Depend on the rating of your LED and how bright that you want it to be. For 20mA 3.5V LED then 470 Ohms is ok.

 

uh i am too lazy to do the calculations but use Ohms law V=IR and use the power one P=VI where v is voltage, I is current and r is resistance....also u might have to do simple algebraic manipulations but it should just take a few seconds to figure it out

 

Yup, this guy's on to something.

we already covered Ohm's law of

E=IR

but we left out his law for power:

P=IE

So reading your pic of the LEDs it says that can handle 3.7v at 20mA

3.7v * .020A == .074W
5v * .020A== .1 W

1/4 watt == .25w, so a quarter watt will more than cover you.

Now where are you going to get your (max 4.5v) from? Since your car runs a 12v system?

If you decide to just run the LEDs off 12v:

E / R == I
12v / 470ohm == .025A == 25mA

Which appears to be 5 mA over the LEDs rated limit. You can generally get away with overvolting stuff, it's amperage (current) which will cause problems.

So I guess if you wanted to use 12 volts, without exceeding the reccomend current rating, you'd do something like this:

E/R==I => E/I==R

E / I == R
12v / .020A == 600ohms

Nobody makes a 600 ohm resistor. So looking at the equation:

E/R==I

You don't want "I" (current) to go up. So you want your denominator (R) to go be larger. (denominator gets bigger, product gets smaller).

Your resistor choices are:

560 - 600 = -40 ohms from target
680 - 600 = +80 ohms from target.

I lied, I just took 2 1.2kohm resistors (1200ohm) resistors, hooked them up in parallel, and that was exactly 600 ohms.

I took some measurments:

600ohms at 12 v was drawing .015Amps of current
470ohms at 12v was drawing .020Amps of current.

theoretically
12v at 600ohms shold draw .020 amps
12v at 470ohms should draw .025 amps

However, all resistors have 10% margin of error (gold band), or 5% margin of error (silver band).

So the LED manufacturers, I'm sure accounted for that using standard industry design practices, when rating their LED (at .020amps).



Here we see the LED running off of 12v, on the left through a 470ohm load, on the right through a 600ohm load.

What does that all mean? You can use a single 470ohm 1/4 resistor, no problemo. 1/2 watt will not give you any extra protection.

Feel free to ask any question (except "why did you make this post so long" )

-PHiZ

 

so yea after you guys lost me with all that math stuff,...what are you guys using LED lights for??? and post pics of what your using them for,..thanks