This is only peripherally Norton related but I think is a general query. I'm building a Tri-Greeves with a minimal (no lights) 12 volt electrical system with small battery, simple full wave rectifier and Zener diode. I remember from my Commando days that these were fitted with 6 volt coils and ballast resistors rather than 12 volt components. Can anybody suggest why this was, and say why it might be a superior setup? I'm using original points on a 6CA backplate.
Voltage Drop
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Thaks both...
.. Yes Jonathan, I remember them being used on Fords in the 1960s for the reasons you suggest. But my Commando was a Mk 2A kickstart....
Tony that makes sense. I've yet to think about where to put them......
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why this was?
Seems no one knows or can speculate.
The ballast/6v coils were engineered into commando for the planned 71 20M3S (starter) which was intended to receive the new starter configuration. Since it was a disaster, the starter was not used but the new ballast/6v coil & wiring change had been allowed to remain. A ballast bypass would normally be used during starter cranking. Ballast bypass is obviously used on MK3 E-Start.
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Sounds reasonable...
... to me. I worked developing computer systems for Austin Rover in the 1980s and they were incredibly cost conscious even going so far as deleting a little light round the ignition switch on one car to save a few pennies. Obviously NVT didn't share this attitude.
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Rubbish
The reply above re size of 6V coils and 12V coils is rubbish. We have always had large 52mm and 40mm coils since the middle 60's. The real reason for 6V coils and ballast resistors is with the electric start when the starter pulls the battery voltage down, the starter solenoid removes the ballast resistor so allowing the 6V coils to get 10V and stand a chance of starting. If you had 12V coils at 10V far less chance of starting. David Comeau answer above is correct. The system is used on cars for many years. Bear in mind once you go electronic with your ignition system then this system is totally lost, but it is very useful to have your two 6v coils in series for wasted spark. Going back to the original post. If you have points and a kick start then there is no advantage for the 6V coils +ballast over 12V. Except that the ballast is yet another electrical item to fail (Modestly unreliable item anyway) so 12V coils would be the way to go.
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Thanks Alan,
That's the route I plan. Space is going to be tight so I was wondering about these coils: said to be suitable for everything (which probably means works on nothing)
https://www.classicbikeshop.co.uk/ignition-coil-for-both-6v-and-12v-points-or-cdi.html
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think ignition not electric
My Sun distributor machine is shown.
Two silver ball on upper right is adjustable spark gap. 0 to 40KV or more
To start an engine, 4-7kv is all that is needed.
Under FULL throttle the nonconductive HIGH pressure air and nonconductive fuel can require 16KV+/-.
On my machine, using an oscilloscope and a 1000:1 probe, the voltage can be measured : open circuit to see what is the maximum potential. 20 to 30 KV or more is common enough.
The 6-12 volt ratings are somewhat vague and indicate the basic current draw of the primary coil. Therefore general heat generated as a result. 6v will run hot on 12v (14.3 regulator limited). 12v is a better choice
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I am no expert but for vehicles with electric start it was a method of keeping the circuit voltage within spec when you had the cranking load on reducing the voltage. When the motor was running the ballast resistor would deal with the excess voltage.
J