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. The reason why the signal current source is better than the voltage source is because the distance between the field and the control room is far, and 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 current source signal is used as the remote transmission. As long as the transmission loop does not branch, the current in the loop does not change with the length of the wire, thereby ensuring the accuracy of the transmission.
2. Reasons for selecting the maximum current of the signal 20MA The selection of the maximum current 20MA is based on: safety, practicality, power consumption, and cost considerations. Safety spark meter can only use low voltage and low current; comprehensively consider the connection distance between the production field instruments, the load and other factors; there are power consumption and cost issues, requirements for electronic components, power supply requirements and other factors. .
three. The reason why the starting point of the signal is 4 mA is 4 mA. The two-wire system 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, and the signal communication and power supply between the field transmitter and the control room meter. Use only 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†is helpful for identifying faults such as power failure and disconnection.
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