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philo

On 2/11/19 12:32 AM, wrote:
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....

https://en.wikipedia.org/wiki/Autotransformer