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Topic: Managed to damage a power supply but can I fix it? (Read 2683 times) previous topic - next topic



I scavenged a power supply from an old printer. It has outputs GND, +5v, -5v and +24v.

I managed to short across one of the +/-5v and the 24v leads. There was a little spark and now the +/-5v leads only output about +/-1-2v.

It's a switching power supply and looking at the board I don't see anything that's obviously burned but one of the regulators seems to be getting abnormally hot.

Does anyone have any suggestions for repair? If I can repair it then I can just use the printer board as-is for driving the steppers that I also salvaged.


If the regulators are 7805 and 7905 you may be able to replace them.
But you probably have something shorting the outputs, pulling the voltage down because the current must be higher than expected, check for electrolytic capacitors and see if removing them helps (and replace them of course)



Good call!

I found a 16v 47uF electrolytic capacitor that's shorted. Now I have to find a replacement!


So it turns out that I need a little more help ...

When I removed the cap that I thought was bad I still get continuity across it's connections on the board (I also discovered that the capactor settings on my multimeter don't work so that was fun.)
After staring at the board for a good long time I also took out the regulator - no change
Then I found a tiny zener diode and when I took that out the erroneous continuity went away.

I don't know much about such diodes but I was able to get continuity in both directions but maybe my multimeter was giving it enough voltage to let current flow backwards.

I put the multimeter on the diode checker setting and it gives me about 0.8 in both directions. According to the internets that means it's bad but how can I know what value zener to replace it with? It's far too tiny to have any markings on it that I can read.


what kind of regulators are they?

how do you know it's a zener and not a standard diode?


The regulator is 29M05 which comes up as a THREE-TERMINAL LOW DROPOUT VOLTAGE REGULATOR. I can't pretend to know what that means exactly but the datasheet says it'll give me 5 volts and I believe it. I also checked it's pins for shorts and it seems OK.

The diode I'm calling a zener is a tiny glass bead with a red tint. I assumed it was a zener because it looks like the ones I've seen online. That is an assumption on my part.

Those tiny little things are hard to photograph with a phone!

This is the best I could do, sorry it's not too clear but it's like 1mm long.


Hard to tell, could be just a diode, if there's no regulator for the -5V, it's probably a 5V zener.

what's the voltage you get get on the input of the 29M05 (left pin when looking at the front)? should be between 6 and 16V, if it's lower than that, something else broke before the regulator.


Frankly any semiconductor involved in the damaged supply rail is a candidate,
replacing them one-by-one till it works (starting with the cheapest) is a possible

Might be easier to get another supply that has current limiting outputs (bomb-proof).
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Might be easier to get another supply that has current limiting outputs (bomb-proof).

You're probably right but I've found that breaking things and forcing yourself to fix them is a great way to learn ;)

(more later)


Jul 03, 2013, 02:10 am Last Edit: Jul 03, 2013, 02:20 am by charlieb Reason: 1
I put the capacitor back and there's 9v on the regulator's input (8v after I re-installed the regulator) and 5v coming out of it.

Oh also I realized how I knew it was a zener diode ... durrr

(sorry for the giant image)
Output is on the left and yes, there is -5.8 backwards across the zener diode.

I guess that means it is the diode that went bad and maybe the capacitor too?


Looks like you're almost good to go, replace the capacitor. If you don't plan to use the -5 output just leave it as is.


Yes! I replaced the diode and the +5v line is restored.
I'm not sure what happened to the -5v line, maybe there's another burned component.

I plugged in the printer's mainboard and I was able to power it up and everything seems to work.

Thanks for all your help. I think I learned quite a few things, chief among them is not to put 24v through a 5v component!

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