# Kettering Ignition Question

Discussion in 'Electronic Basics' started by Clive, Apr 15, 2005.

1. ### CliveGuest

Hi,

Most texts and web sites gloss over the secondary, or HT circuit, in an
automotive points based ignition circuit. Explanations simply say that
voltage produced at the HT post of the secondary winding enables the
spark to jump to ground, but do not explain the circuit from the other
end of the secondary winding to complete the circuit.

The layout of the circuit I am considering is where the -ve terminal of
the coils is connected to both the primary and secondary windings. I
hope the ascii rendering below explains the circuit.

As the points are open when the HT voltage is induced in the secondary,
it seems to me that the only routes are either through the condenser or
the battery.

I have only seen the role of the condensor discussed in terms of the
back EMF from the primary.

Thanks,
Clive.

+ve HT distrib
----------o--c||C--O----------­()------------
| c||C |
| Primary c||C Secondary |
| c||C o
| |--| [] Spark Plug
[+] | . gap
[ ] -ve o |
[ ] Battery | Grnd ---
[ ] |----- -
[-] | |
| Points / === Condensor
| / |
--- Grnd |-----
- |
Grnd ---
-

2. ### John PopelishGuest

Compared to the voltage produced by the HT winding, the battery
voltage is essentially zero, and therefore, the positive battery
terminal is essentially a ground connection.

If the LT coil is phased correctly with the HT coil, it adds another
1% or so to the HT output voltage.
Its primary (excuse the pun) purpose is to control the rate of rise of
voltage as the points open, to minimize arching (which damages the
points and consumes energy that would otherwise go into the plug
spark). It has a minor effect on the spark characteristics at the plug.