09-10-2021, 11:23 AM
Hi Howard
In the past I have measured a standard Austin Seven coil as being nearer to 1.5 ohms when cold. With 6 volts applied, this gives a steady current of 4 amps. Because the points are only closed for about 50% of the time, the average running current is about 2 Amps. This is an approximate value, as coil temperature, battery voltage and inductive effects at higher RPM will all affect the exact value.
You possibly have "sports coils" which were designed for a higher primary current and greater HT output. The only downside is that points wear is increased, and there is more load on the A7's feeble dynamo.
The other possibility is that you have a little error in the resistance measurement, as accurate measurements are more difficult at low resistances. Maybe a quick check on a borrowed standard coil would help for comparison ?
In the past I have measured a standard Austin Seven coil as being nearer to 1.5 ohms when cold. With 6 volts applied, this gives a steady current of 4 amps. Because the points are only closed for about 50% of the time, the average running current is about 2 Amps. This is an approximate value, as coil temperature, battery voltage and inductive effects at higher RPM will all affect the exact value.
You possibly have "sports coils" which were designed for a higher primary current and greater HT output. The only downside is that points wear is increased, and there is more load on the A7's feeble dynamo.
The other possibility is that you have a little error in the resistance measurement, as accurate measurements are more difficult at low resistances. Maybe a quick check on a borrowed standard coil would help for comparison ?