Multiple PC PSU's for high current supply

[mickeyholland]

Hello all,

I have recently "acquired" a 12v DC 24amp pump for keeping our pool topped up. The idea in the future is to invest in some solar panels to drive the motor "free".

Children (of course) do not understand the concept of saving for solar panels, and would like to swim now!

- So, my idea...

It might be crazy, but I have 4 old 250-300W AT PC psu's. Each delivers 12v at 10amp (along with 5v, -5v, -12v).

Could 3 of the psu's be connected (perhaps in series with some big diodes in between) to create a 12v 30amp supply ?

(I have read some articles linked here http://www.rcgroups.com/forums/showthread.php?t=446764 on converting PC psus for bench use, and understand I will also need to add a load resistor/on indicator bulb and connect the 5v supply to the "power good" input on the psu).

Thank you for your advice.
Michael.

[MrAl]

Hi,

You might have trouble getting them all to turn on into a big load like that. If one goes
overcurrent, it may not recover until you turn it off/on.
I guess this is something that has to be tried. Might use diodes on the outputs to make
sure they dont get reverse fed from the other.

[haklesup]

It's worth a try since old ATX power supplies are not very valuable so if you fail, you haven't lost a fortune. I might try running a differnt load like a lamp array instead as a first characterization step. I doubt you can jusr wire the outputs in parallel since there is no sense node to make the voltages all exactly equal but you can probably get away with putting each supply output in series with a low value resistor (0.33 ohms for example) before summing them together. The R can also be used to measure the Current from each supply. Since they're not batteries, you probably won't get back current into any of the supplies thereby not requiring a large blocking diode.

A simpler approach is to use one supply to constantly charge a car battery (or through a charge controller pretending the ATX supply is a solar panel array). Since the pump likely runs intermittantly, you should have enough run and recharge time to make it operate as required. With a battery, large current spikes (in demand) won't be much of a problem. You'll also have a stage of your solar solution in place and debugged.

I'm assuming Solar PV and not Solar hot water though a combination of the two would be prefect for a pool.

I am a little confused, by topped up you mean adding water to make up for evaporation? Wouldn't plain old city water pressure do that job without a pump?

[mickeyholland]

Thank you both for your replies.

haklesup: Clearly I was typing loosely! Topped up was a terrible way to describe the circulating/filtering pump.

In my original post I wrote series by mistake, instead meaning parallel. However, your post hints at another angle -- wiring the 5v outputs in series (instead of the 12v in parallel).

The 5v supply has a higher current rating (about 28 amps from memory). So if I could "series" three 5v supplies that might provide approx. 15v @ 28amp. A big diode or two could be used to drop that voltage down a little to suit the pump. (Although the specs for the pump may allow some tolerance).

This setup might also work well to charge a car battery as you suggest... I like that idea, thanks!
And I think it should also deal with MrAl 's point about any differences in current capabilities of different psu's, which may have occurred with the parallel technique.???

As you say, the psu's are cheap enough to find almost everywhere, so it's a project waiting for the next clear weekend....

I am really glad to receive any tips and advice.

Thank you, Michael.

[MrAl]

Hi again,

The only thing i am worried about is that all three power supplies have to get up and running.
Most likely they will start up one at a time in some random order. The first one up will
go into overcurrent and shut down. If it is foldback current limit it will not come back up
unless it is powered off then back on.

Maybe all three should be brought up into a smaller load, then once all three are running
switch the load on. This means any one will not see much load until they are all already
up and running and putting out the correct voltage.

[mickeyholland]

MrAl,

Is there a "standard" time period I should delay the full load for ? Would it be fair to use a 5 second delay ?

I could put a time-delay-relay between the psu's and the full load, so that on switch-on it would simply illuminate a "booting up lamp...", then the relay would switch over to the full load after X secs.

Seems simple enough- only the timing is the question... Do you think 5 secs is reasonable, or is this really down to the psu - and so some trial-and-error required.

Thanks again for your thoughts,

Michael.

[MrAl]

Hi,


Well, 5 seconds sounds like more than enough time to me. You would have to try it of course.
You would also turn them all on at the same time.

[mickeyholland]

Thanks MrAl

The power-on sync is a good point, so I will make sure I wire all the supplies to one mains on/off switch.

I will give it a go and feedback my final design here.

All the best!

[MrAl]

Hi again,


Thanks, i would like to hear how this project goes and how well it works out.