JUL-AUG 2017

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INTECH JULY/AUGUST 2017 45 Certification Review | association news 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: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006. signal to a digital (D) is required. This class of device is referred to as an "A/D converter." An "I/P transducer" is used to convert an analog current (I) signal to a pneumatic (P) signal, as for actuation of final control elements. A "P/I transducer" is used to convert a pneumatic sig - nal (P) to an analog current (I) signal, as for a pneumatic trans- mitter in a programmable logic controller loop. A "DP transmitter" is a differential pressure transmitter, which can output a pneumatic, an analog, or a digital signal, depend- ing on the model of transmitter used. Reference: Goettsche, L. D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005. CAP question 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 unin- tended 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 electro- magnetic fields when turned on. CAP answer The answer is B, "Short circuit from the load device to ground can cause unintended actuation." An NPN (sinking) open- col lector, when "energized," switches the load side of the re- lay (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. CCST question To change a 4–20 mA analog signal from a transmitter to the sig- nal required by a digital controller, a(n) _____ must be between the transmitter and controller in the measurement loop. A. I/P transducer B. P/I transducer C. DP transmitter D. A/D converter CCST answer The correct answer is D, "A/D converter." A digital controller re- quires a digital signal as its input. A 4–20 mA transmitter outputs an analog signal. Therefore, a device to convert an analog (A) ISA Certified Automation Professional (CAP) program Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems. ISA Certified Control Systems Technician (CCST) program Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumenta- tion for systems that measure and control level, temperature, pressure, flow, and other process variables. +24 VDC GND NPN sinking R

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