A large wire (primary) and a small one (secondary) are connected
together to the ground.
So it makes 3 connections.
The turn ration can easily be about 70 100.
The transformer is generally in an oil can so the isolate either
termally and electrically.
Take care, sparks are above 20,000 or 30,000V.
For old engines, a capacitor was connected in // to the ruptor and it
was this cap which was making such sparks.
At low speed, the ruptor was actuated at about 25 Hz, so when ruptor was
making OFF-ON-OFF, ther was only one spark duting the OFF-ON transition.


a écrit le 11/02/2019 Ã* 07:32Â*:
This may be a litle off topic, but ignition coils are WEIRD, compared to
other transformers. I had to work on the ignition on an old farm tractor
last week. I always thought that the high voltage (secondary) was from
the tip (spark plug wire), to the coil's metal case. It turns out I was
wrong. (I am referring to the old coils which are about the size of a
slim beer can, and have a pointed top).

After a lot of research, I found that the secondary is measured from the
large center terminal, to the ground (-) side of the primary. Once I
learned that, it turns out that the secondary is around 500 to 600 ohms,
and the primary is around 6 ohms. (I compared this to several other
similar coils, and it is about the same).

These ignitions are very simple, but testing them is a challenge, since
they do not follow the (regular) transformer wiring. A regular
transformer will have 2 wires for the primary, and two or more on the
secondary. These transformers only have 3 connections, period....

But this has me puzzled. Since the minus primary connection is used for
the secondary high voltage, how can the circuit work????
Ok, my point is this: The coil's minus terminal is connected to the
ignition points. The points are making a ON-OFF surge repeatedly. (or
creating an AC across the primary). So, how can the coil deliver a spark
during the time the points are open, since the secondary is relying on
the primary (-) to complete the circuit, which is also being switched on
and off by the points. .

This still has me puzzled......

I did find the problem and get the tractor running though. There were
actually two problems. The points had welded together, so I replaced
them. But the spark was still weak which went back to the ignition
resistor. That resistor is supposed to read about 8 volts on the output
side, which feeds the coil. I was reading 4.5 volts, with the battery
fully charged at around 13.5 volts. So I replaced that resistor as well.
Works fine now....