What is 4-20mA? Easy to understand

What is 4-20mA? Easy to understand

It is common in industry to need to measure all kinds of non-electrical physical quantities, such as temperature, pressure, speed, angle, etc., which need to be converted into analog electrical signals in order to be transmitted to the control room or display equipment hundreds of meters away. This physical quantity will be converted into an electrical signal device is called a transmitter. It is widely used in industry to transmit analog quantity with 4-20mA current.

4-20mA, means that the minimum current is 4mA and the maximum current is 20mA .

In the industrial site, to complete the signal conditioning and transmit it over long lines, the following problems arise:

First, the transmission line will be disturbed by noise because the transmitted signal is a voltage signal;

Secondly, the distributed resistance of the transmission line produces a voltage drop;

Thirdly, there is the problem of how to provide the operating voltage of the instrumentation amplifier in the field.

In order to solve the above problems and avoid the influence of related noise, we will use the current to transmit the signal, because the current is not sensitive to the noise. 4-20mA current loop is to use 4mA to indicate the zero signal, 20mA to indicate the full scale of the signal, and lower than 4mA and higher than 20mA signal is used for a variety of fault alarm.

 There must be many friends will ask: why choose 4-20mA instead of 0-20mA? It is very simple, if 0 is the smallest, then open-circuit faults can not be detected!

So why favor 4mA? During normal operation, the current signal will not be lower than 4mA. when the transmission line is disconnected due to a fault, the loop current drops to 0. 2mA is often taken as the disconnection alarm value. There are two reasons for this. One reason is to avoid interference, the other reason is that in the 4-20mA is used in the two-wire system, that is, the two wires that is the signal line, but also the power line, and 4mA is to provide the sensor with the static operating current of the circuit.