It is generally necessary to measure all kinds of non-electrical physical quantities in industry, such as temperature, pressure, speed, angle, etc., which need to be converted into analog quantity electrical signals to be transmitted to the control room or display equipment hundreds of meters away. The device that converts physical quantities into electrical signals is called a transmitter. It is widely used in industry to transmit the analog quantity with 4~20mA current.
4-20MA refers to the minimum current is 4mA, the maximum current is 20mA.
In the industrial field, to complete the signal conditioning and long line transmission, will produce the following problems:
First, because the transmitted signal is a voltage signal, the transmission line will be disturbed by noise;
second, the distributed resistance of the transmission line will produce a voltage drop;
third, how to provide the operating voltage of the instrumentation amplifier in the field is also a problem.
In order to solve the above problems and avoid the influence of related noise, we will use current to transmit the signal, because current is not sensitive to noise. The current loop of 4-20mA is 4mA to represent the zero signal, 20mA to represent the full scale of the signal, and the signal below 4mA and above 20mA is used for the alarm of various faults.
There must be many friends who will ask: why choose 4-20MA rather than 0-20MA? Very simple, if 0 is the minimum, then the open circuit fault can not be detected!
So why 4mA? Under normal operation, the current signal will not be lower than 4mA. When the transmission line is open due to a fault, the loop current drops to 0. 2mA is usually taken as the alarm value of wire break. There are two reasons. One reason is to avoid interference, the other reason is that 4-20mA is used in a two-wire system, that is, the two wires are signal lines, but also power line, and 4mA is to provide static working current for the sensor circuit.