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Can the voltage of photocoupler be reduced if connected to a resistor?

Latest Updated:01/01/2009

Question:

The maximum input voltage of a photocoupler is 5 V in my system. However, none of the input (LED) forward voltages stated in the characteristics tables are more than 2 V.
In such a case, can the voltage applied to the photocoupler be decreased by connecting a resistor to the photocoupler in series?

Answer:

The forward voltage is the voltage generated when the specified current flows between the anode and cathode of the LED. It is not the input voltage.
The photocoupler operates correctly if at least the maximum forward voltage is applied.
However, if the input voltage is too high, the forward current increases, causing the dissipation on the input side to increase. Consequently, by inserting a resistor for controlling the current in series, the system must be designed so that the ratings of the forward voltage and power dissipation are not exceeded.

Suitable Products
Photocouplers / Optocouplers
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