Re: Placement of Ballast Resistor Continued

Posted by Gary Schafer on January 15, 2006 at 16:01:58 from (71.57.149.211):

In Reply to: Re: Placement of Ballast Resistor Continued posted by Thomas Donahy on January 15, 2006 at 12:04:55:

I see that you got many answers to your question.

Some coils may have the bottom end of the high voltage winding grounded to the coil case. Others do not. They are wired with the primary and secondary in series so they look like an auto transformer. They both function in the same manor however. It is only a difference of how the secondary winding gets to ground. In the case of the series wound coil the secondary gets its ground through the battery terminal on the coil. Some of the secondary current does pass through the condenser as well.

The condenser is not there to provide any type of tuned circuit or to cause the circuit to ring for its performance. As soon as the points open the field of the charged core starts to discharge which charges the secondary coil and also the primary coil. The coils are charged to opposite polarity as the magnetic field is discharging.
The condenser is there only to keep the voltage on the primary from arcing over at the points. The primary voltage charges to several hundred volts as the magnetic field collapses.

Yes there will be some ringing due to the capacitor across the coil as it discharges but that is not its purpose. Its purpose is to limit the voltage rise so as not to damage the points. It does this by slowing down the voltage rise at the points while allowing the primary voltage to decay. Its size has to be chosen so as not to discharge too slowly so it allows the magnetic field in the coil to collapse fast enough to charge the secondary. Too large and it would dampen the discharge. Too small and it would allow the voltage across the points to rise too fast before the coil had enough time to diminish its voltage and the points would arc excessively.

As far as the ballast resistor on either side of the coil I don’t see where it would make much difference as the impedance of the primary circuit is so low compared to the secondary of the coil.

On cars with radio interference problems it was common to place a condenser at the positive terminal (battery terminal) on the coil to ground. This kept the return pulse current out of the wiring to the battery and other areas of the car so it didn’t radiate. A shorter path to ground.

As far as polarity of the coil goes I believe that a negative going spark is wanted at the spark plug. This makes the hot spark plug easier to discharge than a positive going pulse.

Regards
Gary

Replies:

• Re: Placement of Ballast Resistor Continued Gary Schafer 19:01:52 01/15/06 (0)

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