Advanced pulse coil question

Windmill John

Windmill John

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Right, can someone answer a question that I don’t need to ask.. Always wanted to know the answer since getting my Tony Tranter Haynes electrical book decades ago…
Now, the answer should be in single syllables 😉 and layman’s English…
I can provide pictures of the system from Tony’s book, but it’s the same diagram in the Clymer manual. And frankly, whoever answers this question will probably know the system.
Okay, question time; be gentle… I understand the layout of the CDI system and understand the principles for obtaining spark. What I don’t understand and cannot get my head round is:
The pick up magnet generates a pulse in the fixed pulser coil. (Now about to quote from the book.) The characteristic of the spark pulser coil is that it gives a broad + pulse which rises steeply as the engine speed rises.
As speed rises, the voltage from the spark advancer coil cuts the thyristor trigger line before the fixed pulser.
How?… By the way, that’s the question 😊
How does it trigger before the fixed pulse coil if that passes the fixed pulse coil first? Is It to do with rise times? No idea why I asked that!
If my question is not clear, I’m trying to ask how the advanced coil triggers before the fixed coil.
I apologise now if you answer and I’m still confused.
 
The image below comes from the FSM. I am not super familiar with this setup, but I'm thinking of it like this: when the rotor moves faster the coil produces a higher voltage. This happens over the same range of angles on the rotor, leading to steeper slopes. The net effect is that the advancer coil produces a pulse that crosses the trigger threshold earlier than the fixed pulse when the speed is sufficiently high.

1654.png

One could make the analogy to valve clearances. When you decrease a valve tappet clearance, the valve opens earlier and closes later. Here, the increase in pulse voltage/slope allows the ignition to trigger earlier.
 
Thanks both, very helpful. I guessing/assuming/hoping/understanding that the advanced trigger coil must have more windings than the fixed pulse pickup. Obviously faster speed would cause both to speed up, so hope it is windings and field etc.
 
I’d forgotten that I’d downloaded the factory manual. In it, it quotes ‘replaced’, see below.
I think I wanted slightly more understanding than ‘replaced’!

From manual:
3. To advance timing, the signal from the fixed pulser is replaced by the output signal from the spark advance pulser.
 
Windmill John said:
Thanks both, very helpful. I guessing/assuming/hoping/understanding that the advanced trigger coil must have more windings than the fixed pulse pickup. Obviously faster speed would cause both to speed up, so hope it is windings and field etc.
Click to expand...
I'm out of my depth here, but I believe the reason the fixed pulse coil is called fixed is because it does not vary with engine speed.
 
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