What Would Fry a Zener Diode?

Fingolfen

Well-known member
So, one of my Thagomizers came back from the wilds with a whine anytime it was plugged in (photos below). The grounding was fine, I replaced the charge pump for good measure, just in case. Plugged it back in - same issue. Mostly on DC plug, but 9V battery was starting to do it as well. I then noticed that the zener diode on the daughter board was hot... I mean, a couple of hobbits were about to throw a ring into it hot. I figured it was toast, so I snipped it out of the board and instead of a 0.7V forward voltage drop and open line reverse, it was about 0.1V forward voltage drop... and about 0.12V reverse voltage drop. This is a 1N4742A zener from Mouser, and I've never had one fail before. Short of it having a defect or being damaged on install (both of which would be firsts), what else could cause this? I'm wondering if it was accidentally put into the wrong power supply at some point...

Edit: After replacing the zener with a new one, the pedal is once again working normally and after 30 minutes of playing (granted I'm not the hottest guitarist out there... nyuk nyuk nyuk...) no issues - new zener is still cool to the touch...

THGB - SN01 - 05.jpg
 
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I lost the 15V zener that was regulating power to the EQ board in my bass amp once. In that case a very very hot signal had fried all the op amps on the board and they had failed short circuit, so there was way too much current being drawn for a split second until the main fuse blew. Too much current then I guess could do it, but if there is nothing else in the pedal that is gone then I doubt that is the cause in this case.
 
I assume this is D2?

The most likely cause would be too much voltage applied. (Above 12V)

Installing it backwards would also do it, but it never would have worked in the first place.
 
Talking to them now they both think they may have accidentally put the 18V in it. Nothing else is fried, though I changed out the charge pump to be sure. The voltage protection diode is okay, so I don't think it got AC.
 
AC shouldn't damage D1, that's the reason for using a series diode.

The diode will block reverse polarity (or the negative swing of an AC voltage), not short it to ground.

D2 on the other hand, dumps any excess voltage to ground, which is why overvoltage is likely to have killed it.
 
Overvoltage leads to overcurrent which in turn causes the zener to dissipate too much power (if the circuit and power source allow it). Too much power causes the zener to get too hot, the die melts and fails shorted. Since zeners fail shorted, they do a good job of protecting things downstream. Hit 'em with enough power and they catch fire and eventually fail open. Don't ask me how I know this.
 
I shared an apt in college. One of the thing I did to amuse myself was pranking my roommates by plugging electronic components into the switched outlets in their rooms. Electrolytic caps would go off like firecrackers. One time, I tried that with a zener. It caught fire and ignited a map hanging on the wall nearby. Chris decided that was enough arson for him and he soon moved into a frat house to get some peace & quiet.

Of course, this is just a story and never actually happened. ;)
 
I assume this is R2D2?
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The most likely cause would be too much voltage applied. (Above 12V)

Installing it backwards would also do it, but it never would have worked in the first place.





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Overvoltage leads to overcurrent which in turn causes the zener to dissipate too much power (if the circuit and power source allow it). Too much power causes the zener to get too hot, the die melts and fails shorted. Since zeners fail shorted, they do a good job of protecting things downstream. Hit 'em with enough power and they catch fire and eventually fail open. Don't ask me how I know this.




With apologies to you both.
 
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