Author Topic: Newbie question: How do coils work?  (Read 372 times)

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Offline roach374

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Newbie question: How do coils work?
« on: March 07, 2020, 08:11:03 am »
While rewiring my bike, and replacing the points with a dyna ignition, I encountered a weird (for me) issue. I turned the bike on an off a bunch of times to test various things (with the headlight never connected), but never started the motor. Eventually the battery died, which I thought was weird.

I suspect that the coils are basically charging up every time I turn on the bike, and drawing a bunch of current. But I want to confirm / refute my hypothesis, and I don't really know how coils work to generate spark. Are they charging up, then discharging every time the points / timing ground them? When I turn on the bike, but don't start the engine, are they continuously drawing current, or only drawing current initially to charge the coils, and then sitting there (because the points aren't grounding them)?

Basically I want to have an intuition of how long I can keep the bike on without running the engine, and not risk killing the battery.

Offline HondaMan

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Re: Newbie question: How do coils work?
« Reply #1 on: March 23, 2020, 05:49:16 pm »
The coils are arranged as a Kettering ignition system. It works like this:
A coil consists of a metal core (iron or steel) with 2 separate sets of wiring wound around it. In our coils, the points-side wiring (primary side) has about 250 wraps (turns) of wire around it. Over that, there are about 3000 wraps of [thinner] wire that comprise the secondary (high voltage) side.
One side of the primary side wiring coil is connected to battery power. The other side is connected to the points. When the points close, the power passes through the primary, then the points, to ground. This current sets up a magnetic field, much like in an electromagnet, in that metal core. The secondary side wiring also 'feels' the magnetic power (called 'flux') as it builds up: it takes a short time to charge this up, which we call the "dwell" time.

When the points open the power is interrupted, which instantly cases a collapse of the magnetic field. This is a very sudden release of the energy that charged the core up: as it collapses it puts the electricity (electrons) back into the windings. The many, many turns of the sparkplug's side become a very high voltage (about 1.2 volts per coil turn of wire, around 7000 volts) and since this end is connected to the sparkplug, it jumps the plug's gap to ground. Out the other side of the coil, the fewer points-side (primary) windings also make a spike of voltage (about 400 volts in our coils) that must be soaked up by the condensor, or else it will arc across the points and start burning them up. When the points close again, this stored-up energy in the condensor gets bled back to ground through those points as the coil begins to charge up its magnetic field again.

That's a little oversimplification, but essentially how it works. When the points close, they 'charge' the coil: then when they open the coil discharges to make a sparkplug fire.
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Offline Tom C

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Re: Newbie question: How do coils work?
« Reply #2 on: March 24, 2020, 03:53:55 am »
It's funny, I was just reading in Hondaman's book yesterday the description of how coils work.  The book has a much more detailed explanation than his reply above.  I put "check plug resistances" on the to do list for my CB550. 

...and when I did so I discovered why my #3 plug was fouling:

Time for new plug caps!  I'll probably replace the coils and wires as well.  Any suggestions?

Hondaman's book is a great coronavirus quarantine read! :)
« Last Edit: March 29, 2020, 09:04:56 am by Tom C »
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