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Transmitter related problems
Release Time:2019-11-25

What is the transmitter's two-wire and four-wire signal transmission methods?

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


    One. What is a two-wire current transmitter?

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

    The two-wire system means that the field transmitter is connected to the control room meter using only two wires, which are both the power line and the signal line. Two-wire system and three-wire system (one positive power line, two signal lines, one of which is common to GND) and four-wire system (two positive and negative power lines, two signal lines, one of which is GND) compared to two The advantages of the line system are:

    1. It is not susceptible to parasitic thermocouples and voltage drop and temperature drift along the wire. It can be used with very cheaper and thinner wires; it can save a lot of cable and installation costs;

    2. When the current source output resistance is large enough, the voltage induced in the wire loop through the magnetic field coupling will not have a significant impact, because the current caused by the interference source is extremely small, and the interference can be generally reduced by using the twisted pair; Shielded wires must be used for the three-wire system, and the shield of the shielded wires should be properly grounded.

    3, capacitive interference will lead to receiver resistance related error, for 4 ~ 20mA two-wire loop, the receiver resistance is usually 250Ω (sampling Uout = 1 ~ 5V) This resistance is small enough to produce significant errors, therefore, can Allowable wire lengths are longer and further than voltage telemetry systems;

    4. Each single reading device or recording device can be switched between different channels with different lengths of wires, without the difference in precision due to the unequal length of the wires, to achieve distributed acquisition, and the advantages of distributed acquisition are: dispersion Acquisition, centralized control...

    5, 4mA is used for zero level, making it easy to judge open circuit and short circuit or sensor damage (0mA state).

    6, it is very easy to add one or two lightning protection surge devices in the two-line output port, which is conducive to safety lightning protection.

    The three-wire and four-wire transmitters are not replaced by the two-wire transmitter. The industry dynamics from abroad and the supply and demand of the transmitter can be seen. When the current transmitter is in use. To be installed on the power line of the field device, the monitoring system with the single chip as the core is located in the monitoring room far away from the equipment site, which is generally tens to hundreds of meters or more away. The environment at the equipment site is relatively harsh. The strong electric signal will generate various electromagnetic interferences. The lightning induction will generate strong surge pulses. In this case, a difficult problem encountered in the single-chip application system is how to make long distances in harsh environments. Reliably transmit small signals.

    The emergence of two-wire transmitters has solved this problem. We designed a small, inexpensive perforated two-wire current transducer with the DH4-20 transmission module as the core. It features low offset voltage (<30 μV), low voltage drift (<μV/C°), and ultra-low nonlinearity (<%). It converts the galvanic isolation of the field device power line into a 4 to 20 mA linear current-changing standard current signal output, and then sends it to the input interface of the monitoring system through a pair of twisted pairs. The twisted pair cable will also be located at the same time. The system's 24V working power is sent to the current transmitter. The measurement signal and the power supply are simultaneously transmitted on the twisted pair, which eliminates the expensive transmission cable, and the signal is transmitted in the form of current, and the anti-interference ability is greatly enhanced. The principle of the two-wire current transducer is shown in Figure 1.


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

    The output of the two-wire current transducer is 4 to 20 mA, which is converted into an analog voltage signal of 1 to 5 V or 2-10 V by a precision resistor of 250 Ω. There are various methods for converting to digital signals if the system is in the environment. Long-term use in harsh industrial sites requires consideration of the safety and reliability of the hardware system. The input module of the system uses the voltage-to-frequency conversion device LM231 to convert the analog voltage signal into a frequency signal, and the photoelectric coupling device TL117 is used to isolate the analog quantity and the digital quantity.

    At the same time, the analog signal processing circuit and the digital signal processing circuit respectively use two independent power sources, and the analog ground and the digital ground are separated from each other, which can improve the safety of the system work. The use of the voltage-to-frequency conversion device LM231 also has a certain effect against high-frequency interference.


    3. What are the advantages and disadvantages of current output type and voltage output type?

    In many applications controlled by single-chip microcomputers, transmitters must be used to convert signals that cannot be directly measured by a single-chip microcomputer into electrical analog signals that can be processed by a single-chip microcomputer, such as current transducers, pressure transmitters, temperature transmitters, and flow meters. Send the device, etc.

   Most of the early transmitters were voltage output type, that is, the measurement signal was converted to 0-5V voltage output. This is the direct output of the op amp, and the signal power is <. The digital signal is converted by the analog/digital conversion circuit for reading and control by the single chip microcomputer. However, in the case where the signal needs to be transmitted over long distances or the grid interference is large in the environment, the use of the voltage output type sensor is greatly limited, and the anti-interference ability is poor, the line loss destroys the accuracy, and the like, and two The line current output type transmitter has been widely used due to its extremely high anti-interference ability.


