4-20 mA Transmitter Wiring Types: 2-Wire, 3-Wire, 4-Wire

A wide range of signal outputs are provided on transmitters. In industrial applications, the 4-20mA analog signal is by far the most prevalent. There are several physical 4-20mA wiring choices available. These choices will be outlined in this advice note.

Table of contents

• 4-20 mA Transmitter Wiring
• Current source transmitter, nonisolated (3 wire)
  • Advantages
  • Disadvantages
• Current sink transmitter, nonisolated (3 wire)
  • Advantages
  • Disadvantages
• Fully isolated (4 wire)
  • Advantages
  • Disadvantages
• Two-Wire Loop-Powered Transmitters
  • Advantages
  • Disadvantages
• Basics of 4 – 20 mA

4-20 mA Transmitter Wiring

The transmitter is the component that regulates the current in the loop. It transforms a physical or electrical parameter into a 4-20 mA signal, with 4 mA representing zero and 20 mA representing full scale. 2-wire, 3-wire, and 4-wire transmitters are the three types of transmitters. The current loop provides the operating power for a 2-wire transmitter. When you don’t have access to external power, this loop-powered instrument is a great option. However, the available power is extremely limited.

This is advantageous while working in dangerous environments when intrinsic safety is required. Limited power, on the other hand, limits the transmitter’s capabilities. It may be unable to drive certain types of sensors due to a lack of power. A loop-powered device’s display will almost always be an LCD. The device will shut down if the loop current falls below 4 mA.

External power is used to power both 3- and 4-wire devices. A separate loop supply is no longer required. Because these devices have more power, they can have more capabilities and features, such as higher sensor excitation, LED displays, relay outputs, and digital communications. However, as compared to 2-wire devices, these units require more cabling, making the design more complicated and the installation cost higher.

Many characteristics, such as pressure, temperature, and flow, can be monitored with industrial transmitters. 4-20mA outputs are available on gas detectors and transmitters, with 4 mA corresponding to a zero reading and 20 mA corresponding to a full-scale reading of the calibrated range. This signal is relayed to a control panel that is positioned far away. This signal is used by the control panel, which activates executive actions.

• As a result, it’s critical to determine if the transmitter or the control system needs to be linked in a specific way.
• It is assumed that both the transmitter and the remote control panel require a 24Vdc supply for the purposes of this guideline note.

Current source transmitter, nonisolated (3 wire)

Modern 4-20mA transmitters are most commonly configured in this way. The same 24V and 0V dc supply connections can be used by the transmitter and control panel. The 4-20mA signal is sent to the controller through the 24V dc line and the signal line.

Advantages

• The transmitter just requires three cable cores.
  Both the transmitter and the control panel can be powered by the same power supply.

Disadvantages

• Electrical interference or pick-up could be conveyed along the signal line, resulting in a false warning in the control panel.

Current sink transmitter, nonisolated (3 wire)

The 0V and 24V dc supply connections can be shared between the transmitter and control panel. The 4-20mA signal is sent to the controller through the 0V dc line and the signal line.

Advantages

• The transmitter just requires three cable cores.
  Both the transmitter and the control panel can be powered by the same power supply.

Disadvantages

• Any electrical interference or pick-up could be relayed along the signal line, resulting in a false warning on the control panel.

Fully isolated (4 wire)

Separate power supplies are used for the transmitter and control panel. Between the transmitter and the control panel, the 4-20mA signal travels through two distinct cable cores.

Of the current-loop sensor transmitter circuit types, the 4-wire sensor transmitter is arguably the least well-known. These transmitters meet market demands for applications that require more transmitter isolation options than 2- and 3-wire transmitters can provide.

In addition, the 4-wire receiver and the power supply do not share a common return (GND). This enables a variety of novel isolation methods, such as fully isolated, power-isolated, and output-isolated transmitters, which build on the input-isolated and non-isolated topologies described before for 2-wire and 3-wire transmitters.

An output-isolated 4-wire transmitter is the first and most basic type of 4-wire transmitter that I’ll cover. The sensor input and power supply share a common GND in an output-isolated 4-wire transmitter, while the output transmitter is powered by an isolated supply generated from the sensor supply.

The power to drive the 4-20mA loop is believed to come from the control panel.

Advantages

• Electrical interference on the voltage supply lines will not be passed to the 4-20mA signal line, lowering the chance of receiving spurious signals at the controller.

Disadvantages

• In comparison to current sink and source options, each transmitter requires an additional cable core.
  Both the transmitter and the control panel require their own power supply.

Two Wire Loop Powered Transmitters

The transmitter and the control panel are connected by a two-wire loop, which provides power and a 4-20mA signal. Not all transmitters can be wired in this manner, and they must be designed expressly for this purpose.

In this case, the transmitter isn’t a true current source in the same way that a 4-wire transmitter is. Instead, the circuitry of a 2-wire transmitter is designed to operate as a current regulator, restricting current in the series loop to a value that represents the process measurement while relying on a remote power source to stimulate the electric current.

Take notice of the direction of the arrow in the dependent current source sign of the transmitter, as well as how it relates to the voltage polarity marks. For comparison, look at the diagram of a 4-wire transmitter circuit. The current “source” in this loop-powered transmitter is an electrical load, whereas the current source in the 4-wire transmitter was a true electrical source.

The working power of a loop-powered transmitter comes from the minimal terminal voltage and current available at its two terminals. The transmitter should always have at least 19 volts available at its terminals, with the normal source voltage being 24 volts DC and the maximum voltage dropped over the controller’s 250-ohm resistor being 5 volts DC.

The transmitter needs always to have at least 4 mA of current to function due to the lower end of the 4-20 mA signal range. As a result, while controlling the current to convey the process measurement to the receiving instrument, the transmitter will always have a specific minimum quantity of electrical power available.

Advantages

• It consumes very little power.
  The transmitter just requires two cable cores.

Disadvantages

• Because this setup continues to draw current in a fault condition, the discrete fault signaling on the transmitter cannot be adjusted to 0mA. This setup is incompatible with control panels that require a 0mA signal for fault detection.
• Due to the narrow range of mA available between a defect and a zero measurement, sub 4mA status signaling is limited.
• Not appropriate for transmitters that require a lot of power, such as catalytic gas detectors or infrared gas detectors that use optical heating components.

See Also: How an Orifice Measures Flow?