This automation industry quiz question comes from the ISA Certified Automation Professional (CAP) certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program.
In a 24 VDC open-collector output sinking configuration, which of the following could occur?
a) configuration increases the risk of electrostatic discharge (ESD) from static electricity
b) short circuit from the load device to ground can cause unintended actuation
c) the triode for alternating current (TRIAC) could remain on when the current (but not voltage) is zero unless a snubber network is used
d) inductive devices, such as motors, can generate back electromagnetic fields when turned on
e) none of the above
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The correct answer is B, “Short circuit from the load device to ground can cause unintended actuation.” An NPN (sinking) open-collector, when “energized,” switches the load side of the relay (at terminal “A”) to ground (terminal “B”). In the drawing, the NPN open-collector is energized at its base with a light-emitting diode in an optically isolated programmable logic controller output card.
If the circuit below “short circuits” from the load device (relay) to ground, it would have the same effect as switching on the NPN open-collector, and the relay (load) would be actuated.
Note: Resistors that are typically installed in these circuits to regulate or limit current have not been shown for simplicity of illustration.
Reference: Nicholas Sands, P.E., CAP and Ian Verhappen, P.Eng., CAP., A Guide to the Automation Body of Knowledge. To read a brief Q&A with the authors, plus download a free 116-page excerpt from the book, click this link.