For the AC discrete input module circuit schematic, what is the purpose of the optical isolator?

• A. To separate the higher line voltage from the logic circuits
• B. To amplify the signal
• C. To filter the signal
• D. To convert analog to digital

Answer: (A)

The key idea is galvanic isolation between the high-voltage line side and the PLC’s low-voltage logic. An optical isolator uses light to transfer the input signal without any direct electrical connection, so dangerous line voltages and transients cannot couple into the logic circuitry. In an AC discrete input module, when the input is present, the LED inside the optocoupler lights up and drives a photodetector on the logic side, producing a clean digital signal for the PLC while keeping the two voltage domains electrically apart. This protection is why the purpose is to separate the higher line voltage from the logic circuits. It isn’t primarily used to amplify, filter, or convert analog signals.