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Choosing a Load Resistor for an Emitter
The simplest way to operate an infrared emitting diode is with a steady-state current. The current to the emitter is controlled by a load resistor in series with the emitter. Each of Vishay’s emitter datasheets includes a graph defining the Forward Voltage versus the Forward Current. In figure 1 the curve for the TSHG6400 is shown. The TSHG6400 an infrared, 850 nm emitting diode based on GaAlAs, double hetero (DH) technology with high radiant power and high speed. It is molded in a clear, untinted plastic package.
Reading the Curve
With a forward current of 100 mA, the forward voltage is approximately 1.5 V. For a forward current of 1 A, the forward voltage is 2.3 V. Driving this emitter with a constant current of 50 mA will result in a forward voltage of 1.4 V. With a supply voltage of 5 V the series resistor is then calculated to be 72 Ω. The nearest value using an E24 resistor table is 75 Ω.
5 V – 1.4 V = (.050 A) ÷ RL
RL = 3.6 V ÷ .050 A
RL = 72 Ω
The curve below is for the VSMY7850X01 surface emitter. The VSMY7850X01 is an infrared, 850 nm emitting diode based on surface emitter technology with high radiant power and high speed, packaged in the low thermal resistance Little Star package. Its 42 mil chip provides outstanding low forward voltage and allows DC operation of the device up to 1 A. Though it is based on completely different technology and can be driven at higher currents, it has similar forward voltage values to the TSHG6400. At 100 mA the forward voltage is 1.2 V and at 1 A, the forward voltage is 2 V. It would use the same load resistor of 75 Ω.
There is not a lot of variability in these curves for infrared emitting diodes.
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