What is the drive voltage for the optocoupler

The drive voltage for an optocoupler is determined by the forward voltage of its internal LED, typically ranging from 1.2 V to 1.4 V for infrared LEDs, and must be applied with a current-limiting resistor to achieve the specified forward current.

LED Side (Input) Considerations

The optocoupler's LED requires a forward voltage (V_F) to emit light, which is usually around 1.2–1.4 V for infrared LEDs used in standard phototransistor optocouplers . To drive the LED correctly, a series resistor (R1) is used to set the forward current (I_F) according to the supply voltage. The forward current is critical because it directly affects the current transfer ratio (CTR), which is the ratio of output current to input current (I_C/I_F) and determines the efficiency of signal transfer . Typical design currents for small-signal optocouplers are in the range of 5–10 mA, but this depends on the specific device and application .

Output Side (Phototransistor)

The phototransistor side operates with a collector-emitter voltage (V_CE), which is usually limited to a safe maximum specified in the datasheet, often around 5–30 V for low-power devices . When the LED is fully on, the phototransistor can enter saturation, with V_CE dropping to around 0.2–0.4 V, allowing maximum current flow through the load resistor . The output voltage swing depends on the supply voltage of the output circuit and the load resistor used.

Practical Design Notes

• Ensure the series resistor on the LED side is chosen to provide the correct I_F without exceeding the LED's maximum rating .
• The supply voltage for the LED side should be higher than the LED forward voltage by an amount sufficient to allow proper current limiting. For example, with a 5 V supply and a 1.2 V LED, the resistor must drop the remaining 3.8 V at the desired current.
• Avoid exceeding the reverse voltage rating of the LED, as this can damage the device .
• The CTR varies with LED current and V_CE, so the drive voltage and current should be selected to ensure reliable operation within the specified CTR range . In summary, the drive voltage is not a fixed value but is determined by the LED forward voltage and the series resistor needed to achieve the desired forward current, typically around 1.2–1.4 V across the LED with a current of 5–10 mA for standard phototransistor optocouplers. Proper design ensures reliable signal transfer while protecting the device from overcurrent or overvoltage conditions.