Temperature Transmitter Design (Integrated 2-wire 4-20mA Output

I. Introduction Two-wire temperature transmitters are matched with thermocouples and thermal resistors, respectively, to convert the temperature signal linearly to a 4 to 20 mA DC standard output signal. The two-wire temperature transmitter should have the following main features: The two lines complete the power input and 4~20mA DC current output, that is, the two lines are both the power line and the 4~20mA standard signal output line. Because the two-wire integrated transmitter is installed in the sensor junction box, it must have good reliability, stability, and a wide temperature operating range (0~85°C) and a small temperature drift, while requiring the smallest possible volume. . A linearization circuit is used in the thermocouple and RTD temperature transmitters, so that the transmitter's 4-20mA output signal is linear with the measured temperature. In the thermocouple temperature transmitter, cold junction compensation is required and the cold compensation range is 0~100°C.

The transmitter is divided into two parts: the range unit and the amplifying unit in the line structure, in which the amplifying unit is universal, and the range unit is different according to the variety and measurement range. Design circuit structure shown in Figure 1.

Figure 1: Circuit Structure In the figure, the thick line is the power line, the thin line is the signal flow, and the two external lines are both the power line and the signal line. The 4~20mA signal system provides a possibility for two-wire design. When the signal under test changes from the lower range to the upper range (0% to 100%), the current on the two transmission lines corresponds to a 4-20mA change; 4mA is used as a transmitter. Circuit operation loss current, but also easy to identify broken power failure. RL is the signal sampling load resistance (RL ≤ 250W). V(AB) must be greater than 12V to ensure the normal operation of the system. Under the condition that the power supply is normal (17~30V), the current I of the loop 4~20mA is determined by the input thermal resistor R or the thermocouple mV signal.
Through the block diagram, we can see that, first of all, the signal generated by the signal source needs to be collected, and then the collected signal is amplified, linearized, adjusted, zeroed and full. Finally, the temperature is linearly reflected by the V/I conversion. The voltage signal is converted into a current signal I1 (0~16mA), and a 4mA quiescent current I2 of the circuit is formed to form a 4~20mA current signal through the two-wire power line output. For thermocouple transmitters, a small CU50 RTD is used to measure the cold junction temperature for cold junction compensation. Both transmitters use the LM124 integrated op amp, which is a four independent high-gain internal frequency compensation op amp. It can adapt to the requirements of the single-supply operation of the circuit. The power supply voltage range is large, the temperature characteristics are very good, and the cost performance is high. The op amps used in the following circuits are all LM124. Second, the design of the thermal resistance two-wire transmitter The thermal resistor Pt100, for example, the two-wire transmitter detailed circuit diagram shown in Figure 2, the following part of the working principle of the introduction.
Fig. 2: Circuit diagram of the thermal resistance two-wire transmitter

Design of Two-wire Transmitter with Thermal Resistance Pt100 as an Example

1, the signal acquisition circuit Thermal resistance is the use of the conductor resistance changes with the temperature characteristics of the measured temperature, commonly used platinum resistance Pt100, Pt10 copper resistance Cu50, Cu100 and so on. Its resistance value and temperature can be queried by the index number table.
In the figure, a Pt100 thermal resistor is used as an example (here, other thermal resistors such as Cu50, Cu100, etc.) can be used. The TL431 is a 2.5V Zener diode. D2 is a protection diode to prevent the input voltage from being reversed. Impact on the circuit or damage. R1 is a current limiting resistor. R2, R3, R4 are used together with R5 (Pt100) to form a resistance measuring bridge. Since the integrated 2-wire RTD transmitter is installed in the junction box, the lead resistance is negligible. R1, R2, R3, and R4 can be determined (see FIG. 2 for their values), in which the thermal resistance R5 changes with temperature. R4 takes different values ​​depending on the thermal resistance index number used. If R4 is taken as 100W when Pt100 is measured, R4 is taken as 50W when Cu50 is measured. The two voltages in the middle of the bridge serve as input signals for subsequent differential amplifiers. They are:

Because R2=R3>>R4 and R5, therefore:

