The application of two wire transducer points1

Two wire transducer is a kind of commonly used products in the transmitter, the two wire refers to the field transducer and contact with only two wire control instrument, the thread is not only the power cord, and signal lines. Engineering to introduce two wire transducer today the basic application of knowledge, hoping to help users better application products.

two wire transducer structure:

on the whole structure of it, two wire transducer is composed of three parts: sensor, modulation circuit, two wire VI converter. Sensors will physical quantity into electric parameters, such as temperature, pressure, regulate the output circuit will be weak or nonlinear signal amplification, conditioning, converted to a linear voltage output. Two wire VI conversion circuit according to the output of the signal conditioning circuit control overall power consumption current; From the loop on the voltage and voltage stability at the same time, for the use of modulation circuit and sensors.

besides VI conversion circuit, the circuit of each part has its own power current, two core wire transducer design idea is the all of the current are included within the feedback loop of the VI transformation.

two wire transducer principle:

two wire transducer principle is to use the 4 ~ 20 ma signal provides power for themselves. If the transmitter power is greater than 4 ma, so will not be able to output the lower 4 ma. So generally require two wire system transmitter power consumption itself ( All circuit), including sensors, No more than 35 ma. This is one of two wire transducer design principles.

the advantages of the two wire transducer:

1, susceptible to parasitic thermocouples, and pressure drop along the wire resistance and the influence of temperature drift, very cheap finer wire are available; Can save a lot of cables and installation costs;

2, when the current source output resistance is big enough, the magnetic field coupling induction in the conductor loop voltage, will not have a significant impact, because of the interference sources caused by the current minimum, general use twisted-pair cable can reduce interference; Two wire system with three wire system must be shielded wire, shielding wire shielding layer shall be properly grounded.

3, capacitive interference can cause the receiver resistance related to error, two wire loop for 4 ~ 20 ma, receiver resistance of 250 Ω (often Sampling Uout = 1 ~ 5 v) This little resistance to the not enough to have a significant error, so, can allow the wire length longer farther than voltage telemetry system;

4, each single reading device or recording device can be in wire length varies among different channels for exchange, not due to wire length range precision difference, realize the decentralized collection, the advantage of the decentralized collection is: decentralized collection, centralized control.

5, 4 ma for zero level, to make judgment, open circuit and short circuit or sensor damage ( 0 ma state) It is very convenient.

6, very easy to add one or two in two line output lightning surge prevention device, is advantageous to the lightning protection explosion-proof safety.

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