Re: Another look at the resistor

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Posted by Bob in KS on January 25, 2019 at 15:35:42 [URL] [DELETE] :

In Reply to: Another look at the resistor posted by Farmer Dan on January 25, 2019 at 07:27:07:

The video gives an excellent description of the resistor bypass starting circuit. However, it is only applicable to a 12 volt system.

The Ford N series front mount distributor operates on a similar principal with it’s 6 volt system.

Ohms law states that current = voltage divided by resistance (do an internet search for further explanation)

Most old tractor distributor ignition coil primary windings are designed to operate at about 4 amps. With a 6 volt battery, a coil with 1 1/2 ohms of primary resistance would give 4 amps. With a 12 volt battery 3 ohms of resistance would give the 4 amps. A coil is a transformer with one side of primary and one side of secondary tied together.

In the front mount N series ignition, the coil is designed to require a special resistor. It is a temperature variable resistor, ie, lower resistance when cold and resistance increases as current through the resistor causes it to heat up. When first started, the resistor with lower resistance would give a slightly higher current to give a hotter spark for starting, then the resistor would increase in resistance and lower the current to 4 amps. The coil can stand this increased current for the shorter starting duration but not over an extended time.

Round can coils are different. A “6 volt” or “12 volt external resistor required” coil have approximately 1 1/2 ohms primary resistance. In a 6 volt system either one will run at 4 amps, no extra resistance required or allowed. They can both be used in a 12 volt system by adding another 1 1/2 ohms of resistance between the output of the switch and input of the coil. 1 1/2 ohms of the coil plus 1 1/2 ohms of resistor equals 3 ohms.

There is also a “direct 12 volt” or “12 volt no external resistor required” coil that has 3 ohms of built in primary resistance. This coil is designed to operate in a 12 volt system with no added resistor. This coil is wound with 3 ohms of resistance in the coil primary, there is no internal resistor in series with the primary.

Some automotive systems have used different coil primary resistances which is why the video is saying 8 – 9 volts . That is true of that coil, but not universally true of all coils.

But here we are talking old tractor systems. The specific coil used will determine if and how much external resistance is required.

Many later tractors did use the starting bypass system described in the video. This would only be used in a 12 volt system with 1 1/2 ohm coil, it would not be used in a 6 volt system. The starting bypass described in the video will not work with a direct 12 volt 3 ohm coil.

For convenience we think of coils in terms of them being 6 volt or 12 volt. Coils really don’t care whether a 6 or 12 volt battery powers the system. Their only concern is with maintaining the 4 amps of current passing through them. The 4 amps is maintained by the combination of battery used and external resistance that may be required for that specific application.


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