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L.E.D's instead of bulbs??


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#1 Hogzi

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Posted 11 November 2005 - 04:47 PM

I want to fit L.E.D’s instead of indicator bulbs. Can I just wire them up or is it more complex than that?

I have seen kits on e-bay but they seem to include some sort of regulator. Has anyone wired any L.E.D’s up and does it work? Can I buy the bits from Maplins??

Cheers all…

#2 Twonk

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Posted 11 November 2005 - 05:05 PM

so long as there 12v you should just be able to wire them in. was thinking about doing this with my brake lights as well

#3 Dan

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Posted 11 November 2005 - 05:05 PM

Much easier to buy LED bulbs than try to build your own. You can get them at lots of Maw-Power type places. Most of these things are not technically legal (in fact none of them are if the regulations are applied fully - strictly speaking bulbs must be incandescent) but that doesn't seem to stop Volvo putting them in their buses!

#4 redfish0

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Posted 11 November 2005 - 05:09 PM

You can probably do it yourself. Get a 220 ohm resistor to and some decent quality, brightest possible (10,000+ mcd/millicandle I think) led's. Do some soldering, mount them, connect them up and jobs done.

But for a propper job you might be better off getting proper ones that fit the bulb holders. Doing it yourself will mean you cant use the bulb holders and makes like more difficult if an led goes out and needs replacing.

#5 dklawson

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Posted 11 November 2005 - 05:32 PM

As Dan said, this is something easier to buy than build.

However, if you're adventurous and enthusiastic, you can buy the LEDs and dropping resistors as Redfish0 describes. However, the resistor you choose will depend on what LED you choose. Don't go out and buy the resistors until you've selected the LEDs and calculated what you need.

Discrete LEDs will have a specified voltage and current draw. They will also list a MCD (milli-candela) value. Bigger MCD = brighter. You want to put a resistor in series with the LED such that the resistor produces the desired voltage drop at slightly below the LEDs rated current. These can all be calculated based on the formula of V = I x R.

For example, let's say you chose a 10,000 MCD LED that specifies operation at 3V and 15 mA (0.015 Amp). Your car really doesn't "run" at 12V, while you're driving around the alternator increases this value. Use 14.5V in your calculations for the car's operating voltage. You need to drop the voltage from 14.5 to 3 (14.5-3 = 11.5V). You know that the LED runs at 0.015 Amps. Derate this by about 10% and calculate a continuous current of 0.015A x 90% = 0.0135A. Now apply the V=IxR rule and you've got 11.5V=0.0135A x R. Solving for R you find you need an 852 Ohm resistor. There probably isn't such a thing but you may find something close and close will do. Pay attention to one more calculation, Power. Power (Watts) = VxI. In our example, Power=11.5Vx0.0135A=0.155Watts. This is more than 1/8Watt and less than 1/4Watt so when you buy your parts look for 1/4Watt resistors close to 850 Ohms. (Use bigger resistors, not smaller).

OK, that's for a single LED wired in series with a dropping resistor. Another way to do this is to wire several LEDs in series without a resistor. Let's use the same sample LED. It has a voltage drop of 3V. Take our supply voltage (14.5v) and divide by 3Volts. 14.5V/3V=4.8 Thus... if you could have 4.8 LEDs wired in series you could safely wire them across your car's electrical system. Round this up to 5 LEDs and you've got a safe solution. Rounding the number up will lower the voltage across each LED slightly and will drop the current a little more.

Well... I've had fun typing this, but you can see why it's easier to buy commercial LED lamps rather than "roll your own".

#6 m1n1

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Posted 11 November 2005 - 07:05 PM

about a year ago i was wandering around halfords, and saw those led brake light bulbs. they do the same job as normal stop/tail bulbs and i bought them specifically to use with my clear rear lenses as the red painted bulbs don't last that long. They work really well and i am very pleased and not dear.
a few months ago i was on ebay and i bought some indicator led bulbs. now these give yellow light, not orange. and they must need some sort of resister in the circuit; due to the fact they use so little power; as the relay does not flash with them in. they just stay on. hazards work tho :grin:

#7 Hogzi

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Posted 12 November 2005 - 11:10 AM

Jesus that's a load of info, thanks!!

So if I go down the route of using a resistor on the following led;

2.4V max
50m/A
Derating factor .04m/A

It would be 11.5x.40m/A = 4.6 watts ??

Or I could just wire up 6 in series;
2.4v x 6 = 14.4v

What if I want to use 5 x 12v led's? Can I wire these all in series or will the brightness of the led be lower than expected?

I think it's easier to wire in series with no resistor. I'm sure I've calculated the above wrong.
Is there any drawback by wiring in series as opposed to using a resistor?

Thanks again
Hogzi




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