Re: [No HA content] Car "alternator warning light"

["Christopher Williams" <ccwilliams@xxxxxxx>]

As I understand it an alternator works like this:

The output uset to charge the battery and run electrics in the car=20
come from the stator of the alternator. ( This is a set of coils=20
arranged around the outside of the altenator, forming a tube. )  This=20
set of coils generates a 3-phase alternating current.  This is=20
rectified by a full wave bridge rectifier ( consisting of 6 diodes )=20
to produce DC. Now to generate this current the rotor in the middle of=20
the alternator has to generate a magnetic field.  This could be done=20
with solid magnets, but in an alternator it is generated with another=20
coil of wires.  To supply power to this coil there is another=20
half-wave rectifier with 3 diodes comming from the stator coils.  It=20
is this supply that is connected to the ignition warning lamp.  To get=20
this supply to the coil on the rotor there is a set of slip rings at=20
the back of the rotor with two brushes rubbing on them.  To control=20
the voltage comming out of the alternator there is a regulator circuit=20
that measures the output from the full wave rectifier and controls
the=20=20
power being supplied to the rotor to keep this constant.

So can this info be used to answer your question?

Two thoughts:

As one side of the warning light is connected to the output from a=20
half wave rectifier and the other to the output from the full wave=20
rectifier, there could be a slight voltage difference between the two=20
causing the dim light.

Secondly if the regulator has decided that the output is sufficient,=20
it will reduce the current going into the rotor, this would increase=20
the voltage drop across the bult slightly.

This is all hypothetical and thought up on the fly!

An interesting asside, I have heard that if the warning light bulb=20
fails open circuit, when you first start the engine, unless ther is=20
sufficient residual magnetic field in the rotor, no output is=20
generated and the alternator appears dead!  New alternator for =A3=A3=A3=20
when a pew p bulb is all that is required.  ( This may have changed=20
with modern alternators. )

Hope that hasn't confused everyone, including myself!

Cheers,

Chris Williams=20=20

--- In ukha_d@xxxxxxx, Keith Doxey <keith.doxey@b...> wrote:
> The alternator warning light is connected beween the output of the
> alternator and the Ingition circuit 12V supply. (12V being the=20
nominal
> voltage - as you point out this will rise to about 14.4)
>=20
> A permanent dim glow can indicate either excessive voltage drop on=20
the
> ignition circuit (disconnect that 3kW Sound system NOW!) or the=20
alternator
> not being 100%.
>=20
> From my experience it has always been the brushes worn in my=20
alternator and
> replacing these for abpout 2 quid cured the problem.
>=20
> The WL (warning light) terminal on the alternator is at 0V when not=20
charging
> so the lamp lights due to the other side being connected to 12V.
For=20
you to
> see the light glowing dimly means that the WL terminal is at about
1=20
or 2
> volts difference to the ignition circuit. If its around 10-11V then=20
the
> alternator isnt charging properly. If its higher than the ignition=20
circuit
> voltage then either there is excessive drop on the ignition side or=20
the
> connection between the main alternator output and the battery is=20
suspect
> with the alternator giving out its proper 14.4v but it is getting=20
"lost"
> between there and the battery.
>=20
> Check for volt drop from alternator to battery. I once had a badly=20
corroded
> main output plug, the result of hitting too many deep puddles :-(
>=20
> HTH
>=20
> Keith
>=20
> > -----Original Message-----
> > From: Geoff [mailto:Geoff@xxxxxxx...]
> > Sent: 27 November 2000 08:04
> > To: ukha_d@xxxxxxx
> > Subject: [ukha_d] [No HA content] Car "alternator warning
light"
> >
> >
> > We are having a discussion on one of the uk car NGs about car=20
ignition
> > warning lights and *exactly* what they mean so I thought I'd
ask=20
you
> > guys as Electronics Consultants   :-)
> >
> > Several of us have had cars where the ignition/alternator/
> > call-it-what-you-will light on the dashboard glows dimly all
the=20
time
> > the engine is running.  It's not mechanical - e.g. slipping belt
-=20
and
> > the alternator seems fine and the battery doesn't run flat.=20=20
Battery
> > voltage is 12.6-13.0 at rest and 14.2 when engine running and
> > on-charge.
> >
> > Change either the battery or the alternator and the problem=20
sometimes
> > goes away.
> >
> > Increasing the load - lights, heater blower, etc - makes little
or=20
no
> > difference to battery state or intensity of the glowing light.
> >
> > So what does this mean?   How do these lights work?  Someone=20
suggested
> > "There are all sorts of types of alternator warning light
> > arrangements, although I *think* the most common sort these
days=20
picks
> > up its (+) from the ignition switch, and its (-) from the
> > regulator/field diode connection. I'd measure the volts at the=20
warning
> > light output from the alternator - this should tell whether
the=20
fault
> > is in the alternator or the lamp wiring."
> >
> > Any thoughts on what these warning lights *actually* mean?  
And=20
how
> > you'd decide whether it was (a) alternator (b) battery (c) duff
> > wiring?
> >
> > TIA
> > Geoff
> >
> >
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