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Why should you always include a resistor with an LED?

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Importance of a current-limiting resistor in an LED circuit


Typical LED ciruit


LED is basically a forward-biased diode:

LED a forward biased diode

Like a revered biased zener, it has a fairly constant voltage drop of 1.5V to 3V.

If you supply more than this voltage, even slightly more, a large current will flow through it, as there is no mechanism to absorb the excess voltage (LED - supply). Result: LED damage from overcurrent.

Mechanism of current limiting resistor: 

A resistor in series absorbs the voltage difference and sets a safe current through it.

A series resistor also has a secondary purpose of setting the light intensity of the LED. Especially in a battery powered gadgets, it is important to conserve battery.

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