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RE: Ballast Resistor

To: "'Steve Laifman'" <SLaifman@SoCal.RR.com>,
Subject: RE: Ballast Resistor
From: "Palmer, Robert L." <RPalmer@brobeck.com>
Date: Thu, 27 Sep 2001 11:52:41 -0700
Steve, Stu,

On a cold winter's morning in Nebraska, the ballast resistor setup might
help. However, I agree with Steve, and all European auto manufacturers, that
even with the reduced cranking voltage there is still enough voltage to the
coil that it is seldom if ever would cause a starting problem. The
approximately 50-100A the starter draws will, however, significantly drop
the voltage due to both battery internal resistance and the resistances of
the cable and connections between it and the starter solenoid. Also, and our
resident electronics expert Theo can usually be relied on to more
authoritatively clarify these kind of questions, there is absolutely no
correlation between the nominal voltage rating of the coil (12V versus 9V)
and the current it draws. If you used a 9V coil and removed the ballast
resistor, then of course it would draw more current. A 12V coil is designed
to draw its design current at 12V, not 9V, so it has a higher resistance.
One could measure the resistance of the ballast plus 9V coil and compare it
with the resistance of a 12V coil to determine what the steady-state
currents would be. To know what the average operating current would be, you
would have to use an oscilloscope, an rms voltmeter, or such device that
would measure the dynamic behavior. However, a priori, there is no reason to
think that any given 12V coil is more likely to damage the Pertronics than a
9V plus ballast. If, on the other hand, you buy a "hot" coil, then
regardless of the voltage rating, it would be expected to draw more current
than a "stock" coil.

Bob



-----Original Message-----
From: Steve Laifman [mailto:SLaifman@SoCal.RR.com]
Sent: Thursday, September 27, 2001 11:18 AM
To: Stu Brennan
Cc: Bob Palmer; CE25593@aol.com; tigers@autox.team.net
Subject: Re: Ballast Resistor


Stu Brennan wrote:
> 
> The stock ignition system shorts out the ballast resistor to provide a
> hotter spark when the starter is dragging the system voltage down.  Do
these
> "12V" coils have a similar feature?  Or are they so "hot" that they still
> have plenty of energy when running on reduced voltage?  I assume that this
> would apply only to a "points" or Pertronics type system, not a fully
> electronic one.
> 
> Stu

Stu,

That explanation of the ballast resistor is a little off. Yes, in the
"start position" the resistor is by-passed and a full 12 volts is
applied to the coil during cranking only. This provides a hotter spark
during start. With the modern 1,000 cranking amp batteries, I do not
believe that the system voltage is noticeably lowered. My voltmeter is
at 14.4 volts during cranking. However, during the run position, the
starter is out of the circuit, and the resistor is in, reducing the coil
to about 9 volts. This prevents overheating the coil, as well as
reducing arcing at the points,

You may be correct in the days of 500 starting amp batteries, but better
are now available.  British cars have not, to my knowledge, over 50
years, ever used a resistor in the start circuit, and only used 12 volt
coils.

The ONLY reason Pertronix ever gave for using a resistor type ignition
coil is to reduce the switching current to within the capacity of the
Pertronix (I) semiconductors. The Pertronix II is a good deal more
robust in this area, and can switch the full amperage of a 12 volt
system continuously. Capacative discharge systems have used 12 volt
coils for a long time, even with points, Unilights, whatever, as the
current through the switching element is significantly reduced to
activating the high energy solid state switches that pour the fast rise
capacative discharge to the coil, directly.

Steve
Steve Laifman        < Find out what is most    >
B9472289              < important in your life     >
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