I have an ATX power supply (400W) and I want to use make a variable power source for 3-20V using a LM317.

I know that the 4 pins connectors (like the ones for the IDE devices) are +12V|GND|GND|+5V, is it possible to put in series 2 of the +12V outputs to get 24V?

  • \$\begingroup\$ How much current will you need? Because going from 24v to 3v on a Linear Regulator like the LM317 will waste 1W of heat from just 50mA! And the LM317 is typically rated for 1.5A with proper heat sinking. \$\endgroup\$
    – Passerby
    Dec 7, 2013 at 10:48
  • \$\begingroup\$ well, I'll need 1A or less .. basically I'm using it to power up Arduino (through the +5V/GND pair) and different sensors and low-power modules/sensors) \$\endgroup\$ Dec 7, 2013 at 11:47
  • 1
    \$\begingroup\$ So why not use the regulated +5v on the atx supply? why bother with 24v or the lm317? \$\endgroup\$
    – Passerby
    Dec 7, 2013 at 12:12
  • \$\begingroup\$ because for some modules I need 18V, for others 15V and for some only 3.3V (but this is also easily taken from the ATX big connector) \$\endgroup\$ Dec 7, 2013 at 16:29

1 Answer 1


No you cannot. Intern there is only one 12V source.
If you try to put in series 2 of the +12V outputs you created a short circuit ( +12V to ground).


As you can see in the comments below @Passerby: sees some dangers in the rest of the answer which I cannot confirm yet.

But inside the power supply there is also a -12V. You can use +12V and -12V to get 24V.

This manual 'Convert a Computer ATX Power-Supply to a Lab Power Supply may be useful for you. Warning: the colors of the cables differe in a lot of ATX Power-Supplies. Always have a look at the connectors to detect which wire is the wanted.

Note from the linked web site:
The voltages that can be output by this unit are:

  • 24V (+12V, -12V)
  • 17V (+5V, -12V)
  • 12V (+12V, GND)
  • 10V (+5V, -5V)
  • 7V (+12V, +5V)
  • 5V (+5V, GND)
  • 3.3V (+3.3V, GND)

P.s. Not every ATX Power Supply seams to have the +3.3V and the -5V

  • \$\begingroup\$ While you might use -12v, the -12v rail is not very strong, and inputing current through it like that is not a good thing to do. \$\endgroup\$
    – Passerby
    Dec 7, 2013 at 10:21
  • \$\begingroup\$ @Passerby: Question was only if it is possible. And it is. \$\endgroup\$
    – MrSmith42
    Dec 7, 2013 at 10:37
  • 1
    \$\begingroup\$ So is taking a bath in irradiated water, you still shouldn't give bad "potentially dangerous to the asker and/or their equipment" advice. \$\endgroup\$
    – Passerby
    Dec 7, 2013 at 10:38
  • 2
    \$\begingroup\$ @Passerby: The ATX Power Supplies are quite save, and I cannot see the 'danger`. If you try to consume too much current the internal fuse will prohibit it. Maybe you can be more specific which dangers you see. I used the 24V some times without any problems. \$\endgroup\$
    – MrSmith42
    Dec 7, 2013 at 11:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.