2. Amplification Circuit and Linearization Adjustment Circuit One of the functions of this circuit is to amplify the collected weak signal and adopt differential amplification in the current circuit. At the same time, a positive feedback nonlinear adjustment circuit is also connected with the amplifier circuit. Its main function is to correct the nonlinearity between the thermal resistor and the temperature resistor to ensure that the output voltage of the amplifier is linearly related to the measured temperature. R7, R8, R9, and LM124 form an amplifier circuit. For this partial circuit, the input signal comes from the collected signals V and V', and the input signals go through the R7 and R8 to the first set of operational amplifiers of the LM124, respectively, to obtain the output voltage V1 (the nonlinear adjustment circuit, ie, the feedback loop, is not considered here. R6 impact on circuit input).
V1=V'+ R9 (V-V')/R8
In addition, there is a very important part in this circuit, that is the linearization adjustment circuit, namely R6 in this circuit. For the process and principle of linearization adjustment, we can use Figure 3 to explain.
Figure 3: The process and principle of linearization adjustment The dashed line in the figure indicates the curve of the output voltage when the output voltage varies with the source temperature when the linearization adjustment is not performed. In the figure, the solid curve represents the specific process of performing the non-linear adjustment of R6. As the temperature increases, the output voltage increases accordingly. Positive feedback The influence is enhanced, as long as the resistance of R6 is appropriate, it can just offset the nonlinear effect of the thermal resistor itself, so that the output voltage and temperature have a linear relationship, which is shown by the straight line in FIG. 3 . According to the principle of linearization adjustment, the feedback voltage V of the linear adjustment resistor R6 is:

Actual output:

Because of the good linearity of the thermal resistance, after calculating and tuning R6=8.2k in this circuit, the non-linear correction of the thermal resistance can reach the precision of two thousandths.

3. Zero-adjustment, power balance, and the circuit for adjusting the zero point of the secondary amplifier circuit are essentially the adjustment of the level of the amplified voltage output of this stage to ensure that the signal source is at zero (R5=100W, the output of the first stage amplifier is zero). The entire loop current I1 = 4mA. It is composed of R10, R16, R13, and W1. It essentially superimposes a zero voltage at the positive terminal of the voltage input to make the quiescent current less than 4mA reach 4mA. In addition, in this circuit, there is a part that is to reduce the impact of power supply fluctuations on the output of the circuit, that is, R15 in the circuit, which can suppress the influence of power supply fluctuations. When the external voltage source fluctuates greatly (or the load resistance RL changes), the quiescent operating current of the circuit changes slightly. We can use R15 to stabilize the output current. One of its operating principles is the increase of quiescent current due to the increase of the power supply. On the other hand, the increase of the power supply is added to the negative terminal of the amplifier through R15 to perform the subtraction, so that the output voltage of the current stage decreases, and the appropriate R15 resistance value is selected. , It can ensure the stability of the output current when the power supply fluctuates within the allowable range. R17 determines the secondary magnification.
4. Full-scale circuit and V/I conversion circuit The full-scale circuit is composed of R18, R20, and W2 and is composed of a voltage divider V2 at the upper stage. Through the adjustment of W2, the final output (the output of the entire circuit when the signal source is the highest input) achieves the desired output result V (W2 middle tap voltage). R21, R22, R23, R24, R25, and op amp make up a V/I conversion circuit. Since R22, R23, and R24 are all large resistors of 200k, R25 is a small 100W resistor, and the entire circuit current outputs I2≈V/R25. R26 is a load resistor.

Third, the thermocouple two-wire transmitter circuit design Thermocouple two-wire transmitter circuit and thermal resistance two-wire transmitter is the main difference between the signal acquisition and nonlinear correction part, the following we will introduce the two separately.
1, signal acquisition and amplifying circuit
Figure 4: Signal Acquisition and Primary Amplification Circuits The thermocouple output is the mV signal that varies with the temperature being measured. The partial circuit design is shown in Figure 4. In the circuit, the role of the TL431 is to output a stable 2.5V. D0 is a protection diode that protects the power input from the adverse effects of the circuit. By dividing the voltage between R3 and TL431, the operating voltage at both ends of the TL431 is kept at 2.5V, and is compensated by the following cold junction to provide DC power for the correction circuit and the zero adjustment circuit. In this circuit, the copper wire wound thermal resistor Cu50 serves as a cold junction compensation. When the thermocouple's thermoelectric potential E12 changes with the temperature of the cold junction, the voltage across the copper resistor Cu50 also changes in the opposite direction. If the resistance of the voltage dividing resistor R2 is properly selected, the voltage change across the Cu50 can be automatically changed. The effect of compensating the change of the cold junction temperature on the thermoelectric thermoelectric power of the thermocouple. According to the definition of cold junction compensation, the voltage difference between Cu50 at 50°C and 0°C should be equal to the thermoelectromotive force at 50°C, and the voltage at the cold junction temperature of 0°C 50/(R2+50)×2500mV. Through the following zeroing circuit solution, take Ni-Cr-Ni-Si (Ni-Al) thermocouple (division K) to measure the transmission range 0~1300°C as an example.
The output thermopotential is equal to 2.022mV at a K index of 50°C:

