnc engraver I'm playing with it would be unlikely they will ever be maxed out. I'm not 100% sure if they spike on start up or not, but if not could I wire both in parallel to the same switch? I was thinking of using the original PC switch and cabinet to house both power supplies. I just like this switch because it has an led indicator built in.

4. each unit has 4 yellows and 11 blacks. Tie the 4 yellows from "A" to the 11 blacks from "B", the remaining 11 blacks from "A" and 4 yellows from "B" will become my 12v battery posts?

Just want to check if we're on the same page.

Thanks for the info.

john5247 true7811 months ago

Hi True - Like I said to the OP Zilver who did the instructable, don't do too much. Take the lid off, put a meter from black to metal box. Remove one screw near the black wires. Lift the board slightly and watch the meter as the circuit breaks. Slide a bit of card or tape under the board to keep the track off the old mounting post.

Cut every wire short close to the circuit board. Leave the green, six blacks, four yellows and one red as long wires. Tape or hot glue the cut ends of all the other red, orange wires. Put the lid back on.

Label one supply A and the other one B. Label the yellow wire bundle of A with black tape and four of the black wires from B with Yellow tape (or whatever you have). Join the tape marked blacks and yellows together, tape over and push out of the way.

Each supply needs its green wire shorted to a black wire to start the supply. You need a switch like a toggle that stays on. The lovely illuminated push button switch is a momentary action type and is no good - sorry. You can probably still make it's LED light up though as an indicator.

The supplies can both go in the same tin box. Both AC cords can be grounded. Go ahead and add fuses, they never do any harm!

And finally, just for info. If you turn over the PCB from one of your trashed supplies you can see the board is split completely in half electrically. All Switch Mode supplies (SMPS) are like this. The high voltage AC side is chopped up at high frequency. It gets to the DC side by magnetic coupling in the transformer. The trafo is given a breakdown voltage protection in kilovolts. 115 VAC isn't crossing this gap! To control the AC power side with whats going on loadwise on the DC side, there is an opto-coupler (an LED and a photo transistor in a chip) This also has a very high breakdown voltage. 12 volts is not going to connect to the AC side, ever.

The only wire from DC side back to AC side is the ground on the circuit board which we have just disconnected by unscrewing and insulating it.

If an AC fault develops and puts 115V on to the case. The ground will catch it and the fuse will blow. If a DC fault develops and 12 volts touches the metal case .... no one cares, no one is going to die so the government and ATX don't regulate or make rules about it.

true78 john524711 months ago

Ok, so I need to unground the 12v dc on one unit only, while maintaining the factory ac cords and ground connections. That does simplify things a lot.

Do you know if the computer power supplies are being switched on from the dc side, or are these 12v switches connecting to a relay that is causing the 115v ac to power up and feed the 12v dc?

Thanks for the time.

john5247 true7811 months ago

"switching on" doesn't really apply to an ATX SMPS. Some supplies have a mains power switch, but many don't (same with 110 - 220 volt changers).

The supply is a complex set of safety circuits. You can "start" the supply by grounding the green wire. If you short circuit any output the supply will "shut down". This is not the same as "switching off".

Think of it like the signal from your battery operated TV remote. The TV is never truly "off", it is just asleep. Off means "unplugged", "disconnected".

It means the PC or the TV remote is no longer in control of the supply.

For info - one of the many supplies inside your ATX box that makes 12, 5 and 3.3 volts for the PC, is a tiny supply that is always on. It is this circuit that responds to the green wire being grounded. It also works the "power good" circuit that keeps the supply from shutting down.

true78 true7811 months ago

I should clarify #3 I meant run the green/black start circuits in parallel from one switch. The power supplies I will still be running in series.

true78 true7811 months ago
As I suspected, I had removed more grounds then I should have, I removed the one for the 12vdc board and had removed the two on the board with the 115v side with the cap. So that supply unit is getting tossed and another one from the pile will be getting just the single 12vdc side isolated. John, you must have done a few of these units before? Good info.
john5247 true7811 months ago

Yes, the high voltage AC side of the circuit board should remain firmly grounded to the case for safety. Otherwise we could just cut the ground wire from the power socket to the metal tag. This would float the DC side to allow series connection to a second or third supply, just like batteries, but the safety ground would be gone. This might cause death and is probably illegal in 57 states.

Incidentally, once the 115VAC has been rectified inside the box it rises to 165V DC. In the UK the 240VAC power rectifies to 340 V DC!!. This is why it is in a grounded steel box and has no vent holes big enough to get your finger in. ATX spec turns a PC into a safe 12volt max system.

and finally, just in case. Laptop PSU's are 19volts so they have extra to charge the batteries and run the laptop at the same time. You can't charge a 12 volt battery with only 12 volts, the charger has to be stronger than the battery .....

john5247 true7811 months ago

You need a double pole switch where each green - black pair is a seperate circuit. You can't parallel them. Each green is independent. Also, don't forget that the A supply black wire and B supply black wire are no longer the same voltage!

B supply black wire is now running at 12 volts due to its connection to the yellow wire from the A supply .....

By the way, in my previous mail I suggested bringing a long red and black wire pair out of the box. This is to add individual dummy lightbulb loads to improve the voltage stability of the PSU. You could also use the 5 volts with a resistor for your LED indicator. You might find the bulbs aren't needed, but it's a pain to get a red wire out of the box later if you find you need it. If you don't use it just tape the end up.

