In a field device powered sensor, which component provides optical isolation to prevent noise transfer?

• A. Opto-isolator
• B. Transformer
• C. Resistor network
• D. Capacitor

Answer: (A)

Optical isolation uses light to transfer the signal between two separate circuits while keeping them electrically apart. An opto-isolator has an LED on the input side and a light-sensitive detector on the output side, with a solid barrier between them. When the sensor output changes, the LED’s brightness changes and the detector converts that light back into an electrical signal on the isolated side. This light barrier prevents any direct electrical path, so noise, ground loops, and voltage transients on the power side can’t couple into the sensing circuitry, and vice versa. That separation is exactly what protects the control electronics in a field-device powered sensor in noisy environments.

Transformers provide isolation through magnetic coupling, which is a different mechanism and not optical. Resistor networks and capacitors don’t offer true isolation; they link the circuits and can pass noise.