In electronics, an LED circuit is an electrical circuit used to power a light-emitting diode (LED). The circuit must provide sufficient current to light the LED at the required brightness, but must limit the current to prevent damaging the LED. The voltage drop across an LED is approximately constant over a wide range of operating current; therefore, a small increase in applied voltage greatly increases the current. Very simple circuits are used for low-power indicator LEDs. More complex, current source circuits are required when driving high-power LEDs for illumination to achieve correct current regulation.
The simplest circuit to drive an LED consists of a voltage source and two components connected in series: a current limiting resistor, sometimes called the ballast resistor, and an LED. Optionally, a switch may be introduced to open and close the circuit. Although simple, this circuit is not the most energy efficient circuit to drive an LED, since energy is lost in the resistor. More complicated circuits improve the energy efficiency.
An LED has a voltage drop specified at the intended operating current. Ohm's law and Kirchhoff's circuit laws are used to calculate the appropriate resistor value to obtain the desired current. The value is computed by subtracting the LED voltage drop from the supply voltage, and dividing by the desired operating current. If the supply voltage is equal to the LED's voltage drop, no resistor is needed.
This basic circuit is used in a wide range of applications, including many consumer appliances such as mobile phone chargers.
The voltage versus current characteristics of an LED are similar to any diode. Current is approximately an exponential function of voltage according to the Shockley diode equation, and a small voltage change may result in a large change in current. If the voltage is below the threshold or on-voltage no current flows and the result is an unlit LED. If the voltage is too high the current exceeds the maximum rating, overheating and potentially destroying the LED.
It is therefore important that the power source provides an appropriate current. LEDs should only be connected to constant-current sources. Series resistors are a simple way to passively stabilize the LED current.