What usually fries when electronics are supplied with reverse voltage? - diyAudio
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Old 17th May 2013, 08:57 AM   #1
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Default What usually fries when electronics are supplied with reverse voltage?

My friend lended his GPS tracking device to another friend and this guy manages to connect it with reversed voltage. It's supplied by 12V off a car.
So I opened it up and there's some obvious damage on a zener (looks blown). A rather large resistor has obviously gotten quite hot, as the PCB has gone a little brown close to it. I plugged it in for some quick testing and what I found out was that the large resistor gets hot, too hot. The voltage regulator IC doesn't get 12V, it gets about 4 volts and outputs 1.5 volts. The part number of the voltage regulator is 78m05 so it seems that it should maybe be outputting 5 volts (not quite sure on this). I measured the blown zener, another zener close to it and identical looking ones further away on the board. The results were that the ones that were probably ok measured 1900 and 639 on the diode-testing-mode of my multimeter (I guess these are ohms?), and the obviously blown one 730-ish one way and no reading the other way around. The polarity is so that this 730 was the "same" reading as the 639 on the others. The one that might be blown measured 639 and no reading. I don't really know what values I need for a replacement zener, can you guys help me by telling how to measure these? If you can give some insight on what else is probably fried on the board that would be of great help
(If this is an inappropriate board for this message I'm sorry )
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Last edited by Skiivari; 17th May 2013 at 09:02 AM. Reason: forgot pictures
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Old 17th May 2013, 09:41 AM   #2
DF96 is offline DF96  England
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Any semiconductors, including probably all chips. Electrolytic capacitors. Anything else which suffers knock-on effects due to excessive current in the wrong place.

The safest assumption is that the board is toast. Repairing it could be more expensive than buying a new one.
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Old 17th May 2013, 01:27 PM   #3
macboy is offline macboy  Canada
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It's too bad that the designers of this thing didn't include reverse-voltage protection. It is so simple, a fuse (or even just a SMD "0 ohm" resistor) followed by a reverse-biased diode across the supply. If the supply is connected backward, the diode conducts fully and blows the fuse or fries the resistor, opening the circuit and hopefully protecting everything else.

A friend of mine plugged the wrong power supply into an external hard drive and blew the thing. After pulling the drive mechanism, I inspected it and found a couple fried 0 ohm resistors near the power connector. After I replaced them, the drive worked and his data was saved. So even on a very tight margin product like a hard drive that has polarized connectors which should be fool proof, they included a few extra components for this protection and it saved some hardware and some data that day.

Back on topic, the diode may not be a zener. If it is a simple diode, then replacment is easy; if it is a zener, then you need to know what voltage it is, and that is hard to know now that everything is SMD without full markings. That resistor may be a voltage-dropping resistor ahead of the regulator (to reduce power dissipated in the regulator). If it is too hot, and the reg is only getting 4 V at input, then something is shorted somewhere. It could be the regulator itself. You are correct, it is a 5 V one.

Sadly, DF96 may be right, it may take more time/money to fix this than to replace it.
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Old 17th May 2013, 01:36 PM   #4
DF96 is offline DF96  England
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The diode test in a DMM often puts a small current out and measures the voltage. If so, 638 probably means 0.638V, which may be OK for a forward biassed diode. 1900 on reverse means that it is a zener, or damaged or both. A good rectifier or small signal diode will give no reading (off-scale) on reverse. 730 may be OK for a forward reading too - perhaps a much smaller diode?
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Old 18th May 2013, 07:41 AM   #5
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I've changed the diode that was toast and checked the 7805, it's working. I found out that the big capacitor was shorted, so I'll be replacing that next week. I think there was some sort of protection in place, having to do with the diode blowing jos but i'm not 100% sure. I'll update this if i still have some questions. Thank you guys
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Old 18th May 2013, 08:10 AM   #6
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It's very unlikely that any manufacturer of electronics powered by a single pin AC supply wouldn't include some form of reverse bias protection.

Replace the diode and cap and you should be good. The resistor will be fine.
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