AJH wrote:
We seem to have less electronics expertise than in the past, what
hppened to Andrew Gabriel?

I have one of the cheap 2T 650W gensets based on the Yamaha 50cc
capacitively excited generator handed me by a mate to fix, a NUTool .

It won't start because the HT is too weak to produce a spark and I
currently put that down to loss of magnetism in the flywheel magnets.

These things are cheap low quality copies and generally not worth
repairing but I also have my first machine saw from 1974 that exhibits
the same problem. They are both capacitor discharge ignition.

Any suggestions for a simple battery powered circuit which could sense
the trailing edge of the pulse to the coil circuit I and use that to cut
the current running through the coil and produce a decent spark?

CDI Capacitor Discharge Ignition Circuit Demo

https://www.youtube.com/watch?v=0yK3Opq_i0M

Primary side V
| ___
| ___| |
| ___| |
| ___| | ___
| ___| \ ___|
| ___| | ___|
| __| \_|
+-------------------------------------------- Time
excitor charges C, ^
SCR dumps to coil |
spark on
falling edge

The excitor cannot raise the capacitor voltage too
high, or the output of the ignition coil could rise
to too-high a voltage and damage the insulation in
the ignition coil. As drawn in the Youtube video,
the circuit does not have any regulation and runs
open loop. And relies on the ratio of excitor pulses
to firing pulse, for correct operation. There are
ways to fix this (add some regulation), but those
ways would also add failure modes to the design.

Since the SCR drains the capacitor exactly (SCR
only stops conducting at a low voltage on that
falling edge), the capacitor starts from a
known state after a firing. It can't "drift off"
or anything. The charging stairstep starts
at close to zero.

So then the question would be, what needs fixing
in the circuit ? Which part has failed, and what
do we need to fix ? If a semiconductor fails short
(as diode-like devices tend to do), a failure to
function could just be a semiconductor problem.
It doesn't always have to be a magnetics failure.

It's the kind of circuit, you'd need to see an oscilloscope
picture of it working, to know how to deal with failures later,
as the picture would tell you what a "normal" peak voltage
on the primary is. The ignition coil has some turns ratio.
You know you need a certain voltage to make a sufficiently
hot spark, and so on. But seeing the waveform, and
comparing to the current waveform, hints at how big
a "fix" is needed, and what's going on. If the thing
remains "flat" as you try to start it, then something
has failed short.

I've been thrown across the basement floor by a
spark coil, so you don't have to tell me about
safety or anything :-) When I came to, my first
question was "how did I get over here?". That's
how far the shock made me jump. Stay safe...

Paul