An LED circuit is any electrical power circuit used to power light-emitting diodes, or LEDs. At its simplest, an LED circuit comprises a voltage source, a resistor to reduce the voltage, and an LED. The LED circuit may contain other components too, such as switches and sensors, but these don't affect the basic principles involved in powering an LED.
LEDs are cheap, reliable, robust, and energy efficient. The LED circuit has become a common feature in electronic devices, automobiles, and appliances, where they serve as indicator lights and for minor illumination. Larger LEDs have been developed that have home applications, and these will no doubt become more important in the future, but this discussion will concentrate on the elements of a the LED circuit as it applies to smaller LEDs.
LEDs are simple, solid-state diodes. A diode is a device that allows electrons to flow in only one direction by interposing a bit of semiconductor material in the current's path at what's known as the junction. In LEDs, this semiconducting material is specially treated to emit photons of light when current passes through it. When electrical current passes through the LED in the right direction, it's said to be forward biased and the semiconductor material in the junction glows.
When the flow of current is reversed, a condition known as reverse bias, no electrons flow through the semiconducting material and no light is emitted. Excess reverse bias voltage will easily damage the junction, so this reverse breakdown voltage must be guarded against. For this reason, direct current is preferred to alternating current for powering LEDs, making batteries ideal power sources.
Excess forward bias voltage may also damage the LED junction, so a resistor is almost always used to reduce the voltage to the device. LEDs are rated by the manufacturer regarding both their current draw and their resistive voltage drop under forward bias, or Vf, and these must be taken into consideration when choosing a resistor for an LED circuit. The value of the necessary resistor can be calculated by subtracting the LED's resistive voltage drop from the power supply voltage, and dividing the results by the LED's listed current rating.
The danger of reverse breakdown voltage makes it critical that the LED be placed in the LED circuit with the correct polarity. LEDs allow electrons to pass from their cathodes to their anodes, so LEDs must be wired so that their cathodes connect to the negative pole of the power supply and their anodes to the positive side. Manufacturers indicate the polarity of the LED in several ways, notably by making the cathode wire shorter, flat, black, or by flattening or notching the cathode side of the LED case.
The typical LED circuit as used to power a small indicator light will use a 1.5 to 9 V battery as a power source. It will have a typical forward voltage drop, about 1.8 to 3.3 volts, depending on the color. The LED will draw perhaps 5 to 20 mA of current.