are zener diodes common failure?

[jeepster]

I have come across many defunkt UPS (uninterruptible seems a bad
adjective for these). Anyway, I opened up one that lasted 10 years
before crapping out, and noticed printed on the PCB:

When the VCC short to GND
first check up this one

And it was pointing to a zener diode. Now the manufacturer must
have seen more than one of these fail, to print a helping hint for
the service tech!
(It was made by Main Power Elec. Co. of Taiwan.)

[Th3_uN1Qu3]

Re: are zener diodes common failure?

noticed printed on the PCB:

When the VCC short to GND
first check up this one

And it was pointing to a zener diode. Now the manufacturer must
have seen more than one of these fail, to print a helping hint for
the service tech!

Now that is nice. Zeners will short out in the event that there is an overvoltage on that rail. By shorting they take down the rail and blow a fuse upstream of that power supply (or something else ), so that the device connected to that power rail is protected from overvoltage.

[999999999]

Re: are zener diodes common failure?

If a zener is used to be a protective measure, yes it will sacrifice itself because some other fault is present. Otherwise, they don't tend to fail from regular service like you see with some poor quality or defective capacitors... but everything fails eventually.

However, you wrote that the UPS was dead, but not that you (yet) have evidence the zener failed. Sadly seeing the silkscreened tip on the PCB, while nice/better than nothing, would seem more like they knew the design had a weakness and instead of engineering out the weak link they chose the cheap way out, and I can't entirely fault them for that, in the end the bean-counters decide how much the build budget is regardless of how nice or ideal the engineer would like to make it.

[Longbow]

Re: are zener diodes common failure?

These are general remarks only: if there are protective zeners across the output lines, these parts ARE prone to short even if no other psu problems exist. A typical example would be a 6.3v protective zener across a 5v line. Or a 13.8v zener across a 12v line. Not much room for error. And, a quick resistance check will find problems like this. I don't know if the output lines spike during startup or if the zener avalanche voltage changes with heat and time. Naturally, you will want to take the opportunity to use your ESR meter now that the supply is apart.