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A simple warning light like that will often not show a failed alternator. If the brushes fail, the most likely symptom is that the light won't come on when the ignition is on but the engine isn't running. A complete failure of the regulator that stops the alternator working may cause the same symptom
If the alternator tries to overcharge the battery because the regulator has failed, the LED won't show that.
The alternator warning light will reliably show a failed drive belt.
Many engines have run that sort of circuit with no problems, so unless you have good reason to have some more sophisticated diagnostics, the warning light is fine.
I replaced the alternator bulb on my boat with an Led and a shunt resistor, I found that the Led flickered a little bit after the engine had run for a few hours and the battery was full.
If you really want to detect charging problems fit a voltmeter (not an amp meter not a good idea), that will tell you a lot if you know what to look for.
Ok, guys. Now I am considering the LED again. Main reason: I have replaced all the other lights on dash with LED's.
Now I have this ugly old fashioned bulb light tucked away at bottom of dash.
I can only get 6V led's in my town.
Can you guys pls check that my specs will work with 6v led.
Someone also suggested rather go for 100ohm/3watt resistor because it will dissipate heat better?
A resistor running at its full power will get really hot so it is a good idea to have a resistor of a larger rating.
The resistor in parallel with the alternator warning lamp is needed to get the alternator started, and 100 Ω sounds good for that. It will dissipate around 1.5 W when on, so a 3 W resistor is a reasonable rating, and it will still get hot if left on.
If you have a 6 V LED, that will almost certainly be an LED with a resistor in series to allow it to run on 6 V. You should measure the current it takes at 6 V, and then you need a resistance of 6 divided by the current, and put that in series. So if it takes 20 mA you need 6/0.02 = 300 Ω.
(The 6 in the calculation is the extra voltage that you need to loose. It is calculated from 12 V (battery) - 6 V (LED) = 6 V (extra) and it just happens to be the same number as the LED voltage).
The lamp current sets the minimum field current to the alternator, and also provides the initial excitation current to guarantee the alternator starts operating correctly.
It's functionally connected _across_ the alternator regulator output.
Once the battery is fully charged, the regulator will reduce the field current and the voltage across the regulator will increase. With a typical LED that will start to draw some current at around 2V, you will get some output in normal operation.
I'd connect a 5.6V or 6.1V zener plus a moderate value (100R ?) resistor in series with the 6V (internal resistor type) LED lamp.
That will mean the LED cannot illuminate until you get at least 8V, which is a more reasonable range for this use.
You may need a power resistor across the whole thing to guarantee proper excitation, if the LED current is too low when initially starting up.
I'd add a value to give around another 50mA or so, eg. around 240 Ohms rated at least 1W. Two 120R 1W in series should be suitable and give a safe power & heat margin.
A lucas acr alty usually has a 1.2w pilot bulb in the warning circuit, a 100 ohm 3w resistor does the job that the bulb would do nicely, put the 100 ohm inline with the igntion switch & alty, the Led & resistor across that.
A 6v Led would be Ok if you put something like a 220 ohm 1/4w in series with it, if you can get one that is.
Open up an old pc power supply or something and pull one out, colour code red red brown.
Having done a bit more research it appears that now there are a lot more variations of alternator regulator setups than when I was actively messing with such things & the lamp may be wired in any of several ways.
I'd still advise the zener in series with the LED to increase the voltage needed before it starts to take any current though, if it's flickering on an otherwise-working charging system.