PLC inputs are often protected from hostile voltages by ______.

PLC inputs are often protected from hostile voltages by ______.

• A. Optocouplers
• B. Relays
• C. Zeners
• D. Capacitors

Answer: (A)

Optocouplers provide galvanic isolation between the external input and the PLC’s internal circuitry. By transmitting the signal through light rather than a direct electrical connection, they keep high voltage spikes, ground shifts, or interference on the outside world from reaching the PLC logic. An LED on the input side is energized by the external voltage, and its light drives a photodetector on the PLC side, which then triggers the internal logic. This separation protects the PLC and improves safety and noise immunity. Other options don’t offer true isolation. Zeners or capacitors can clamp or filter voltages but don’t prevent a voltage difference from appearing across the PLC’s input circuitry, while relays provide physical separation but are slower and bulkier. The key protective mechanism here is the electrical isolation achieved by the optocoupler.

Optocouplers provide galvanic isolation between the external input and the PLC’s internal circuitry. By transmitting the signal through light rather than a direct electrical connection, they keep high voltage spikes, ground shifts, or interference on the outside world from reaching the PLC logic. An LED on the input side is energized by the external voltage, and its light drives a photodetector on the PLC side, which then triggers the internal logic. This separation protects the PLC and improves safety and noise immunity.

Other options don’t offer true isolation. Zeners or capacitors can clamp or filter voltages but don’t prevent a voltage difference from appearing across the PLC’s input circuitry, while relays provide physical separation but are slower and bulkier. The key protective mechanism here is the electrical isolation achieved by the optocoupler.