Alternator Led Warning Light

[Guest mcramsay]

I have been reading up on why my led alternator light will not get the alternator charging. It seems there is not enough current being pulled through the led to excite the alternator, looking online if I wire a resistor in parallel with the led it will allow the current required to be pulled through the resistor, I'm trying to work out what size Resistor I need so I am doing the following sums: assuming the bulb on the dash would be around 4w

 

P=IV so I=P/V

I=4/12 = 0.33

 

Then

V=IR R=V/I

R= 12/0.33

= 36.3 ohms

 

the nearest value I can find on eBay is a 39ohm 5w resistor, will that do the job?

[Jamiep]

You could always wire the original lamp behind the dash in parallel. Sounds daft but the simple cheapest option if you already have it.

[Guest mcramsay]

Don't have the lamp. And to be honest a resistor is only around £2!! Can't really get it much cheaper

[fry61]

With a resistor in parallel Craig the volt drop across the LED will be almost zero -- so no glow !! & the dash lamp is in series with the alternator so the volts at dash will be lower -- depends on alternator winding resistance.

[Guest mcramsay]

Does that mean my suggestion won't work?

[Guest chris brown]

It will work but a tungsten bulb instead of the led is the best answer.

[Guest mcramsay]

why is that the best answer?

[Jez Morton]

I found the lamp to be around 1.2watt in the sierra dash. So I think you need something around 120 ohm, and at least 1.5 watt. The resistor in parallel with the Led won't dim the led the volt drop will be the same, like the light bulbs in your house they don't dim with more bulbs on?

[fry61]

I found the lamp to be around 1.2watt in the sierra dash. So I think you need something around 120 ohm, and at least 1.5 watt. The resistor in parallel with the Led won't dim the led the volt drop will be the same, like the light bulbs in your house they don't dim with more bulbs on?

The circuit for the alternator is 2 resistances in series, so current is same through-out but voltage drops across each resistor (lamp) dependant on each resistance in series.

[fry61]

I am wrong & Chris is correct, the circuit will work because the alternator is a higher resistance than the dash lamp/resistor & in effect acts as a false earth for the dash lamp until the alternator is generating , which then extinguishes the dash lamp due to + 12volt being present on either side.

Edited May 3, 2016 by florin metal works

[rthomas69]

Does this help?

[Guest chris brown]

What is the series diode there for?

[sj-bradley]

I have been reading up on why my led alternator light will not get the alternator charging. It seems there is not enough current being pulled through the led to excite the alternator, looking online if I wire a resistor in parallel with the led it will allow the current required to be pulled through the resistor, I'm trying to work out what size Resistor I need so I am doing the following sums: assuming the bulb on the dash would be around 4w

 

P=IV so I=P/V

I=4/12 = 0.33

 

Then

V=IR R=V/I

R= 12/0.33

= 36.3 ohms

 

the nearest value I can find on eBay is a 39ohm 5w resistor, will that do the job?

 

Yes, that will work, and if you want to get the LED to light as well, you need the following info.

 

39 ohms * 033 amps = 12.87 volts across the resistor and power dissipated = 12.87 * 0.33 = 4.24 Watts, so a 5 watt resistor is cutting it fine and will get quite hot. Since it will not be running constantly though, that should be fine.

 

You will also get 12.87 volts so you could wire the indicator in parallel with the resistor provided that the indicator has a series resistor built in (as it's in the dash, that's almost certainly the case). This will give a little extra current to the alternator and should work as expected.

 

The resistor is equivalent to wiring a filament lamp in parallel except it should last longer than a lamp would.

 

Hope that helps?

 

Simon.

[rthomas69]

What is the series diode there for?

The diode lets current flow in one direction only & protects the led

 

This explains it - http://www.shoptalkforums.com/viewtopic.php?t=125353

Edited May 5, 2016 by rthomas69

[Longboarder]

 

why is that the best answer?

Because a tungsten bulb is a two way flow indicator and a diode is only one way.

Voltage difference battery to alternator will light a tungsten or diode. ie low alternator output.

A failed regulator giving excessive alternator output and flow the other way will also light a tungsten but not a diode.

That's why it's the best answer.

 

Nigel