LED resistors
LED resistorsAuthor: NajeebThe need of a resistor in an LED is to limit the current in the LED to a safe value. LEDs are semiconductors or diodes in particular. The current flowing in an LED is an exponential function of voltage across the LED. As a result what happens is that a small change in voltage can produce a huge change in current. That is the most important concept of why resistors are used in LEDs. Resistors have specific properties. 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. Because of this, you can’t say that LEDs have resistance.
Resistance is defined as the constant ratio of voltage to current in a resistive circuit element. Even worse, there is no real way to figure out exactly the relationship between current and voltage for any given LED across all possible voltages other than direct measurement. The exact relationship varies among different colors, different sizes, and even different batches from the same manufacturer. When you buy an LED, it should come with a rating. That rating will look like something 5.5V @ 20 mA. That gives you a general point on the operating curve. This is normally a safe operating point. In addition you may get a maximum rating.
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