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What resistor will i need for these leds? 

 

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 Post subject: What resistor will i need for these leds?
Posted: Sat Jan 07, 2012 9:38 pm 
Oompa Loompa
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I have 6 leds that I want to wire into the headlight ciruit (Auto-Shifter Light) and was hoping someone could tell me what kind of resistor i need? The leds are rated at 0.26Watt@12.8v

Sorry for my complete lack of knowledge in the area.
Thanks in advance, any replies are immensely appreciated :)

 

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 Post subject: Re: What resistor will i need for these leds?
Posted: Sun Apr 15, 2012 12:31 am 
Getting Side Ways
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Smithysix wrote:
I have 6 leds that I want to wire into the headlight ciruit (Auto-Shifter Light) and was hoping someone could tell me what kind of resistor i need? The leds are rated at 0.26Watt@12.8v

Sorry for my complete lack of knowledge in the area.
Thanks in advance, any replies are immensely appreciated :)


It is basic Ohms law, you need to know the voltage that you want to get rid of 14.4 - 12.8 = 1.6V, the current 0.26/12.8 = 20mA, Ohms law states R=V/I so 1.6/0.02 = 80 Ohms....

Now what you can also do is if you were planning to run these all together whenever you plan to use them is to use some diodes as a diode has a forward voltage drop of between 0.5 - 0.6V so 2 in series would give 1.0 - 1.2V drop which may be enough to suit es[ecially if your car is running at less than 14.4V, if it were only 14V then that would be perfect...

 

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 Post subject: Re: What resistor will i need for these leds?
Posted: Sun Apr 15, 2012 1:30 am 
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Smithysix wrote:
I have 6 leds that I want to wire into the headlight ciruit (Auto-Shifter Light) and was hoping someone could tell me what kind of resistor i need? The leds are rated at 0.26Watt@12.8v

Sorry for my complete lack of knowledge in the area.
Thanks in advance, any replies are immensely appreciated :)


For starters there is no such thing as a 12.8V LED... they're typically no higher than 5V DC... And very rarely do you see them rated in Watts... While you can work out current from that I'd sooner it from the datasheet for the LED...

For example:

http://www.jaycar.com.au/productView.as ... rm=KEYWORD

Note the table in the page...

Quote:
Specifications
Size (mm) -> 5
Colour -> White
Lens -> Waterclear
Viewing Angle (Degrees) -> 15
Wavelength (nm) -> 0.29/0.29
IF Typical (mA) -> 30
VF Typical (V) -> 3.2
IV Typical (mcd) -> 45000
IF Max Continuous (mA) -> 30

Preferred Series Resistor (ohms)
5VDC -> 62
9VDC -> 200
12VDC -> 300


And Jaycar even gives you the resistor size...

So these are a 3.2V LED and need a forward current of 30mA to achieve max brightness... Assuming you want 3.2V across the LED, there is 13.2 nominal (all cars vary, but nominal is cell voltage x 10%)... Therefore you have 10V to many... And the current limit factor in the branch must be 30mA... Because you want that resistor to drop the 10V and also limit the branch current to 30mA you use the eqn above R=V/I 10/.03 and you get 333.33ohm... You should never use a value less than this because the current flow through the diode will be too high and you will over heat the junction... And LED's brightness varies with the current flowing through it, not the voltage placed across it... If you pass 30mA through it, that LED will always drop 3.2V... That is the characteristic of a diode... All diodes when operating at their maximum current will drop a certain voltage... Most power diodes drop less than a volt...

Other factor is the wattage of the resistor you need...

W=V*I so 10V dropped accross the resistor, and 30mA through it so 10*.03=.3W so you want a resistor that is bigger than .3W otherwise you'll be heating the resistor beyond its rated properties... Most resistors of the "common" size you see are .25W and while you could probably roll with one of these, Jaycar stocks resistor which are the same physical size, but rated at .5W

First off for you is to double check your specs on your LED's...

If it is indeed rated at 12.8V then it sounds to me like it already has a resistor with it... How about attaching a photo of the said LED too?

Cheers,
Tim

 

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 Post subject: Re: What resistor will i need for these leds?
Posted: Sun Apr 15, 2012 1:06 pm 
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Smithysix wrote:
The leds are rated at 0.26Watt@12.8v
If this correct, your LEDS already have resistors.
If you do a google search, there's plenty of converter websites where you can enter your info, and it'll calculate what you need - there's some where you can arrange in series/parallel configurations.

 

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