This automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.
How can a transmitter’s current output signal be converted to a voltage-input signal, as required by an electronic controller?
a) a resistor is placed across the input terminals of the controller
b) all wiring in the loop is tied positive-to-negative
c) a forward bias diode is placed between the transmitter and controller
d) capacitor is placed across the output terminals of the transmitter
e) none of the above
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Answer B is a true statement for a current loop, but is not the way that a current signal is converted to a voltage signal that is required by the controller. This answer indicates the driving force for direction of current flow.
Answers C and D would electrically modify the behavior of the circuit, but would not convert current signals to voltage signals. A diode across the controller could be used to prevent current flow in the reverse direction, or could be used with an LED in an optical isolation circuit. A capacitor in a current loop could be used to suppress surges at the transmitter terminals.
The correct answer is A, “A resistor is placed across the input terminals of the controller.” A 250Ω resistor in a 4-20mA DC current loop will produce a 1-5VDC signal, as indicated in Ohm’s law: E = I · R, where E is voltage, in volts; I is current, in amps; and R is resistance, in ohms. At 4mA (0.004A), E = 250Ω x 0.004A = 1V. At 20mA (0.020A), E = 250Ω x 0.020A = 5V.
Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition