Theo is ecentially correct but let me explain it a little further
to emphise the function of the idiot light in a modern automotive
alternator system.
On start up the regulator (internal or external) is connected to
the battery only through the idiot light. The initial current to
get the alternator field powered flows through the light bulb. As
the alternator starts generating current, a separate set of diodes
inside the alternator take over supplying the regulator and there
by the field current, from the alternator not the battery.
When this separate diode bridge can supply all the necessary field
current no current flows through the bulb and it goes out.
You might think that if the bulb were burnt out, or disconnected,
the alternator would never start In reality the field has some
residual magnitism and will eventually get the regulator powered
but will require a much higher RPM to start up. For this reason
many factory installations parallel the bulb with a resistor to
supply some starting current if the bulb fails.
Finally for those of us still running the Lucas 10AC stock
alternators, the warning light serves a couple purposes, Yes it
warns that you may not be charging the battery if it remains on
while the engine is running But much more important it warns you
that the ignition and alternator field are powered when ever the
ignition switch is on but the alternator is not turning. If the
10AC is not turning, the old design regulator is applying full
current to the field, because it thinks the output voltage is too
low. But no air is flowing to cool the field winding which will in
short order burn it out.
The new design, which is quite a bit cheaper to build (no relays)
solves this problem by only allowing the limited bulb current to
flow until the alternator is turning.
Steve F
*******************************************************************************
Steve Finberg W1GSL w1gsl@mit.edu
PO Box 397082 MIT Br Cambridge MA 02139-7082 617 258 3754
*******************************************************************************
Theo Smit wrote:
>I'll take a swing at it.
>
>First off: Read Larry's post carefully: One wire to the battery/starter
solenoid, and another wire to the light. The one-wire alternator has TWO
wires. My Toyot alternator has three: the output wire (big bolt-on post)
and two spade-lug connections, one for the ignition and the other for the
light. The "one-wire" alternator combines the function of the last two
connections, and Steve L's description of its operation is pretty much
correct: When the ignition is on but the engine isn't running, the
alternator doesn't put out any current, and its internal regulator
pulls some current from the 'light' connection to turn it on.
>When the engine is running the alt output is above some threshold, and the
internal regulator turns off its current switch for the 'light' output.
When the ignition is off the voltage at the light is zero, and the
alternator's regulator takes that as a cue to do nothing. If your charge
light is on when the ignition is off then either the light is wired to
the wrong side of the switch, or there is some internal fault with the
alternator that is allowing the current flow in the wrong direction
(some older cars had the diode Steve and Tim Ronak referred to on the
instrument panel circuitry but this newfangled alternator shouldn't
need it).
>So we should have
>12V ---> ignition switch---> (run position) -->light--->alternator 'red'
>wire->
>
>alternator.
>Clear as mud? Let me know - I can try and draw you a schematic and email it.
>
>Theo
|