McFadyean wrote:
>
> ...
> Denied of the swamping and stabilising capacity of a battery, alternator
> output voltage is likely to increase and/or become erratic. Without a
> voltmeter, the first indication could be all the avionics frying! Rotax
> tell us not to run the charging circuit with the battery disconnected.
To add to that, I think the avionics likely won't get fried but can be
zapped. The perm magnets and the low power of the Rotax alt should
keep steady state voltage well in check (unlike a field-coil type alt
that can go wild, capable of 70A when you need only 20). The Rotax
alt is probably badly "spikey," and maybe that's what that odd
25,000uf capacitor is there for, in case batt loses it's ability to
stomp on the spikeys.
Mr. Nuckolls does describe a "crowbar overvoltage protection" circuit,
saying you need a fast acting circuit for spikes, but saying its for
runaway voltage. But elsewhere he says, spikes, shmikes, don't
worry. Don't happen, and avionics are built to RTCA Doc standards for
transient immunity anyway.
Confused, so I have this 16v, 50-watt Zener diode to sit across the
buss. It can at least turn big spikes into little ones, acceptable as
long as I think it will doing anything. Some production A/C mfrs have
one, but in the service manual, they don't say how to test it, nor
what the symptoms are when it goes bad!
Best,
Fred F.
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