   The voltage output type transmitter has extremely poor anti-interference ability, and the line loss is not high. Sometimes the output DC voltage is superimposed with AC component, which causes the MCU to misjudge and control the error. Will damage the equipment, the output 0-5V can not be far-distance transmission, after the long-distance transmission line pressure drop, the accuracy is greatly reduced. Now many ADC, PLC, DCS input signal ports are made into a two-wire current output type transmitter 4-20mA, which proves the inevitable trend of the voltage output type transmitter being eliminated.


    What are the general methods for the 4. 4 ~ 20mA current output type to the interface?

    The output range of the current output type transmitter is commonly used in 0 to 20 mA and 4 to 20 mA. When the current transducer outputs the minimum current and the maximum current, it represents the minimum and maximum rated output values calibrated by the current transmitter.

    The following is an example of a current transducer with a measurement range of 0 to 100 A. For the 0mA to 20mA transmitter 0mA current corresponding to the input 0A value, the output 4 to 20mA transmitter 4mA current corresponds to the input 0A value, the 20mA current of both types of sensors corresponds to the 100A value.

    For the transmitter output 0 ~ 20mA, we only need to select the appropriate step-down resistor in the circuit design, directly convert the 0-5V or 0-10V voltage on the resistor to the digital signal at the input interface of the A/D converter. Yes, circuit debugging and data processing are relatively simple. However, the disadvantage is that it is impossible to discriminate the damage of the transmitter, and it is impossible to distinguish the open and short circuit of the transmitter output.

    For the output of 4 ~ 20mA transmitter, circuit debugging and data processing are more cumbersome. However, this kind of transmitter can judge whether the circuit is faulty, whether the transmitter is faulty when the transmitter line is not connected, when the short circuit or when it is damaged, whether it can detect the current in the normal range (the minimum value is 4 mA at normal time). Damaged, so it is more widely used.

    Since the 4~20mA transmitter outputs 4mA, the voltage on the sampling resistor is not equal to 0, and the digital quantity directly converted by the analog-to-digital conversion circuit is not 0. The single-chip microcomputer cannot be directly used, and the calculation by the formula is too complicated. Therefore, the general processing method is to eliminate the voltage drop generated by the 4 mA on the sampling resistor through the hardware circuit, and then perform A/D conversion. This kind of hardware circuit is the first RCV420, which is a kind of precision I/V conversion circuit, and I/V conversion circuit which uses LM258 self-contained circuit. This circuit is 4~20mA current and 24V generated by two-wire current transducer. And the sampling resistor forms a current loop, which generates a 1-5V voltage drop across the sampling resistor and inputs this voltage value to pin 3 of amplifier LM258. The resistor divider circuit is used to generate a fixed voltage value at pin 2 of the integrated circuit LM258 to cancel the voltage drop generated by the 4 mA current across the sampling resistor. Therefore, when the two-wire current transmitter has a minimum value of 4 mA, the voltage difference between pins 3 and 2 of the LM258 is substantially 0V. The LM258 and its connected resistor form an adjustable voltage amplifying circuit, which amplifies the voltage of the two-wire current transducer current on the sampling resistor and outputs it to the analog/digital conversion circuit through the 1 pin of the LM258 for reading by the CPU of the microcontroller. The 4-20 mA current of the two-wire current transducer is displayed on the LCD/LED screen by a data processing method in the form of a 0-100 A value.

    5. What are the six comprehensive protection functions of the two-wire current transmitter:

    (1) Input overload protection;

    (2) Output overcurrent limit protection;

    (3), the output current is short-circuited for a long time;

    (4) Two-wire port transient induction lightning and surge current TVS suppression protection;

    (5), working power supply overvoltage limit protection ≤ 35V;

    (6), the working power supply reverse connection protection.

    6. How to distinguish between true and false current and voltage transmitters?

    After the marketization of production materials, the fierce competition is intensified, the true and false are difficult to distinguish, and the transmitter is a marginal subject. Many engineering designers are unfamiliar with this. Some manufacturers are confused with industrial grades and commercial commercial grade indicators (industrial grade). The price is 2-3 times that of commercial commercial grade. Some manufacturers can make a transmitter with a few cents of LM324 and LM431. If you don't believe it, you can open it and see if you bought it for a few hundred yuan. The LM324 and LM431, such a transmitter to send you, you dare not use it!

