Getting a -12v +12v range from ATX power supply

[IngerAlHaosului]

I have a old PC power supply that outputs +3.3v, +5v, +12v and -12v, i want to convert it into bench top power supply. I want to have one of the outputs to   have a +12v -12v range. I was thinking to use a potentiometer connected to the + and - rails and connecting the middle contact to the output of the power supply. I used a circuit simulator to check my idea and it appears to work however i don't know if this is ok to do IRL. If anyone know or has better idea please tell me.

[jeroen79]

Try simulating that with a load (a resistor to ground) connected to the output.

[mariush]

The -12v output on atx power supplies is very weak, usually under 0.5A, and it's not very well regulated. 

It's not suitable for anything serious.

[IngerAlHaosului]

Ok, another reason i wanted to do this was to be able to set the adjust pin of lm317 from +12 all the way down into the negative

[mikerj]

Ok, another reason i wanted to do this was to be able to set the adjust pin of lm317 from +12 all the way down into the negative

An LM317 is a positive voltage regulator.  Applying a negative voltage on the adjust pin will not magically make it output regulated negative voltages. 

If you want a single bi-polar output that is continuously adjustable between +12 and -12v w.r.t 0v then the LM317 simply won't do it.  If you want a positive and negative regulated output then you could use a LM317 for the positive rail and an LM337 for the negative rail.  However, they both have a fairly high drop out voltage (1-2.5v depending on current and temperature), so your output won't go all the way to the +-12v rails.

[IngerAlHaosului]

An LM317 is a positive voltage regulator.  Applying a negative voltage on the adjust pin will not magically make it output regulated negative voltages. 

With a adjust voltage of 0 the LM317 output is 1.25v but with a negative adjust you get all the way down to 0

A for regulation i want to regulate the other +12v rail to go from 0 to at least 10

[Simon]

Then it seems you need a negative voltage regulator to produce -1.25V that will take the ref pin and your output down to 0V. This should work despite the -12V not providing current as your output would be from the 0V to +12V output, I think.....

[SeanB]

If you want to do that use a TDA2030 power applier, which can act as a voltage follower at DC for your pot. will source and sink 2A or so, though the sink capacity into the -12v rail will limit the current it can sink a lot.  You will have a lot of protection though on the variable supply.