W. Ray Gibbons wrote:
>
> 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:
>
> > 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?
**** Sure. The *magic* thing is that with the you get to run a _6V_ coil
off the 12V. Just for a moment, til it charges up.
> 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.
**** Nope. You can make a 12V coil, sure. But no way will it charge up
as fast as the 6V coil/resistor combination. Remember, the coil looks
like an open circuit at the moment that the points close!
>
> 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?
**** Well, there's cost, for one thing. Also, if you take apart a
"12V" coil, you may well find that there's a resistor built into it.
And who ever said that car makers always use the best systems? For example,
my TR2 uses a worm & pin steering box, even though rack & pinion is
clearly better for a sports car. Even then, overhead cams were known to
be superior for high-revving motors, yet both my `2 and MGA have pushrod
engines. One could go on & on....
> Either there is something more subtle involved here, having to do with
*** Nope.
> 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.
*** Yes, but it would have to be a ****TWELVE VOLT**** coil. And it would
charge slower than a 6V one running off a current source.
>
> > 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.
*** No its not, Ray. But it's a lot better current source than the
positive terminal of a battery. And its a good enough current source to
charge that coil fast enough to keep up with our engines.
If you don't believe me, maybe this will help:
Here's a quote from "Automotive Test Equipment You Can Build" by
A. Edward Evenson ( SAMS, 1972 ) page 35:
"The function of the ballast resistor, on those cars so equipped, is
somewhat of a mystery to many people. Actually, it serves two functions:
it REDUCES COIL SATURATION TIME (my emphasis) important for high speed,
and, when bypassed during cranking, increases coil output for starting."
He goes on to say:
"The ability of the ballast resistor to improve saturation time is
less obvious. It is well known that when resistance in series with
an inductor is increased, the current buildup or saturation time is
reduced. This is the familiar L/R time constant relationship. The old
6-volt coils generally had ample output voltage; their problem was in
not saturating quickly enough at high speeds. By retaining basically
the same coil design, adding a resistor to limit the current to a safe
level, and feeding this from a 12-volt system, considerable ignition
improvement can be made. Such a 12-volt ignition system can build up
in much less time than a comparable 6-volt system."
( end quote )
Another area where current sources were used to charge inductors
quickly was the teletype machine. These had a pair of electromagnets that
were pulsed by an incoming logic signal, and drove a really amazing
Rube Goldberg collection of wheels, bellcranks and code bars. The
voltage required to charge the "selector magnets" was 9V or so.
But simply applying the 9V directly to the magnets resulted in them
charging up too slowly, and common practice was to drive them off a
120V DC source with a big resistor in series.
- Jerry
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