Zener Diode Failure Mode (troubleshooting SX-1080)

[crooner]

I was wondering... What's the most common failure mode for voltage regulation zener diodes?

I am troubleshooting a SX-1080 with power supply problems. Most of the electrolytics on the power supply board have been replaced. Regulator transistors have been checked and found good. There are no drifted resistors. However, checking the zeners on the board one of them shows resistance (using an ohmmeter) on both directions. This zener is connected to a resistor that is overheating on the board.

I am suspecting the zener. Is this a common mode failure for these devices? It is not shorted completely, but rather is acting as a resistor instead of conducting in one direction.

Thanks in advance,
crooner

 

[EchoWars]

Need to pull it out to check. Use a diode test, not the ohmmeter. Ought to measure just like a regular diode.

As an aside, most zeners are about $0.10 each. When I'm in doubt, I pop a new one in.

 

[ferninando]

zeners don't commonly fail unless zapped with too hi a voltage causing brakedown.
Be sure to get the CORRECT zener voltage to replace and replace the R too.

 

[ilimzn]

zeners don't commonly fail unless zapped with too hi a voltage causing brakedown. Be sure to get the CORRECT zener voltage to replace and replace the R too.

Actually, almost anything can fail if overheated...

N.

 

[ferninando]

true, but diodes are pretty tough critters and can take alotta heat before fail. Would you say 100deg C is hot? His zener probably didn't get that hot. A resistor got hot and probably caused a voltage spike, enough to violate the zener breakdown voltage. And lo and behold, it broke down. That's a more likely senario to me. But that's just me.

 

[sanyofreak]

zener failure

Only from experience, I will answer the question as originally posted in the thread, I have repaired a fair amount of power supplies and there is one in particular that comes back on a regular basis - it is a zener failure, the design of the supply is solid, and the zener is used to clamp max voltage on a control IC, it is biased well under it's half watt rating, but the moral of the story is every one failes in the same mode - dead short - reduced to a hunk of wire every time, ohm check verifies this in both directions and diode check also shows short (0V drop fwd or rev). Perhaps they fail in other ways, but this is my 2 cents.

 

[ferninando]

My question would be did they just fail on their own or did something else cause the failure? Once you replaced the zener did everything work ok without further zener failures? Just curious.