Pxista john524712 months ago

can you please detail your project ?? is exacly i need...

john5247 Pxista12 months ago

Hi Pxista,

All we are doing is disconnecting the black wires on the ATX PSU off the metal case. Take off the case lid - with the power off of course! Look at where all the DC output wires are coming out of the circuit board. All the black wires are connected to the metal case of the power supply. The connection is made with one of the screws that hold the circuit board down.

If you connect your test meter to the metal case and the black wires it will beep or indicate a short circuit. Loosen one corner screw and move the circuit board up a little - away from the case. The connection between the black wires and ground should be gone.

The screw to remove is always on one of the corners near all the red, yellow and black output wires. DO NOT remove the screws from the mains power end of the PCB. The screw you take out has to stay out. You also need to slip some insulation under the PCB in the corner where the screw used to be to prevent the metal touching again.

What you have now is a "floating DC ground" on the power supply, which means it can be connected to a second supply in series to increase the voltage. IT WILL NOT increase the current. That will still be the maximum available from the smallest of the two supplies.

The mains power safety ground is still on the metal case for your safety. Switch mode power supplies use two completely electrically seperated circuits on one circuit board. The two parts talk to each other through a transformer and opto-isolators. So the utility power is magnetically and optically isolated from the low voltage DC part of the supply.

There is no real need for a DC power supply to have a ground connection. Most HiFi and audio equipment is built that way to keep ground currents out of the audio. The utility company safety ground is there to stop power that might turn up on the metal case if a wire falls off inside . If you look at a lab bench power supply you will see that grounding the DC negative is an option on the front panel. The same goes for oscilloscopes and other test equipment.

Pxista john524712 months ago

thank you dude!! i did it and worked fine.. now i will put the cases together and organize the wires and stuff.. Many thanks !!!

Zilver (author)  john52471 year ago
Hey john5247, thanks for the kind words and constructive criticism, i really appreciate it. If i re do the instructable, which i very well might, since i'd like for a cleaner setup, much like yours. I that case I'll properly end up reusing you ideas, i remember when i did this mod, i just wanted to be sure not to short anything from PSU B, think I'll do a delayed start for PSU B aswell controlled by PSU A, but more on that if i ever get to it :).

Thanks anyways, and happy holidays!
russ_hensel2 months ago

Just a note to let you know I have added this instructable to the collection:
Encyclopedia of ATX to Bench Power Supply Conversion
Take a look at about 70 different approaches to this project.

NICOLAS_FR2 years ago
Hello, I like this idea to get the 24 v and use old PSU.
However in the second PSU i think the capacitor need to be changed. Must be 16V. need to improve to minimum 25v in the line of 12V.( I like 50V)
I am not specialist it is just what i think on the moment about this project.
By the way Thanks for your job and share.
xslr NICOLAS_FR9 months ago

That is not necessary. The voltage output from each PSU is still 12V. Therefore the caps will still be operating within their intended specification.

MMFahmy1 year ago
What are the chances that anyone has a clue what maybe causing power supply B no to start at all when its ground is connected to the 12v of psu A.
On the other hand, if you started psu B first it would start normally, but powering up psu A will cause psu to shut down immediately
Pxista MMFahmy12 months ago

mine do the same.. i read something about disconnect ground for the second psu, but i didnt understood.. if someone can help us please....

Hi, does anyone know if I can use 3 power supplies to make a 36 volt charger capable or charging a 18v NiCd battery from a cordless drill? Thanks.

robthepyro2 years ago
To anyone that reads this: BEWARE.
depending on the internal configuration of your powersupply, it may not be mains isolated, thus the PSU ground may be referenced to mains neutral. There are two immediate issues with this, 1: connecting the supplies in series could cause the second supply to short directly to gnd through the mains neutral wire (although the supply -should- have protection and shut down in this case..)
2: everything may work perfectly, until you touch your '24v' output wire to some other device that is grounded on the same mains system, and suddenly wonder why there is 30A + flowing...
this is a hack and do-able, but make sure you know what you're doing or ask someone that does..
-friendly internet EE guy..
I tried this, and both my PSU's had 0V connected to earth, (which is connected to Neutral here in Australia).

If the PSU's have an earth connection, it seems dangerous to disconnect it, as its there for a reason.

So I'd advise anyone not to use this method unless you have some knowledge or training in mains electrics and electronics, as you could quite easily end up with 24V connected to mains Live or Neutral, which would not immediately be obvious unless you touched the 24V output and another metal object which is earthed.

I'd not advise anyone to use this method to make 24V.
jasshopper1 year ago
could this be used for powering a zvs driver?
sie222 years ago
can any one help me i have tryed this but when i put the yello and the black wires to gether the one psu shuts down and the other way round
OmnivoreNZ3 years ago
This site has a similar idea, stacking 6 PSUs to get 30V at 40A.  - but it looks much messier than the way you've done it. I hadn't thought of simply throwing away the metal cases which allows a much cleaner and better looking unit. Thanks!
This is beautiful!!!
Karakia2 years ago
Clue Gun: Also known as the 'Clueless Gun' - noun. An imaginery weapon, most often used when dealing with one's manager.

Source: Dilbert's Concise Dictionary of Business Terms.
I did something similar,but instead of addig a vent to the front I turned one fan around so one sucks in & one blows out of the cabinet. Seems to work just fine. Cheers!
Great Job! I need to make a 24 volt power supply for my CNC machine and this is the design I will probably use.
icatchem9253 years ago
excellent Instructable. Needed a 24v PS for project, this worked perfectly. Easy build with no problems. Thanks.
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