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# Do I subtract the voltage? Answered

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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## 5 Replies

Orngrimm (author)2012-12-25

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)2012-12-21

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?

steveastrouk (author)2012-12-22

Correct.

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

So V battery = I x R + Vled.

HavocRC (author)2012-12-22

Hey thanks!