Basic LED circuit
The simplest circuit to drive a LED consists of a voltage source and two components connected in series: a current limiting resistor, sometimes called the ballast resistor, and a 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 a LED, since energy is lost in the resistor. More complicated circuits improve the energy efficiency.
A 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.
Power source considerations
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 or equal to the threshold, 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, but energy is wasted in the resistor. For higher supply voltage or higher current an active constant current regulator needs to be used.
Miniature indicator LEDs are normally driven from low voltage DC via a current-limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficiency tends to reduce at low currents, but indicators running on 100 μA are still practical.
Series resistor calculation
The formula to calculate the correct resistance to use is:
- Vs is the power supply voltage, e.g. a 12-volt battery.
- Vf is the LED forward voltage (also referred to as the “voltage drop”) across the LED.
- If is the desired current of the LED.
Typically, the forward voltage of an LED is between 1.8 and 3.3 volts. It varies by the color of the LED. A red LED typically drops 1.8 volts, but voltage drop normally rises as the light frequency increases, so a blue LED may drop from 3 to 3.3 volts.
This table shows some typical Vf values vs LED colors: