Coils (6V /12V) in series or in parallel

Classic Norton Commando Motorcycles.

Coils (6V /12V) in series or in parallel

Postby pierodn » Tue Nov 12, 2013 1:48 am

I found on Commandos some Lucas coils both by 6V and other both by 12V.
My knowledge is that the electrical Commando system is 12V has the 6V coils placed in series (+-+-), while those from 12 V must could be put in parallel (++--).
It is so or 12V coils should not be used?.
The electronic injection (Boyer/Pazon) can be used with both coils from 6V or 12V too?.
I have some confusion because I'm studying now the electrical wiring system of the BSA 1969 Rocket 3 and I understand that it has 3 x 12V coils in parallel (right?) but if you want to fit the electronic  injection must replace them with 3 x 6V (in parallel = 6V!!!!) or (3 x 4V in series = 12V)
I think I need your help.
Thank you.
Ciao.
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Re: Coils (6V /12V) in series or in parallel

Postby fredful » Tue Nov 12, 2013 3:45 am

Standard on my MK2A with boyer ignition, six volt coils in series, has worked good for 15 years, dont know but parralell for twelve volts sounds right, the missus Trident had to be changed to six volt coils with a boyer system, works good in conjunction with three phase alternator, will still start easily with flat battery, got the pazon with single small twin terminal coil (very neat instalation) on the racer running total loss with shorai battery, works well, thats all I know for now.
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Re: Coils (6V /12V) in series or in parallel

Postby YING » Tue Nov 12, 2013 4:54 am

Is there an advantage to replace 6V coils with 12V?
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Re: Coils (6V /12V) in series or in parallel

Postby dave M » Tue Nov 12, 2013 5:41 am

6volt coils will more or less produce the same spark when fed 6volts as a 12 volt coil will produce when given 12 volts. With a points ignition setup each coil is effectively a separate ignition system and fires alternately with the other coil, if you disconnected the left side cylinder points or left side coil for example, the right side would still fire.

With electronic ignition the coils are wired in series and both spark at the same time, with the cylinder that is on the compression stroke firing when it reaches the top of the stroke and the cylinder that is on the exhaust stroke recieving a spark but not firing due to having no compressed gasses or flamable mixture - this is generally known as "a wasted spark". This system is fed with 12 volts but as two 6volt coils are generally used this is the appropriate voltage for two 6volt coils in series.

Nortons with points that use 6volt coils utilise a balast resistor in the circuit before each coil to reduce the 12 volts of the electical system down to a 6volt feed for the coils. I must admit I don't know why this was done, although it is common practice in automobiles to run a 6v or 9v coil with a ballast resistor which is bypassed while the starter circuit is engaged. This will give a bigger spark to aid starting, but cuts off the moment the key is moved from the cranking position to the ignition position. If a 6v or 9v coil is run with 12v for any reasonable length of time, it will likely overheat and burn out.

In summary, I don't think there is any advantage to using a 6volt coil instead of 12 on a Norton with points ignition, but if you use two 12 volt coils in series in a system that uses electronic ignition the spark will be weak.

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Re: Coils (6V /12V) in series or in parallel

Postby DogT » Tue Nov 12, 2013 6:57 am

To add confusion to Dave M who is correct, the early Commandos had 12V coils in parallel (separate spark with points for each side), starting in 71 went to 6V coils in parallel with ballast resistor (separate spark with points), like Dave, don't know why. Pazon recommends using 6V coils in series (wasted spark), no ballast so that's what I'm using. Means I had to buy 2 new 6V coils with Pazon to replace AAU and points. Pazon says using their EI in parallel with 12V coils is erratic. I think it's in their instructions.

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Re: Coils (6V /12V) in series or in parallel

Postby worntorn » Tue Nov 12, 2013 7:53 am

The Ballast resistor was added to make the points live longer. Arcing at 6 volts is less destructive than arcing at 12 volts. And the 6 volt points setup will last a very long time. I pulled some out at 23,000 miles, they were only slightly pitted. After a brief but unsuccessful trial of some Japanese made replacements, the original Lucas sets were cleaned up and reinstalled. Still going nicely at 37,000 miles.

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Re: Coils (6V /12V) in series or in parallel

Postby PeterJoe » Tue Nov 12, 2013 8:59 am

The stock Norton Commando that uses 6 volt coils has a ballast resistor in series with the coils. However, the coils are wired in parallel. I honestly believe that Norton used this ballast resistor set up because the Norton is already wired for a starting circuit. Within the starting circuit there is a ballast resistor bypass when the starter relay would have been energized.

