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Something you don't know about transmitters

Time:2020-10-13 10:49:46 Number of hits:

Transmitter related problems


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What are the two wire and four wire signal transmission modes of transmitters?


In the two-wire transmission mode, the power supply, load resistance and transmitter are in series, that is, two wires simultaneously transmit the power supply and output current signal required by the transmitter. At present, most transmitters are two-wire transmitters. In the four-wire system, the power supply and the load resistance are connected to the transmitter respectively, that is, the output signals of the power supply and the transmitter are transmitted by two wires respectively. . Look at the transmitter eight questions eight answers.




one What is a two-wire current transmitter?


What is a two-wire system? What are the advantages of two-wire system?


The two-wire system means that only two wires are used to connect the field transmitter with the instrument in the control room. These two wires are both power lines and signal lines. Compared with the three-wire system (one positive power line, two signal lines, one GND in total) and the four-wire system (two positive power lines, two signal lines, one GND in total), the two-wire system has the following advantages:




1. It is not susceptible to parasitic thermocouples and resistance voltage drop and temperature drift along the wire, and thinner wires can be used as very cheap ones; Can save a lot of cable and installation costs;




2. When the output resistance of the current source is large enough, the voltage in the wire loop can be induced by the magnetic field coupling, which will not have significant influence. Because the current caused by the interference source is very small, the interference can be generally reduced by using twisted pair wire; Two wire system and three wire system must be shielded wire, the shielding layer of the shielded wire should be properly grounded.




3. Capacitive interference will lead to receiver resistance-related errors. For a 4-20mA two-wire loop, the receiver resistance is usually 250Ω (Sampling Uout=1 ~ 5V) This resistance is too small to produce significant errors, therefore, the allowable wire length is longer and farther than the voltage telemetry system;




4, each single reading device or recording device can be exchanged between different channels with different wire lengths, without the difference in accuracy caused by different wire lengths, to achieve decentralized collection, the advantages of decentralized collection is: decentralized collection, centralized control...




5. Using 4mA for zero level makes it very convenient to judge open circuit and short circuit or sensor damage (0mA state).




6. It is very easy to add one or two lightning protection and surge protection devices at the two-wire outlet, which is conducive to safety, lightning protection and explosion protection.




Three wire system and four wire transducer are not the above advantages is replaced by two wire transducer, from outside the industry dynamic and transmitter heart the quantity of supply and demand can be slightly know, current transducer when use to install the equipment at the scene of the power line, and with the single chip processor as the core of monitoring system is located in the far away from the scene of the equipment in the control room, both general dozens to hundreds of meters apart even further. The equipment site environment is relatively harsh, strong electric signal will produce a variety of electromagnetic interference, lightning induction will produce strong surge pulse, in this case, the SCM application system encountered a thorny problem is how to reliably transmit small signals in a bad environment in a long distance.




The emergence of two - wire transmitter makes this problem better solved. We have designed a small and inexpensive perforated two-wire current transmitter with the DH4-20 converter module as the core. It has a low misalignment voltage (< 30 & Mu; V), low voltage drift (< & Mu; V/C°) , ultra-low nonlinearity (< %). It converts the current isolation of the field equipment power line into a standard current signal output with a linear scale of 4 ~ 20 mA, which is then sent to the input interface of the monitoring system through a pair of twisted pair wires. The twisted pair also sends the 24V working power supply located in the monitoring system to the current transmitter. The measurement signal and power supply are transmitted simultaneously on the twisted-pair cable, which not only eliminates the need for expensive transmission cable, but also the signal is transmitted in the form of current, which greatly enhances the anti-interference ability. The principle of two-wire current transmitter is shown in FIG. 1.




Ii. How to convert the 4-20mA output of the current transmitter?


The output of the two-wire current transmitter is 4 ~ 20 mA, through 250 Ω The precision resistance is converted into an analog voltage signal of 1 ~ 5 V or 2 ~ 10V. There are many ways to convert digital signals. If the system is used for a long time in a harsh industrial environment, the security and reliability of the hardware system should be considered. The input module of the system USES voltage and frequency conversion device LM231 to convert analog voltage signal into frequency signal, and USES photoelectric coupler TL117 to isolate analog and digital data.




