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What resistance will I need?
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.
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.
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