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Do I subtract the voltage?

So I was reading in my electronics for dummies book and it said there is about a 1.2 volt drop when using a standard LED. So, when calculating a resistor, with say a 9v battery, in my equation, do I make it R = (7.8 - 3) / 0.02 ? Instead of using 9v in my equation, do I have to subtract the voltage drop first??


Thanks!

~electricloser

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Orngrimm4 years ago
Different colors have different drop-voltages.
White has a very high one with around 3-3.6V
Check http://en.wikipedia.org/wiki/LED_circuit#Series_resistor_calculation for tha formula.

Also check http://en.wikipedia.org/wiki/Light-emitting_diode#Colors_and_materials for the different voltage-drops.
You probably want R = (9V - 1.2V) / 0.02A. If the circuit only consists of a battery, LED, and resistor this will give you the appropriate resistor value. I assume your original equation had a circuit element with a 3V drop, which you would replace with the 1.2V drop of your LED.
HavocRC (author)  advancedMischief4 years ago
Alright so if my LEDS are rated at 3v, then there would be a 3v drop? Alright, I think I see, you don't need to subtract the drop from the initial voltage. It's just what you put in for V2 in the equation.

Correct?
or
NO?
Correct.

Consider ALL the voltage drops, they HAVE to add up to the battery voltage.

So V battery = I x R + Vled.
HavocRC (author)  steveastrouk4 years ago
Hey thanks!