The circuit must provide sufficient current to light the LED at the required brightness, but must limit the current to prevent damaging the LED. Very simple circuits are used for low-power indicator LEDs. More complex, current source circuits temperature sensor circuit diagram pdf 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.