4-wire and 2-wire current loops are two common configurations used in industrial process control for transmitting analog signals, such as temperature, pressure, or flow measurements, from field devices (sensors or transmitters) to control systems (PLCs, DCSs, etc.). Both configurations utilize current loop communication, typically with a 4-20 mA signal, but they differ in their wiring setup and power requirements.

1. 4-Wire Transmitter Current Loop:

• Wiring: In a 4-wire current loop configuration, there are four wires: two wires for power supply (usually labeled "+V" and "-V") and two wires for signal communication (usually labeled "I+" and "I-").

• Power Supply: The transmitter is powered by an external power supply connected to the "+V" and "-V" terminals. This separate power supply provides power to the transmitter and isolates it from the control system.

• Signal Communication: The analog signal representing the process variable is transmitted over the "I+" and "I-" wires. The current flowing through these wires is proportional to the measured value, typically ranging from 4 mA (minimum value) to 20 mA (maximum value).

• Advantages:

  • Better accuracy and stability due to separate power and signal wires.
  • Reduced susceptibility to interference from power supply fluctuations.
  • Ability to provide power to the transmitter from a separate, dedicated power supply.

• Applications:

  • Suitable for applications where accuracy and reliability are critical.
  • Commonly used in process control systems with long distances between the transmitter and control system.

2. 2-Wire Transmitter Current Loop:

• Wiring: In a 2-wire current loop configuration, there are only two wires: one wire for power and signal (usually labeled "I+") and one wire for the return path (usually labeled "I-").

• Power Supply: The transmitter is powered by the same current loop that carries the analog signal. The loop is typically powered by the control system, which provides a constant current source.

• Signal Communication: The analog signal and power are transmitted over the same wire ("I+"), while the return path is provided by the "I-" wire. The transmitter modulates the current flowing through the loop to represent the measured value.

• Advantages:

  • Simplicity and cost-effectiveness due to fewer wires and components.
  • Suitable for applications where wiring is limited or expensive.
  • Can be powered directly from the control system, eliminating the need for a separate power supply.

• Limitations:

  • Limited power available for the transmitter, which may affect performance or require low-power designs.
  • Susceptible to interference from power supply variations and voltage drops along the loop.

• Applications:

  • Commonly used in field devices with limited power requirements, such as temperature or pressure transmitters.
  • Suitable for short-distance applications or installations where wiring is constrained.

In summary, 4-wire current loops provide better accuracy and stability, especially over longer distances, while 2-wire current loops offer simplicity and cost-effectiveness, making them suitable for applications with limited wiring options or power requirements. The choice between these configurations depends on factors such as distance, accuracy requirements, and cost considerations.