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How Does LED Brightness Vary by Currents?

Diodes are semiconductors designed to limit current flow to one direction. LED is an acronym for light emitting diode, and an LED is optimized to emit light when it is given a voltage. An LED̵7;s brightness increases as the amount of current it receives increases. To control the current level, most LEDs inside a circuit must be paired with a resistor. Resistors are components that resist or limit the flow of current.

LED Operation
- A typical LED has two leads. It has a positive lead called an anode, and a shorter negative lead called a cathode. When current flows from the anode to the cathode, the LED is forward-biased. This means that it is placed inside the circuit so that its positive lead is indirectly or directly connected to the positive side of the battery source. When current flows in the other direction, the LED is called reverse-biased, which means that it is placed backward inside the circuit. An LED will emit light when it is forward-biased. Typical colors include red, green, yellow, and orange.
Power Ratings
- LEDs are rated for voltage and current. The voltage rating is the actual voltage drop that occurs when the component is inside the circuit. The current rating is the most important, because it tells the maximum amount the device can receive before it becomes damaged. It is also the level at which the LED is at its brightest. The LED becomes dimmer as the level is decreased. Most LEDs are rated between 10-20mA (milliamps). A red LED may have a rating of 2.2V (volts) and 10mA, for example.
LEDs and Ohm̵7;s Law
- The power ratings can be used with Ohm̵7;s law to determine the minimum size the current limiting resistor should be. Ohm̵7;s law gives the relationship between the voltage, current and resistance. It is V = IR, where V is voltage, I is current, and R is resistance. For an LED with a current a current rating of If, a voltage rating of Vf, with a battery V, the resistor R = (V ̵1; Vf)/If.
Brightness Experiments
- You can study the relationship between LED brightness and current with a circuit composed of two AA batteries, a resistor and a red or green LED. The two AA batteries provide 3.0V. For an LED with a rating of 2.2V and 15mA, convert the current to amps, and use Ohm̵7;s law. Then R = (3V ̵1; 2.2V)/0.015A = 253 ohms. Since resistors are sold in standard values, and the nearest one above 253 is 270, use 270 inside the circuit. Place the components on a breadboard and connect them in series. The resistor should be connected on one side to the positive side of the voltage, and on its other side to the LED̵7;s positive lead. The LED̵7;s negative lead should be connected to ground, which is the voltage source̵7;s negative side. Substitute resistor values of 330, 470, 1K and 10K ohms. The LED will grow dimmer as the resistance increases, and at 10K it will not light.

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