PC ps into testbench supply?

[charmrus]

I see alot of at and atx power supplies for next to nothing and I'm wondering how difficult would it be to transform one of them into a supply I couuld use for all my projects.<p>What Im trying to figure out is how can I combine the +5 and +12 supplies and make them adjustable volt and adjustable amp.<p>any thoughts?

[haklesup]

AT power supplies are no problem, they power up as soon as you plug them in but ATX power supplies have a sense line ( blue wire I think but it may be the orange one)and it has to be connected to ground (I believe but it may be one of the red 5V lines) before it will power up at all. Normally this is plugged into the motherboard and so is the power switch.<p>As for making it variable Voltage, you may find a potentiometer inside for adjusting the voltage output but it probably won't give you full range. You could hovever feed the output to a 3 pin adjustable voltage regulator to get the voltage you want. Current limiting is another story, not simple to implement. A fuse on the output may be your simplest option

[MrAl]

Hi there,<p>I used an old AT style ps for mine.
Made a quick supply with outputs of
+5, -5, +12, and -12 volts.
Didnt make it variable though, and was
thinking about adding an adjustable
regulator to the output +12 supply.
This wouldnt be hard to do, and since it
would be external, if anything went wrong
with the ps, i could easily replace it with
another without going through the work of
figuring out how to make the new one adjustable
internally also.<p>Good luck with your circuits,
Al

[Chris Smith]

Lm 317, 339, and a few others are good for variable voltage. The lm 7805, 12, 18 etc can be configured to run as a variable current limiter.

[MrAl]

Hi again,<p>Sounds good Chris.
The only thing i dont like about using
a 7805 configured for a current limiter
is that it will require a 5 volt voltage
drop wont it? This would mean the 12 volt
supply would only deliver +7 volts to the
output around the point just before it
goes into current limit.
I was thinking maybe one of the 1.2 volt
ref diodes connected to a transistor?
This would provide about 10.8 volts just
before it goes into current limit. Should
be enough for most of my tests.
I guess if i wanted the full 12 volt output
id have to go into a zero voltage drop
current limit circuit.<p>Take care,
Al

[Chris Smith]

Actually if you float the ground through a resistor on the 78xx series, you get more voltage. <p>I suggest the 317 configured first to get any variable between 1.2 and 37 volts depending on the inputs limit. <p>Then configure another 317 for curren regulation, after the first if need be. <p>But because the original suupply usually only supplies just below 12 volts, [11.98] you wont get 12 volts through any regulator, with a 1.2 volt loss minimum leaving you with around 10 point 8 volts starting voltage. <p>Even if you use the diode and transistor approach, it still applies. <p>For the 12 full volts, just by pass the reg circuit.<p>I have BMPs and JPGs of all the circuts.<p>[ September 22, 2002: Message edited by: Chris Smith ]</p>

[haklesup]

you could use the +12V and -12V supplies together to get an effective 24V source, then use the -12V output as your new ground. one limitation is that you can't use the 5V outputs at the same time since there ground is at a different potential. Another concern is that on many supplies, the -12V supply has much less power than the +12V one does. <p>Another gotcha I noticed was that on some power supplies (and some of their outputs) there is a minimum current spec on the sticker. I assume that this means that voltage is not regulated for loads less that the minimum.

[wd5gnr]

Have a look at this article for another idea. I never did post the pictures, but if you really want to see it, drop me a note and I'll try to rustle up a photo:<p>http://www.wd5gnr.com/power.htm