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

What is a Transmitter?

A transmitter is a device that converts the small signal from a sensor into a representative signal of the measured variable. Transmitters come in various shapes and sizes and can interface with different types of sensors. The output signal, reflecting the measured variable, is typically in the form of voltage or current.

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.

Transmitter Analog Output Loops

An analog output loop for a transmitter consists of the transmitter, a power supply, and a receiving device, such as a PLC or DCS. Like all instruments, transmitters require a power supply to function. But is it a 2-wire or 4-wire transmitter?

The specific wiring connection between the transmitter and the power supply depends on the type of transmitter.

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

What are the types of transmitters used in process instrumentation?

Two types of transmitters are used in process instrumentation

Pneumatic and electronic transmitters

Electronic transmitters are classified into analog or digital transmitters

Why pneumatic transmitters were replaced by electronic transmitters in process instrumentation?

SL NO	PNEUMATIC TRANSMITTERS	ELECTRONIC TRANSMITTERS
1	A lot of moving parts	Less number of moving parts
2	Slow response	Quick response
3	Pressure signals are used for transmission	Electrical signals are used for transmission
4	Not suitable for long-distance transmission	Suitable for long-distance transmission
5	There will be mechanical vibrations and noise	No mechanical vibrations and noise
6	Low accuracy	Good accuracy
7	Frequent maintenance would be needed	Low maintenance
8	Non-linear and non-repeatable	Good linearity and repeatability
9	It does have external leakage and hysteresis	Doesn’t have any external leakage or hysteresis

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

Two-wire transmitter configuration

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.

Advantages of two-wire transmitters

• 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.

Disadvantages of two-wire transmitters

• 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 configuration

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.

Advantages of three-wire transmitter

• 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

Disadvantages of 3 wire transmitters

• 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 configuration

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.

Advantages of 4 wire transmitter

• 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.

Disadvantages of 4 wire transmitters

• 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.

2-wire Transmitter Current Loop Diagram

A typical 2-wire current loop is depicted in a loop diagram to illustrate its configuration.

In this setup, the 24-volt power supply is connected in series with the differential pressure transmitter and the PLC’s analog input card. This arrangement ensures that the current flowing through the loop represents the measured variable.

Two Wire Loop Powered Transmitters

In this configuration, both the power supply and the 4-20 mA signal are carried over a two-wire loop connecting the transmitter to the control panel. Only transmitters specifically designed for this setup can operate in this format.

Advantages of 2-Wire Loop-Powered Transmitters

• Low Power Consumption: Efficient operation with minimal energy usage.
• Simplified Wiring: Requires only two cable cores, reducing installation complexity and costs.

Disadvantages of 2-Wire Loop-Powered Transmitters

• Unsuitability for Certain Control Panels: Not ideal for systems that rely on a 0 mA signal to indicate a fault.
• Fault Signaling Limitation: Cannot signal a discrete fault at 0 mA because the transmitter still draws current even in a fault condition.
• Reduced Sub-4mA Signaling Range: Limited ability to signal conditions below 4 mA, restricting the range available for fault and zero readings.

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

SL NO	2- Wire transmitters	3 – wire transmitters	4 – Wire transmitters
1	Energized by the loop current	Energized by the supply voltage in the transmitter	Powered by external power source
2	Less number of wire runs are needed when compared to the other two types	Less number of wire runs are used  than 4 wire type but it has more number of wire runs than the two-wire type	More wires are used when compared to the other two types
3	Not suitable for long-distance transmission	Transmission distance is better than 2 -wire type	Suitable for long-distance
4	Low accuracy when compared to the other two types	Higher accuracy than the 2 -wire transmitter	Good accuracy than the other two transmitters
5	2 wires are used for both the signal and the power supply	3 wires are used commonly for the signal and the power supply	2 wires are used for the data signal and the other two wires are used for the power supply
6	Signal and power supply are isolated	Signal and power supply are not isolated	Signal and power supply are isolated

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

FAQs: 2-Wire Transmitter

What is a 2-wire Transmitter?

A 2-wire transmitter is a loop-powered device, meaning it derives power from the same DC power supply (typically 24V) used for signal transmission. It operates within a 4-20 mA current loop, where the current represents the measured variable. The power and signal are carried over the same two wires connected to a centralized system like a Distributed Control System (DCS) or data acquisition hardware.

What is the difference between a 2-wire and a 4-wire transmitter?

• 2-Wire Transmitter: Powered through the signal wires (loop-powered). These are simpler to install and widely used for remote locations.
• 4-Wire Transmitter: Requires a separate pair of wires for power, in addition to signal wires. These are often used for devices requiring higher power or where additional functionality is provided.

A 4-wire transmitter operates using a 110V or 220V power supply, enabling it to directly control devices such as relays, pumps, or solenoids. However, in hazardous environments where flammable vapors, gases, or dust may be present, this setup poses a risk of ignition under certain conditions.

In contrast, a 2-wire transmitter is loop-powered, utilizing a low-voltage, low-current DC power supply, typically 24V. It transmits a 4-20 mA current signal, typically supplied by a distributed control system (DCS) or other centralized data acquisition hardware, over two wires. Unlike a 4-wire transmitter, a 2-wire transmitter does not directly control relays or pumps and is designed to minimize the risk of sparking in case of a malfunction, making it safer for hazardous locations.

What is a 2-wire connection?

In telecommunications, a two-wire circuit supports bi-directional transmission over the same pair of wires. In contrast, a four-wire circuit uses separate pairs for transmitting and receiving signals. In instrumentation, a 2-wire transmitter uses the same two wires for power and signal transmission.

Is a 2-wire transmitter active or passive?

A 2-wire transmitter generally has a passive output. It depends on the external loop power supply to generate and transmit its signal. To determine if the transmitter needs an external power supply:

Disconnect the signal acquisition loop.

Measure the voltage across the two wires using a voltmeter.

If the voltage is present, the transmitter is active. If not, it is passive and requires an external power source.

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