November/December 2012
Certification Review

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

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

CCST answer

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.

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.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments & Systems, 2nd Edition (2005), ISA Publishing