Im looking for some help from you LED guys. I want to built some tails and use these LED's
http://cgi.ebay.com/100-x-10mm-SUPER-BRIGHT-RED-LED-70-000mcd-F-Resistor_W0QQitemZ120028148839QQihZ002QQcategoryZ26207QQrdZ1QQcmdZViewItem But I want to know how many I can run in a row without a resistor? Ill be running 9 in one section and 18 in another. Or if I need a resistor what would I need?

 

Source Voltage divided by the Forward Voltage.

14.4/1.9 = 7.5 LEDs without a resistor.

 

Thanks for the reply! So how do you know what size resistor you'll need if you go over the seven?

 

if I was to run a lot of LEDs i would run it in parallel because of one goes out they all won't go out.

here's a good tutorial with a calculator link near the bottom of the page:
http://www.theledlight.com/ledcircuits.html

almost all vehicles i've seen with LED tails use these type of LEDs, much brighter and can still be seen with the sunlight on it. (flux leds)


along with a reflector bowl..

 

7 leds @ 1.9v = 13.3v voltage drop across leds

14.4v - 13.3v = 1.1v voltage drop across resistor

V=IR : R = V/I = 1.1v/0.020A = 55 ohms. (using 20mA current from specs)

this is for 7 leds wired in series with a dropping resistor inline.

if you use 8 leds in series then you'll be driving them at only 1.8v and they will probably be a little dimmer, but no resistor will be needed.

what ever you do don't over drive them. they will initially look very bright, but after a couple of months they will begin to dim and eventually go out.

 

Oh God I'm back in Physics class all over again.

 

I noticed that the Chrysler Tomahawk motorcycle (sic?) uses a bunch of 5 watt leds for head lights, has anyone tried this?

 

I use the LED wizard...learned about through PeteNJ who had posted it in his LED sidemarker DIY...

http://led.linear1.org/led.wiz