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Optocoupler module input output

Fibre interconnection and data room reference Published 25 Feb 2026

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An optocoupler transfers an electrical signal from its input to its output using light, isolating the two circuits electrically.

Input Side

The input of an optocoupler consists of a Light-Emitting Diode (LED). When a voltage or current is applied to the input terminals, the LED emits infrared light proportional to the input signal. The input can be digital or analog, depending on the application.

For digital signals, the LED turns on or off, representing logic high or low. For analog signals, the LED intensity varies with the input current, allowing proportional signal transfer. A current-limiting resistor is typically used in series with the LED to control the forward current and prevent damage.

Output Side

The output of an optocoupler is a photosensitive device, most commonly a phototransistor, though photodiodes, photo-SCRs, or photothyristors can also be used. The phototransistor detects the light emitted by the LED and converts it back into an electrical signal.

When the LE

D is on, the phototransistor conducts, allowing current to flow through its collector-emitter path, effectively acting as a closed switch.

When the LE

D is off, the phototransistor stops conducting, acting as an open switch. The output can be configured in saturated (digital) mode or linear (analog) mode, depending on whether the application requires a logic-level signal or a proportional analog signal.

Key Parameters

Current Transfer Ratio (CTR): The ratio of the output current (phototransistor) to the input current (LED), which determines the efficiency of signal transfer. Isolation Voltage: The maximum voltage difference the optocoupler can safely withstand between input and output, often several kilovolts. Response Time: The speed at which the output responds to changes in the input, influenced by the type of photodetector and circuit configuration.

Applications

Optocouplers are widely used to isolate high-voltage circuits from low-voltage control circuits, protect microcontrollers from voltage spikes, interface different logic levels, and prevent noise from propagating between circuits. They are essential in switching power supplies, sensor interfaces, and communication circuits where electrical isolation is critical. In summary, the input of an optocoupler is an LED that converts electrical signals into light, and the output is a photodetector that converts light back into an electrical signal, providing safe electrical isolation between circuits.

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Optocoupler module input output
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