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

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 analogue 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.

4-20 mA Transmitter Wiring

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

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.

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.

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.

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