In the process of selecting and using transmitters, the signal output and wiring method are crucial considerations. The terms “two-wire,” “three-wire,” and “four-wire” refer to transmitters with various direct current (DC) signal outputs.
The issue of how many wires a transmitter uses fundamentally stems from differences in working principles and structure, not merely the form of wiring.
The so-called “two-wire system” means that the power supply and load are connected in series, with a common point. Only two wires are used for both the power supply and signal transmission between the field transmitter and the control room instruments. These two wires serve as both the power supply line and the signal line. Since the starting signal current for two-wire transmitters is 4mA DC, it provides the transmitter with a static working current.
Additionally, the electrical zero point of the instrument is 4mA DC, which does not coincide with the mechanical zero point. This “live zero” helps to identify faults such as power failure or disconnection. Furthermore, the two-wire system facilitates the use of safety barriers and promotes safety in explosion-proof environments.
Due to the widespread adoption and application of the 4-20mA DC (1-5V DC) signal system, control systems require signal uniformity to simplify connections.
Therefore, some instruments that are not electrically powered, such as online analysis instruments or instruments measuring mechanical quantities or electrical quantities, are expected to adopt the 4-20mA DC signal system.
However, due to the complexity of conversion circuits and high power consumption, it is difficult for all instruments to meet these requirements and still operate on a two-wire system.
As a result, some instruments can only achieve a four-wire system by using an external power supply to output a 4-20mA DC signal.
Some instrument manufacturers have switched from a 220V AC power supply to a low-voltage DC power supply, such as a 24V DC power supply box, in order to reduce the size and weight of the transmitter, improve anti-interference performance, and simplify wiring. This switch to low-voltage power creates the conditions for a shared negative line, leading to the development of three-wire transmitters.
Therefore, due to the different working principles and structures of various transmitters, different products and wiring systems—two-wire, three-wire, and four-wire systems—have emerged.
For users, when selecting a transmitter, it is important to consider the specific circumstances of their organization, including signal system uniformity, explosion-proof requirements, the needs of receiving equipment, and budget, in order to make a comprehensive decision.