On Tue, 08 Aug 2006 13:14:49 GMT, Arfa Daily wrote:

"Dave Plowman (News)" wrote in message
...
In article ,
Arfa Daily wrote:
Most reverse protection diodes do fail short-circuit. It's unusual to
find one that survives when it does its basic job.

Eh? The usual way is a series diode and that won't fail if the battery is
reverse connected. Be a bit silly if it did...


Sorry to disagree with you on this one Dave, but that's not the case. All
car radios have a diode connected in shunt with the supply. When you connect
the radio backwards, the diode conducts heavily and blows the fuse. Usually,
the diode itself fails also. The same applies to CB radios, ham radios and
PMR radios. Likewise, a couple of pieces of portable audio equipment whose
schematics I just pulled at random, also have a shunt diode. A series diode
is seldom found because apart from anything else, it would have to be rated
to carry the maximum continuous current of the equipment, and would also
cause a voltage drop, which is wasteful of power, and not desirable for
battery powered equipment. In any case, the principles are not in question
here. This meter *did have* a shunt protection diode, which had failed as I
would expect it to. The OP said so.

What the OP did not mention was whether or not the DVM could also run
off of a wall wart -- or, at least, make use of a wall wart-style
charger. *That* could well be why the diode was toasted....

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
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