WHY DOES LED NEEDS A RESISTANCE?

The short answer: to limit the current in the LED to a safe value.

The long answer: LEDs are semiconductors, diodes in particular. The current flowing in an LED is an exponential function of voltage across the LED. The important part about that for you is that a small change in voltage can produce a huge change in current. That is the most important concept of this article. Resistors aren’t like that. The current and voltage in a resistor are linearly related. That means that a change in voltage will produce a proportional change in current. Current versus voltage is a straight line for a resistor, but not at all for an LED.

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regards

Ramesh