After following this for a loooong time, I feel the need to add my $0.02
worth. partially because nobody has really explained the functions of
the components. Not being a designer of ignition systems, but of power
supplies, let me try my efforts to explain in some simple terms, what’s
going on here. Its not unlike a flyback switchmode power supply.
Using the schematic and waveforms on Barney’s site, follow along with
my version of sparks and other interesting phenomenon. do not follow the
MGA schematic.
To start with, for these purposes, I’ll use current flow (from + to -)
theory. Also please understand that the purpose of the coil is to store
energy, not the capacitor. In order fo the waveforms to be proper, the
windings are wired with starts ( or finishes) at the same point, the
"points" terminal of the coil. Therefore it is not used as an
auto-transformer, even though it may look like one.
Starting out just after the first plug firing, the points close. Voltage
across the points becomes zero. This causes the voltage to be imposed
across the primary of the coil. The current begins to flow from the +
terminal of the battery, thru the ignition switch, thru the primary of
the coil and thru the closed points to ground. And then to the - term of
the battery. The current (and magnetic field) builds throughout the
dwell time until the points are opened. The reason that you see the coil
current taper off at about 5 Amps is due to the ballast resistor and
primary resistance. These resistances limit the current in the circuit
to the voltage divided by resistance or 12/2.5 or 4.8 Amps. In our LBC’s
the coil has a ballast resistor built internally. (Perhaps it is just
wound with higher resistance wire.) During this charge time, there is a
voltage at the HV terminal of the coil which is approximately the turns
ratio times the applied voltage: 100*12v or 1200 volts. Don’t chastise
me for the use of 12 v here. It’s only for explanation. This 1200 volts
is not enough to arc over the gap between the rotor and distributor cap
terminal AND the spark plug gap. Assuming 0.010 gap at the cap and
0.025" at the plug, arcing would take somewhere around 70,000 volts per
inch in free air or 2400 Volts ( higher under cylinder pressures).
When the points open, with no more potential applied to the primary,
the field begins to collapse. This collapsing field induces a voltage in
BOTH the primary and the secondary windings. The voltage rises at the
open points and falls at the HV terminal of the coil. The voltages
continue to rise (fall) until something fails. In this case, it is the
insulating gap at the points and rotor. They "break down" or spark over
at about 30,000 volts. This causes the voltage at the points to be 300
volts due to the 100:1 turns ratio of the coil. Energy is depleted
during this arc-over until the magnetic field is collapsed
The purpose of the capacitor is limit the rate of rise and fall of the
voltages on the coil and consequently the points, NOT to provide the
energy to fire the plug. This reduces the sparking across the gap as
they open and thus extend the useful life of same. A properly designed
capacitor / coil will be designed for a specific time constant do do
some wave shaping. Too low a value of capacitance will eventually create
a metal spike on one terminal while too high a value will create a spike
on the other.
I do not claim to be a physics expert on the dynamics of the spark and
how long, or how much energy it contains, however the energy definitely
decreases as engine speed increases.
Please note that changing the plug gap wider without any other changes
will provide a higher voltage spark but also a higher secondary voltage
on the coil, perhaps exceeding the design limits of it’s insulation
system. Also it provides a higher voltage on the capacitor possibly
causing early failure. I have known some early CD ignition systems (Mark
III I think) to have so high a voltage as to be un-useable during damp
weather conditions. The distributor cap does not support the higher
voltages and shorts out. The CD ignition systems, when wired out of the
circuit, started the cars just fine!( they used the existing points.)
Sorry if this was too long an explaination but I put it in as simple of
terms that I can to describe what is happening. If I misspelled a few
words, then so be it. If I stepped on a few toes, it was not my
intention.
This will be my one and only discussion on this subject. Next lets talk
about why the points in the fuel pump burn out and why the capacitor
fixed it somewhat but the anti-parallel diode fixes it better. And how
about adding a transistor switch as a final solution.
Of course, this is just my opinion. yours may be different.
Chuck Scahefer
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