A 24 VDC PLC digital I/O circuit is connected to a device by physical wiring, using a 24 VDC power supply to provide power to the circuit. The PLC I/O card backplane typically operates on 5 VDC or 12 VDC that is managed by the PLC through its own, connected power supply. The devices commonly used to protect these circuits from external electrical threats are provided by:A. Junction FETs
B. Bipolar SCRs
The answer is C, “Optoisolators.”
An optoisolator is an electronic component that transfers electrical signals between two isolated circuits by using light. In the case of a programmable logic controller (PLC) digital output card, the card turns on an LED inside the card, which switches on the field current using a phototransistor. For a PLC digital input card, the two components are in the opposite position. Photo isolation keeps the PLC backplane circuit isolated from the field circuit, protecting both circuits from each other due to short circuits, surges, and other electrical threats.
Reference: Sands, Nicholas P. & Verhappen, Ian, A Guide to the Automation Body of Knowledge, Third Edition, ISA Press, 2019.
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