Re: coils and resistance

[farna@xxxxxxx]

I'm just stepping all over myself on this one! I KNOW it's 1.0-1.35 ohms, not K ohms, but wrote it with a "K" anyway!! A ballast resistor/resistance wire the only application I'm familiar with that isn't in hundreds or K (thousands) ohms -- habit prevailed!! 

On November 8, 2005 Tom Jennings wrote:

> 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.


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