Jim Muller writes:
> Some time ago, maybe a year?, I saw an explanation of why there is a ballast
> resistor in the coil circit. I thought the explanation was here in SOL.
>
> I am just curious, no other reason, but I remember it was more than just the
> obvious explanation of "well, you need to cut back the current or it will
> overheat the coil". This explanation was more along the lines of why you
> had to use a coil "intended" to run on a lower voltage than 12 anyway when
> the rest of the car is a 12 volt system. Could someone post that again?
It was in January 1991. I saved 4 articles, they are attached below.
Unfortunately, I didn't save the original article which contained the
Midget wiring diagram that is referred to.
---------------------------------------------------------------------
T.J. Higgins 205-730-7922 | higgins@ingr.com (Internet)
Mapping Sciences Division | uunet!ingr!higgins (uucp)
Intergraph Corp. M/S IW17A6 Huntsville, Alabama, U.S.A. 35894-0001
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Date: 2 Jan 91 15:35:02
From: Jerry Kaidor <uunet!engtwomac.synoptics.com!Jerry_Kaidor.ENGINTWO>
Subject: RE>Re- ballast resistor
Reply to: RE>Re: ballast resistor
>QUESTION :
>Does anybody know hoe the ballast resistor is wired in ignition
>circuit of a late Midget ?
I don't know what kind of electronic ignition system is involved here, but
with Capacitive Discharge ignitions, a ballast resistor should not be
necessary. See, the idea of the ballast resistor-lower-voltage-coil
combination is to get the coil to saturate faster. The ballast resistor
converts the 12V "Voltage Source", into a half-assed "current source". The
idea is to cram maximum operating current into the coil as fast as
possible when the points close.
A coil of wire ( an inductance ) has the property that it wants to keep
a constant current flowing through it. If the current flowing through it
previously was ZERO, the coil would like to KEEP it that way. It does this
by behaving like an open circuit at the instant that voltage is first applied.
Ballast-resistor systems make it possible to get the coil up to current
faster. When the points first close, the coil seems to have high resistance.
Therefore the current through the coil, and through the ballast
resistor, is small, and the voltage drop across the ballast resistor is also
small ( voltage-across-resistor = current THROUGH resistor * resistance OF
resistor ). So for that tiny fraction of a second, the FULL TWELVE VOLTS is
applied across the coil, bringing it up to saturation much faster than if you
had a higher-voltage-coil straight across the twelve volts. This system would
work even better if you had a higher voltage and a bigger resistor, but 12V is
what we've got, and it's adequate.... When the
coil saturates, its current stabilizes at the design point, which is determined
by the internal DC resistance of the coil, and the value of the
ballast resistor.
The ballast resistor is shorted out while starting because the battery
voltage typically goes down to 8-9 Volts anyway, due to the enormous
load imposed by the starter motor. Also, the motor is turning very slowly,
and there is plenty of time for the coil to come up to saturation anyway.
Now, the way CD ignitions work, is, they use a DC-DC converter to boost
the 12V to some 400V DC. They use this 400V to charge a capacitor. When the
points open, a transistor/SCR switch connects the capacitor to
your ignition coil, and dumps its charge into the coil. This typically occurs
an order of magnitude faster than the constant-current-ballast-resistor system
can charge the coil. Also, the current output is limited by the charge in the
cap, and the internal resistance of the ignition system. If you use a ballast
resistor with a CD
ignition, you will probably lose the main benefit of the ignition, which is
the extremely quick dump of energy into the coil. This fast energy dump
translates into a fast spark that tends not to "foul out". Of course, I don't
know if the ignition in question is a CD unit, or just a set of "electronic
points". In the latter case, the resistor might be necessary, and for the
exact same reason as with the mechanical points.
