For an ignition coil to have uniform secondary voltage capability over
a wide range of engine speeds, complete saturation of its magnetic fields
must be developed at these varying engine speeds. Magnetic saturation
depends on the amount of voltage applied to the coil and the amount of
current flowing in the windings. It also depends on the length of time
during which the current flows.
An Automobile engine does not operate with uniform current or voltage.
When Cranking the voltage drops to ~10V. At high RPM the charging
system voltage is about 14V and the coil primary winding current flow
time is reduced to a few milliseconds. Uniform coil saturation must
develop under extremes of low voltage and short current flow. To
achieve this most ignition coils are designed to operate on 9-10 ohms.
To keep the primary voltage at the desired level a ballast resistor is
used to compensate for changes in temperature current flow and
voltage.
(someone else posted and IT'S TRUE) At low speeds the current flow time
is long and the resistor heats up, increasing the resistance thus
limiting the current. At high speeds the resistor cools and the
resistance drops. Higher voltage is applied to the coil, but the
shorter current flow duration results in about the same magnetic
saturation of the coil.
During cranking the ballast resistor is bypassed because cranking drops
the voltage to the desired level.
I hope this information helps. Comments welcome!
Mark Jurras jurrasm@torrington.com = =o&o
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