Under the ballast resistor heading within the factory service manual it states:
(The ballast resistor) permits the use of 6 volt coils in an otherwise 12 volt system. The two coils are wired in parallel with the resistor in series. When the battery voltage has dropped due to heavy current taken such as by the use of an electric starter, the ignition coils are fed direct, bypassing the resistor thus enabling the coils to work at their approximate potential (voltage).


Apparently Norton's electrical division was up and ready for the Norton with a starter motor. However the mechanical guys were falling way behind the power curve in that department.

There are advantages to using 6 volt coils in conjunction with a ballast resistor for engines that run at very high rpm. The reason is that the spark coils can saturate quicker than the 12 volt coil. However I don't think a Norton will ever fall in that category especially since each cylinder had its own ignition system (that is using the stock point system).

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Re: Coils (6V /12V) in series or in parallel

Postby hobot » Tue Nov 12, 2013 10:32 am

Note, on electronic ignitions its not the coil voltage that matters so much as their ohm resistance, so 2 coils in series or dual coil pack, must be in the ohm range the ignition system is compatible with. The coil voltage addition-substraction 'logic' of initial post does not compute or make sense electrically so forgetabout that in deciding coil voltage. Coil feed voltage ratting only has to do with what voltage a coil can stand being fed for a long period. 6V coils would over heat-burn up if not throttled back soon by ballast resister. Coil out put voltage only means what a coil could produce and stand but usually jumps spark gap at rather lower voltage than advertised.
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Re: Coils (6V /12V) in series or in parallel

Postby L.A.B. » Tue Nov 12, 2013 11:01 am

pierodn wrote: I found on Commandos some Lucas coils both by 6V and other both by 12V.
My knowledge is that the electrical Commando system is 12V has the 6V coils placed in series (+-+-), while those from 12 V must could be put in parallel (++--).
It is so or 12V coils should not be used?.


6V or 12V coils are only wired in series when the original Commando points ignition has been replaced with a "wasted spark" electronic ignition system (the ballast resistor also has to be disconnected).

Both "2x 12V coil" and "2x 6V coil + ballast resistor" Commando ignition systems have coils wired in parallel (compare the pre-'71 and '71-on wiring diagrams, both diagrams show the coils in parallel).


pierodn wrote:The electronic injection (Boyer/Pazon) can be used with both coils from 6V or 12V too?.
I have some confusion because I'm studying now the electrical wiring system of the BSA 1969 Rocket 3 and I understand that it has 3 x 12V coils in parallel (right?) but if you want to fit the electronic  injection must replace them with 3 x 6V (in parallel = 6V!!!!) or (3 x 4V in series = 12V).


3x 6V coils in series will probably work well enough, but 3x 4V coils in series is to be preferred (3 x 4 = 12).

dave M wrote:I must admit I don't know why this was done, although it is common practice in automobiles to run a 6v or 9v coil with a ballast resistor which is bypassed while the starter circuit is engaged.


Therein lies the clue, as Norton did intend to fit an electric starter to the Commando for 1971, the same year the ignition system changed from two 12V coils to two ballast-fed 6V coils. The electric start 850 MkIII did make use of the ballast bypass system for starting (as did the electric start Triumph T160 which had 3x 6V coils with ballast resistor).
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Re: Coils (6V /12V) in series or in parallel

Postby dave M » Tue Nov 12, 2013 6:21 pm

Thank you Worntorn and L.A.B. this discussion has been most illuminating and I have learned something from it.

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Re: Coils (6V /12V) in series or in parallel

Postby Matt Spencer » Wed Nov 13, 2013 12:43 am

" Coils (6V /12V) in series or in parallel " / best in garbage bin , actually . Theyre pathetic wimps of things , particularly the 6V .
Though had a chap tell me he had a genuine motorcycle Lucas ' Sports ' Coil ,

This is BAD : theyre so old theres NO picture on Google . They had a blue checkered band painted aroud them .
They threw a half inch blue/ white spark . WHICH IS ESSENTIAL to a resonable performance .
A weak 10 mm yelloow / red sparks pathetic .

Newer coils may be slightly improved, perhaps . Cycle Worlds ' big coil test ' went for 10 dollar K Mart coils as best , for their Ducati S S Cook / Nielson thing .
Lucas coils rated worst & weakest of all tested .

Which explains those farty old unit Triumphs . :P :x :wink:
The Japanese response to ' styling ' , was to add more .
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