From this we can find: R2 = 13k.
In the circuit, the voltage of the thermocouple mV signal and cold-cold-copper resistors are summed and input to the first-stage amplifier of LM124 through R4. According to the working principle of the amplifier, we can obtain the output voltage (including the sum of thermocouple and cold-junction. The input signal is V)V1=V(1+R6/R5). The design considerations are such that when the temperature of the thermocouple reaches a maximum (1300C corresponds to a thermoelectric potential of 52.398 mV), the amplifier's output voltage is 2.5V. That is, the voltage at the thermocouple cold junction temperature of 0°C plus the thermoelectric thermocouple's maximum thermopower, multiplied by the amplification factor, should be equal to 2.5V, ie:

Among them, K is the magnification of LM324, from which K=40 can be calculated. If R4=R5=5.1k, then R6 should be 180k.
2. Linear adjustment circuit and secondary amplifier circuit The partial circuit (output V2 of this stage) is a very important part of this circuit, and it is also a difficult link. Because it involves the linear adjustment of the entire circuit. The magnified part has already been described above, and now the linear adjustment problem is elaborated. The specific circuit is shown in Figure 5 (the circuit in which several diodes are connected is a linear correction circuit). R9, 10 in the circuit
R11, R13, R14, R15, R16 are all disconnected. We only add this resistor when needed.
Figure 5: Linearization and secondary amplifiers This circuit uses a non-linear amplifier circuit to correct the nonlinear characteristics of the measured parameters. The principle is that the polyline parallel branch consisting of the diode compensation resistor plays a role in different positions of the input signal, so that the amplifier has different positions in the signal size. With different magnifications, the nonlinear characteristics are just opposite to the nonlinear characteristics of the thermocouple being measured. In this circuit, six break points (three positive three are negative) are used. The position of the break point can change the branch diode conduction voltage adjustment. Adjusting the fold line branch resistance can change the slope compensation slope. In the actual design process, several points may be taken for correction. For the K degree (detection range 0 to 1300°C), it is first assumed that the range is approximately 0 to 100°C, the nonlinearity error is negligible, and another 500°C is taken. 900 °C, 1300 °C as a correction detection point, when the detection point value above the required linear value, it means that the output value is too large, which requires reducing the output, the specific measure is to connect one of the D7 ~ D12 adjustment circuit; otherwise, connect One of the stages D1~D6 adjusts the circuit. The inflection point selection diode in the circuit may use a silicon tube or a germanium tube according to the need for correction. The adjustment method is as follows: First, the temperature is adjusted to 0.degree. C. by adjusting to 1000.degree. C., and then the calibration is repeated in the following order:
A. For the non-linear adjustment of 100°C~500°C, we can connect D1 or D12, then adjust the R9 or R16 resistance to change the amplification of the amplifier to reach the specified output value. If it is detected that the output value is too small, select R9 D1 and calculate and adjust the resistance of R9 to make the amplifier amplification of this section rise until the output voltage increases to the required linear value. If we detect a large output value, we need to select R16 and D12. And adjust the resistance value of R16, which promotes the decrease of the amplifier output voltage of the segment to the required linear value.
B. When adjusting the non-linear adjustment from 500 °C to 900 °C, we can connect D2, D3 or D10, D11, and then adjust the size of R10 or R15.
C. For the non-linear adjustment between 900 °C and 1300 °C, according to whether the output value of the detection point 1300 °C is too large or too small, it is decided which one of the two polyline compensation branches (three diodes) is left. The method is the same as above. .
As with the RTD, the role of R12 in this circuit is to correct the effect on the entire circuit when the power supply fluctuates. Prevent 4~20mA fluctuations caused by unstable voltage source. The zero-adjustment full-scale and V/I conversion circuits are also the same as the thermal resistance and are not described here.

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