Using old PC psu.

[valveaudio]

Like me there must be lots of posters on this forum that must be totally fed up with the number of "wall warts" they have to use, also I imagine that most posters have redundant PC psu's.
I thought about mounting a psu in a box which could have a number of switched output sockets each with an LED and maybe a current limiter.

Do you think there is any problem using a switched mode PC psu with these light loads? I seem to remember someone saying that the regulation may be poor under these conditions.

Trevor

[Skywave]

Yes, indeed. Some PC SM PSUs do require a minimum load so that they will work. However, a quick bench test will establish if a dummy load is necessary and what size that load needs to be.

If I have not misunderstood you, though, I think I see a conceptual problem with your idea. Many devices that use wall warts need d.c. voltage supplies that are different to the nominal d.c. voltage outputs of a PC SM PSU. Although there are ways of overcoming that (internal modifications to the PSU or external 'add-ons'), either approach does raise the question of 'effort to benefit ratio' - if you see what I mean. There is also the question of a 'noisy supply', a PC SM PSU being a 'switcher': some loads might get unhappy about that & some 'wall-warts' are not 'switchers'.

Having said all that, let's see what others think: I might have missed something relevant.

Al.

[McMurdo]

Not only do some PC power supplies (eg At, ATX) need a minimum load to actually fire up (or they just sit there in hiccup mode) but usually a couple of the rails don't work until the main rail is loaded, so for example the +12V rail might only give you 11 until the 5V rail has (say) 20% loading. But it's easy to experiment

[Refugee]

The minimum load is what it takes to get a stable 50Khz series of very short pulses on the anode of the rectifiers.
A 12 volt lamp on the 5 volt rail will give you a nice neon look as a power indicator.
It should be between 1 and 5 watts.

[Lucien Nunes]

A specific caution if you want to power multiple interconnected devices, that would otherwise be powered from individual PSUs: Beware circulating ground currents interfering with signal interconnects. It is easy to get noisy DC current flowing through audio cable screens for example, due to the number of parallel paths that can exist back to the common negative terminal of the supply. This is an extremely common situation in some of the equipment that I build and it is not unusual for me to have to mutually isolate the loads on a common DC supply with multiple DC-DC converters, or simply use separate wall-warts built into the rack, to prevent digital and SMPSU nasties appearing in the signal path.

[MrElectronicman]

I recently used two PSU's from my old printers that put out 30 Volts DC at 1 amp. Connected them to a couple of DC-DC converters that supply 5 Volts DC for my Raspberry PI's and I have 4 outputs of 5 Volts from each setup. Appears to work very well, no noise and no problems with the PI's, despite some of the earlier Pi's which are very sensitive to the input voltage.

The printer PSU's are quite small, not much bigger than a wall-wart, and the converters are even smaller, so the whole arrangement is better than an old PC PSU.

I have tried PC PSU's, One is an ATX type that can provide 18 amps at 5 volts, works well too, but because of the fan, it's noisy. Guess I could disable the fan since I would not be using anywhere near 18 amps, but it does take up some desk space.

[Andrew B]

You could use a Peltier device as a load on one of the rails and use a smaller fan? The Peltier device would give a little cooling enabling you to use the smaller fan to remove the heat from the Peltier device... a perpetual motion heat pump?

[paulsherwin]

I've never had any trouble getting ex PC PSUs to work, even with completely unloaded power rails. They are big and noisy unless you need a lot of power though.

[Andrew B]

I hang around R/C model / electronics / electric racing bicycle forums and they use them to "put fire" into LiPo batteries, they join 2 or 3 in series for big lipo packs.

[Oldcodger]

I assume the OP knows that PC PSU need a start up signal (S/C) between two pins (16 & one other) to soft start the PSU. I haven't seen it mentioned, but thought it might be useful

[Refugee]

A quick look at a motherboard pin out will reveal the power button and enable pins.

[andrewn]

I built one up and fitted 5V & 12V terminals on the top with the minus rails brought out on wires. The 3.3V is left unused. If the 12V line only is loaded it tends to drop to 11.5V when a few amps are drawn but I don't find that a problem for my use.

[paulsherwin]

It goes without saying that people should be very careful when modifying switch mode power supplies. They are potentially dangerous. Many forum members have the expertise required but those that haven't would be better off leaving them alone internally.

You could use a Peltier device as a load on one of the rails and use a smaller fan?

Not really, the peltier watts will need dissipating too ending in a bigger fan.

[Radio Wrangler]

If you do use a PC power supply lump, you may want to open it up and fit all the EMC filter components that got bridged-out to optimise profit.

David

[dave cox]

PC supplies range from very good quality (with decent regulation right down to zero amps) to near trash! Peeping through the air vents will give a good indication of general quality but testing unloaded is well worthwhile. Every supply I have tested worked unloaded without problem although the efficiency may not be that good.

dc

[Tractorfan]

Hi,
I've used an ex desktop PC PSU for many years as a power supply for our camping coolbox. It takes about 4.5 amps at 12 volts and I brought the 12volt feed out of the PSU to a heavier pair of leads and a lawnmower type Duraplug two pin rubber connector. It was a freebie as opposed to 30-odd euros for a 'proper' camping power unit.
I just had to jumper two points inside to make it fire up.
Cheers, Pete.