Hi Glyn
Yes, that should be ok.
So long as you realise that the capacitor won't run the lights for
long without the engine running.
It won't run them for very long at all without the engine running.
In fact the capacitor may run out of charge before the engine has
come to rest after switching off.
The 'lighting coil' gives quite a 'lumpy' output, generally these
run of the same magnet assembly that the ignition system uses,
and for those who don't know, its not evenly spaced around the fly
wheel. Its very much all over one side, the fly wheels are usually
cast with a shape that will even out the weight distribution and also
machined to balance up. The magnet assembly is actually two magnets
positioned near together so that between them there is a fairly high
magnetic field. It is this that is used, in conjunction with the points
to make the ignition spark. Thats why the flywheel has to be positioned
with respect to engine tdc.
I mention this to show that the lighting coil, which uses the same
magnets, gets a very lumpy voltage. What it gets is a sort of spike,
every revolution of the engine. Now when its driving filament lamps
that matters little, it will even out as an RMS load lighting the
lamp. When the engine revs are low, like tick-over the lights can be
noticeably flickering.
If you want to fit a capacitor on this i would mention two things,
firstly with the lights off, or failed lamps, the cap will charge far
too highly. I suggest a ten or twelve volt zener as limiter, so that
it should not charge to an awkwardly high voltage, which could pop
the lamps if switched on when its charged, or blow the capacitor if
not restrained.
Secondly i suggest you use a diode bridge rather than one diode so
that you get the best use of the output from the lighting coil. Using
just a single sided output will give a higher spike to output ratio.
You will probably find one side of the lighting coil is grounded, so
to use a diode bridge you would have to free that end, to make the
coil 'floating' that is with both ends free.
Using a capacitor will give a much better light from the lamps,
i think that this is something that a lot of people do with these
types of bikes.
Best of luck with it, John
just seen russlk's post, he has done this too with 3000MFD, i would
have guessed at more like 10,000, but see how it goes - John