Tag: current loops

A fundamental principle in instrumentation system troubleshooting is that every instrument has at least one input and at least one output, and that the output(s) should accurately correspond to the input(s). If an instrument’s output is not properly corresponding to its input according to the instrument’s design function, there must be something wrong with that…

Some self-powered (4-wire) analog electronic transmitters are designed to behave as electrical loads rather than as electrical sources. Such transmitters are commonly referred to as having passive or sinking 4-20 mA outputs, as opposed to the active or sourcing 4-wire transmitters previously described: At first this seems needlessly confusing. Why build a self-powered transmitter requiring a second power supply in the circuit to…

It is possible to convey electrical power and communicate analog information over the same two wires using 4 to 20 milliamps DC, if we design the transmitter to be loop-powered. A loop-powered transmitter connects to a process controller with only two wires, which is why loop-powered transmitters are synonymously known as 2-wire transmitters: Here, the transmitter is not really…

DC electric current signals may also be used to communicate process measurement information from transmitters to controllers, indicators, recorders, alarms, and other input devices. Recall that the purpose of a transmitter is to sense some physical variable (e.g. pressure, temperature, flow) and then report that quantity in the form of a signal, in this case a 4…

The simplest form of 4-20 mA current loop is the type used to represent the output of a process controller, sending a command signal to a final control element. Here, the controller supplies both the electrical power and signal information to the final control element, which acts as an electrical load. To illustrate, consider the…

A 4 to 20 mA current signal represents some signal along a 0 to 100 percent scale. Usually, this scale is linear as shown by this graph: Being a linear function, we may use the standard slope-intercept linear equation to relate signal percentage to current values: Where, y = Output from instrument x = Input to instrument…

An “analog” electronic signal is a voltage or current proportionate to the value of some physical measurement or control quantity. An instrument is often classified as being “analog” simply by virtue of using an analog signal standard to communicate information, even if the internal construction and design of the instrument may be mostly digital in…