What resistance will I need?

Question

I have a BMW 316i (Shape: E30) and I need to know what resistance I'll need from the battery to a strip of 10mm superbright LEDs.

Here's a link to the page I'll be buying from, just need to know what resistor I'll need for a circuit of maybe 10 - 15 of these LEDs.

Not sure what the difference is of the 2 LEDs on that page but the bottom one looks like it might be brighter so could anyone tell me what resistor I'll need for that one?

http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Optoelectronics&tier3=10Mm+Leds&tier4=Superbright+10mm+LED&moduleno=64871

Sorry forgot the link.

Answer 1

The datasheet indicates a typical forward voltage drop of 1.85V for all of them with a current of 20mA. When your engine is running, your battery voltage is actually about 13.5 to 14V. If you do one dropping resistor for each, you'd want a resistor of

(14V - 1.85V)/20mA = 608 ohms. Use 620 ohm resistors. Of course, one LED is in series with one resistor, and all the pairs are in parallel.

Alternatively, you could do 4 LEDs in series, with one resistor of

[14V - 4(1.85V)]/0.020A = 330 ohms

I wouldn't use a single dropping resistor with all the LEDs in parallel, I think you might see significant difference in intensity between them.

Answer 2

Each LED will need its own resistor. You need to look up the current because you want a resistor that will drop the difference between the battery voltage (12-14 volts) and the 5 or so volts that the LED needs.

One problem you will have if you buy resistors by the onesies is that they are probably not going to match each other in brightness (due to variations in production). You may end up with a freaky looking combo if some are significantly brighter than the others. Manufacturers of light strips that contain multiple LEDs buy them in sets with matching brightness and pay a premium for that selection by the LED suppliers..

Answer 3

Same resistor whichever one you use. You need a separate resistor for each LED used (parallel leds will not current share equally). The value is 13.8v (the supply voltage) - 1.8v (the voltage across the LED)/0.02A (the current through the LED)=600 ohms (any value between 560 to 680 will do).
Note that waterclear leds have a higher light output but a narrower field of view than red filtered leds, which are more diffuse.

Answer 4

at 5v with 30mA
5/(30*10^-3)
=167Ω
put them in parrelell Maybe on a vero board,
with the resistor in series with the parrelell network

This guy puts about 100 LED's in paralell but his experimeting says for better results you 1 resistor per 1-10 leds. He uses 150Ω but different types of leds have different needs and different colours etc also 150-166 is a small difference and both would work equally well.
http://www.instructables.com/id/Darth-Maul-LED-light-saber/

Answer 5

I agree with wolfman go with flameproof 1/4 to 1/2 watt resistors.

Answer 6

I'd try series and parallel with about 4 resistors