4..20mA Current Loops: 2, 3 and 4 Wire Transmitters

Posted 26 Sep 2017 by Nikolay
2, 3 and 4 Wire loops
2, 3 and 4 Wire loops

2, 3 and 4 Wire loops in their vary basics. Description and wiring examples.

Two-wire 4-20mA loops (loop powered device). All 4-20 mA current loops require a power supply in order to operate. A lot of loop setups will utilize an external power supply in order to power the devices which are attached to the current loop.

It is possible, however, to power the device with the power supplied for the loop itself. These devices are referred to as loop-powered. The 2-wire transmitters used in these loops draw their operating power from the current flowing through them and the voltage across them.

Figure 1. 2 Wire transmitter

Loop-powered devices come with two input connections: a positive (+) and a negative (-). The current signal enters through the positive (+) terminal and leaves via the negative (-) terminal. Therefore the term ‘loop-powered’ is synonymous with the term ‘2 wire’ (meaning that only 2 connections/wires are involved in any of the connections between the transmitter, the power source and the output device).

The current is sourced by the power supply, flows in controlled fashion through the transmitter, then into the receiver, and returns to the power supply. The current flowing through the receiver produces a voltage that is easily measured by the analog input of a controller (like PLC) or monitoring device.

Benefit of loop-powered 2-wire transmitters is that they have their power supply and signal sharing the same pair of connection wires. This simplifies installation considerably, and the low DC transmission levels permit the use of small, inexpensive copper wiring.

The Transmitter is not the source of current, but simply regulates the flow and magnitude of the current through it.

Four-wire 4-20mA loops. A four-wire connection uses the current loop as a means to transmit the 4-20 mA process signal only. This type of connection will not draw the power it needs from the current loop. It will create a voltage drop on the loop, but this is minimal when compared to that of a loop-powered device. The power, four-wire devices need, is instead provided by an external power supply. This can be either an alternating or direct current power supply because the device is powered independently from the direct current loop.
Isolated four-wire connected devices “float” within the current loop. This means that the common, or the return process signal wire from the device does not connect to the power supply ground. As may be apparent from the name “four-wire,” two wires connect the power supply to the device and two wires connect the process signal to the device. Isolation, therefore, is built into the system. There is no electrical connection between the power supply and the process signal.

Four-wire transmitters can be AC or DC powered. In fact, 24VAC is a common power voltage for AC powered 4- wire transmitters. The following figures show basic transmitter connection:

Figure 2. 4 Wire transmitter

3-Wire devices. The standard also describes a Type 3 connection type, or 3-wire transmitter loop, where the Transmitter and Receiver share a ground connection with power, and the transmitter uses a third wire to connect to power outside of the current loop.

Figure 3. 3 Wire transmitter

Three-wire devices are commonly found to be lower cost than four-wire primarily because they do not feature isolation. They can also be slightly easier to install because they require less wire and, in the case where wire needs to be run through conduit, this wire can often be run along the same channels because they are already electrically connected.
In contrast to four-wire, three wire devices do not feature isolation because of the fact that the power supply common and the process signal return share the same wire. When dealing with complex 4-20 mA signal networks, an installer must be very careful while wiring the devices to avoid crossing current paths. Any grounds, commons or returns that cross paths with the process signal loop will cause current to travel into different circuits and the process signal will no longer provide predictable, usable current values. Three wire devices cannot be powered by an alternating current (AC) power supply. Four-wire devices can be powered via AC current because the connection powering the device is completely separate from the connection to the process signal. Three-wire devices do not feature this isolation, so all of the power in the system must be direct current (DC), just like the process signal loop.

Practical diagrams. 4 – 20 mA loops are built from in series connected elements. Shielded twisted pair wiring is often used to connect the longest distance between the field transmitter and remote receiver. The receiver device is commonly the input channel of a Programmable Logic Controller (PLC), a Digital Control System (DCS), or a panel meter. Some receivers already provide excitation for the transmitter and these are referred to as “sourcing” inputs. Other receivers that do not provide excitation are referred to as “sinking” inputs and these will require that a separate power supply connect in the loop. Here are example transmitter connection diagrams for “sourcing” and “sinking” receiver types:

Figure 4. Sourcing receiver

Figure 5. Sinking receiver

On the image below is shown a possible wiring of power supply, transmitter, sensor and display.

Benefits of 4..20mA current loops. The current loops have low sensitivity to electrical noises. This is very important for long distance transmission in harsh industrial environments. The accuracy of the signal is not affected by the voltage drop in the interconnecting wiring. This allows the signal transmission to occur over long distances, with varying conductors.

Compare this to voltage signals, which will always have an associated signal loss related to the length of the wires—the 4-20mA signal current does not exhibit any signal losses under this same scenario.

The 4mA “Zero-Offset”, “Live Zero”, or “Positive-Zero” is Failsafe. The use of 4mA as the starting point for our transmitted signal is useful in trouble-shooting, as signal integrity is verified with 0% of input and output signal. A failed current loop due to a lead break or open device can be immediately discerned as zero current flow, which is a fail-safe level outside of the signal range.
By offsetting the signal from zero, some transmitters will define an alarm limit just below 4mA and different from zero, allowing a receiver to detect other failures in the system, like an input sensor lead break. Having a live zero in your control system also allows you to set the “zero” of your controlled device (i.e. an actuator valve or other device) just a little bit below 4mA to hold it completely OFF. You wouldn’t be able to do that with a zero-based 0-20mA output. A “Live Zero” of 4mA also permits the two-wire current loop to power the transmitter, simplifying installation and reducing costs.

Related articles

  4..20mA Current Loops





Join the discussion