----- Original Message -----
From: "Aaron Whiteman" <aaronw@wsu.edu>
To: <mgs@autox.team.net>
Sent: Monday, February 18, 2002 12:18 AM
Subject: Mystery of 2 resistors solved (Was: Lucas Sport)
> ... Whatever, I know this must be right, the
> coil
> needs 12 volts, and a resistor isn't going to give it 12 volts, right?
> Err, yeah,
> but what about that alternator?
Er, what about the alternator?
> 2 random comments cropped up over the last couple weeks.
> 1. Mallory ships a ballast resistor with their coils (or at least some
> of them).
> It is to be installed "in all cases". Even the 12V (which mine was)
> has one.
> 2. A running car is not a 12V system. It is 14V.
>
> So, with this in mind, I started playing with the voltmeter. Sure
> enough, while the
> car is running, the coil is getting 14V. Hmm. I install the extra
> resistor that
> came on the car, this time to the white wire I am running to +ve (rather
> than the
> resisted wire in the loom). Plug it all together, I get 12V at the
> coil. Perfect.
>
> So, the moral of the story: If you have a 12V coil, you _still_ might
> want a
> ballast resistor (though not the stock one of course), because that coil
> getting
> 14V (which may be too much).
Many components on a car are labelled '12v' and as you say the actual
running voltage is more like 14v, but all these other components don't need
resistors in series with them, the '12v' is just a nominal designation to
differentiate from '6v' and '24v', for example. Even a '12v' battery isn't
12v given that it has 6 cells each of a nominal 2.2v.
You can't use a voltmeter - analogue or digital - to measure the voltage at
the +ve terminal of a coil that has a ballasted voltage supply and expect to
read anything specific. At best an analogue meter will display an average
voltage, a digital will probably never give a settled reading at all. This
is because the points are opening and closing so the the coil, and hence the
ballast resistor, will only be drawing current intermittently. A voltage is
only developed across a resistance when it is drawing current, so when the
points are open you will see exactly the same voltage on the coil +ve for a
6v coil with loom ballast as on a 12v coil without. When the points close,
however, you will see the nominal ballasted voltage which will be about 6v.
Factor in the running supply of about 14v and you will get 14v at the coil
+ve with the points open and 7v with them closed. The average voltage that
an analogue meter will display with the engine running will be something
between these two voltages, exactly what will depend on the relative lengths
of time that the points are open and closed i.e. the dwell. Now the dwell
(points closed) is 60 degrees on a 25D distributor (51 degrees on a 45D)
which on the 4-cylinder car equates to 67% (57%). So for 67% of the time
there will be 7v on the coil +ve (points closed) and for 33% of the time
there will be 14v (points open). 33% of 7v (the difference between the
closed 7v and the open 14v) is 2.3v, which when added to the 'points closed'
voltage gives us 9.3v as a measured average. For a 45D you would get an
average voltage of 10.3v. Coils vary in resistance, also according to
temperature, and the ballast resistances probably do as well, so these
voltages are approximate.
I can't comment on Mallory coils except to say that if they supply ballast
resistors to be used in series with a particular coil then that coil is
obviously not a 12v coil, and one should use the supplied ballast otherwise
you can expect an overheated coil and excessive burning of the points. A
measurement of the primary resistance of one of these coils would be
useful - 12v coils are 3 ohms, 6v coils 1.5 ohms, and a typical 12v sport
coil is 2.4 ohms. You *will* need a digital meter to measure this.
Factory 12v coils don't need an external ballast, factory 6v coils do and it
must be either the supplied loom ballast or an equivalent. IMHO it is
highly unlikely that any 3rd party coil should be supplied from the factory
loom ballast, whether it is supplied with its own ballast or not.
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