<Dd> R = V s − V f I (\ displaystyle R = (V_ (s) - V_ (f) \ over I)) </Dd> <P> where V is the power supply voltage, e.g. a 9 - volt battery; V is the LED forward voltage (also referred to as the "voltage drop") across the LED; and I is the desired current of the LED . Both V and the maximum recommended value for I are obtained from the LED manufacturer's specifications . The above formula requires the current in amperes, although this is usually given by the manufacturer in milliamperes, such as 20 mA . Many circuits operate LEDs at less than the recommended maximum current, to save power, to permit the use of a standard resistor value, or to reduce brightness . </P> <P> Typically, the forward voltage of an LED is between 1.8 and 3.3 volts . It varies by the color of the LED . A red LED typically drops 1.8 volts, but voltage drop normally rises as the light frequency increases, so a blue LED may drop from 3 to 3.3 volts . </P> <P> The formula can be explained considering the LED as a V f I Ω (\ displaystyle (V_ (f) \ over I) \; \ Omega) resistance, and applying Kirchhoff's voltage law (KVL) (R is the unknown quantity): </P>

What is the voltage drop across an led
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