Coils and resistors, was Re: Part needed, Ballast Resistor? no, radio noise suppressor

[Tom Jennings <tomj@xxxxxxx>]

On Fri, 19 Aug 2005, Sandwich Maker wrote:

> nb. i've never taken an old coil apart, but i don't think it'd have an
> internal resistor because of the heat factor.  i've always assumed
> that the primary was just wound with more turns, for more resistance
> and less current draw at 12v.  the spark wouldn't be as hot as a
> regular coil at 12v but it could match the regular coil through the
> regular ballast.

It's just more turns of wire on the primary to make a higher
resistance.  There's no internal resistor or ballast for the
reason Andrew says.

When designing (or choosing) a coil, there are a bunch of factors
to juggle.

"Low resistance" coils have fewer primary windings, they draw more
current on average (points-killer), they charge up faster so
they're better at higher RPMs, and it's easier and cheaper to make
a high primary:secondary ratio for higher voltage.  The catch is
they eat a lot of power and get really hot.

Coils take time to build up the energy that makes the spark; fewer
turns --> lower inductance --> faster charge time. That part is
good. But at lower RPMs, after the coil charges up, the rest of
the "on" time of the ignition just causes heat.

A compromise for designers is to use a low-resistance coil with a
resistor. The coil still charges quickly (low inductance) but once
it's charged up, the additional current from low RPM "on" time is
a lot less, and much of it makes heat in the resistor instead of
the coil.

For low speed putt-putt motors, where 4000 rpm means a missed
shift, a high-resistance coil, often with a resistor or resistance
wire in the harness, is "good enough" and is more reliable due to
the lower currents and temperatures.


Coils get their hottest with the engine at low RPMs; the coil
charges up, the rest of the dwell time is spent making heat.

Coils run their coolest at high RPMs, where the coil has just
enough time to charge up, and nearly all of the energy stored in
the coil ends up in the spark plug gap. That could be 90%
efficient at a really narrow RPM band.

Smart ignitions could do essentially a constant dwell time to
charge the coil at low speeds and avoid the excessive heat; it
would mean picking the coil ON time backwards in time (eg. prior
to the next cycle, not a time machine :-) so that at "points open"
time the coil is 100% charged without all the stupid waste of
heat.

That'll happen when power budgets get tight enough. My TFI
Motorcraft ignition consumes about 60 - 70 watts; half that goes
to heating the passenger side spring tower via the resistor. I'd
guess 20 watts more is heating up the coil and Duraspark box, and
the remainer making a spark.


A 'ballast' or 'ballast resistor' is a resistor that changes
resistance with temperature; usually the resistance increases the
hotter it gets. Some metals do this.

THe reason is that when the engine is cold, there's more spark
(for the bad old points days). As the ignition stays on longer the
ballast gets hotter, current drops.

The "resistance wire" buried in the harness by cheapskate
manufacturers (no one we know of course :-) might be a ballast,
but I doubt it. I'm not sure any car after the 1950's has an
actual ballast.

Nearly all resistors for ignitions are plain, fixed, resistors.
Around 1 ohm plus or minus 1/2 ohm. The word "ballast" is buried
deep in car culture so it won't go away any time soon, but
basically it's just a resistor.