At the same time, analog signal processing circuit and digital signal processing circuit use two sets of independent power, analog and digital ground are separated from each other, which can improve the safety of the system. LM231 also has some anti - high - frequency interference effect.




Iii. What are the advantages and disadvantages of current output mode and voltage output mode?


In many applications controlled by microcontroller, transmitters are used to convert signals that cannot be directly measured by microcontroller into electrical analog signals that can be processed by microcontroller, such as current transmitters, pressure transmitters, temperature transmitters, flow transmitters and so on.




Most of the early transmitters were voltage output type, which converted the measurement signal to 0-5V voltage output, which was the direct output of the operational amplifier, the signal power <, and the digital signal was converted through analog/digital conversion circuit for the MCU to read and control. But needs in long distance transmission signal or the occasion, with large environmental eccn interference voltage output type sensor use is limited by a lot, exposed the anti-interference ability is poorer, line loss destroyed the precision shortcomings and so on and so on, the two wire type output current transducer with high anti-jamming ability has been widely applied.






Voltage output type transducer anti-interference ability is poor, the destruction of the line loss, not how high precision, is sometimes superimposed on the dc voltage output have communication components, make SCM make wrong judgments, error control, serious when still can damage equipment, 0 to 5 v never far eastone, remote transmission, line drop after precision. At present, the input signal ports of many ADCs,PLC and DCS are made into 2-wire current output transmitters of 4-20mA, which proves the inevitable trend that voltage output transmitters will be phased out.




Iv.4 ~ 20mA current output to interface what are the general treatment methods?




The output range of current output transmitters is usually 0 ~ 20mA and 4 ~ 20mA. When current transmitters output minimum current and maximum current, they respectively represent the minimum and maximum rated output values calibrated by current transmitters.




The following is an example of a current transmitter with a measuring range of 0 ~ 100A. The 0mA current of the transmitter which outputs 0 ~ 20mA corresponds to the input 0A value, the 4mA current of the transmitter which outputs 4 ~ 20mA corresponds to the input 0A value, and the 20mA current of both types of sensors corresponds to the value of 100A.




For transmitters with output of 0 ~ 20mA, in the circuit design, we only need to select the appropriate step-down resistance and directly convert the 0-5V or 0-10V voltage on the resistance into digital signals at the input interface of A/D converter. Circuit debugging and data processing are relatively simple. But the disadvantage is that the transmitter can not distinguish the damage, can not distinguish the transmitter output open circuit and short circuit.




For transmitter with output of 4 ~ 20mA, circuit debugging and data processing are quite complicated. However, this kind of transmitter can be used more widely because it can detect the current in the normal range (the minimum value is also 4mA in normal time) when the transmitter line is not connected, short circuit or damaged.




Since the voltage on the sampling resistance is not equal to 0 when the 4-20mA transmitter outputs 4mA, and the digital quantity converted directly by the ANALOg-to-digital conversion circuit is not equal to 0, the single chip can not be directly used, and the calculation by formula is too complicated. Therefore, the general treatment method is to eliminate the voltage drop generated by 4mA on the sampling resistance through the hardware circuit, and then perform A/D conversion. Indisputably RCV420, this kind of hardware circuit is a kind of precision I/V transform circuit, and the application LM258 since I/V transform circuit, the circuit produced by two wire current transducer with 24 V and 4 ~ 20 ma current sampling resistance to form current loop, thus on the sampling resistor to produce a 1-5 V voltage, and voltage input to the amplifier LM258 3 feet. The resistance divider circuit is used to generate a fixed voltage value on the 2 pins of the IC LM258 to offset the voltage drop generated by the 4mA current on the sample resistance. Therefore, when the two-wire current transmitter is the minimum value of 4mA, the voltage difference between 3 and 2 pins of LM258 is basically 0V. LM258 in connection with the resistance of the structure can adjust the voltage amplification circuit, the two wire current transmitter current on the sampling resistor voltage amplification and output through LM258 1 foot to analog/digital conversion circuit, for single chip microcomputer CPU reads, through the data processing method two wire current transducer 4-20 ma current on LCD/LED screen displayed in the form of a value of 0-100 - a.






V. What are the six overall protection functions of the two-wire current transmitter:


(1) Input overload protection;


(2) Output overcurrent limit protection;


(3) Output current short-circuit protection for a long time;


(4) TVS suppression protection for transient inductive lightning and surge current at two-wire port;


(5) Overvoltage limit protection of working power & LE; 35 v.


