>> My electronics friends tell me that a 22,000 uf/25 volt capacitor is huge.
>> I ended up using a 2,200uf/25 volt capacitor.
>> I have absolutely no alternator hum in my radio.
>The capacitor is not for eliminating hum in your radio, but to protect its
>input circuits from too big differences of voltage.
I presume we're talking about the capacitor shown on Rotax
wiring diagrams. It's connected directly across the output
of the alternator's rectifier/regulator assembly.
The unlike the automotive 3-phase systems, output of the Rotax
alternator is from a single phase winding and is MUCH
noisier. The capcitor is intended to reduce the noise
by providing some electrical inertia between the widely
spaced pulses of energy from the alternator's rectifier.
>It's a kind of voltage regulation, and it also helps the output
>transistor of your regulator to work not too far from the same voltage.
>If you switch all your gadgets on (radio, landing lights, strobe, etc) with
>a 22,000uF capacitor, you will be surprised to see - with a scope - how
>far from flat is the bus voltage.
Not a quantified statement. 3-phase alterntors have a peak-peak
ripple on the order of 5% of the dc output rating. E.g. a 14
volt system has about 700 mV peak to peak. A single phase alternator's
ripple is 100% of output, E.g, with no filtering provided by either
battery or a capacitor, the ripple from a 14v output machine
is 14v pk-pk.
There are thousands of airplanes flying single-phase alternators
with NO capacitor using only the battery for smoothing and
getting satisfactory results. Some PM alternators will run
with NO battery on line IF you include the capacitor in the
system . . . don't know if the Rotax system operates this way.
>Remind your electronic friends that the more amps they draw the bigger the
>capacitor should be.
The general rull of thumb for 60 Hz power supplies is 1,000
uFd per amp of power supply capacity. Applying the same rule
to the 18A Rotax alternator puts the 22,000 uFd device right
in the ballpark.
>Also put in series with this capacitor a 12V - 20W bulb : if the capacitor
>goes short, you will protect your regulator, keep your 12V power, and also
>immediately know it. At power on, when the cap is discharged, the bulb will
>shortly illuminate : you know the capacitor is OK.
NOT a good idea. The capacitor does it's job by looking as
if it were a dead short to the AC voltages on the system.
ANY device connected in series with the capacitor raises
it's equivalent series resistance value and degrades it's
ability to do the job. A 10A fuse is a good idea . . . capacitors
rarely short but it's not impossible. Better to open a fuse
that blow the end out of the capacitor . . . they stink
and are really messy.
>Electrolytic capacitors are fragile, the best place for them is not under
>a cowl, with heat and vibrations, so I think a bulb is a good protection.
>It's better than a fuse because it shows you immediately if the capacitor
>is OK or not.
Capacitors rated for operation at 85 degrees C are standard.
Further, they don't have to be big either. A 33,000 uFd,
16 volt device can be had with an envelope of 1.4" diameter
and 2.13" long. You want a computer grade device with 10-32
screw terminals. These have the lowest internal resistance
and are most suited for attaching the fat wires used to wire
the alternator system.
If there's enough interest in this part, I'll add them to
our inventory and website catalog. They'll cost about $15.00
each.
Bob . . .
http://www.aeroelectric.com
|