# LED's



## omega (Jun 27, 2002)

Does anyone here have much experience with lighting with LED's or cold cathodes? I'm not a big lighting nut but could be I suppose. Post any picks if you have them.


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## Ninety-Nine SE-L (May 5, 2002)

I'm going to end up making this post a 101 class, LOL. If you happen to know this already, sorry if I seem to make this sound elementry. 

I don't have LEDs on my car right now but I'm pretty good with electronics and LEDs, what do you need help with? 

I'll start you off with a little formula that should help with resistors. All LEDs need resistors when you hook them up. You can either tie a resistor to each LED or tie a resistor to a string of LEDs (I recommend a resistor for a string 5 or less LEDs b/c of the strain you will end up putting on the resistor) (if you have more...like say 15, you will have either 1 resistor per led....15 resistors...or 1 resistor for every 5 LEDs...3 resistors...)

the formula is:
*Ohm=(V-1.7)*1000/mA*
>V is the voltage on your car. The voltage in a car is actually closer to 14V than 12V, so you want to say 14 in the formula.
>mA is the power draw on the LED (milliamp or 1/1000 of an amp). Most LEDs run 20-30mA and you will find this number somewhere on the packaging of the LED (if you happen to purchase at Radioshack or something). (IMPORTANT: if the LED says 20mA, you will enter 20 into the formula and you will get the resistor rating for that INDIVIDUAL LED. If you want to do 1 restor per string of 5, that's 5 LEDs or 20mA*5=100. You will enter 100mA into the formula and that will be the resistor rating for that entire string)
>the final# is in Ohms. This is the rating of the resistor that you want to tie onto the LED(s). If you get a strange number like 603, you want to round UP to the next highest your supplier has. if the formula calls for a 603 ohm resistor and your supplier carries a 580ohm and a 630ohm, get the 630ohm.

The resistors will limit the flow of power to the LED. If you have too much power, it will be brighter but the life will be reduced, if you have too little power, the life will be increased and brightness decreased.

to make things a bit simpler, I will do some calculating for you (>= means 'greater than or equal'):

if the LEDs say 10mA:
single.....>=1230ohms (1.23k ohms)
string of 5....>=246ohms
if the LEDs say 20mA:
single....>=615ohms
string of 5....>=123ohms
if the LEDs say 30mA:
single....>=410ohms
string of 5....>=82ohms

here is a picture to sum it up. I used 20mA as an example:









I hope I answered your question in some form.


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## DwnShftngIsBad (Feb 11, 2003)

dude, are u still talking about cars?


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## Ninety-Nine SE-L (May 5, 2002)

hehe, Iono. pretty much, you can put leds just about anywhere, it's all about wiring them up 

peace..


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## san_fran_b14 (Oct 25, 2002)

owww..my head


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## nismoprincess (Sep 27, 2002)

most led's now come ready to install just wire to ground(black) and power (red) 
very simple


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## Ninety-Nine SE-L (May 5, 2002)

where r u looking? almost every led I've seen (that isnt some car novelty item) requires resistors. Unless it's some kind of light bar or scanner, or those little leds on ebay that are way overpriced, LEDs usually come alone b/c their application may vary. some leds are used on 3VDC while others may be on 120VAC while others are in strings of many rather than just one. LEDs need different types of resistors depending on where they are going to be installed.


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## omega (Jun 27, 2002)

Well looks like I've found two good places, if I buy the LED's elsewhere and some resistors from another place, I can get 20 of the LEDs and resistors for a dollar a piece.


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## cranium (Oct 8, 2002)

thank you for that post i needed to know what that formula was so i can do it myself. and it seem that at some point that with a 30mA led you would not need to run a resistor. am i correct?


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## Ninety-Nine SE-L (May 5, 2002)

no prob, I picked up that formula somewhere on the internet and I memorized it b/c I like working with LEDs.

you are right, there is a point where a 30mA LED does not need a resistor (2 cases):
1. if you are running the LED off of a ~1.33V system (I think). (R=(*1.33*-1.3)*1000/30=*1ohm*)
2. on a ~14V system like on our cars, if you ran a string of ~410 LEDs in a row. (R=(*14*-1.3)*1000/(30**410*)=1ohm)


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## 99.se.ltd (Jun 21, 2002)

superbrightleds.com, autodynamic.com
both are great places to get replacement led bulbs for auto's. just get them in whatever bulb size you need.

yes they are plug and play....they have the resistors built in so you cannot reverse the polarity. if they are wedge-type (194), and they don't work when you put them in, just turn them around and put them in the oppisite way. my whole interior and some of the outside is lit with led's, and i've NEVER wired any resistors. of course they don't make led replacements for EVERY bulb size....and in that case, you'd have to wire resistors in. but for the majority, they have led replacements you can buy.


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## Ninety-Nine SE-L (May 5, 2002)

Well, you can do that if you are replacing the bulbs. I think he's looking to do custom stuff. you can hide LEDs anywhere and if you want' you just get the LEDs and the resistors and do your own custom thing.


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## omega (Jun 27, 2002)

Yes I do a lot of custom work and like the effect LEDs have over huge neon tubing, so no bulbs but it's nice to know that formula, I am a little bit curious as to why the formula calls for 515 ohm on a 20 ma led at 12 volts, but everyone suggest the 470, any reason for this?


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## Ninety-Nine SE-L (May 5, 2002)

470 is about what you should use on a 30mA one and you should always calculate it at 14.0-14.4V b/c that's about what our cars actually run at.

I've always heard about 600 myself for the 20mA and that's about what it comes out to. You don't want too little resistance b/c that will shorten the life of the LED alot.


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