How to Build a 24V Power Supply From 2 ATX PSU





Introduction: How to Build a 24V Power Supply From 2 ATX PSU

About: Working on a 40W laser, repraps, and IoT

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.

Step 1: Theory / Build

Since there more and more computers laying around the junkyard theres more and more ATX PSU to get for free.

The higher the wattage  described on the PSU the more amperes it can pull when done.
Small PSU will be suitable for testbeds where bigger could be used to weld or power a CNC (as I'm doing).

Before going any further if you scavanged your PSU like I you should test them out before spending more time with them.

Short PIN 14 (on standard ATX PSU) to any ground wire. It will make the PSU turn on, test with a multimeter. For any iregular PSU search around for howto-short-a-PSU. For pictures (i forgot to document that step) just take alook around instrucables and you'll be able to see how to do that step. Theres different way of doing this, but they all do the same job.

There also alot of chatter about how you keep the woltage steady without spikes. Again see other instrucables since most PSU are different an all have different build in features.

I went with a KISS solution.

The two PSU is now called PSU A and PSU B

Since theres some capacitors, inside the cabinet, which can still have a charge in them, wait a while before working with the PSU if they have been powered recently.

Now tear apart both of them. You do this to clear any ground connection there might be, remember to remove the ground (GND) from the mains line you are powering the units with, then you are sure that part of the project don't go FUBAR on you. I would not reuse the metal cabinets, period. Theres way the much risk in them making a contact with any loose wires. Which could result in frying you/electronics.

We can then procced to connect the +12 V wire from PSU A to PSU B GND wire, that will give PSU B GND a +12 V DC as GND then you add the additional +12 V DC it produces on it's own when powered on. To utillize the +24V DC you've created between the PSU you need to use PSU A GND wire  and PSU B +12V DC wire in operation and between those to wire to be more excact theres +24V DC @ the amperes that one would have given.

I also slashed all wires i wouldn't use for this project to inhance airflow in the small cabinet i build. Again think twice cut once.

Step 2: Leach the Beast

Create a box and make it just as safe as you treassure your own life.

Hint: I'm refering to the danger of electrocution.

When you are working with tree, in my case MDF, make sure you dismantle the PSU from the mockup board. Before cutting and clean all pieces very good. Just to minimise the risk of a fire turned on by any sparks.

Since all PSU are different i had to make seperate on/off switches to PWR them on. First i PWR PSU A then PSU B. Before i put switches in it only sporadickly PWR both on and only gave +12V DC out because PSU A didn't function properly.

Step 3: Enjoy the New Tool.



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We have a be nice policy.
Please be positive and constructive.




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 ......

19 replies

If the power cord for ps1 was coupled with another and the ground was cut out on the second cord, you can power both units using the modified cord without having to open either of them up as long as they are isolated from each other and chassis don't touch you're good to go


It was interesting reading your comment on 2 atx for 24v dc. I am trying to make one. Will yo be kind to provide me detailed wiring in form of a sketch diagram. I dot want to go wrong and make mistake. Your help will be useful

Hi John! I would like to ask you about putting a start wire switch in PSU B. I have wired up the PSU's with both start wires connected permanently to their relative earths, and as long as there is no significant load, PSU B almost always starts, but if there is any significant load (eg a stepper motor driver board) sometimes PSU B doesn't start. Would putting a switch in the start lead for PSU B fix this?

How are you switching the mains side? If you wire the mains to one switch so both supplies get power at the same time should help. Are the ATX boxes identical? If not try reversing boxes A and B. Finally you could get a double pole switch so the green - black link is switched for both boxes at once. DON'T MIX your blacks, the one from A is a real zero volts, but the black from B is at +12V from the A supply yellow. A double poe switch has two isolated electrical switches under one mechanical button.


Hi John! In the end I put a switch into the soft start of PSU(B) and bingo! No more problems.

Excellent! Now you can add supply number 3 and go for the whole 36volts .... or not. You should continue to collect a spare ATX as they come your way though. They don't live forever.


I'm very interested in this design but there is one problem that I
see with it: the parts of these psu were rated for a specific amount of
Watts. By putting the second one in series, I'd be doubling the voltage
and the overall wattage that's being loaded on it. These actually
expensive psus I'm working with and I can't afford to blow one up. How
does one go about verifying if the PSU can handle the doubled wattage?

Hi Fadi,
Your 2 power supplies can be seen as a pair of 12 volt batteries when the DC ground is isolated. If one battery is a large car battery and the other one is a smaller motorcyclel battery you can only draw the amount of energy out that the smaller battery can provide.

Even if you have 2 similar car batteries, one might be older or weaker than the other. The weaker battery will provide as much power as it can and no more. The extra capacity of the bigger battery (or PSU) counts for nothing when they are connected in series.

Don't forget your power supply provides the power and the load takes what it needs - the power supply can never push extra power into a load and damage it.

Please take care with the high voltage side of these power supples.
Electricity only kills you once.

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.

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

Yeas, but even though the amperage stays the same, the total wattage is doubled b/c voltage is doubled, if at some point down the line he halfes the voltage with a resistor, he'll have double the amperage.

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.