Which protective device is commonly used to electrically isolate PLC inputs from higher voltages?

Which protective device is commonly used to electrically isolate PLC inputs from higher voltages?

• A. Optocouplers
• B. Relays
• C. Transformers
• D. Diodes

Answer: (A)

Protecting the PLC from higher voltages relies on providing galvanic isolation, so the control circuitry never shares a direct electrical path with the field voltage. An optocoupler achieves this by using light to carry the signal across a nonconductive barrier. The input side has an LED that turns on when the field device energizes; the output side contains a photosensitive element (like a transistor) that detects that light and switches the PLC input, all without any direct electrical connection between the two sides. This arrangement lets the PLC safely interact with higher-voltage equipment while remaining protected from voltage spikes, noise, and ground potential differences. Optocouplers are favored here because they offer solid-state, fast, compact isolation with a defined isolation rating. Relays can provide isolation via separate contacts but involve moving parts and slower actuation; transformers isolate via magnetic coupling but add bulk and require extra drive circuitry; diodes don’t provide true isolation—they simply steer or block current.

Protecting the PLC from higher voltages relies on providing galvanic isolation, so the control circuitry never shares a direct electrical path with the field voltage. An optocoupler achieves this by using light to carry the signal across a nonconductive barrier. The input side has an LED that turns on when the field device energizes; the output side contains a photosensitive element (like a transistor) that detects that light and switches the PLC input, all without any direct electrical connection between the two sides. This arrangement lets the PLC safely interact with higher-voltage equipment while remaining protected from voltage spikes, noise, and ground potential differences. Optocouplers are favored here because they offer solid-state, fast, compact isolation with a defined isolation rating. Relays can provide isolation via separate contacts but involve moving parts and slower actuation; transformers isolate via magnetic coupling but add bulk and require extra drive circuitry; diodes don’t provide true isolation—they simply steer or block current.