Design of Fundamental LED Circuits

The IR LED emitter is commonly continuously powered, forcing a constant DC current through the LED. The LED is connected to the DC supply via current limiting resistor (See schematic Fig.1). The value of this resistor is determined by the following equation :

RIN = (VCC – VF) / IF

VCC = supply voltage
VF = LED forward voltage
IF = LED forward current

To set the desired current IF, read the typical VF from the Typical Electro-Optical Characteristics graphs on the datasheet and calculate RIN.

Example :
For IF = 5mA, the graph shows VF = 1.2V.
Assuming VCC = 5V, then: RIN = (5V – 1.2V) / 0.005A = 760 ohms.


Fig 1

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