(6) Reverse connection protection of working power supply.




How to distinguish the true and false advantages and disadvantages of the current voltage transmitter?


Means of production market, increased competition, true and false quality failure, and because of transmitter is edge discipline, this is strange, many engineering technicians some manufacturers products are industrial level and the level of civil commercial index confusion (the price of industrial grade 2-3 times that of civil commercial grade) some manufacturers use a few cents LM324 and LM431 can make a transmitter, not letter you open and see, hundreds of yuan to buy if you use LM324 and LM431, such a transmitter to send you, you dare to dare not to use!




The author tries to take the current and voltage transmitter of common grade accuracy as an example, from the following methods to distinguish the pros and cons.


(1) The datum should be stable,4mA is the corresponding input zero datum, the datum is unstable, and the accuracy linearity should be discussed. The zero drift of 4mA should not exceed % within 3 minutes after cold starting; (i.e.), load 250Ω Expensive energy gap reference is used in foreign IC cores, and the temperature drift coefficient varies by 10ppm per degree.


(2) The total consumption current of the internal circuit is <4mA, which is equal to after setting. Moreover, the consumption current of the active rectifier filter amplifier constant current circuit will not change due to the change of the original input, and the foreign IC core chip adopts constant current power supply;


(3) When working voltage and full range, the reading of full range will not be caused by load 0-700Ω Change for change's sake; Change no more than %;


(4) When the full range is reached, the load is 250Ω , the reading of the full range will not change with the change of working voltage; Change no more than %;


(5) When the original side is overloaded, the output current should not exceed +10%, otherwise the 24V working power supply for the transmitter in PLC/DCS and the A/D inputclamp circuit will be damaged due to excessive power consumption. In addition, the transmitter output in the transmitter will also be damaged due to excessive power consumption, and the A/D inputclamp circuit will suffer more damage.


(6) The transmitter shall not be damaged when the working voltage is 24V reversed, and must be protected by polarity;


(7) Clamp the transmitter when the inductive lightning and inductive surge voltage between the two lines exceeds 24V without damaging the transmitter; Generally, one to two TVS transient protection diodes in parallel between two lines can suppress the impact of positive and negative pulses of 20 ms pulse width once every 20 seconds, and the transient state can withstand the impact power.


(8) Whether the linearity % of the product label is absolute error or relative error can be distinguished by the following method at a glance: conforming to the following indicators is true linearity %.


When the original input is zero, the output is 4mA plus or minus %(), and the load is 250Ω The pressure drop is zero


When 10% is input on the original side and output A plus or minus %(), the pressure drop at 250 ohms is


When the original input is 25%, the output is 8mA plus or minus %() with a load of 250Ω The pressure drop is zero


When the original input is 50%, the output is 12mA plus or minus %() with a load of 250Ω The pressure drop is zero


When the original input is 75%, the output is 16mA plus or minus %() with a load of 250Ω The pressure drop is zero


When the original side input is 100%, the output is 20mA plus or minus %() load is 250Ω The pressure drop is zero


(9) Original input overload must limit current: When original input overload is greater than 125%, output overcurrent limit is 25mA+10%() load is 250Ω The pressure drop above is;


(10) Unclamping discrimination when induced surge voltage exceeds 24V: connect two wires at two output ports and an AC 50V pointer meter head to touch the two output ports instantly to see whether there is a clamp, the clamping number of volts can be clear;


(11) To distinguish whether there is polar protection or not: use pointer multimeter Ω Multiply by 10K and measure the two output ports, there is always one Ω Infinite resistance, there is polarity protection;


(12) Long time short circuit protection with non-polar output current: 250&Omega will be loaded when the original input is 100% or when the overload is greater than 125%-200%; Short circuit, measure whether the short circuit protection limit is 25mA+10%;


(13) Discrimination between industrial grade and civil and commercial grade: The working temperature range of industrial grade is -25 degrees to +70 degrees, and the temperature drift coefficient is 100ppm per degree change, that is, 1 degree change of temperature per degree, and the accuracy change is 1/10,000; The operating temperature range of civil and commercial grade is 0 degrees (or -10 degrees) to +70 degrees (or +50 degrees), and the temperature drift coefficient is 250ppm per degree of change, that is, 1 degree of change in temperature per degree, and the accuracy change is 2.5 parts per million; The temperature drift coefficient of the current and voltage transmitter can be tested in a thermostat or a high and low temperature chamber.


