In a message dated 97-06-21 16:02:36 EDT, tboicey@brit.ca (Trevor Boicey)
writes:
> The catch is, the faster you bring the voltage from 12 to zero,
> the higher that voltage will be. The idealized system with
> no resistance and no capacitance will produce an infinite voltage,
> the real world is much less.
>
> The electronic ignition can make a cleaner wave edge from
> 12V to zero. No mechanical system like points can keep up. What
> happens is that the points together are a good conductor, the
> points apart are a good insulator, but in the microseconds they
> are seperating, there is partial contact from fuzz, ionized
> air (spark) and so on. This means that the transition from
> current flow to no current flow is not ideal.
Trevor:
Close, but not quite!
First, if you take out the capacitor, the coil will produce a very weak
spark. So weak, in fact, that the engine will not run. The capacitor is a
vital part of the circuit, not just to prevent point burn. While it is true
that the faster the primary winding field collapses, the higher the output
voltage from the secondary, you reach a point of diminishing return as the
collapse time drops. If the decay time is too fast, a lot of the energy in
the output is in the form of RF energy, which is dissipated in radiation,
rather than in the spark. The coil and capacitor combination is "tuned" to
provide a definite collapse time, much slower than the theoretical limit.
Even though electronic components may be able to switch faster than points,
there is no need to. The points are more than fast enough.
At the instant the points open, the capacitor looks like a dead short to the
coil, so it doesn't even know that the points have opened. From then on,
until they close again, the points might as well not be there. The capacitor,
not the opening time of the points, controls the decay time.
If you replace nothing but the points with an electronic module, the only
benefit, albeit significant, is timing accuracy and longer maintenance
intervals. To get a performance boost, you must swap out the "system."
We beat this subject to death over on the Triumph list a while back. Bob
Sykes and I wrote a brief summary of the operation of the ignition sytem as a
result. If you would like to see it, go to:
http://www.vtr.org/maintain/ignition.html
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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