Charging light resistor

[Griffer]

I have always had a lazy alternator light on start up on my early precat which needs a good blip on the throttle to put the light out, has always been so for the last 15 years in my ownership and have never had charging issues. I remember in an old post that this could be fixed by using a resistor on one side of the alternator LED but cant find my notes on this. I plan to take the dash out at the weekend so can anyone help as to resistor value, may be about 150 ohm? I presume that this goes to the battery +volts side rather than the alternator trigger side of the LED?
Thanks in advance, Steve

[Griffer]

Got the dash out and have a 470R resistor in parallel across the charge LED, looks like a later addition, is that correct? Sure I saw someone on here post up a circuit diagram with an additional resistor in series.

[spend]

Griffer said:

Got the dash out and have a 470R resistor in parallel across the charge LED, looks like a later addition, is that correct? Sure I saw someone on here post up a circuit diagram with an additional resistor in series.

Thats standard.

Essentially circuit does 2 things...

Ignition (battery) powers up field coils so alt has magnetic field as soon as it spins (they will self generate magnetic field once they reach @2000 engine rpm ~ aka self exciting)

Bulb between battery & alt sense will light if the PD (voltage diff) is enough to light a bulb (note a bulb WILL work fine ~ just LEDS are much more sensitive to small voltage differences).

Resistor in // is normal so that field coils get power if bulb blows (broken circuit if no resistor), adding resistor in series with LED is required to restrict the sensitivity which IIRC would glow with a forward voltage of just 2v! Not uncommon battery to be at 14V & alt at 12V giving false alarm, the one I couldn't fathom was why with battery low & high rpms the light would glow faintly...


I gave up trying to solve both issues & glued a proper bulb behind a LED lens, which works perfectly in all conditions (still with // resistor in place)

ETA: all this thread is specific to pre-serp charging arrangements, the serps don't have any such glitches with the different alternators AFIK.

Edited by spend on Friday 6th June 13:06

[Hoover.]

My light always used to go out until I changed the alternator, then I had a garage have play around which resulted in resistors being installed.

The TVR S's charging light doesn't out until the car hits 1500 rpm, so if it's doing something like that not an issue