In electronics, an LED circuit or an LED driver is an electrical circuit used to power a light emitting diode (LED). The circuit must supply enough current to illuminate the LED to the required brightness, but it must limit the current to avoid damage to the LED.
The voltage drop on an LED is approximately constant over a wide range of operating current; therefore, a small increase in the applied voltage increases the current. Very simple circuits are used for low power indicators. To obtain a correct current regulation, more complex current source circuits are required when controlling high power LEDs for lighting.
The simplest circuit to drive an LED consists of a voltage source and two components connected in series: a current limiting resistor, sometimes called a ballast resistor and an LED. Optionally, a switch can be inserted to open and close the circuit. Although simple, this circuit is not the most energy efficient circuit to drive an LED because energy is lost. The most complicated circuits improve energy efficiency.
An LED has a voltage drop specified to the desired operating current. The law of Ohm and the laws of the Kirchhoff circuit are used to calculate the correct value of the resistor to obtain the desired current. The value is calculated by subtracting the voltage drop from the supply voltage and dividing the desired operating current. If the supply voltage is equal to the LED voltage drop, a resistor is not necessary.
This basic circuit is used in a wide range of applications, including many consumer devices, such as cell phone chargers.
Multiple LED strings are normally connected in series. In one configuration, the source voltage must be greater than or equal to the sum of the individual LED voltages; Typically, LED voltages reach up to about two-thirds of the supply voltage. A single current limiter can be used for each string.
Even the parallel operation is possible, but it could be more problematic. The parallel LEDs must have near direct voltages (Vf) to have similar branching currents and therefore a similar light output. Changes in the manufacturing process can make it difficult to achieve satisfactory operation when connecting certain types of LEDs in parallel.
