Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10 5/3/83; site rabbit.UUCP
Path: utzoo!decvax!wivax!linus!allegra!eagle!alice!rabbit!jj
From: jj@rabbit.UUCP
Newsgroups: net.auto
Subject: Ballast Resistors-Trust your neighborhood wookie
Message-ID: <1537@rabbit.UUCP>
Date: Fri, 3-Jun-83 09:57:59 EDT
Article-I.D.: rabbit.1537
Posted: Fri Jun  3 09:57:59 1983
Date-Received: Wed, 8-Jun-83 23:13:23 EDT
Organization: Bell Labs, Murray Hill
Lines: 41


Well, I should know better than to get into a debate in net.auto,
where superstition runs rampant, but alice!wookie was right about
the ballast resistor.

The gentleman who recently pointed out that the ballast resistor
is switched out during starting is also right, but only with some cars,
and some years.  While all cars (except some of those with electronic
and/or capacitive discharge ignitions) have ballast resistors,  all
cars do NOT switch out the ballast resistor during starting.
While examples no longer occur to me, since I've had no standard
ignition systems in the last 6 years, several of the cars I've owned,
including an early model Datsun B-210 <ecch, for other reasons>, did
not switch out the ballast resistor at any time.

While the engine is not turning, the ballast resistor is indeed essential
to prevent the coil from drawing an enormous amount of current.
(Unless, of course, you have one of the GM cars with the ballast resistor
built in to the coil. <Cars with this type of coil, by the way, do
not switch out the ballast resistor, except in the earlier versions
with 3 primary terminals. The reason should be obvious.>)
While the engine is running,the ballast resisitor still limits the current
through the coil at low RPM's, much more than it limits the current at
high RPM's.
Why?  Because the longer the points are closed, the more current flows through
the coil.  <This is, in fact, the standard defination of inductance, an
attribute commonly associated with coils of all sorts.>  At low RPM's,
the points are closed longer, the peak current is greater, the ballast resistor
heats up more, and as a result, since it is deliberately constructed
of a type of wire, usually nichrome, which has much higher resistance when
hot, limits the coil current.
At high RPM's, conversely, the peak current is less, the ballast
resistor cools down, its resistance lowers, and the coil then charges
(yes, increasing the current in an inductor is "charging" it)) more than
it would have without the ballast resistor.  Since the peak current through
the coil and the spark voltage are proportional, the ballast resistor
helps (and does a reasonably good job) keep the spark voltage constant.

have a nearly acceptable day, all

rabbit!<don't mail me>jj