[alfa] RE: Glimmering Alternator light

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi All,

Yes they do seem to do this on the early alternators.  Not really noticable in bright
sunlight but after dark it becomes a noticable dim glow.  Well, you have to understand
what the ignition light aka alternator warning light does.

The alternator regulator IC module, before the alternator is spinning fast enough to
generate its own supply, needs  a current bleed from the battery to start the regulator
working.  This is gets via the alternator light.  The lamp is a resistance that only
allows a small amount of current to flow to the internal voltage regulator in the IC to
"boot" it up.  When the alternator is first powered up it needs a small current to flow in
the field winding (the excitation winding) to allow the main stator winding to start
generating the AC voltage.  (The alternator is an AC type of machine, the diode rectifiers
in the bridge turns the AC into a raw form of DC which then charges the battery etc).

The regulator cannot start working until the internal reference voltage is established.
This is provided by the current bleed via the alternator warning light.  Once the
alternator output exceeds about 12V the regulator then runs off this voltage and hence the
light should go out.  On my old Bosch alternator this only happens once you get the engine
rpm above a certain point, around 2000rpm.  Hence, a quick blip on the pedal makes the
alternator light go out and stay out.  It is a peculiarity of the early Bosch alties.  I
think that what happens is that within the internal circuitry is a voltage detector that
needs to see the battery voltage go above a certain trip-point before it unlatches the
lamp current.

I have found two simple ways to stop the alternator lamp from dimly glowing.

The first method is probably the simpler of the two and requires a higher wattage
bulb/lamp to be fitted.  The normal bulb IIRC is a 6W 12V beast.  If you replace this with
a 12W bulb the small current that still flows when the alternator is up and running is
insufficient to cause it to glow.

The second method achieves the same but in a different way.  You need to shunt some extra
current into the IC regulator.  By fitting a resistor in parallel with the bulb the bulb
has less current flowing in it and hence appears to go out.  I can't remember the exact
value resistor I used, it was quite a low value.  Maybe a 22 ohm 1W resistor would be a
good starting point.  Simply connect this across the bulb.

I hope this helps.

John
Durban
South Africa
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