wrote in message
oups.com...
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 check for only very basic diode functionality, in circuit, and
without the unit powered. You cannot check for zener voltage, and any which
were open circuit or leaky, may be difficult to spot. Short circuit ones
should show up ok, but bear in mind that you will probably have a decoupling
cap directly across the zener, and the zener may be strung directly across a
transistor junction if it's part of a 'conventional' linear regulator. If
either of those additional components was leaky or short, the reading across
the zener will be directly influenced. On the other hand, zeners that are
perfectly ok may show what appears to be reverse leakage, but is actually
the influence of attached circuitry, and if any are very low voltage
zeners - say 3v3 - depending on the meter's test voltage, they may *just*
start to conduct in the reverse direction, giving the impression that they
are reverse leaky, when they're not. Is there a big problem with powering
the unit ? Do you suspect that it has failed zeners ?

Arfa