# Variable power source from an ATX power supply?

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?

• 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. Dec 7, 2013 at 10:48
• 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) Dec 7, 2013 at 11:47
• So why not use the regulated +5v on the atx supply? why bother with 24v or the lm317? Dec 7, 2013 at 12:12
• 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) Dec 7, 2013 at 16:29

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).

## ? WARNING ?

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

• 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. Dec 7, 2013 at 10:21
• @Passerby: Question was only if it is possible. And it is. Dec 7, 2013 at 10:37
• So is taking a bath in irradiated water, you still shouldn't give bad "potentially dangerous to the asker and/or their equipment" advice. Dec 7, 2013 at 10:38
• @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. Dec 7, 2013 at 11:17