LED Charge Warning Light

Shaun

Supporter
Morning all
Ok so here is one for the group, my engine now runs and I will be updating the build log been far too long, bit I have no alternator charge, all wiring looks ok so did some digging and it seems the alternator needs the warning light to energise up the coils until the alt takes over, simples with an incandescent bulb BUT I have LED lights and I have seen a few places stating they will not work.
Thoughts? Is this similar to the turn lights when LED causes the flasher relay to go ape shit as there is deemed to be not enough load.
Would a load resister resolve? Or perhaps an standard bulb in parallel to the LED concealed behind the dash?
Cheers
 

Ed McClements

Supporter
I think you need a decent current flow, which allows the windings to build up a magnetic field, then the alternator can start charging. Trouble is...surely adding a resistor will reduce the current, not increase it? I'm tempted to suggest just switching to a filament bulb...but that might not meet your aesthetic requirements!
 

Shaun

Supporter
Indeed Eddy that was a thought but all others are LED plus I have a bleeding great hole in the dash now if I did, odd they sell charge warning lights in LED if they cause problem but in essence they just change the lens.
The resistor was to bridge the LED and allow some current through as when the alternator spins up I think it back feeds 12v to the light hence negating the voltage difference and light goes out, although some also say it sometimes does not as LED's don't need a lot to drive them.
This is a image I found and they are my same lights
1612356951880.png

Just looks a bit of a bodge but mine would be neater!!
Also found this as a indication
"If you find your alternator does need load, fit a 5 watt, 68 ohm resistor across the LED for load. Be sure to leave some space around it as it will get hot but only when the LED is lit up "
Plus this diagram which futher shows the need for a diode
1612357354265.png

Should have fitted bulbs !!
 

Shaun

Supporter
Mick at SGT suggested connecting the yellow / white to permanent 12V and he says that does the trick, in my diagram it had it to switched live and in another said it was only needed if it was overcharging, Mick said when he switched live as per my diagram it hit 17v!! Still will light the way home I guess!!
Will give it a go and feedback for the next poor soul who comes across this.
 

Neil

Supporter
It might be the characteristic if a tungsten filament; when it is first switched on the filament is cold so its resistance is low and high current flows through it. As the filament gets hotter its resistance increases and the current flow drops markedly. It is probably the turn-on surge current through the filament of the light bulb that polarizes the alternator.
 

Mike Pass

Supporter
The LED bulb does not allow enough current to energise the alternator as it's effective resistance is too high. To allow enough current to the alternator you need to place another path for current to flow by soldering a suitable resistor in parallel (side by side) with the bulb which will allow enough current to flow to the alternator. This is done as per your pic showing the resistor soldered across the terminals of the bulb. This resistor will let through enough current for the alternator. The value needed for this resistor can be calculated and will depend on the actual LED bulb used. Because this resistor will take enough current for the alternator it needs to be of a high enough wattage. The resistance will be in the 68 to 100 ohm range and at least 2 Watt rating. I would suggest a resistor of 68 ohms and and 5 watt rating soldered across the LED bulb as per the pic. Keep the resistor away from the holder as in the pic as it may get warm. It doesn't matter which way round it is attached as it is only a resistor.
Suitable resistors can be bought from Ebay or one of the many electronics supplies
e.g.

Cheers
Mike
 

Shaun

Supporter
Thanks chaps
Mike that sort of re-enforces the strands I found on the web but confirms the Ohm value needed and Neil that also makes sense, progress to LED's not quite in line with the good old alternator technology, still wont need them soon we will all be run by batteries!!
 

Ian Anderson

Lifetime Supporter
I am no sparky, but could you run a wire from the switched side of the starter motor solenoid to the alternator, it would only be live during cranking by which time the alternator should be generating power?

ian
 

Shaun

Supporter
Ian that's a good idea and easy to test, Mick said he wired to the starter could well have been that and luckily its close.
Paul did not know you used LED's? What size did you use? 68 Ohm 5W as per Mike's suggestion?
Meant to get up there tonight but on bloody calls with off shore dev teams don't these clients know I have a damned car to build!!! Selfish the lot of them, mind did win the lottery last night...£2.80 still it would buy a resistor....
 
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