If you’ve ever sat down and designed (from scratch) a discrete 4-20 mA control loop, you may have found it a bit more challenging than it appeared. First, current loop communication (both analog and digital) have been around for many, many years. It dates back to the 1950s when it was introduced to replace the pneumatic 3-15 psi control systems.
Today, these control loops are still very much in use. A simple example loop is shown in the figure below. Several factors are required to make this work. First, you need to make sure that the sending voltage (VSEND) is greater than the total voltage drops of all series impedances when 20 mA of current is present in the loop. For instance, if the wire impedance represents 100 Ohms, and the sensing resistor is 250 Ohms, then the total drop is (100 + 250) * 0.020, which equals 7 volts. For two-wire senders (sensors) such as a temperature sensor, the remote device may require some working voltage, which means the sending voltage needs to be higher to overcome the resistive losses.
This is a very simple example. What becomes more difficult is building a variable current sender along with a reliable receiver. This could be done with discrete operational amplifiers and discrete components, which was the traditional way of building these systems. However, there are highly integrated 4-20 mA transmitters and receivers available that greatly simplify the design as well as provide additional features. For example, the XTR105 and RCV410 make building sensor senders and receivers much simpler.
The example in the diagram shows how simple a 4-20 mA RTD temperature sensor can be built along with a fully isolated receiver using the XTR105 (transmitter) and RCV420 (receiver). In addition to the 4-20 mA loop transmitter, the XTR105 also provides second order RTD linearization for more accurate measurements. The entire sensor only requires a few discrete components and is completely powered from the sending voltage (receiver) side. This example also includes an isolation amplifier (ISO122) to completely isolate the loop from the rest of the system. For more details on why that’s important, check out my article on Electronic Design, “Protect PLCs from Transient Surges.”
So, the next time you need to design an industrial 4-20 mA loop system, check out TI’s entire family of transmitters and receivers specifically designed for this purpose. It will make your design job much easier! Till next time…
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