paul stewart wrote:
> Control vision on their web site say that the EXP bus now provides
> overvoltage protection. My understanding is that the Rotax has a PM
> alternator and so no field circuit to simply 'turn off', thus the OV
> protection requires a relay capable dumping the whole alternator
> output to ground (as per the club mod). Their website suggests if
> using an alternator without a field input that the circuit may be
> used for any 5 amp application. I'm not clear that the EXP bus does
> provide the OV protection to prevent boiling the battery in the
> event of regulator failure.
I find it hard to believe that one should short either the regulator or
generator output to ground. Good grief!
EXP's Rotax schematic says the relay is connected to what I guess is a
control voltage to the regulator. The innards of the regulator are not
in the public domain, so we'll have to take EXP's word for it that this
arrangement works.
"Boiling the battery" is really not the concern. It will take a while
to do that, and in fact in the short run it's beneficial in that it's a
recommended method of desulfating the plates! The concern is avionics,
but I've not seen a schematic on modern avionics where overvoltage will
quickly cause damage such that instant protection is needed. Except
maybe some 30-cent illuminating lamps, and on some VHF comms please
don't key the mic during a severe OV condition.
Bob Nuckolls thus has writ that for Rotax PM generators (not an
alternator!), it's sufficient due to limited output to provide for panel
indication of overvoltage with visual/aural alarm. The overvoltage
problem on mechanical alternator regulators has to do with frozen
"buzzer" contacts, but of many dozens of solid state power supply
failures I've seen they've all gone open and thus to zero output. At
least I've never found any anecdotal report on the net that the Rotax
Reg fails in overvoltage.
My OV scheme for the avionics and electric gyros is just a 17V, 50W
stud-mount, heat-sinked Zener diode across the bus. The theory is that
most of the generator output is being sapped by devices on the bus, so
there's plenty finite time before that $12 puppy blows to allow for an
aural overvoltage indication to dictate corrective action.
Regards,
Fred F.
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