PC power supplies

[richiereynolds]

Hallo, I've got a couple of old PC PSUs. What's their suitability for powering a microcontroller and other hobby robotics stuff for testing without mobility (rather than having to keep charging batteries)?
I know they've got various output voltages available but are they stable enough? Or does the stability a PC needs get taken care of by circuitry on the average PC motherboard?

Cheers, Richard.

[corrado33]

You, my friend, have stumbled across an almost perfect lab bench power supply.  Check out these sites.

http://www.instructables.com/id/Convert-an-ATX-Power-Supply-Into-a-Regular-DC-Powe/

http://www.instructables.com/id/ATX--%3E-Lab-Bench-Power-Supply-Conversion/

Hmmm this one seems interesting.  It has a variable voltage supply circuit in it.

http://www.instructables.com/id/DIY-PSU-into-Bench-Power-supply/

Oh and as for stability, I'm pretty sure it's take care of in the PSU itself.  If you open it up there's a bunch of capacitors and inductors, and I can't think of any other reason to have them other than to stabilize the output.  I actually built one of these bench power supplies using a few parts from radio shack, and a pair of pliers, so it's really not that hard.  Just be careful not to touch anything in the PSU (especially the caps), besides the wires.  The real trick is that there is a "sensing" wire that is used to tell the PSU to turn on.  If this wire isn't connected to GND, the PSU will never turn on.  You just have to figure out which wire it is.  On newer PSUs, it's always the same color, but on older ones it's pretty much a guess.  (I think mine was grey or something.)  Once this wire is connected, all of the yellow wires are +12V and all of the red ones are +5V.  You simply make those things easily accessible and you have yourself a power supply.  Also, you can make these things give bunch of different voltages, but honestly I find +5v and +12v sufficient.  Also, most of the other voltages on the PSU (like 3.3V) are only able to provide a very small amount of current, so there is no sense in using them. 

[Cristi_Neagu]

Also, most of the other voltages on the PSU (like 3.3V) are only able to provide a very small amount of current, so there is no sense in using them. 

Actually, because they are of lower voltage, you get more amps out of a 3.3V wire than of a 12V. I am looking on some thermaltake PSU, and while the 12V supply is rated at 14A, you can get 24A out of 3.3V. Remember that 3.3V is used to power the processor, and that is a big power hungry beast, not your average ATMega640...

Depending on the manufacturer, PC PSU's are very well built, and excellent for lab work and such, or just for powering 1 robot. They have current limiting features (if something shorts out somewhere, the PSU won't blow), and are very efficient (they are switching mode after all).

Anyways, just expect a post from Soeren... He'll shed more light on the subject than you can shake a stick at.

Good luck.

[z.s.tar.gz]

I saw one once that separated the connectors and the psu and connected them via the psu's native 20pin (or whatever) plug.

With this method you can actually combine several psu's for high voltage or high amperage testing.

Just a little food for thought.

[richiereynolds]

Excellent, sounds just the job then!   

Thanks for all the info guys. So glad I didn't bin them now 

[corrado33]

Actually, because they are of lower voltage, you get more amps out of a 3.3V wire than of a 12V. I am looking on some thermaltake PSU, and while the 12V supply is rated at 14A, you can get 24A out of 3.3V. Remember that 3.3V is used to power the processor, and that is a big power hungry beast, not your average ATMega640...

Oh... I always thought that there weren't many things using the weird voltages, so the PSU wasn't build to deliver a lot of current to them.  I guess I was wrong!  

[tim_wang]

I actually use the exact same setup, except mine is not connected a variable voltage regulator. Another good weekend project.

[SmAsH]

Beware if its old, i once did this and well... It exploded while powering a fan >.<

[corrado33]

Hmmm... I built mine from an old one (I'm guessing it was built around... 90-95) and it works fine.  Powers my triton 2 battery charger just fine.   

But I can see where somethings could go wrong.

[Soeren]

Hi,

You, my friend, have stumbled across an almost perfect lab bench power supply.  Check out these sites.
[Snip]
Hmmm this one seems interesting.  It has a variable voltage supply circuit in it.

To be a lab supply, it has to be variable at the very least.
To really carry that title, a variable current limiter should be included as well.

I'll recommend fusing the low voltage lines. Car fuses are good for this and the regular blade type fuses can be attached with spade terminals.

Oh and as for stability, I'm pretty sure it's take care of in the PSU itself.

Yes, as long as the load is within what it's designed for (be carefull with inductive loads).

The real trick is that there is a "sensing" wire that is used to tell the PSU to turn on.

A couple more points worth noting:
- Most PC PSUs still need a certain amount of load on the 5V line to keep in regulation.
- Never ever remove any point of the feedback wiring - it's an important part of the regulation loop.

A conversion from an old 200W+ PC PSU to a 0V to 30V, 0A to 5A (or whatever max. is attainable with a particular supply) lab supply with linear post smoothing taking less than 10% of the available power, would be a nice way of using some of the now obsolete supplies.

They'd make really nice chargers for automotive batteries. and you can lift them with one finger, while my present car charger needs both hands and a hint of biceps to haul around.