What is the difference between 2-wire, 3-wire, and 4-wire transmitters?

1, Overview
2-wire transmitter:
A 2-wire transmitter is the simplest type, which uses only two wires to transmit power and signals simultaneously. One of these two wires is both the positive pole of the power supply and the output line of the signal; And the other one is the negative pole of the power supply and the return line of the signal. The 2-wire transmitter utilizes a 4-20mA current signal to provide power to itself. The power supply is introduced from the outside and connected in series with the load to drive the load. Due to this design, 2-wire transmitters have significant advantages in situations such as long transmission distance and explosion-proof.
3-wire transmitter:
The 3-wire transmitter adds a grounding wire (COM terminal) on top of the 2-wire to balance voltage and enhance signal stability. The positive terminal of the power supply and the positive terminal of the signal output are separated, but they share a common COM terminal. The power supply of 3-wire transmitters is mostly DC24V, and the output signal can be DC420mA or DC010mA. Due to the addition of grounding wires, 3-wire transmitters can better solve signal attenuation problems during signal transmission, making them more suitable for scenarios where signal attenuation issues need to be addressed.
4-wire transmitter:
A 4-wire transmitter completely separates the power supply and signal, using two separate wires for transmission. The main purpose of this design is to reduce signal errors and interference, and improve the stability and reliability of signal transmission. The power supply for 4-wire transmitters is mostly AC 220V or DC 24V, and the output signal can also be DC420mA or DC010mA. Due to the complete separation of power and signal, the 4-wire transmitter provides higher flexibility and stability, making it suitable for applications that require high precision and reliability.
2, Working principle and characteristics
working principle:
2-wire transmitter: The working principle of a 2-wire transmitter is to use a 4-20mA current signal to provide electrical energy to itself. The current signal is not easily disturbed during transmission, and the internal resistance of the current source is infinite. The series connection of wire resistance in the circuit does not affect accuracy. Therefore, the 2-wire transmitter can transmit hundreds of meters on ordinary twisted pair cables and can detect wire breakage.
3-wire transmitter: The working principle of a 3-wire transmitter is similar to that of a 2-wire transmitter, but due to the addition of a grounding wire, it can better balance voltage and enhance signal stability. The presence of grounding wires enables 3-wire transmitters to better resist external interference during signal transmission, improving signal accuracy and reliability.
4-wire transmitter: The working principle of a 4-wire transmitter is to completely separate the power supply and signal, and transmit them using two separate wires. This design enables 4-wire transmitters to significantly reduce interference and errors during signal transmission, improving signal stability and reliability. Meanwhile, due to the separation of power and signal, 4-wire transmitters can also provide more power options and more flexible signal processing methods.
Characteristic comparison:
Wiring method: The 2-wire transmitter only has two wires connected, making the wiring method simple and cost-effective; The 3-wire transmitter has added a grounding wire to balance voltage and enhance signal stability; A 4-wire transmitter completely separates the power supply and signal, using four wires for transmission.
Signal transmission method: The signal and power supply of the 2-wire transmitter share the same wire, which is susceptible to interference and error; Due to the addition of a grounding wire, the 3-wire transmitter can better solve the problem of signal attenuation; The 4-wire transmitter completely separates the power supply and signal, reducing interference and errors.
Error and interference: Due to the signal and power supply of the 2-wire transmitter sharing the same wire, it is susceptible to interference sources such as environmental noise and parasitic thermocouples; The 3-wire transmitter, due to the addition of a grounding wire, can better resist external interference; A 4-wire transmitter reduces interference and error by completely separating the power supply and signal.
Power supply method: The 2-wire transmitter can only be powered by a power supply instrument; A 3-wire transmitter can also be powered by a power supply instrument, but other power options can also be considered; The 4-wire transmitter can be powered by an external power source and fully utilize the power options of the power supply instrument.
3, Application scenarios and advantages and disadvantages
Application scenarios:
2-wire transmitter: 2-wire transmitters have significant advantages in situations such as long transmission distance and explosion-proof. It is widely used in the chemical industry, medical industry, metallurgical industry and other fields. For example, in harsh environments such as chemical plants and oil fields, 2-wire transmitters need to be used as safety protection; 2-wire transmitters also play an important role in monitoring various departments of hospitals, medical and health equipment, and patient monitoring. In addition, 2-wire transmitters can also serve as a convenient way to connect safety barriers and other devices, achieving automated monitoring and timely response to prevent safety accidents such as fires and explosions.
3-wire transmitter: 3-wire transmitters have a wide range of applications in the field of industrial automation. It is mainly used to measure and control physical quantities such as pressure, flow rate, temperature, and liquid level in industrial production processes. In the petrochemical industry, power industry, steel industry, and water treatment industry, 3-wire transmitters have a wide range of applications. Due to the addition of a grounding wire, a 3-wire transmitter can better solve the problem of signal attenuation, making it more suitable in scenarios where signal attenuation needs to be addressed.
4-wire transmitter: 4-wire transmitters are suitable for applications that require high precision and reliability. It is widely used in situations that require long-distance transmission, high stability requirements, or flexible power options. For example, in fields such as precision measurement, control systems, and remote monitoring, 4-wire transmitters can provide stable and reliable signal transmission and data processing capabilities.
Advantages and disadvantages:
2-wire transmitter: The advantages are simple wiring and low cost; The disadvantage is that signal transmission and power supply share the same wire, which is susceptible to interference and errors, and the transmission distance is limited.
3-wire transmitter: The advantage is the addition of a grounding wire to balance voltage and enhance signal stability; The disadvantage is that compared to other types of transmitters, the cost is slightly higher, and in certain specific applications, it may not be as stable and reliable as a 4-wire transmitter.
4-wire transmitter: The advantage is that the power supply and signal are completely separated, reducing interference and errors; The disadvantage is that the wiring method is relatively complex and the cost is high. However, for applications that require high precision and reliability, these drawbacks are acceptable.