coils and resistance

[Tom Jennings <tomj@xxxxxxx>]

Um, here's some poop on ignition coil resistance and resistance wire  
in general:

Modern coils need to charge up quickly to operate at high RPMs. To do  
so they have fewer turns on the primary; at low speeds, once the coil  
is charged up, the excess current turns to heat -- lots of it. The  
series resistor does not slow down the quick charge-up, yet limits  
the excess current at low speeds.

Many cars jump around the resistor during cranking to give a little  
boost to the spark. It does pass excessive current through the coil,  
but not for long enough to do any harm.

(The value of these resistors is around 1 (one) ohm, not K, which is  
"1000". Most I've seen are 0.85 or 1.35 ohms.)

With a points car, if you turn the ignition ON (engine not running)  
and the points are closed, you'll see something 5V - 8V on the coil  
"+" terminal. That's because about 10 milliseconds after you turned  
it on, the coil was fully charged -- what you're seeing is the steady- 
state voltage, which doesn't even matter.

What you are NOT seeing is the current flow (not voltage, current)  
through the coil, and you can't see the total energy stored in the  
coil as a magnetic field, and that's all the matters.

Current flow through inductors (coils) and capacitors/condensors is  
dynamic, you can't see it with a voltmeter simply because it happens  
too quickly. You can see it on an oscilloscope.

I teach and demo this stuff as a part of my job.



Coils charge up when the points (or electronic equivalent) are  
closed, making current flow through the coil, building up a big  
magnetic field.

Ignition coils discharge, into the spark plug, when the points (..)  
open.