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3 minutes to show you why the standard signal is 4-20mA?

by:Rika Sensors     2021-11-25
3 minutes to show you why the standard signal is 4-20mA?
In the industrial production control process, especially in the continuous production process, some physical quantities such as temperature, pressure, flow, etc. are often required to be controlled. These physical quantities all change continuously with time. In the field of control, these physical quantities that change continuously over time are called analog quantities.

4-20mA is a commonly used analog current signal, which is a standard signal. Means that the minimum current is 4mA and the current is 20mA. When transmitting signals, because there are resistances on the wires, if voltage transmission is used, a certain voltage drop will be generated in the wires, and the signal at the receiving end will have a certain error, so current signals are generally used as the standard transmission of the sensor.

Because the current signal has strong anti-interference ability, is not easy to be interfered by external factors, and can be transmitted remotely, it is widely used in industrial sensors. In order to adapt to different situations in the industrial control industry, 4-20mA current sensors generally have four-wire, three-wire, and two-wire systems.

The reason why the current source for remote signal transmission is better than the voltage source

Because the distance between the field and the control room is longer, and the resistance of the connecting wire is large, if the voltage source signal is used for remote transmission, because the wire The voltage divider between the resistance and the input resistance of the receiving instrument will cause a large error, and a constant current source signal is used as a remote transmission. As long as the transmission loop does not branch, the current in the loop will not change with the length of the wire, thus ensuring The accuracy of the transmission.

The reason why 20mA is selected for acquisition signal current

The safety spark meter can only use low voltage and low current. The spark energy caused by the on-off of the 20mA current is not enough to ignite the gas, which is very safe; the current 20mA choice is Based on safety, practicality, power consumption, and cost considerations.

Why the starting point of 4-20mA is not 0mA

When the sensor is connected, it may cause poor contact or disconnection due to some reasons. After the disconnection, the receiving device cannot collect the current signal, then the real 0mA and disconnection are confused and cannot be judged. If the starting point is set to 4mA, it is easy to judge the disconnection. At the same time, setting to 4 mA also has many benefits for two-wire signal transmission.

Advantages of current signal (4~20mA signal)

1. 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 will be divided when the wire itself has resistance. Pressure, resulting in inaccurate measurement. Generally, the farthest transmission distance of 4~20mA is controlled within 100m, and it is recommended to use digital signals (485 communication) for further distances.

2. Current signals generally use two-wire systems, and voltage signals generally use three-wire systems. In contrast, Compared with the three-wire system, the two-wire system saves materials and reduces the cost.

3. Compared with the more complicated field conditions, the anti-interference ability of the current signal is stronger than that of the voltage signal. In working conditions with interference, it is recommended that the user select 4-20mA signal.

4. The current signal can appropriately exceed the range of the range, and no signal is output. For example, a pressure transmitter with a range of 1MPa and an output of 4-20mA can output 24mA when it exceeds the range.

The voltage signal is slightly different depending on the power supply. For example, a pressure transmitter with a measuring range of 1MPa and an output of 0~10V, when the power supply is 9V, it cannot output a signal above 9V.

5. When the wire material is not the same (such as copper, nickel), a magnetic field will generally be generated, and the voltage signal will have errors in the occasions with higher accuracy requirements, but the current signal will not.



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