2001Millrunner

what resistor do i use to use LEDs in 12v(automobile) settings for interior lighting? i think most LEDs are at either 1.5v or 3.0v. I just went to radioshack and had no clue because i guess resistors are catagorized by wattage and % rate.

WATRD

I believe this is what you are looking for...

http://home.cogeco.ca/~rpaisley4/LEDcalc.html

MTL_4runner

Just remember V = I x R which is ohm's law.....never fails.

So you also need to know the current and voltage on the LEDs to figure out the resistance. If you have no idea if the current on the LEDs, get a potentiometer (maybe 1K ohm) and hook it inline to the power source and the LED. Then vary the resistance until you get the proper lighting. Unhook he pot and measure the resistance with a multimeter, then use a resistor of that value in the circuit.
Then to get the resistor at rat shack use the following:

Just remember wattage W = V x I

The wattage on the resistor must always be higher than what you calculate you need.
The % rate is just telling you the tolerance on the resistance try to get 5% or lower if possible.

This is just a typical case:
A forward-biased LED will have a voltage of about 2V. Forward current for a
> typical LED is about 10 mA. So the correct resistor would be:
>
> R = (9-2)/0.010 = 700 Ohms
>
> 680 Ohms is a standard value and close enough.

2001Millrunner

WOW! too complicated for my simple mind. I'll ponder on this when I get home and try to figure this out. Thank you Rob&Mtl_Runner!!

Rob ur truck is lookin awesome, now that you've done everthing, what is next on the list?

LSUMatt1514

2001,
Check out Wild Bill's website. I think he used 470 ohm resistors. Dude, if you're serious about trying this, let's give it a go together because I've already got my LEDs in hand and the dash totally disassembled.

Matt

2001Millrunner

I tried Stained glass paint(double coat) just now, and I'm gettin the teal-blue color as well. It looks almost identical to the 90 ish honda car' gauge colors.........I think im gonna call it good for now...i already blew 2 twist-socket type interior bulbs which I think can only be ordered through Toyota and LEDs have not worked for me....as it is way too hard to disperse lights evenly throughout the gauge clusters. Flat-tipped LEDs do not work either as it does the same thing

bulldog-yota

How many LED's do you want to connect to a single resistor? WHat type of LED are you talking about, cause there is a whole variety of them these days?

Typical values for the bright ones seem to be 3V and 20mA.

That means 450 Ohm or close enought to it. Also your resistor will need to handle 200mW per led, so if you get 5 leds you will need a 1W resistor. However in my experience I found always get at least twice the wattage on a resistor than what you need as they get pretty hot at their wattage limit. SO I would say get 500mW per LED. In short 4 LEDs get a 2W 450 Ohm resistor (I think 470 Ohm is a very standard value).

For you other electric and electronic engineers dont take 12V for a car as it normally use 13.8V as the upper level, and is the best for this application to use that voltage. For amps with switch mode PSU, I used to use 11.2 as the bottom and 14.5 as top.

ALso dont waorry about the % as it indcates tolerance, a 5% is normally good enough for this application and should be cheap.

Hope this helps.

SO short answer for bright LED's that require 3V and 20mA use 470 Ohm resistor(s) with a rating of 500mW per LED and tolerance of 5%. If the Radio shack guy doesn't get this just him.

bulldog-yota

Oh forgot and for every LED you add to the same resistor you need to brings the resistor's Ohm value down in proportion.

i.e. 1 LED = 470 Ohm
2 LEDs = 230 Ohm
3 LEDs = 150 Ohm
4 LEDs = 112 Ohm.

Probably just best to do a single resisitor to LED.

Duffdog

your talking about wiring leds in series right? because in parallel that wouldnt work, you would still have the same voltage drop no matter how many led's you have. I would just use a fricking 500ohm pot and be done with it.
T

bulldog-yota

Nope I'm talking in parallel, every LEDs add to the current which will increase the voltage drop over the resistor. The voltage on the LEDs will remain the same. A pot is a good solution provided you get a pot that can handle the wattage required. If you have 5 LEDs drawing 100mA it can kill the little carbon pots quickly. I would recommend wire wound, if you want to use a POT. A POT will abviously give the ability to set the brightness of the LEDs within limits which can be a cool feature if needed. But I will still add a resistor in series, cause people tend to pop the LEDs if you turn it all the way up.

Duffdog

"every LEDs add to the current which will increase the voltage drop over the resistor. The voltage on the LEDs will remain the same"

that doesnt make sense, so your saying that if you increase the current, you also increase the voltage drop? doesnt jive with ohms law -- if you have more current draw, that is because you have less resistance (more paths to ground)--so where is the increased voltage drop coming from if the leds have a common power source and a common ground? if you have 2 50w lights, you have 2x the current of 1 50w light--and no change in voltage ( unless they are in series, at which point the resistance of the two lights is additive and you now have the same amount of power as 2 25w lights or 1 50w light ) but still, the voltage across the entire load is the same right, only between the two nodes is there a drop.

T

bulldog-yota

Remember a LED does not work like a resistor, it tries to achieve a constant voltage drop. It will also try to achieve a constant current. SO if you put two LEDs in parallel it will try to draw twice the current with the same voltage drop. If you supply voltage stays the same (big fat car battery), it will mean that your resistor will have to compensate for it. WHich means if the current double the voltage drop on you resistor will go up, which means less volts on your LEDs. DO an expirement where you put one LED in with the appropiate resistor, then keep the resistor the same and add a second LED to first in parallel. Notice how much it output of the LED drops, add a third and probably by the time you add a fourth you will have very little light coming out of the LED's. Thats why you need to drop the resistance of the resistor as you add LEDs in parallel.

Hope this makes it clearer.