Weak current College】 【current transmitter problem solving】.

<br> <br> 1. What is the current two-wire transmitter? <br> <br> Two-wire system refers to the field transmitter and control room gauges with only two wires, this two lines are the power cable and signal .wires. Two-wire system and three-wire system (a root is the power cord, two signal lines, one common GND) and four-wire system (both positive and negative power cable, two signal lines, one root GND) compared to .the advantages of the two-wire system are: <br> <br> 1, vulnerable to parasitic thermocouples and resistance along the wire pressure drop and temperature drift, the available very cheap fine conductors; can save a lot of cables and installation costs; .<br> <br> 2, in the current source output resistance is large enough, the magnetic coupling loop induction to the conductor, the voltage will not produce a significant impact, because the interference caused by the current, generally use twisted pair can reduce .interference; two-wire system and three-wire system must use shielded cables, shielded cable's shield to proper grounding. <br> <br> 3, capacitive interference can cause the error, the receiver resistance for 4 ~ 20mA two wire loop receiver resistance .usually 250 Ω (sampling Uout = 1 ~ 5V) this resistance as small enough to make a significant error, therefore, permissible cable length voltage telemetry system more farther; <br> <br> 4. each single shows reading device or recording device can .wire length of different channel for exchanging, not because the cable length of inequality caused by the difference in accuracy, enabling decentralized collection, the benefits of decentralized collection is: decentralized collection, centralized control ....< br> <br> 5, .4mA to zero level will be to determine the open circuit and short or sensor damage (0mA) is very convenient. <br> <br> 6, two-wire output is very easy to add one or two only lightning surge protection devices, conducive .to safe atmospheres. <br>.