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mike

wrote:
I have a circuit that I built years ago. Rather than just replacing a
bunch of zener diodes, is there a way to test them in circuit, without
powering on the circuit?
I know that surface mount zeners fail shorted, but they can open if
there is enough current.
Can I just do a diode check at each polarity with a DMM to detect a
bad zener?
Thanks

You can find a shorted circuit node with an ohm-meter.
If you've got stuff in parallel that could be shorted, you have
to determine where the current is going. Easiest thing is to
unsolder something. If you put a bunch of current into a shorted
node, the offending surface mount will desolder itself...sometimes
explosively. Or use a high-tech infrared thermal imager. Used that
back in the day to find internal circuit board shorts. Low-tech
non-contact thermometer works if you can find one with a very narrow
beam width.

Another approach is to use a HP current tracer
and HP current pulser to determine which way the current is going
in a trace/wire.

Open zeners are a bit more difficult. Easiest thing there is to power
the circuit and measure the voltage across the zener. If you can't put
the circuit in a mode that turns on the zener, it's not likely the cause
of your symptom...or you didn't need one in the
first place. A curve tracer can often tell you a lot about the
condition of your zener, but it depends heavily on whether you can
reach zener voltage without blowing up something else. YOu can make
one from an AC wall wart, a resistor and scope. Modems often have AC
wall warts.

Would be interesting to know why you can't power it on??

If you built the circuit, you should be able to get close to the problem
by analysis of the symptoms.

And zeners do fail in modes whereby the small signal behavior
is more like a resistor.
mike

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