auto ignition coil: why autotransformer?

Using an autotransformer saves a small amount of copper.

Cheers,

Phil Hobbs

Yep. It minimizes the chance that the two windings will have a high potential difference. This was much more important back when insulation wasn't so great, but it works so why change it?

George wrote in news: snipped-for-privacy@4ax.com:

yes you are. The spark return is through the block since it is at ground potential and so is one side of the coil....

spark o | c c sec c

12v- | c c pri c | gnd

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In , me wrote (edited for space):

(I SNIP the alternative circuit as part of editing for space)

I thought that usually neither end of the primary went to ground, but one end did usually go to +12V (as shown in the above circuit that I consider to be the traditional one, although omitted is the condenser in parallel with the points.) But out of 20,000 volts secondary voltage, how much difference does

12-14 volts make when we are eliminating a terminal and an external wire connection? Especially since the secondary winding can be oriented for that 12-14 volts to be adding to its EMF?

I think the main reason for making an ignition coil 3-terminal is to save a terminal and an external connection. A few cents saved per car adds up to a lot of money!

- Don Klipstein ( snipped-for-privacy@misty.com)

*** ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^--- *NOT* part of harness, all of this is the auto chassis and enginr block.

The circuit is wrong in a standard ignition set up. The primary is connected to 12V or a ballast resistor to 12V. The other end of the primary goes to a switch to ground, and a capacitor. When the switch opens the voltage at that end of the primary goes up above the 12v rail. This is the end connected in series with the secondary so that its output goes very high to jump the plug gap. This allows a smaller turns ratio than would be required with a transformer configuration with seperated windings. Hope that helps. Jim Stockton

Thanks Jim, now when you add the capacitor across the points you shunt out the spark there and push it down the secondary too. Basically all the energy stored in ther primary ends up in the secondary because of one end being common on each.

Thank you both.

George