It is generally required in the industry to measure various types of non-electrical physical quantities, such as temperature, pressure, speed, angle, etc., which need to be converted into analog electrical signals and transmit to control rooms or display devices hundreds of meters away. This device that converts physical quantities into electrical signals is called transmitter. The most widely used in the industry is to use 4 to 20 mA current to transmit analog quantities.
The reason for using the current signal is that it is not susceptible to interference. And the internal resistance of the current source is infinite, the wire resistance is connected in series in the loop will not affect the accuracy, and can be transmitted hundreds of meters on the ordinary twisted pair. The upper limit is 20 mA because of the requirements of explosion protection: the spark energy caused by the current interruption of 20 mA is not enough to ignite the gas. The reason why the lower limit is not taken as 0mA is to detect the disconnection: it will not be lower than 4mA during normal operation, and the loop current will drop to 0 when the transmission line is disconnected due to a fault. Usually take 2mA as the disconnection alarm value.
The current-type transmitter converts the physical quantity into a 4~20mA current output, which must be supplied by an external power supply. The most typical mode is that the transmitter needs two power lines, plus two current output lines, a total of four lines, called a four-wire transmitter. Of course, the current output can be shared with the power supply (common VCC or GND), which saves one line and is called three-wire transmitter. In fact, you may notice that the 4-20mA current itself can power the transmitter.
The transmitter is equivalent to a special load in the circuit. The special point is that the current consumption of the transmitter varies between 4 and 20 mA depending on the sensor output. The display meter only needs to be stringed in the circuit. This transmitter requires only two external wires and is called a two-wire transmitter. The industrial low current standard has a lower limit of 4 mA, so the transmitter has at least 4 mA of power as long as it is within the range. This makes the design of the two-wire sensor possible.
In industrial applications, the measuring points are generally on site, while the display or control devices are typically located in the control room or control cabinet. The distance between the two may be tens to hundreds of meters. Calculated by the distance of one hundred meters, the elimination of two wires means that the cost is highly reduced ! Therefore, two-wire sensors are definitely the first choice in applications.