Simon,
Nothing like a good discussion, ;^)
Simon Matthews wrote:
>
> I don't think anyone has got the full explanation of the role of the
> capacitor quite right.
>
SNIP
> Now before anyone says: 'but this means if you have a steady DC primary
> voltage, then you will have a steady DC secondary voltage', I will say:
> true,
Uh, nope. Given a steady current through the primary (steady state,
points closed), there's no induced voltage in the secondary.
SNIP
>
> As the points open, the primary current suddenly begins to drop.
Nope, not until the condenser is almost charged up to the supply
(read battery) voltage.
SNIP
> As the capacitor charges, the voltages rise in both primary and secondary.
Nope, The voltage across the primary was already at the supply
potential. No place to go but down. Er, until the back EMF takes over.
> As the voltage across the capacitor rises, the RATE of CHANGE of current
>increases.
Fair enough, another way of saying the current thru the primary begins
to decrease rapidly.
> This continues until there is a spark across the plugs. Current can now
> flow through the plug to ground. The supply path is ultimately via the
> battery,
Dunno how you can say this when the primary circuit to ground is
opened and the back EMF has risen to a value higher than the battery
voltage. Take a look at the voltage waveforms on Barney's homepage.
> The Points Capacitor may supply some of the current, but there
> is no reason why it should supply all.
Sure there is: The coil secondary and condenser form a series-tuned LC
circuit which is in its own little universe after the points open. The
spark lasts until the tuned circuit's energy is dissipated below a
certain point. After the points open yer looking at an AC circuit, not
DC. It has laws of its own. And things are happening fast, according to
the LC time constant of the circuit. Take a look at the current
waveform on Barney's HP; you'll see that the primary current actually
reverses. The small oscillations at the end are open-circuit ringing,
the spark is over.
Damn, I wrote another book. (G)
CR
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