> But why does it work? I don't understand why increasing the resistance
> across the 'charge light' circuit makes it kick in sooner.
>
> Right after start up, there's enormous demand for juice, as the battery
> has just shot the wad starting the car. I didn't know that the charge
> light circuit had anything to do with when the regulator engaged.
In order to generate electricity, alternators (and generators for that
matter) also consume electricity, known as field current for generators,
rotor current for alternators. Most are set up so that they supply their
own field/rotor current, but that leaves a "chicken and egg" type problem as
to how they get 'started'. With generators, since the field is also part of
the heavy iron housing, the housing becomes magnetized to some extent,
enough to generate the small amount of current necessary (since the
generator output is not connected to the battery at that point).
Most alternators cannot do this, and so need an external source of current
to get started. That 'starting' current is supplied by the 'ignition' lamp.
Adding the resistor actually lowers the resistance (since it's in parallel
with the lamp), which increases the current through the rotor when the lamp
is on, and allows the alternator to get started at a lower rpm. Once the
lamp is out, the resistor no longer makes any difference (there's no voltage
across it so no current flows), thus it doesn't actually increase the
available output at low rpm, just ensures that you get whatever output the
alternator is capable of.
BTW, I don't know how they do it now, but early AC alternators (like those
used to generate power for your home) also required DC excitation as it's
called. They used to actually run a DC generator from the same power
source, whose only purpose was to supply the rotor current to the
alternator. The field current to the generator controlled the rotor current
to the alternator, which controlled the main output voltage.
Randall
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