??? to the LED guys here???

nharkey85 · Dec 7, 2009

yeah i was afraid of that. damn chevy. thanks guys

yosh2000 · Dec 12, 2009

PreRunnerSeth said: ↑

yes you will need resistors. Heres how to figure it out. First in the specs of the LED you will see Vf (forward voltage) and IF (forward current).

Example PLCC2 White LED. Vf = 3.2V IF = 20mA

First you need to know how much voltage to drop across the resistor.
V = Voltage at the bulb 12-14V for a car.

V - Vf = drop across resistor.

14V - 3.2V = 10.8V (the voltage needed to drop across the resistor)

Now you need the resistance of the resistor needed to drop that voltage at 20mA (the rating of the LED)

R = E (V) / I (If of the led)

R = 10.8V / 0.020A (mA converted to A) = 540 Ohms

You will need a 540Ohm resistor to get 20mA on the LED at max voltage of 14V.

Now you need to know what power rating resistor you need.

P = E (voltage across resistor) x I (Current through resistor)

P = 10.8V x 0.020A = .216W or 216mW you will need a 1/4 (.250)W resistor.

Now If you want to put 2 LEDs in series. just add two Vfs together and use that value as Vf in the first equation.

3.2V + 3.2V = 6.4V Now use 6.4V as the Vf in the first equation.

Note: always use the high end of your voltage range for this calculation. If you use the low end then at higher voltage you will overcurrent the LED.
Click to expand...

here is also an easy site that will help design and tell you all the wattage rating resistors you need...just remembered about it today.

PreRunnerSeth · Dec 12, 2009

yosh2000 said: ↑

here is also an easy site that will help design and tell you the wattage rating resistors you need...just remembered about it today.
Click to expand...

haha.. i knew there were sites.. but thats cheating.. thought it better to explain how its calculated.

yosh2000 · Dec 12, 2009

true...you explained the theory, i provided the crutch!!

BTW, picked up a weller WP25 iron this week and just did a cluster w/ it today, so much easier than the iron i had before! thanks again for all your soldering help and suggestions, it is much appreciated.

PreRunnerSeth · Dec 12, 2009

yosh2000 said: ↑

true...you explained the theory, i provided the crutch!!

BTW, picked up a weller WP25 iron this week and just did a cluster w/ it today, so much easier than the iron i had before! thanks again for all your soldering help and suggestions, it is much appreciated.
Click to expand...

no problem.

nharkey85 · Dec 12, 2009

PreRunnerSeth said: ↑

haha.. i knew there were sites.. but thats cheating.. thought it better to explain how its calculated.
Click to expand...

stumbled across that exact site few days ago. and speaking of new equipment, i need a new tip for my unit like a dog needs a bone.

Think the next work i will be doing is pink idiot lights in the cobalt

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