3 minutes let you know why the standard signal is 4 ~ 20mA

- Nov 07, 2017 -

4 ~ 20mA.DC (1 ~ 5V.DC) signal system is the International Electrotechnical Commission (IEC) process control system with analog signal standards. Our country from DDZ-Ⅲ-type electric instruments began to adopt this international standard signal system, the instrument transmission signal using 4 ~ 20mA.DC, contact signal using 1 ~ 5V.DC, that is, the use of current transmission, voltage receiving signal system. So why the standard signal should be set at 4 ~ 20mA? Today, Xiaobian come and tell you about the reason.

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One. The reason why the far-end signal current source is better than the voltage source


Because the distance between the scene and the control room is far, when the resistance of the connecting wire is large, if the voltage source signal is far-away, a large error will occur due to the partial pressure of the wire resistance and the receiving meter input resistance, Current source signal as a remote, as long as the transmission circuit does not appear branch, the loop current will not change with the length of the wire, thus ensuring the accuracy of the transmission.


Two. The reason why the maximum current of the signal is 20mA is selected


The maximum current 20mA choice is based on safety, practical, power consumption, cost considerations. Safety spark instrument can only use low voltage, low current, 20mA current on and off caused by the spark energy is not enough to ignite gas, very safe; comprehensive consideration of the connection distance between the production site instrumentation, the load and other factors; there is Power and cost issues, the requirements of electronic components, power supply requirements and other factors.


Three. The reason why the signal start current is 4mA


4 ~ 20mA transmitter two-wire system mostly two-wire system that is the power supply, the load in series with a common point, and the site transmitter and control room signal communication between the instrument and power supply only two wires. Why is the starting signal is not 0mA? This is based on two points:

   1. Transmitter circuit no static operating current will not work, the signal starting current 4mA.DC is the transmitter of the static operating current.

   2. At the same time instrument electrical zero 4mA.DC, do not coincide with the mechanical zero, this "live zero" is conducive to identify power failure and disconnection and other failures.

standard signal

Four. Current signal (4 ~ 20mA signal) advantages


1. The current signal is suitable for long-distance transmission, because the current signal is not affected by the resistance of the wire, and the voltage signal is divided under the condition that the wire itself has resistance, resulting in inaccurate measurement. General 4 ~ 20mA farthest transmission distance control within 100m, and then it is recommended to use digital signals (485 communications)

2. Current signal generally two-wire system, the voltage signal is generally three-wire system, in contrast, two-wire system to save material than three-wire, reduce costs.

Relative to the scene conditions more complicated, the ability of current signal anti-interference stronger than the voltage signal. In case of interference, it is recommended to select 4 ~ 20mA signal.

4. The current signal can be properly out of range, the output is not accurate signal, such as the range of 1MPa, output 4 ~ 20mA pressure transmitter, over-range can output 24mA.

The voltage signal, then according to different power supply is slightly different. Such as the pressure range of 1MPa, 0 ~ 10V output pressure transmitter, when the power supply is 9V, you can not output more than 9V signal.

5. When the wire material is not the same time (such as copper, nickel), the general will produce a magnetic field, high precision voltage signal will have occasions error, the current signal will not have.


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