Feature: Sensors


Figure 3 Typical current loop using a four-wire transmitter


Field wiring Field wiring conductors are used in the loop to connect the transmitter to the process monitoring or control hardware. It is important to see them as an element of the loop because they have resistance and produce a voltage drop, just like any other element in the loop. If the sum of all the voltage drops is higher than the loop power supply compliance voltage, the current will not be proportional to the measured parameter and the system will produce unusable data. Te distance between the sensor-transmitter combination and


the process controller or readout can be several hundred feet or more. Te resistance of the field wiring conductors is normally given in Ohms per length, typically Ohms per 1000 feet, so the total resistance is the product of this value times the length of the wires divided by 1000. Note that the wire length includes the loop conductor going out and the loop conductor for the current return, which is twice the individual conductor length. Te total wiring resistance is represented by the symbol Rw


, as is shown


in Figure 1. Te voltage drop due to the field wiring is given by Ohm’s law:


Vw = IRw


Receiver or process controller Aſter the loop current is generated, it must usually be further processed in the system; for example, the current could be used as feedback to a valve controller to open, close, or modulate the valve in order to initiate or control a process. It is easier to perform control functions with a voltage rather than a current, and the receiver is the part of the loop that converts current into voltage. In Figure 1, the receiver is a simple resistor in series with the


loop, so from Ohm’s law the voltage developed across it is directly proportional to the measured physical parameter, or measurand. Te load resistor used in a 4-20mA current loop is not an


arbitrary value. For any specified compliance voltage, there is a maximum loop load resistance that will permit full current to be developed in the loop. Exceeding the maximum loop resistance, which must include the resistance of the field wiring, prevents the system from providing the full 20mA output current in the loop. In the case of a typical current output sensor, whose loop load


32 April 2021 www.electronicsworld.co.uk


graph is shown in Figure 2, at 18V input, the total loop load can be as high as 550 Ohms. At 24V input, total loop load can be as high as 850 Ohms, and at the system’s maximum input of 32V, the total loop load can be 1200 Ohms.


Choosing the right loop load resistor Te choice of loop load resistor usually depends on the input signal voltage the receiver system requires for good resolution. A 4-20mA loop current will develop 2-10VDC across a 500- Ohm load resistor (E = IR). If the receiver system will work satisfactorily with a lower input voltage, the 4-20mA loop current will develop 1-5VDC across a 250-Ohm load resistor, which is the most common loop load. Note that a loop load resistor is quite oſten already built into the receiver input terminal connections. Check the specifications of the receiving device to determine if this is the case. It is very important that the loop load resistor power rating


is sufficient to ensure that any heating caused by current flowing through it won’t change its value and thereby the voltage developed across it. Recall that the wattage dissipated by the resistor is I² x R. For a 500-Ohm load resistor, the power dissipated at 20mA is 0.2W. A good choice for the resistor’s power rating is at least 2W because such a load will not heat up very much. Even at full loop current, there won’t be a voltage change across the load resistor due to heat from power dissipation instead of actual loop current changes. Wire-wound resistors are preferred here, since they usually have lower temperature coefficients than metallised resistors.


Transmitter types Tere are several different varieties of current transmitters used for 4-20mA current loops. In general, they conform to the following categories, delineated by the number of connections required for operation: • Two-wire transmitters, which usually function as loop-powered current sinking devices.


• Tree-wire transmitters, which are independently-powered loop current-sourcing devices.


• Four-wire transmitters, which are normally independently-powered devices used when loop isolation is needed for noise or ground loop


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