[DML] "Alternator Light"
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[DML] "Alternator Light"


Just to clarify wording regarding the indicator light for the alternator.
It serves two functions:

	1.  The primary function is to indicate that the alternator has

	2.  The secondary function is get the alternator started generating power
when required.

Discussion of 1. The indicator lights when the ignition is on and the engine
had not been started, and thus the alternator is not functioning OR when the
engine is running and the alternator is not supplying power to the system
(not functioning).  The indicator is wired between the 12 V bus, which is
supplied by the battery when the ignition is "on" and the field (rotating
portion) of the alternator.  As the field has little or no residual
magnetism, the alternator will not supply power to the system at the time
the engine starts.  The regulator for the alternator is powered by the
output of the alternator output but is isolated from the battery by diodes
which will only allow power to go the regulator when the alternator is
making some.  Thus the second function:

Discussion of 2.  When the engine is first started, without the indicator to
provide some small amount of current to "excite" the field, the alternator
would not provide power to the system.  When the field starts to have
magnetism due to the excitation of the indicator lamp, then the regulator
begins to function and control the field and thus the alternator's output!
The field of the alternator has little or no residual magnetism.  .

Clarification of terms:  The field of an alternator is the rotating element,
and the armature is the wiring in the body of the alternator.  As opposed to
a generator where the field is the non rotating component, and the armature
is rotating.  Hence, alternators have brushes which contact slip rings, and
generators have brushes which contact commutator rings (which act as

Why this long discussion?  The indicator light does not provide regulation
for the alternator, and in fact is excluded by diodes from doing more than
Function 1 once the engine is on and the alternator is functioning.    I
want to make clear that the engine will run for some time without the
alternator (depending on the battery's condition and functions being used).
Thus if the indicator lamp or bulb were not there at all, the engine would
still start;  it just would run for a long time, (depending on the
electrical load and battery state) probably hours.

Now as to the type of indicator.  I have what papers to be a factory
supplied Motorola alternator in my car and I have an LED indicator in the
"alternator"  position of my dashboard.  I have driven the car for many
thousands of miles under all kinds of battery load condition, without having
to recharge the battery.  Therefore I conclude that the Motorola alternator
starts and  will work fine with an LED in that situaton.  That is to say,
the relatively low current that can flow through the LED is sufficient to
start generation.

John H. and I have had lots of discussions about this with regard the extra
power alternators which he sells.  He claims that those alternators will not
work with an LED in the indicator socket.  I can't understand why not, but
concede the point to him.  A resistor to shunt the LED and thus provide
sufficient current to excite the alternator might be a solution.  But also
defeat the purpose of the LED.

Another couple of comments.

The regulator for the alternator does not control the current output.  It
only controls the voltage output.  This is fine, as the diodes isolate the
alternator from the system if its voltage output is below the battery
voltage.  The only current regulation is provided by the resistance of the
fixed armature windings.  When the current output from the alternator
increases, the resistance of the windings increases.  At some point the
regulator cannot provide enough field current to over come the armature

As I know that many of the people who look at this forum has other cars,
some of which have generators for the charging supply.  These devices were
generally designed before the wide availability of solid state devices.
Thus the generator is controlled by electro- mechanical means.  Another way
to look at the situation is that the generator is really and alternator
which uses the commutator to convert the generated AC voltage to DC.  I
mention generators only because, they were self starting as the field
winding was static and wound on a core which had residual magnetism.  Thus
when converting from positive to negative ground the field had to be
"sparked" (thus polarizing the core correctly) to get the generator to be a
source of power as opposed to a sink.

Please refer comments and corrections  directly to me so as to not take up
forum time and space.

Al Roberts
VIN 16049


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