The remote signal must use a current source. Why is the starting point not 0mA?

It is often necessary in the industry to measure non-electrical quantities such as temperature, pressure, and wind speed. These usually need to be converted into analog quantities to be transmitted to the control room or display device. For some current output Sensors , 4-20 mA DC is generally selected. Current output, then why not choose other output values?

4-20mA.DC (1-5V.DC) signal system is the International Electrotechnical Commission (IEC): analog signal standard for process control systems, such as connecting instruments, transmission equipment, control equipment, computer sampling equipment. China has adopted this international standard signal system from DDZ-III type electric instrument. The instrument transmission signal adopts 4-20mA.DC, and the contact signal adopts 1-5V.DC, which is a signal system using current transmission and voltage reception.

First, why should the current source be used for remote transmission signals?

Usually, the distance between the measurement site and the control room is relatively long. If the voltage source signal is transmitted remotely, when the resistance of the connecting wire is large, a large error will occur due to the partial pressure of the wire resistance and the input resistance of the receiving instrument. The reason for using the current signal is that it is not susceptible to interference, because the amplitude of the noise voltage in the industrial field may reach several V, but the power of the noise is very weak, so the noise current is usually less than the nA level, so the error caused by the 4-20 mA transmission is very Small; the internal resistance of the current source tends to infinity, and the wire resistance is connected in series in the loop without affecting the accuracy. Therefore, it can be transmitted for hundreds of meters on a common twisted pair. As long as the transmission loop does not branch, the current in the loop does not vary with the length of the wire. Change to ensure the accuracy of the transfer.

Second, why is the standard current signal 4-20mA?

The design of the 4-20mA two-wire sensor uses 20mA to indicate the full scale of the signal, but why is the starting signal not 0mA? The current type transmitter converts the physical quantity into a 4-20 mA standard current signal output, and it must be powered by an external power supply. The 0-4 mA current is required to supply the sensor's own minimum operating current. If 0-20 mA is used, the signal is used. At a very young age, the sensor cannot get enough working energy. At less than 4 mA, signals above 20 mA are used for various fault alarms, such as no less than 4 mA during normal operation. When the transmission line is broken due to a fault, the loop current drops to 0, so 2 mA is often taken as the disconnection. Alarm value. At the same time, the instrument's electrical zero point is 4mA.DC, which does not coincide with the mechanical zero point. This "live zero" is useful for identifying faults such as power failure and disconnection.

The choice of 20mA maximum current is based on safe and practical considerations. The spark energy caused by 20mA current interruption is not enough to ignite gas.

Therefore, the general sensor is 4-20mA signal output, so the signal is not easily interfered and safe and reliable, the two-wire 4-20mA output can save the sensor cost, etc. These reasons make the current output mainly pressure sensor, temperature Products such as sensors are commonly used in the industry for two-wire 4-20 mA output.

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DC current range: 0-20mA, support GS standard current signal input, range is 4-20mA.

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