From: Julian Burke (julian@knology.net)
Date: Mon Apr 04 2005 - 07:41:42 PDT
Someone asked:
"What's the job of the capacitor in the M151, is that
to filter A/C and only allow D/C current to flow?"
Which capacitor are we talking about here?
The one in the distributor is to keep the points from arcing.
Any other is a "hash" filter used mainly for the radio systems because
of generator or alternator whine. If an alternator had a filter cap in its'
system, would be no need for hash filters. For all practical purposes,
radios have built in filtering systems and it would add extra cost to the
alternator that is not needed in most applications. Capacitors BLOCK DC but
will pass AC through charge and discharge. When they charge, they kind of
"fill up" to make it simple and will not do anything more until they
discharge. CAPACITORS WILL NOT ALLOW DC CURRENT TO "FLOW" THROUGH THEM!!!
If they do, you have a shorted capacitor. Remember, the capacitor is always
connected across the B+ (to ground) and never in series. CUCV's use what
appears to be a bank of 3 capacitors in series with the windshield motor.
They are actually some form of a capacitor but actually a choke instead. (a
coil) Remember, caps will NOT pass DC! A choke coil will do the same thing
to filter out unwanted generator brush hash or alternator whine due to
"notch" filter theory.
If a little more theory is needed here tube circuits use "bypass" caps
between stages (screen or supressor grid) to bypass unwanted noise or
oscillation between stages. They will block the B+ (no short to ground)
but will pass the noise in the frequencies they are specified for.
Remember, noise, music or talking is "AC" modulation. Old radios seldom
ever work when plugged in because these old wax capacitors short out and
take B+ straight to ground and the radio blows fuses or damage the power
transformer. Julian Burke
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