A two-color LED (with series resistor) would theoretically work fine here.
I recommend 2 bright red LEDs soldered next to each other in a reverse
polarity configuration. This way you can put them in the stock bulb holder.
If you used a Bi-color, you would never see the green one because of the red
filter on the dash. Several problems with this idea though, one is that the
resistance needs to be lower, because the 20ma of the LED is not generally
sufficient to "prime" the exciter. The second is that the LEDs need to be
REALLY BRIGHT to be visible through the existing dash filter.
I have replaced a number of stock dash bulbs with ultra-bright LEDs with
success. I never bothered with the Alternator bulb because it is quite
hefty. It's the same bulb used in the marker lights rather than like the
rest of the dash. This is done probably to lower the resistance for proper
start-up.
Here is how the alternator works: (assuming a "hole flow" theory for all you
"purists" ;-)
When you first turn on the key, the current flows from the battery through
the ignition switch and the bulb. It goes from there directly to the slip
ring brushes, through the exciter, and then the regulator "grounds" it.
(back to the battery) This current flow illuminates the light while
energizing the exciter somewhat. (why the low resistance of the bulb is
important!)
Once the engine is running the alternator field starts making 3-phase AC and
the 3 small "freewheeling" diodes take over powering the "light" circuit.
Because the 3 diodes and both sides of the lamp circuit are at about the
same potential (voltage), no significant current flows and the light goes
out.
Once the regulator senses the output is up to around 14 volts, it begins to
open the ground to the exciter, and eventually settles on a balanced
condition keeping the output voltage constant. There are 6 heavy diodes
rectifying the 3-phase AC to the large terminal on the back which is
connected directly (according to Peugeot Electrical vagaries) to the
battery.
If the alternator stops turning, the freewheeling diodes cease to have
potential again and the light comes back on. If the regulator gets "stuck
on" the alternator will produce a higher potential than the battery, so the
light will again illuminate (albeit dimly). Also, if one or more of the
main diodes (typical) or the freewheel diodes fail there will be a potential
difference enough to illuminate slightly the light one way or the other. So
several no-charge or over-charge scenarios can illuminate the light. If
your regulator fails open, or the slip rings or brushes fail you will not
see a light however! Also, despite the light bearing the international
symbol for "battery", it has nothing to do with the condition of the
battery. I suspect a little icon of the alternator on the dash would just
confuse people. =)
-Phil
----- Original Message -----
From: "William Heacox" <heacox@cox.net>
To: <peugeot-L@yahoogroups.com>
Sent: Thursday, March 31, 2005 7:12 PM
Subject: [Peugeot-L] Re: 505GL wagon: Voltmeter hookup?
>
>
>
>>
>> A bulb with a tungsten filament is of course not the best thing for
>> reliability in an environment with a lot of vibrations. I was
> thinking
>> that modern LEDs have incredible lifespan, particularly when
>> underdriven. Some also are bicolor/bidirectional. Might be an
>> interesting thing to replace that bulb with one of those.
>>
>> Francois
>
> Unfortunately, a regular LED won't work here. An LED is basically a
> diode- a one-way electricity valve. A tungsten bulb doesn't care what
> pole is positive or negative, which is why they can be used here.
> LEDs are specific which pole is what, and cannot be a 2-way path.
> Looking at it a little differently, when you turn the ignition on,
> the light comes on because the regulator hasn't tripped on yet, and
> grounds the bulb. Start the car, and the lamp (usually!) goes out,
> because the regulator has come on, and the bulb now has power on both
> sides. The path is from the ignition switch, to the bulb, and on out
> to the regulator. Because of the one-way nature of an LED, this path
> is interrupted, keeping the regulator off. NOW.... with all that
> said... that comes from experience with streetrodders who thought
> they knew more. A bi-directional may work. I'd have to look into that
> and see.
>
>
>
>
>
>
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Received on Fri Apr 1 10:19:06 2005
