Electronics > Repair

Shorted industrial controller

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MultiMike:
I'm a millwright and an electrician (millwright first).
I do a lot of repairs, and I do major electrical installations.
I'm getting into electronics as a hobby.
I'm trying to fix an industrial controller stopped working.
The controller has one major board which takes various AC voltages from a small multi-tap transformer.  These are fed through a bank of diodes acting as half-wave rectifiers.  There is a 7805 and a 7815 on a heat-sink.   My problem is with the 5V bus (of course).   
The output from the 7805 is only about 2V.  So I immediately replaced the 7805, which was not the right call (as most of you probably know).  The new VR does the same as the old, because there is a short somewhere on the 5V rail.   Now I need to find it.   I'm trying to decide how to do that.

I've been beating my head against it at home, because my electronic stuff is there.   Now I think I've hit a wall and I need advice on how to find this short.   I'm going to check my meters at work to see if I have one that will do at least 0.01 ohm resolution.   Might have one.   I'd love to build a Shorty but there is a long lead time on the chips from the supplier I checked.   I have a power supply at home, and a thermal camera at work.

Any thoughts?

Mike

coromonadalix:
you have to find the "short"  by isolating sections ....  or if you have access to a thermal camera it could help ?

kripton2035:
my shorty "clone" uses standard IC that are available AFAIK
http://kripton2035.free.fr/Projects/shorty-display.html

TheMG:
Since it's not a dead short and you're still getting 2V out of the 7805, something has got to be getting warm (other than the 7805 itself which is of course going to be quite hot).

As long as there's no high voltages lurking about on the circuit board, you can always use you fingers to feel the temperature of various components, ICs especially but don't overlook any MLCC bypass caps I've seen those go bad and load down a 5V supply.

Another trick is to dab some isopropyl alcohol on the components, it will evaporate far more quickly from any hot areas.

PaulAm:
If you have access to a milliohm meter, you can put one lead on ground and follow the +5 around until you find a  minima, which will be at the problematic part.  Dead shorts are a little easier, but it should still work.

If you don't have one of those, it's probably worth building the shorty mentioned by kripton2035.  It's a pretty simple project and will make finding the problem a whole lot easier.  If you don't want to mess with a microcontroller, there are discrete milliohm meter projects available online.

Otherwise, it's divide and conquer.  Isolate the problem to a board by disconnecting things or using thermal cues and persevere.  The right tools can make finding the problem an easy 15 minute task.  Without them, it just feels like this:  |O

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