Arfa Daily wrote in message
news

"Jim Land" wrote in message
. 3.44...
Ken Weitzel wrote in
newshRBg.332437$Mn5.252184@pd7tw3no:



me wrote:

Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and
found that there was a power to ground short. The short is being
caused by a SMT diode that is across the 9v battery wires on the PCB.
I lifted one side of the diode and the diode is shorted and burned
pretty badly.
I powered up the DMM briefly and everything seems to be working ok.
I
want to know if anyone has any suggestions on what type of diode
should I use to replace the shorted diode. It doesn't have to be a
SMT diode.
I am guessing that is a protection diode and not a zener since most
of
the ICs run off of 9v according to their datasheets. Any help would
be greatly appreciated. Thanks in advance.


Can't think of any reason you'd need one at all, unless you plan
on somehow installing the battery backwards

Sounds like someone momentarily touched the battery terminals backward
and the diode forward-conducted and blew out. Conclusion: a surface-
mount diode doesn't work for backwards-connection protection, it just
self-destructs. Unsolder the diode and leave it out of the circuit.



Most reverse protection diodes do fail short-circuit. It's unusual to find
one that survives when it does its basic job. The battery must have been
connected backwards for the original to have failed, and as the rest of
the
meter seems to work ok, then IMHO, it has done the job it was designed-in
for. For the minimal cost, and the protection afforded, I really can't see
any reason NOT to replace it ... ?? Any old diode will do from a 1N4148
up.

Arfa



I've asked this before here , but again.
Does anyone know of series of diodes that are guaranteed to go open circuit
on excess forward current or guaranteed go short circuit on reverse voltage
overload.


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