I have a general-purpose 12V, 25 amp power supply I bought from Radio Shack. Last night I turned it off and about 2 minutes later accidentally shorted the terminals with an alligator clip with the load still connected. There was a bright spark and an audible whining noise for a second or two.
The power supply turns on but produces erratic, very low voltages.
Two questions:
Did I fry the power supply or is there likely to be a fuse or other safety device somewhere I can reset?
Is it possible I did any damage to the load (a DC motor)?
sounds like it didn’t have over-current protection. If there’s no fuse (either in a holder on the back panel or on the circuit board inside) then it’s dead.
Yup, dead it is, now it depends on what it is as to whether its repairable.
My guess is that the regulator will be fried and perhaps the rectifier and these are pretty easy to swap. There is often something like an inline diode that could be cooked, easy to sort out.
1) Did I fry the power supply or is there likely to be a fuse or other safety device somewhere I can reset?
Usually if there is a blown fuse you’ll get no output at all. The erratic voltage means that something is still connected to the output, so it’s not a fuse problem. I don’t know how this particular power supply is designed, but with a little luck all you did was fry the final output drive component, whatever it happens to be. It could be a single transistor or it could be some kind of integrated power module. As the previous posters said though it could be other things. The regulator could have died or the transformer windings could have melted.
If/when you fix it, make sure you put a fuse in it to prevent this from happening again.
2) Is it possible I did any damage to the load (a DC motor)?
There are all kinds of different ways that it could have failed, so it’s not impossible for the power supply to have briefly supplied a large overvoltage to the motor. It’s much more likely I think that the voltage was just clamped low by your unintentional short. Even if there was a brief overvoltage to the motor, most motors are reasonably robust. Even if you hit them with a quick overvoltage it’s usually not enough to overheat the motor windings and make them fail.
I won’t 100 percent guarantee you that your motor survived, but chances are it’s fine.
Hmm, that’s interesting. If the power supply was off, and the load (DC motor) was still connected, I wonder why the power supply’s output caps were still charged? Unless, for some reason, the motor was an open circuit. Even then, the supply should have incorporated a bleed off resistor across the output caps.
A whining noise? Hmm. Sounds like the power supply’s output capacitor, which is electrolytic, may have burst.
Take off the cover and look around. Look for any signs of overheating such as charring, discoloration, etc. Take a close look at the output capacitor(s) and output transistor (the latter will be mounted to a heat sink). And stick your nose in there and sniff around.
I’ve never seen a capacitor burst from being shorted out, even mains-type capacitors which can store dozens of joules of energy and make a really big bang if shorted, e.g. 560 uF at 400 volts, plus there should have been a big cloud of smoke (amazing how much smoke/vaporized electrolyte an electrolytic capacitor can produce). Perhaps the sound was the motor, which if it was still spinning, would be producing back EMF, up to the input voltage (but after 2 minutes I’d think there would be no output). Or it was just ringing in the ears from the sound of the spark (which I can relate to, see above). But again, there shouldn’t have been any voltage on the output after 2 minutes; are you sure it was turned off?
Also, by any chance is the power supply you have this one (it says 13.8 volts at 25 amps, couldn’t find a 12 volt version)? Although this is a switching power supply, which makes even less sense for there to be significant output voltage after 2 minutes, due to the much smaller output capacitors (filtering 10-100 kHz instead of 50/60 Hz, this also means not much energy is stored in them), plus they should have overload/short protection.
Actually, come to think of it, a SMPS can produce a whining sound if shorted (switching frequency drops into the audible range), which goes back to whether it was really turned off. Odd though that it still produces some output since failure is usually complete (no output at all).
I found the manual for the power supply. It says it has automatic short-circuit protection. Sure enough, it works fine this morning. I had already removed the cover and did not see any signs of damage inside.
Thanks for the help, everyone. I got the specs on the power supply wrong (the labeling has worn off) - it is actually this model:
The power supply was definitely turned off, but in retrospect, I had actually turned the motor off first, so there was an open circuit at the time I turned the supply off. I know from past experience that the output capacitors do store a decent bit of energy, since the supply will often continue to power small loads for up to 30 seconds after I shut it off.
Well, I guess it does not say short-circuit protection, just “overload” protection, which I assume just means excessive current draw. In any case - it still works.
I agree, but I was just trying to come up with some plausible explanation for the sound. His description sounded like something was outgassing, and the only thing I could think of was electrolytic caps.
I am curious about the origin of the noise too. It was definitely a whine, clearly audible from a few feet away, diminishing in intensity, lasting about 1-2 seconds. It was a relatively “pure” tone - it didn’t sound like something outgassing, more like an electronic component - similar to the sound made when a camera flash charges. The power supply has never made any other noises.
Since the power supply seems to have survived I suppose I could do this again. But I likely will not.
That’s a switching power supply, if not the same model I found (based on the current rating you initially gave). Of note, while the output capacitors don’t store much energy in a SMPS, the input capacitor does (about 1.44 joules/watt-seconds per 100 uF at 120 vac, equivalent to about 15,000 uF at 13.8 volts), and can keep the supply running with no/low load for some seconds after removal of input power, which is likely what happened in your case (I’m still thinking the sound came from the power supply as the switching frequency dropped into the audible range, the sound would come from the transformer, similar to the humming 50/60 Hz transformers can make; the sound you mention from a camera flash is the same thing as well).
Yea, I would agree with you. If it was a pure tone, then it was probably a magnetic component. Most likely the switching transformer if it’s a SMPS. They tend to do that when the current is really high or the frequency is way off.