Electrolytic capacitors are the exception to the rule that says you can't go
wrong by using a higher rating than specified for the circuit. By all means
use a larger capacitor than 22,000 uF (the Rotax 912 installation manual for
1989 shows 47,000 uF), but keep the voltage rating at 25.
Unlike other types of capacitor, electrolytics need to have a working
voltage applied to keep them in good condition. If they are unused for long
periods time (many years) it is possible for them to blow up if they are not
"re-formed" before having the circuit voltage fully applied. It has been
known for higher-voltage-rated electrolytics to fail when operated in
lower-voltage circuits.
Take Bob's advice and use a computer-grade electrolytic suitable for a
rugged environment, with screw terminals, a high ripple-current rating (e.g.
10 amps) and a low high-frequency impedance. Avoid having long connecting
leads to the voltage regulator, as they could radiate interference and their
inductance would raise the high-frequency impedance. It should not be
necessary to shunt the electrolytic capacitor with a small high-frequency
non-electrolytic capacitor.
Hope this helps
Mike
-----Original Message-----
From: Fred Fillinger
Subject: Re: Re: 22,000uF capacitor
Robert L. Nuckolls, III wrote:
> 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.
Makes sense. Need quality here, on the premise
that a big, electrolytic cap explosion uner the
cowl is highly undesirable. I can't match Bob's
expertise here, but I would suggest (to play
safe), don't cheat on the 25v rating (higher is
harmless, just heavier and costlier), but higher
capacitance may not be (increases initial inrush
current - can the solid-state Rotax regulator take
it?). I bought mine (new, screw-terminal,
computer grade) from a computer surplus store for
$4 USD (same place as my $7.50 pressure sensor pot
for the AOA indicator). But Mouser
(www.mouser.com) has reasonable prices, and
probably Digi-Key too (www.digi-key.com).
Regards,
Fred Fillinger, A063
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