Signal generator capable of outputting and measuring 4-20 mA signals
The 4~20mA.DC (1~5V.DC) signal system is an analog signal standard for the International Electrotechnical Commission (IEC) process control system. 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 the signal system using current transmission and voltage reception. So why is the standard signal set to 4~20mA? Today, Xiaobian will come to talk to you about the reasons.
one. Reasons why the source of the remote signal is better than the voltage source
Because the distance between the field and the control room is far, when the resistance of the connecting wire is large, if the voltage source signal is transmitted remotely, due to the partial pressure of the wire resistance and the input resistance of the receiving instrument, a large error will be generated, and the constant error will be generated. As the remote source transmits the current source signal, as long as the transmission loop does not branch, the current in the loop does not change with the length of the wire, thus ensuring the accuracy of the transmission.
two. Reason for the maximum current of the signal to choose 20mA
The choice of maximum current 20mA is based on safety, practicality, power consumption, and cost considerations. Safety spark meter can only use low voltage, low current, spark energy caused by 20mA current on and off is not enough to ignite gas, it is very safe; comprehensively consider the connection distance between the production field instruments, the load and other factors; Power consumption and cost issues, requirements for electronic components, power supply requirements and other factors.
three. Reason for starting 4 mA of signal starting current
The 4~20mA transmitter is mostly two-wire system. The two-wire system is that the power supply and the load are connected in series, and there is a common point. The signal connection and power supply between the field transmitter and the control room meter only use two wires. Why is the starting signal not 0mA? This is based on two points:
1. The transmitter circuit will not work without static working current, and the signal starting current is 4mA.DC is the static working current of the transmitter.
2. 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 point" is useful for identifying faults such as power failure and disconnection.
Signal generator capable of outputting and measuring 4-20 mA signals
four. Advantages of current signal (4~20mA signal)
1. The current signal is suitable for long-distance transmission, because the current signal is not affected by the wire resistance, and the voltage signal is divided under the condition that the wire itself has resistance, resulting in inaccurate measurement. Generally, the longest transmission distance of 4~20mA is controlled within 100m, and it is recommended to use digital signal (485 communication).
2. The current signal is generally two-wire system, and the voltage signal is generally three-wire system. In contrast, the two-wire system saves materials and reduces costs compared with the three-wire system.
3. Compared with the on-site working conditions, the ability of the current signal to resist interference is stronger than the voltage signal. In the case of interference, it is recommended that the user select a 4~20mA signal.
4. The current signal can be out of the range of the range, and the output is not accurate. For example, the range is 1MPa, and the pressure transmitter with 4~20mA output can output 24mA in over-range.
The voltage signal is slightly different depending on the power supply. If the measuring range is 1MPa, the pressure transmitter of 0~10V is output. When the power supply is 9V, the signal above 9V cannot be output.
5. When the wire material is different (such as copper, nickel), a magnetic field is generally generated. When the accuracy is high, there is an error in the voltage signal, and the current signal will not be present.
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