Which component provides isolation between high voltage and the PLC logic in input circuits?

• A. Optical isolator
• B. Resistor
• C. Capacitor
• D. Diode

Answer: (A)

The essential idea is to keep the high-voltage field side separate from the PLC logic so dangerous voltages or transients can’t reach the control electronics. An optical isolator accomplishes this by sending the input signal across a non-conductive gap using light. The LED on the high-voltage/input side activates a photodetector on the PLC side, so there’s no direct electrical path between the two realms. This provides strong isolation, improves noise immunity, and protects the PLC from voltage spikes while allowing the signal to be translated to a safe logic level.

Resistors just limit current and don’t stop voltage from crossing domains. Capacitors can couple signals and noise between sides, not isolate them. Diodes allow current flow in one direction but also don’t provide true isolation between the high-voltage and low-voltage circuits.