    The author tries to use the current level and precision current and voltage transmitter as an example, and start from the following methods to distinguish between good and bad.

    (1) The reference should be stable, 4 mA is the corresponding input zero reference, the reference is unstable, and the precision linearity is determined. The zero drift of 4 mA in the cold start is not more than % within 4 minutes; (ie), the load is 250Ω. The pressure drop is that the foreign IC chip uses an expensive energy gap reference, and the temperature drift coefficient varies by 10 ppm per degree;

    (2) The total current consumption of the internal circuit is <4mA, which is equal to after the addition and setting, and the active rectification and filtering amplification constant current circuit does not change the current consumption due to the input change of the primary side, and the foreign IC chip is powered by constant current;

    (3) When the working voltage is at full scale, the full-scale reading will not change due to the load 0-700Ω change; the change does not exceed %;

    (4) When the full scale is used, when the load is 250Ω, the full-scale reading will not change due to the change of the working voltage; the change does not exceed within %;

    (5) When the primary side is overloaded, the output current does not exceed +10%. Otherwise, the 24V working power supply and A/D input clamp circuit for the transmitter in the PLC/DCS are damaged due to excessive power consumption. The output of the device is also damaged due to excessive power consumption, and the A/D input clamp circuit is even more devastating;

    (6) When the working voltage is 24V, the transmitter must not be damaged and must have polarity protection;

    (7) When the two lines are clamped due to inductive lightning and induced surge voltage exceeding 24V, the transmitter should not be damaged; generally 1-2 TVS transient protection diodes can be connected in parallel between the two lines to suppress every 20 seconds. The impact of the positive and negative pulses of a 20-millisecond pulse width at a time interval, the transient is subjected to the impact power;

    (8) The linearity % of the product label is absolute or relative error, which can be discerned by the following method: it is true linearity % according to the following indicators.

 When the primary input is zero, the output is 4mA plus or minus %(), and the voltage drop across the load is 250Ω.

 When the primary input is 10%, the output A is positive and negative, and the voltage drop at 250 ohms is

 When the primary input is 25%, the output voltage of 8mA is positive or negative.

 When the primary input is 50%, the output voltage of 12mA is positive or negative.

 When the primary input is 75%, the output voltage of 16mA is positive or negative.

 When the primary side loses 100%, the output voltage of 20mA is positive or negative.

 (9) The primary side input overload must be limited: when the primary input overload is greater than 125%, the output overcurrent limit is 25mA + 10% () the voltage drop on the load 250Ω is;

   (10) If there is no clamp when the induced surge voltage exceeds 24V: In the two-wire output port and an AC 50V pointer type head, connect the two lines with AC 50V to instantly touch the two-wire output port to see if there is any Clamping, how many volts can be clamped at a glance;

    (11) Discrimination with or without polarity protection: use the pointer multimeter Ω multiplied by 10K file to measure the two-wire output port positively and positively. There is always an Ω resistance value infinitely large, and there is polarity protection;

    (12) With or without output current for long-term short-circuit protection: when the primary input is 100% or the overload is greater than 125%-200%, the load is short-circuited by 250Ω, and the short-circuit protection limit is measured at 25mA+10%;

    (13) Identification of industrial grade and commercial commercial grade: industrial grade operating temperature range is -25 degrees to +70 degrees, temperature drift coefficient is 100ppm per degree change, that is, temperature changes by 1 degree per degree, and accuracy changes by one ten thousandth; The commercial commercial temperature range is 0 degrees (or -10 degrees) to +70 degrees (or +50 degrees), the temperature drift coefficient is 250ppm per degree, that is, the temperature changes by 1 degree per degree, and the accuracy changes to two ten thousandths. 5. The temperature drift coefficient of the current-voltage transmitter can be tested and verified by an incubator or a high-temperature box.

 The above 13 methods can also be used to distinguish the advantages and disadvantages of other transmitters.


   Seven. Can you give an example of the technical parameters of a class of industrial grade level accuracy current transmitter?

   1. Accuracy: better than %;

   2. Nonlinear distortion: better than %;

   3. Rated working voltage: +24V±20%, limit working voltage: ≤35V;

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

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

   6. Perforated core hole diameter: 8, 9, 12, 20, 25, 30mm;

   7. Output form: two-wire system DC 4 ~ 20mA;

   8. Output current temperature drift coefficient: ≤ 50ppm / °C;

   9. Response time: ≤ 100mS;

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

   11. Output

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