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Principle of two-wire transmitter

by:Sure     2021-08-09
The following are some of my views on the two-wire transmitter. If there is something wrong, you are welcome to correct it: The two-wire transmitter is a transmitter that provides drive power through the signal line. The use of a two-wire transmitter can save power cords. . The two-wire transmitter was first adopted by the differential pressure transmitter. Differential pressure transmitters are generally distributed in huge factories, using two-wire transmitters. The following are some of my views on two-wire transmitters. If there is something wrong, please correct me: Two-wire transmitter It is a transmitter that provides drive power through a signal line. The use of a two-wire transmitter can save power lines. The two-wire transmitter was first adopted by the differential pressure transmitter. Differential pressure transmitters are generally scattered in huge factories, and the use of two-wire transmitters greatly reduces construction costs. The difference between the two-wire transmitter and the 4-wire transmitter is that it can greatly save the cost of the power cord. For example, in a tank farm with a large number of storage tanks in the open air, because the storage tanks are scattered and many cannot be connected to the power supply, it is very convenient to use a two-wire transmitter that does not require power supply. The signal of the two-wire transmitter is the familiar DC4-20mA DC current signal. The DC4-20mA direct current signal has strong anti-noise ability, and it can ensure the stable transmission of the signal even in an environment with power equipment. If a voltage signal is used, for example, a 10V noise voltage 10V is added to the signal line of DC0-10V, the signal on the signal line will be doubled. The output terminal of the two-wire transmitter measured the internal impedance value of the transmitter to be at least 1MΩ, so the current value I of the noise voltage 10V added to the received impedance: I(A)u003d10(V)/10^6 u003d10^-5 (A)u003d0.01mA, relative to the range of 20mA, the effect is only 1/2000. This is the advantage of the two-wire transmitter. The wiring of the two-wire transmitter is shown in Figure 1. The signal loop is a 4-20mA current signal. The two-wire transmitter uses the input signal 0% corresponding to 4mA DC current action. When the signal is 100%, the transmitter adds 16mA to 4mA to control the loop signal to 20mA. The internal control of the two-wire transmitter makes the input signal proportional to the 4-20mA output signal. Because the two-wire transmitter itself has a voltage drop of V1, the maximum value of the trusted impedance in the signal loop is (DC24V-V 1)/20mA (Figure 1). For how the two-wire transmitter realizes the linear correspondence between the input signal and the output 4-20mA, you can refer to the block diagrams of some foreign current loop chips. Here are the block diagrams of several current loop chips of BB company (now part of TI company) for your reference when designing or applying. Those who are interested can use the knowledge of analog electronics to first analyze the principles of the following block diagrams, and then use discrete components to build the circuit test according to the diagram, so that you can have a more thorough understanding of how the two-wire transmitter performs signal conversion. . It is recommended to start the analysis from the XTR115 block diagram, and then analyze the XTR105 and XTR101 block diagrams. The related pictures of this topic are as follows, click on the picture to enlarge: the internal block diagram of XTR105. The disadvantage of this isolation method is that it requires a set of isolated dual power supplies to supply power to the input terminal. This will increase the cost and the system will be more complicated. The cost saving and cost reduction are the opposite. Now I recommend a domestic two-wire isolated power distribution chip RS 1119. Using this chip can simplify the isolation circuit of the two-wire transmitter, greatly reduce the isolation cost of the two-wire transmitter, and improve the reliability of the entire system. Under the circumstances, reduce the cost. The two-wire isolated power distribution chip RS 1119 is very convenient to use. You can directly change the circuit shown in Figure 1 to the connection shown in Figure 2 to achieve signal isolation, and the linearity and accuracy can be guaranteed within 0.2. Since RS 1119 itself consumes energy, a voltage drop of 3~4V will be generated on the current loop, so the maximum value of the trusted impedance in the signal loop is (DC24V-4V-V 1)/20mA (Figure 2). Generally, the V1 of the two-wire transmitter can work normally at 12V, so the maximum resistance that the two-wire circuit can drive after adding the two-wire isolated power distribution chip RS 1119 is 400 ohms, which is much larger than the standard 250 ohm. Therefore, the use of the two-wire isolation power distribution chip RS 1119 can reduce the isolation cost of the two-wire transmitter and make the entire acquisition system simple and easy to maintain.
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