>Large value electrolytics have significant self inductances ie. impedance to
>high frequencies. The Rotax regulator is a switching device which shorts
>half cycles to ground every now and again (on low load) and produces high
>frequency, high voltage spikes in so doing. (This is the cause of most RF
>interference). This combination can easily exceed even the 25v value for
>very short periods. Whatever, a 16 volt cap. blows up nicely after a very
>short time - and very spectacular too. Been there.
Not sure about the Rotax regulator but all of the PM
alternator regulators I've opened now use a full wave bridge
rectifier with a silicon controlled rectifier in two
legs to provide SERIES switching for control of output.
This is in contract with older SHUNT switching regulators
popular on small motorcycles of decades past. In
any case, it's not voltage spikes that eat the
capacitor but ripple current at the 2x the AC output
frequency of the alternator.
A number of my customers have blown some rather
small capacitors (1.2" x 1.5") rated at 22,000 uFd
and 25v with small solder terminals intended for
mounting to an etched circuit board. These capacitors
are NOT suited to this task at ANY voltage rating.
. . . the capacitors I recommend are the so called
"computer grade" electrolytics with fat, 10-32 threaded
screw terminals. I believe the smallest one I could
offer was on the order of 1.3" x 2.5" and have a
ripple current rating of well over 10 amps.
Bob . . .
http://www.aeroelectric.com
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