Power supply voltages go down after shorting.

[DJduck]

Hi, i have an old power supply that i think is from an old Olivetti machine. I want to use it for burning some stuff, but whenever the output is shorted, the 12v goes down to 4v and 5v goes down to 2v. I looked inside, and there is no PWM or protection chips. If i turn it off for 5-7 seconds and turn it back on, then the voltages are right again. Is there a way to short that thing without voltage dropping? I can provide pictures of it if needed It is built like a tank, huge caps, all nichicon and panny. I think that the voltage drop is a protection?

[tom66]

Re: Power supply voltages go down after shorting.

No, it's probably just unable to supply that.

Let's work out the current at 4V:

Zero ohm short, current = infinity. And the power required = 4V * infinity = infinity. (You have successfully incinerated us all as infinite power = infinite heat.)

OK, realistically, the wire will have some resistance, maybe 0.1 ohms. Current will be limited, to around 30~40A, simply by the design and topology. So the supply is probably hitting an internal limit (transformer saturation, coupling capacitor, primary drop-out, etc.)

If you want a supply you can short, I have used two 12V 98A power supplies in parallel and managed to drive that into a piece of AWG26. It did not last long at all. The power supplies did not break a sweat.

[DJduck]

Re: Power supply voltages go down after shorting.

Thanks for the info dude! Is there a way to remove SCP from a pc psu? I have an old Dell psu that i use for burning crap, but it shuts down after aout 2 seconds of short circuit. Or is it the same tihing?

[PeteS in CA]

Re: Power supply voltages go down after shorting.

Wow! Some folks are well in touch with their Inner Pyro! (Hope that joke works for our Esteemed Estonian Friend)

It sounds like DJd is using a multi-O/P P/S, using one O/P for his pyro purposes and measuring the voltage on another O/P. If shorting the high current O/P trips the current limit protection and the supply either folds back or is of the shutdown-and-retry type, some voltage will appear on the unloaded O/P. If you know what the current limit circuit is you may be able to defeat it, BUT you will probably hear a loud noise, see a bright flash and smell a bad smell the next time you short an O/P.

AWG #26 is good for an amp or two. Even one 98A supply would have sufficed to melt the wire, though the burnt PVC smell and glow might have lasted a little longer. An engineer I worked with liked to connect AWG #18 magnet wire across the O/P of a 300A switcher he was developing. Burnt magnet wire varnish doesn't smell too good.