How to select precision resistors for 4–20 mA current loop
The 4-20 mA circuit is widely used in many monitoring and control circuits. It is often used in many industrial applications where processes are remotely monitored and controlled from a central station upwards of 1000 feet from the processes themselves. However, it is also used in applications where the monitors are very close to the process being monitored. This is accomplished with a closed loop circuit that includes a DC power supply to power the loop, a sensor/transducer/transmitter that inserts the sensor-related current into the loop, and a read-out or control device at the other end of the loop.
Generally, analog sensors with just microvolt level outputs continuously detect changes in any significant stage in the process, e.g.: temperature in containment vessels, flow rate in pipes, positioning of control equipment, pressure in a boiler, etc. The analog sensor output is often amplified to a convenient working level, processed, converted to an output current and distributed over a long distance to a remote facility where a monitor or control circuit indicates a set of readable characters representative of the output of the sensor.
The low output voltage level of the analog sensor and the long distance to the control room together constitute significant system design challenges. To begin with, the processing of the microvolt level sensor output must be brought up to a suitable level for processing with minimal loss or distortion. This is generally accomplished by superimposing the low-level sensor output signal onto a higher-level carrier signal through a bridge network, with its output feeding directly into a differential amplifier to extract and amplify the lower signal while rejecting the common carrier signal completely. The differential amplifier requires a set of precision resistors for complete common mode rejection. The surviving amplified sensor output voltage is then converted to a current representative of the sensor output signal.
The current signal must then be transmitted to a distant location with minimal distortion, suppression, interference, noise, or attenuation. Wireless transmission is subject to noise and interference so the transmission is accomplished via fixed wire connection to the monitoring station. The long transmission line and all the components along the circuit may change due to heat, cold, stress, and drift, but the sensor-related current injected into the loop must remain the same throughout the loop regardless of changes within the loop (Kirchoff’s law.) The 4 mA minimum current provides a stable zero-signal base circulating current not influenced by any resistance changes in the circuit. The 4 mA is used as the zero-signal so that there can be a definitive differentiation between this zero-signal and an open circuit while the sensor-related current injected into the loop is added to the minimum 4 mA base current.
Finally, there must be an accurate and reliable means of pulling the actual sensor-related signal from the line, from the amount of current above the zero-base 4 mA current. The system’s maximum signal is generally 20 mA because this amount of current conveniently converts to a 5 V level with the selection of a suitable sense resistor, with 4-20 mA producing the 1 V to 5 V commonly used for metering and control circuits.
A 250-Ω foil resistor is often used as the sense resistor in the current loop. This is a critical resistor for system operation. Because the voltage across this resistor is fed directly to the meter, any error in this resistor would present a wrong voltage to the meter and the system would erroneously indicate that the sensor is reading a condition that it is not actually reading. The process control correction then made on this basis would then cause the system to operate at less than optimal conditions and reduce its process efficiency, which could result in increased operating costs, environmental pollution, hazardous spills, associated fines, and poor product quality.
Because the sensor output is often amplified from a microvolt level and converted to the current output of the transmitter, the transmitter output—while being representative of the sensor output—does not necessarily have a linear relationship to the sensor output. Therefore, a percentage error in the sense resistor may represent a much larger misrepresentation of the sensor output than that same percentage. So the stability of the sense resistor is a critical factor in the monitoring and control of the system.
Figure 1. Examples of common current loops circuits
Critical Factors in the Stability of the Sense Resistor
Some process controls are not very critical but many, many are—particularly when a process is operating near a tipping point where it could get out of control quickly if not well monitored. Entire production batches have been lost or suffered reduced reliability when critical parameters were not kept within narrow limits. One thing that can cause this to happen is changes in the precision sense resistor over time. Reference points in the control process thus become less and less reliable. Repeatability of the process from batch to batch begins to drift. The process is changing while the monitors appear to be holding it within specified limits because the sense resistor is producing a different output voltage than it was in previous runs for the same sensor output. So the process appears to be under control when, in reality, it is experiencing an undetected drift.
Long-term stability is thus one of the considerations that drive the selection of which resistor technology to use in the application. Wirewound resistors might be considered for leaded configurations, though they are much less often used in modern assembly practices. As for surface-mount chip resistors, the options are thin film versus foil.
Thermal Electromotive Force (EMF)
In a resistor, the resistance is composed of a resistance element of one material and two terminations of a different material. When the junction of the element and the termination is heated in a closed circuit, there is a DC voltage generated in the circuit (see Seebeck and Peltier Effects). The polarity of the voltage is in one direction moving from junction A to junction B, but is the opposite polarity when moving from junction B to junction A. Hence, if both termination junctions of the resistor are at exactly the same temperature the voltages cancel each other out and there is no net thermal EMF voltage generated in the circuit due to thermal EMF error voltages in the resistor.
In fact, however, the terminals are very seldom at the same temperature because their temperatures are influenced by uneven power dissipation within the resistor, differential heating from other components on the board, and heat conducted along the board itself. Obviously, in a sense resistor that’s supposed to accurately convert a current to a voltage, the presence of an extraneous thermal EMF voltage could constitute a significant error source in the system. So designers should choose a resistor with a low thermal EMF voltage across the element to termination junction.