LED dash bulbs

Shunt it. What current flows through the original bulb ?

Steve

I reckon you could frig the alternator one with a resistor of the correct value.

Depends on what wattage the original bulb was. R = V(sq)/P

So if it was a 2w bulb, a 75ohm resistor will provide the same current.

Although that is only a 160mA current, would think an alternator will need a bit more than that. Could be a 5w bulb which will require a

Alex

The current flow from the battery to alternator field is via the warning light so you need to make your LED draw the same current by fitting a parallel resistor across it and its series resistor.

To work out the value you'll need to find out the wattage of the bulb. If say 2.2 watts it goes like this.

Watts = Volts x Amps (W = V x I) W so I = --- V 3 I = --- 12 = 0.18A

The LED will take probably 0.02 A so we're looking at 0.16A via the shunt resitor.

From V=IR, V R = -- I

So 12 -- = 75 ohms. 0.16

This isn't a preferred value of resistor, so use a 68 ohm 3 watt type.

I think it will be OK as once the alternator gets up to battery voltage it supplies its own field current and no current runs through the warning light.

AJH

On or around Wed, 26 Dec 2007 14:19:10 +0000, Steve Taylor enlightened us thusly:

could do, but it's not a big issue having a bulb there. and not a lot, it's only about 1.2W. It's more down to reference voltage for the alternator, I think.

It's current rather than voltage that matters.

There may be another advantage to sticking with a bulb for the charge light. LED's are pretty much on/off devices with nothing in between so you won't get the typical "dimly on/flickering" effect that you'd see with a bulb if, say, your alternator belt were slipping.

That's simply not so - unless you're driving them off a constant current source. Otherwise their light output is proportional to the voltage same as any other bulb - and I'd guess more linear than tungsten.

Dave, you're so right. I have no idea what I was thinking when I wrote that drivel. Excuse me while I get my coat :-)

You could have been thinking of HIDs - since those are being discussed here too. They need special techniques to dim - and can't be dimmed fully.

But high power LEDs are best driven using a pukka driver which produces constant current - and that effectively prevents them being dimmed by a simple voltage dimmer.

You need a constant current source and a PWM drive.....

Steve