Gee, this is kind of fun. Hope others agree, because I am learning all
kinds of stuff--now if I just believed all of it.
It is a case of the halt leading the lame, perhaps.
On Thu, 19 Jan 1995, Jerome Kaidor wrote:
> **** Uh, no. That's not how it works at all. Listen carefully,
> boys & girls:
>
> When you first hook up a voltage source to an inductor, it looks like
> an open circuit. This is because a reverse voltage (AKA counter-emf )is
> produced by the expanding magnetic field. As the magnetic field nears
> its full expanse, the counter-emf gets less & less, and the coil starts
> to look like a short circuit.
>
> If the coil is not given enough time to charge up, you don't get a
> spark. Remember, it looks like an open circuit when the points first
> close. How can we stuff those electrons into it faster?
>
> We can stuff electrons faster into most anything by using more voltage.
> Suppose we applied a 400V source to the coil? Bet we could saturate it
> real quick!
>
> Well, we don't have 400V available. But we do have 12V. And we can
> choke it down to 6V with a resistor, so the coil doesn't burn up.
Could we stop right there? Is there something magic about 6V, which won't
burn a coil up, and 12V, which would burn it up and therefore has to be
choked down to 6 to *avoid* burning the coil up? I'm not an EE, I dropped
out when left hand rules became more than my poor mind can absorb.
HOWEVER, I should think it is perfectly feasible to design a coil that
will not burn up when the primary is run on a constant 12V (the obvious
way would be to cause the primary wire inside the coil to have a higher
resistance than that of coils designed to operate with a ballast
resistor), and that such a coil could be made to "charge up" at least as
fast as a series combination of a resistor and a 6V coil.
I would appeal to your common sense--if there is some major theoretical
value to the external resistor, why are any coils designed to operate
without the external resistor?
Either there is something more subtle involved here, having to do with
phase angles, back emfs, power use, etc., which may well be the case, or
the main advantage of having the external resistance is simply that it is
easy to bypass it during starting.
Jerome continued thusly:
> However, at the moment when we first connect up the coil, and it looks
> like an open circuit, the RESISTOR DOES NOT DROP ANY VOLTAGE WHATSOEVER,
> AND WE HAVE A FULL 12V ACROSS THE 6V COIL. Remember, resistors drop voltage
> by the virtue of current flowing through them ( V=I*R ). No current flowing
> through, no voltage drop. This makes the coil "start to charge" much faster.
Yes, but if there were no external coil, we would also have a full 12V
across the primary when the coil is first connected.
> Us electronics weenies would say that the resistor converts the car's
> 12V into a "current source". In other words, a source that wants to deliver
> a specific value of _current_, unlike the car battery, that really wants
> to deliver 12V regardless of the current ( or lack of same ) pulled out
> of it.
Not much of a current source, Jerry.
> Bet everybody's *really* confused now :-).
You got me to open my physics book. I burned my EE books. Now if you can
explain why inductive light dimmers use less energy than resistive light
dimmers, I will be impressed.
> - Jerry Kaidor ( jerry@tr2.com )
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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