Message text written by INTERNET:Ct54531@aol.com
>My TR4A doesn't have an alternator but plenty of other TR's do so maybe
that
will count for LBC content in this question (which is in reference to a 95
Corolla).
What is the electrical dash light registering/reacting to? Is it the
voltage
coming from the alternator? Going into the battery? Draw out of the
battery?
Just wondering.
<
An alternator develops alternating current power in the windings (three
phase) and uses silicone recitfiers to convert the AC to DC. There is an
extra set of diodes that develop a power supply just for the field winding
and regulator circuit. This is called the excitation power supply. The
voltage of this power supply should be the same as the battery voltage
within 1 volt or so. The lamp is connected between this supply and the
switched battery voltage from the keyswitch.
If the key is off and the engine is not running, both are at zero volts and
the light is off. If the Key is on and the engine is running and the
alternator is producing power, both points are at 14 volts (more or less)
and the lamp is out (or glowing very dimly if there is sufficient
mismatch). When the Key is switched on but the engine is not running the
switched circuit is at battery voltage and the excitation power supply is
at zero and the lamp will glow brightly. The current going through the
lamp supplies a boost to the excitation power supply and helps the
alternator bootstrap itself into operation.
You may have noticed that when you switch off the lamp will come on
momentarily. This is because the switched circuit voltage becomes zero but
as long as the alternator is turning it is producing voltage and the the
excitation power supply voltage remains high enough to light the lamp.
This is the most common configuration. There are alternatives. The late
60's Jag XKE has a different deal altogether involving a relay (or two) to
switch on the dash indicator light. But that was the early days of
alternators and not typical of today's practices.
Cheers
Dave
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