This assumes that the installer did not fuse the + lead to the radio.
Normally a 25 to 35 watt radio has a 15 amp fuse but you might be
surprised how many times the power cable has been cut and no fuse.
Anyhow, without a fuse, the reverse protection diode or transorb in the
radio tries to short and shunt the reverse current from the radio. Most
of these can handle about 20 amps for a few seconds before the smoke
gets out. Then the reverse protection diode is essentially gone and
next in line us usually the audio power amp, and RF PA. While the radio
tries to act like a short (for a few brief moments during cranking with
the first battery dead the starter will draw hundreds of amps (or try
to) thru the radio and protection diode. This doesn't continue very
long at all. If the radio is properly fused, then the diode may short
and blow the fuse, but without a fuse, the radio gets blown.
Steve
Dave Gomberg wrote:
> At 05:22 3/25/2008, Steve S. Bosshard (NU5D) wrote:
>
>> any protection diodes - 10 amp protection diode against 400 amp starter
>> current - no contest.
>>
>
> Steve, this confuses the dickens out of me. I now understand the
> reverse polarity part, but won't the protection diode be reverse
> direction, and therefore the only relevant stat is the reverse
> voltage, 12v, and therefore presumably within ratings?
>
> I don't see what the current has to do with it because the diode will
> be non-conducting. The current rating is only for the conducting
> direction, right? (with one battery dead, the other battery will cause
> reverse current thru the first battery during cranking and at that time the
> protection diode will be briefly conducting).
>
>
>
>
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