The above 13 methods can also be used to distinguish true from false with other transmitters.

Seven. Can you give an example of the technical parameters of a certain brand of industrial grade precision current transmitter?


1. Accuracy: better than %;

2. Nonlinear distortion: better than %;

3. Rated working voltage: +24V& PLUSMn; 20%, ultimate operating voltage: & LE; 35 v.

4. Power consumption: static 4mA, equal to loop current when dynamic, internal limit 25mA+10%;

5. Rated input: 5A...... 1KA (38 specifications);

6. Diameter of perforating core round hole: 8, 9, 12, 20, 25, 30mm;

7. Output form: two-wire DC4 ~ 20mA;

8. Temperature drift coefficient of output current: & LE; 50 PPM / ℃;

9. Response time: ≤ 100 ms;

10. Input/output insulation isolation strength: >AC3000V, 1min, 1mA;

11. Output load resistance: RL = V+-10V/(Ω );

Note :(1) the load impedance is 700&Omega at standard V+24V; ;

(2) the RL = 250 & Omega; Convert 1 ~ 5V resistance + total copper resistance of two transmission lines.

12. Input overload protection :30 times 1min;

13. Output overcurrent limit protection: internal limit 25mA+10%;

Note :(1) International standard output overcurrent limit protection: internal limit 25mA+10%;

(2) Can be customized according to customer requirements: internal limit 22mA+10%, 24mA+10%.

14. TVS inhibitory protection capability of two-wire port transient inductive lightning and surge current: TVS resists impulse current 35A/20ms/;

15. The two-wire port is provided with +24V power reverse connection protection;

16. The output current is limited by a long short circuit protection; Internal limit: 25mA+10%;

17. Working environment: -40℃-80℃, 10%-90%RH;

18. Storage temperature: -50℃-85℃;

19. Executive Standard: GB/T13850-1998;

20. Series models, specifications, wiring diagrams, product shapes, product photos, safety precautions.


8. Can you give an example of the main features of a certain brand of industrial grade precision current transmitter?


1. True RMS two-wire transmitter specially designed for power automation 50/60Hz AC current measurement;

2. Adopt the single-turn perforation through core structure, combine the current transformer and current transmitter into an integrated design;

3. It has six comprehensive protection functions:

(1) Input overload protection;

(2) Output overcurrent limit protection;

(3) Output current short-circuit protection for a long time;

(4) TVS suppression protection for transient inductive lightning and surge current at two-wire port;

(5) Overvoltage limit protection of working power & LE; 35 v.

(6) Reverse connection protection of working power supply.

4. Two-wire output connection is the most advanced output mode in the current analog serial port, with 6 advantages;

(1) Less susceptible to parasitic thermocouples and resistance voltage drop and temperature drift along the wire, finer twisted pair wires can be used at very low cost;

(2) When the output resistance of the current source is large enough, the voltage in the wire loop induced by the magnetic field coupling will not have a significant impact, because the current caused by the interference source is very small, in general, the use of twisted pair wire can reduce the interference;

(3) Capacitive interference will lead to receiver resistance-related errors. For 4-20mA two-wire loop, receiver resistance is usually 250Ω (Sampling Uout=1 ~ 5V) This resistance is too small to produce significant errors, therefore, the allowable wire length is longer and farther than the voltage telemetry system;

(4) Each single reading device or recording device can be exchanged between different channels with different wire lengths, and the difference in precision will not be caused by the difference in wire lengths;

(5) Using 4mA for zero level makes it very convenient to judge open transmission line or damaged sensor (0mA state).

(6) It is easy to add surge prevention and lightning protection device at the outlet of two-wire transmission, which is conducive to safety, lightning protection and explosion protection.

5. Primary and secondary side are highly insulated and isolated;

6. High reliability, high stability, high cost performance;

7. Especially suitable for intelligent monitoring system of load current of generator, motor, low-voltage distribution cabinet, air conditioner, fan, street lamp, etc.

8. Ultra-low power consumption. In a single static state, the full-range power consumption is, and the output current internal limited power consumption is.


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