- Jerry Kaidor
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Date: Wed, 2 Jan 91 13:23:43 EST
From: uunet!bu.edu!Stardent.COM!jmuller (Jim Muller)
Subject: ballast resistor
Gerry,
Your ballast resistor picture is not quite correct. It would typically be in
series in the primary circuit. Since most systems have a feature to bypass the
resistor during starting, they would typically look like:
key resistor + -
+12 o--->|-----o\/\/\/\/o------o o--------o--->|-----(earth)
| | .!----!. points
| | | | in
o--->|--------------- |______| distributor
| starter coil
| position
---->|----------------->to starter solenoid
The idea is to limit the current in the coil during normal running, but not
during starting.
An electronic ignition module is just an ordinary electronic device that
wants to have its full allotment of voltage, so you want to feed it an
"unobstructed" 12 volts. Whether you still want to protect the coil with
a ballast resistor is another story, probably depending on the coil. If
the Allison system includes a coil and it still says no resistor, then
don't use one at all. Otherwise your picture should be ammended to be:
-------- Black
|Elec |-------------------------------|
|Ign | Red |
| |------ |
-------- | |
| | |------>--|+ve | -ve
| | | ---------------
| | ---- | |
Earth | | | | |
| | | | Coil |
| | | | |
| | | ---------------
| | |
| ---- <--ballast resistor
| ^
| |
-----o
|
12Volts
The same starter switch arrangement should also be provided to bypass the
resistor, giving full voltage to the coil during starting.
Perhaps (probably!) someone will give particulars for that year Midget, but
this is how a ballast resistor would typically apply to a normal distributor.
I am guessing at how the electronic ignition module wants to work, but this
makes sense, especially with your quote from Allison. It is quite possible
that there should be none at all, even wired coil-only. But you certainly
don't want the resistor in the 12 volt line feeding the E.I. module.
Jim
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From: "barry klawans" <uunet!sybase.com!barry>
Date: Wed, 2 Jan 91 14:12:10 PDT
Subject: Re: ballast resistor
>QUESTION :
>Does anybody know hoe the ballast resistor is wired in ignition
>circuit of a late Midget ?
>I have looked at several wiring diagrams and none show a ballast resistor.
>The coil says to use one and the Allison Elec ign instructions
>say "NEVER RUN WITHOUT A BALLAST RESISTOR" - yet I can not find
>a wiring diagram anywhere !!!!!
Well, I can't speak from experience about a Midget, but I have run into this
installing a Lumenition ignition on both my TR-8 and a friends Lancia
Scorpion. (The
Lancia has convinced me that there are worse things in this world than Lucas
electronics.) Most post 1960 cars (which may not include a Midget) use an ~8
volt ignition system, and shunt 12 volts to the coil only when starting. The
purpose
of the ballast resistor is to drop the voltage from twelve to eight volts. If
the
Allison ignition is similar to the Lumenition one, it needs twelve volts to run
on,
but is designed to drive an eight volt coil. (It also sounds like your coil is
designed for eight volts - thus the warning sticker.) There is a way to
determine
whether your current coil is designed for eight or twelve volts by measuring its
resistance, but I'm afraid I can't remember what range to expect for each coil
type.
Maybe someone else can help out.
Here is my version of your diagram. Note the change in where the ignition gets
it power.
-------- Black
|Elec |----------------------------------------------|
|Ign | |
| | Red |-------------------| |
| |-------------| ballast resistor |--| |
-------- ^ |-------------------| | |
| | |+ve | -ve
| | ---------------
| | | |
| | | |
Earth 12 Volts | |
| Coil |
| |
| |
| |
---------------
Does your Midget have a ballast resistor? If not, you may have trouble
converting to
the Allison unit.
-Barry Klawans
barry@sybase.com
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Date: Wed, 2 Jan 91 17:22:09 EST
From: uunet!bu.edu!Stardent.COM!jmuller (Jim Muller)
Subject: ballast again.
I reread my note and gerry's, and I musta's been thinking his note said the
Allison note said *don't* use a ballast resistor. It actually said *do*
use one. I think what I wrote still holds. The ballast resistor is supposed
to limit the current in the coil, not reduce the voltage to the module itself.
Just setting the record straight. Someone else have anything to add?
Jim
(Sorry for the misteak. mistake? miztake?)
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