Dear List,
I am *truly* sorry if you are tired of this. I want to assure you of
this: it has nothing to do with a reluctance to concede I am wrong. (I
have been wrong before--at least once or twice). No, it is worse than
that, I want to understand.
On Thu, 19 Jan 1995, Jerome Kaidor wrote:
> 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."
OK. He is comparing a 6V coil supplied by 12V through a ballast, to a
6V coil supplied by 6V through no ballast. I agree, the former will
charge faster, according to the following reasoning:
An induction coil consists of two coupled coils, 1 and 2. Where dI1/dt is
the rate of change of current in coil 1, E2 is the EMF generated in coil2
and M is the coefficient of mutual induction,
E2 = M(dI1/dt)
So for any given physical relationship between the primary and secondary
coils, i.e. for any given M, the faster the current changes in the
primary, the higher the EMF generated in the secondary.
If we consider coil 1 as an inductor in series with a resistance, the
current as a function of time after a switch is closed connecting the
series combination to a voltage is:
I = (E/R){1 - exp[(-R/L)*t]}
where (L/R) = the time constant of the system, Tau. The mathematically
challenged need to realize that the term in the curly brackets is 0 at
t=0, and 1 at t=infinity. So the current goes exponentially from 0 to a
final value of E/R. The time constant Tau is the time it takes the
current to go to approximately 2/3 the final current.
Suppose a 6 V coil has a resistance R = 1.5 ohms and inductance L henries,
then:
Tau = L/1.5 seconds
The current in that coil goes from 0 to a final value of 4 amps with a
time constant of L/1.5.
If we take that same exact coil, and supply it with 12V through a 1.5 ohm
external resistor, then R = 1.5 + 1.5, and:
Tau = L/3 seconds.
The E/R term in the equation is the same, so for this situation, the 6V
coil driven by 12V through a 1.5 ohm ballast goes from 0 to a final
current of 4 amps, with a time constant half that required if the same
coil were driven by 6V with no ballast.
So Evanson's analysis (and Jerry's) appears correct, if one compares just
those two situations--a 6V coil driven by 6V with no ballast, vs. the same
exact coil driven by 12V through a ballast.
My argument, however, has never been with this comparison. Rather, I'm
saying start over, and design a coil with the same old inductance L, but
give it an internal resistance of 3 ohms. How? Just use smaller wire.
Now what do you have?
Well, I cannot see any way that this is different from the second case
above. R = 3 ohms. The final current is 12/3 = 4 amps. When a switch is
closed, the current goes from 0 to 4 amps with a time constant L/3. As
far as I can see, such a coil will have the same di/dt as a 6V coil in
series with a resistor, and therefore will generate the same EMF in the
secondary.
Aside from being unable to increase voltage during starting, the only
disadvantage I can think of to making the resistance part of the coil
winding is that there is more heat to be dissipated inside the coil.
Maybe a coil with greater internal resistance fails more often. Maybe
there is some subtle difference between having more of the resistance
residing within the wire of the coil rather than externally, but that does
not seem likely. If that were the problem, why not make 3 V coils by
using larger primary wire, and drop 9 V through the ballast? Wouldn't
that be even *better*?
I would guess that the primary reasons for the ballast were easier
starting, because the ballast could easily be bypassed when cranking the
engine. Perhaps it reduced heat in the coil. It may have allowed 6V
coils to be used interchangeably between 6V and 12V systems at a time when
there were a lot of 6V cars still on the road. But I *still* cannot see
why it would work better or different from starting with a clean sheet of
paper.
Sorry, list. You can imagine what being married to me must have been
like. Jerry, if you reply at all, maybe you should do it off the list.
People have probably had enough of this.
Ray
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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