Common 4-20mA transmitters are divided into two-wire system (power line, signal line), three-wire system transmitter (current output and power supply share one line) and four-wire system transmitter (two power lines, two current output lines)
What is a two-wire transmitter？
Two-wire transmitter: the transmitter is a converter that converts the output signal of the sensor into a signal that can be recognized by the controller. It is used in industrial field and can measure various non-electrical physical quantities, such as temperature, pressure, speed, angle, etc.
Advantages of two-wire pressure transmitter
1) It is not easy to be affected by parasitic thermocouples and resistance voltage drop and temperature drift along the wire. A very cheap and thinner wire can be used, which can save a lot of cable and installation costs;
2) When the output resistance of the current source is large enough, the voltage in the wire loop induced by the magnetic field coupling will not have a significant impact, because the current caused by the interference source is very small, and generally the interference can be reduced by using twisted pair;
3) Capacitive interference will lead to errors related to the receiver resistance. For 4~20mA two-wire loop, the receiver resistance is usually 250 Ω, (sampling Uout=1-5V) which is too small to produce significant errors. Therefore, the allowable wire length is longer and farther than the voltage telemetry system;
4) Each single readout device or recording device can be switched between different channels with different wire lengths, which does not cause differences in accuracy due to different wire lengths, and can realize decentralized acquisition and centralized control;
5) Using 4mA for zero level makes it very convenient to judge open circuit and short circuit or sensor damage (0mA state);
6) It is very easy to add one or two lightning and surge protection devices at the two-wire output port, which is conducive to safe lightning protection and explosion protection.
Structure of two-wire pressure transmitter
Two-wire pressure transmitter is composed of three parts: sensor, conditioning circuit and two-wire V/I converter.
Principle of two-wire pressure transmitter
The principle of two-wire transmitter is to use 4-20mA signal to provide power for itself. If the power consumption of the transmitter is greater than 4mA, it is impossible to output the lower limit 4mA.
Consideration of stability and safety of two-wire pressure transmitter
The industrial environment is harsh and requires high reliability, so the design of two-wire transmitter needs to consider certain protection and stability enhancement measures.
What is the three-wire system of sensor?
Three-wire sensors are separated from the positive end of the power supply and the positive end of the signal output, but they share a COM terminal.
What is the four-wire system of sensor?
The four-wire system refers to two power lines and two signal lines. Power and signal work separately. That is, on the basis of the three-wire system, the signal line has its own ground and does not share the ground with the power line.