Wondering how to wire led series

[darrylmcl]

Hi there, I have a series of LEDs that I need to get running. i plan to use this series in my car as a cutom LED gauge cluster (you know, speedometer, fuel, engine heat). Anyhow, I've got 17 white LEDs

Specs:
Voltage Supply = Unkown
17 LEDs
3.1 Voltage
20 - 30 mA

I was wondering if anyone may know the amount off voltage needed to power something like this and also the best way to wire them (i was thinking series).

Using a resistor calculator I've been told that I need 100 oHm resitistors. (this was with a 12V example) which doesn't make sense as the 12V's couldn't posibly illuminate this seires.

Thanks in advance

Darryl

 

[Bob S]

Hi Darryl-
You have to design this string to work on about 14 volts, the maximum voltage when the engine and alternator are on. My guess is that you want 4 LEDs and 100 ohm resistor in a string. There will be 12 volt drop across 4 LEDs in series, and 2 volts across the 100-ohm resistor. You could also do 3 LEDs and a 240 ohm resistor in series.
Bob S