Difference between 2 wire & 4 wire transmitter current loops

The current loop uses DC power because the magnitude of the current represents the signal level that is being transmitted. If AC power was instead used in the loop, the magnitude of current would be continuously changing, making it difficult to discern the signal level being transmitted.In this session we are gonna discuss about Difference between 2 wire & 4 wire transmitter current loops

2 wire transmitter current loop

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 in the following manner:

2 wire transmitter current loop

Here, the transmitter is not really a current source in the sense that a 4-wire transmitter is. Instead, a 2-wire transmitter’s circuitry is designed to act as a current regulator, limiting current in the series loop to a value representing the process measurement, while relying on a remote source of power to motivate current to flow. Please note the direction of the arrow in the transmitter’s dependent current source symbol, and how it relates to the voltage polarity marks. Refer back to the illustration of a 4-wire transmitter circuit for comparison. The current “source” in this loop-powered transmitter actually behaves as an electrical load, while the current source in the 4-wire transmitter functions as a true electrical source.

A loop-powered transmitter gets its operating power from the minimum terminal voltage and current available at its two terminals. With the typical source voltage being 24 volts DC, and the maximum voltage dropped across the controller’s 250 ohm resistor being 5 volts DC, the transmitter should always have at least 19 volts available at its terminals. Given the lower end of the 4-20 mA signal range, the transmitter should always have at least 4 mA of current to run on. Thus, the transmitter will always have a certain minimum amount of electrical power available on which to operate, while regulating current to signal the process measurement.

4 wire transmitter current loop

The simplest form of 4-20 mA measurement loop is one where the transmitter has two terminals for the 4-20 mA signal wires to connect, and two more terminals where a power source connects. These transmitters are called “4-wire” or self-powered. The current signal from the transmitter connects to the process variable input terminals of the controller to complete the loop:

4 wire transmitter current loop

Typically, process controllers are not equipped to directly accept milliamp input signals, but rather voltage signals. For this reason we must connect a precision resistor across the input terminals to convert the 4-20 mA signal into a standardized analog voltage signal that the controller can understand. A voltage signal range of 1 to 5 volts is standard, although some models of controller use different voltage ranges and therefore require different precision resistor values. If the voltage range is 1-5 volts and the current range is 4-20 mA, the precision resistor value must be 250 ohms. Since this is a digital controller, the input voltage at the controller terminals is interpreted by an analog-to-digital converter (ADC) circuit, which converts the measured voltage into a digital number that the controller’s microprocessor can work with.

In some installations, transmitter power is supplied through additional wires in the cable from a power source located in the same panel as the controller:

4 wire transmitter current loop

The obvious disadvantage of this scheme is the requirement of two more conductors in the cable. More conductors means the cable will be larger-diameter and more expensive for a given length. Cables with more conductors will require larger electrical conduit to fit in to, and all field wiring panels will have to contain more terminal blocks to marshal the additional conductors. If no suitable electrical power source exists at the transmitter location, though, a 4-wire cable is necessary to service a 4-wire transmitter.

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