What is a 2-wire, 3- wire, and 4 – wire transmitter, and how are they different from each other?

A transmitter is an electronic device that would convert the process variables such as frequency, pressure, temperature, and flow to a 4-20mA signal. So this device would convert the sensor input signals to a control signal which is capable to regulate the current flow. The 2,3, and 4 wire transmitters play a major role in the 4-20mA signal transmission system. The output of the transmitter would be current. In the transmitter, there won’t be any degradation of the signal output with distance. So the transmitters are capable to do long-distance signal transmission. Mostly the transmitter current loop won’t be affected by the noise. The transmitter would convert the value of the measured variable to a standard signal and this can be easily sent to other devices, a transmitter can be described as a combination of the transducer, amplifier, and signal conditioning.

What are the major functions of a transmitter?

In an industrial process, the sensor would transmit a physical signal like pressure or temperature and this signal will be converted to an electrical signal. The electrical signal will be transmitted over a long distance without any signal loss to the controller. So a transmitter does the transduction, amplification, and also signal conditioning. A transmitter would sense the process variable with the help of a sensor and the output value of the transmitter would vary as a predetermined function of the process variable.

Why analog signals are used in transmitters and what is an analog signal in instrumentation?

An Analog signal is an electrical signal and its current or voltage magnitude corresponds to sensors inputs such as temperature, flow, etc. The 4-20mA current signal is the most common way of transmitting the analog signal. So in this transmission, the 4mA would represent the lowest measured value and the 20mA represents the highest measured value.

Click here to know why the 4-20mA signal is used for analog transmission

Two types of transmitters are used in process instrumentation

Pneumatic and electronic transmitters

Electronic transmitters are classified into analog or digital transmitters

2 wire, 3 wire & 4 wire Representation
1A lot of moving partsLess number of moving parts
2Slow responseQuick response
3Pressure signals are used for transmissionElectrical signals are used for transmission
4Not suitable for long-distance transmissionSuitable for long-distance transmission
5There will be mechanical vibrations and noiseNo mechanical vibrations and noise
6Low accuracyGood accuracy
7Frequent maintenance would be needed  Low maintenance
8Non-linear and non-repeatableGood linearity and repeatability
9It does have external leakage and hysteresisDoesn’t have any external leakage or hysteresis
Comparison of electronic and pneumatic transmitters

What are 2-wire, 3-wire, and 4-wire transmitters?

2 wire transmitter

The 2 – wire transmitter would receive the power and transmit data signal through the same pair of cables. These two wires are used to power the transmitter and also to transmit the output signal.

Two-wire transmission is only possible in the current transmitters. These transmitters are energized by loop current, these transmitters depend upon the loop current from the receiving device. This transmitter would convert the process signals to 4-20 mA DC so that this signal can be transmitted without any loss. The loop current would energize the two-wire transmitter and there won’t be any external power source required for this process. So the two-wire transmitter is dependent on the loop power.

  • Simplicity and Cost: Easier and cheaper to install due to fewer wires.
  • Reduced Wiring Complexity: Minimizes wiring errors and reduces the need for extensive wiring infrastructure.
  • Power Efficiency: As they use loop power, there is no need for an additional power supply line.
  • Low impedance capability is the major limitation of the two-wire transmitter
  • Limited Power Availability: The power available for the transmitter is limited by the current loop, which may restrict the types of sensors that can be used.
  • Signal Interference: Sharing the same wires for power and signal can make them more susceptible to noise and interference.
  • Lower Performance: May not be suitable for applications requiring high accuracy or high-speed signal transmission due to power constraints.
Three-wire transmitter

The 3 -wire transmitter would transmit the data signal and the power with respect to the common ground. The three-wire transmitters are energized by the supply voltage in the transmitter and the transmitter would source the loop current. The receiver common is connected to the transmitter common. In this transmitter, the current loop can also be operated on a measuring instrument that has high input impedance. The three-wire arrangement is not widely used but it would deliver more power to the module electronics.

  • Higher accuracy than the two-wire transmitter
  • Better Signal Quality: Separation of power and signal lines reduces the potential for noise and interference.
  • Greater Power Availability: Can provide more power to the transmitter compared to 2-wire systems, supporting more demanding sensors.
  • 24v DC power supply; Effective for different power problems
  • It can use higher Ohmic loads
  • More Complex Wiring: Requires an additional wire, increasing installation complexity and potential for wiring errors.
  • Higher Cost: The additional wire increases material and installation costs
Four wire transmitter

In the case of a 4- wire transmitter, two wires are used for the data signal and two wires for the power supply. This is the simplest form of the 4-20mA measurement loop in which the transmitter has two terminals for the 4-20mA signal wires to connect and it also has two more terminals where the power source will be connected. In this arrangement the current signal from the transmitter would connect the process variable input terminals of the controller to complete the loop. 4 wire transmitters are powered by the external power supply at the transmitters.

  • These transmitters have higher signal side impedance and due to this it has better loading and signal-transmission distance characteristics
  • Maximum Power Availability: Independent power supply lines can provide ample power for high-performance sensors and transmitters.
  • Highest Signal Integrity: Complete isolation of power and signal lines ensures the least interference and noise, suitable for high-precision and high-speed applications.
  • Flexibility: Can accommodate a wide range of sensor types and applications.
  • Most Complex Wiring: Requires the most wiring, leading to higher installation complexity and cost.
  • Higher Material Costs: More wires mean increased cost of materials and potentially higher maintenance costs.

Comparison Between 2-Wire, 3-Wire, and 4-Wire Transmitters

SL NO2- Wire transmitters3 – wire transmitters4 – Wire transmitters
1Energized by the loop currentEnergized by the supply voltage in the transmitterPowered by external power source
2Less number of wire runs are needed when compared to the other two typesLess number of wire runs are used  than 4 wire type but it has more number of wire runs than the two-wire typeMore wires are used when compared to the other two types
3Not suitable for long-distance transmissionTransmission distance is better than 2 -wire typeSuitable for long-distance
4Low accuracy when compared to the other two typesHigher accuracy than the 2 -wire transmitterGood accuracy than the other two transmitters
52 wires are used for both the signal and the power supply3 wires are used commonly for the signal and the power supply2 wires are used for the data signal and the other two wires are used for the power supply
6Signal and power supply are isolatedSignal and power supply are not isolatedSignal and power supply are isolated
Comparison of 2, 3, and 4 wire transmitters

Choosing the right type of transmitter depends on the specific requirements of the application, including factors such as power needs, signal integrity, installation complexity, and cost.

What are the advantages of using current as an electric signal for transmission?

  • We can connect many loads at a different location in series, to a specified limit on the total resistance while the transmitter behaves as a current source
  • The resistance and length of the transmission circuit won’t affect the sensitivity of the signal
  • Electrical signals would travel a greater distance with a low time lag
  • Multiple input signals can be handled
  • These signals will be compatible with the digital computer

Click here for Why not use 0-20mA & 0-15psi instead of 4-20mA & 3-15psi?

Click here for How to do troubleshooting of a 4-20mA loop?


post-graduate in Electronics & communication.

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