CN217358783U - A two-wire thermocouple temperature transmitter with cold junction temperature compensation - Google Patents

Abstract

The utility model discloses a two-wire thermocouple temperature transmitter with cold end temperature compensation, which comprises a cold end temperature compensation circuit and an XTR101AP conversion circuit; the A end of the thermocouple is connected with the XTR101AP conversion circuit, the B end of the thermocouple is connected with the cold end temperature compensation circuit and the XTR101AP conversion circuit, and the XTR101AP conversion circuit is also connected with the cold end temperature compensation circuit. The utility model discloses a 4 ~ 20mA two-wire system special chip XTR101AP of high accuracy low drift to innovatively add high accuracy current type temperature sensor AD590 and carry out the design of thermocouple cold junction precision compensation circuit, form the two-wire system thermocouple temperature transmitter of the high accuracy precision cold junction compensation of unique excellent performance.

Description

A two-wire thermocouple temperature transmitter with cold junction temperature compensation

technical field

The utility model relates to a two-wire thermocouple temperature transmitter with cold end temperature compensation, belonging to the field of temperature detection.

Background technique

Thermocouple is a widely used temperature sensor. It generates thermoelectric potential due to the temperature difference between the hot end (in the measured temperature field) and the cold end (in the external environment). The thermoelectric potential can be amplified by using a temperature transmitter. After the conversion, the current signal of 4-20mA is output to facilitate transmission and measurement. In order to make the output of the temperature transmitter only related to the measured temperature, it is necessary to take cold junction temperature compensation measures. XTR101AP is a commonly used high-precision and low-drift 4-20mA two-wire transmitter dedicated chip. The conventional application scheme is to use diodes in the peripheral circuit of XTR101AP to achieve cold junction temperature compensation, but due to the temperature characteristics of diodes (voltage drop The relationship with temperature) is non-linear, therefore, the accuracy of diodes used as cold junction temperature compensation is not high, resulting in a large measurement error of the temperature transmitter. Therefore, it is necessary to provide a temperature transmitter capable of realizing high-precision cold junction temperature compensation.

SUMMARY OF THE INVENTION

The utility model provides a two-wire thermocouple temperature transmitter with cold junction temperature compensation. The thermocouple cold junction compensation circuit is designed through the XTR101AP chip and the high-precision current-type temperature sensor AD590, and a precision cold junction is formed. Temperature compensated two-wire thermocouple temperature transmitter.

The technical scheme of the utility model is: a two-wire thermocouple temperature transmitter with cold junction temperature compensation, comprising a cold junction temperature compensation circuit and an XTR101AP conversion circuit; wherein, the A terminal of the thermocouple is connected with the XTR101AP conversion circuit, and the thermoelectric The B end of the pair is connected with the cold junction temperature compensation circuit and the XTR101AP conversion circuit, and the XTR101AP conversion circuit is also connected with the cold junction temperature compensation circuit.

It also includes: a current expansion circuit and an anti-reverse connection circuit, the current expansion circuit is connected with the XTR101AP conversion circuit and the anti-reverse connection circuit, and the anti-reverse connection circuit is also connected with the XTR101AP conversion circuit.

The cold junction temperature compensation circuit includes a resistor R1, a resistor R2, a resistor R3 and a temperature sensor AD590; wherein one end of the resistor R3 and the B end of the thermocouple are connected to pin 10 of the XTR101AP in the XTR101AP conversion circuit, and the other end of the resistor R3 is connected. The connection between one end and one end of the resistor R2 is connected to the negative electrode of the temperature sensor AD590, one end of the resistor R4 in the XTR101AP conversion circuit, the connection between the positive electrode of the temperature sensor AD590 and one end of the resistor R1 is connected to the 11 pin of the XTR101AP in the XTR101AP conversion circuit, and the The other end is connected to the other end of the resistor R2 and connected to pin 3 of the XTR101AP in the XTR101AP conversion circuit.

The XTR101AP conversion circuit includes XTR101AP, resistor Rs, resistor R4, capacitor C1 and capacitor C2; wherein, pin 3 of XTR101AP is connected to the connection between the resistor R1 in the cold junction temperature compensation circuit and the resistor R2 in the cold junction temperature compensation circuit, and the capacitor One end of C1 is grounded, the other end of capacitor C1 is connected to the connection between pin 4 of XTR101AP and end A of thermocouple, both ends of resistor Rs are connected to pins 5 and 6 of XTR101AP respectively, and one end of resistor R4 is connected to the cold junction temperature compensation circuit The connection between resistor R3 and resistor R2, pin 7 of XTR101AP is connected to the other end of resistor R4, capacitor C2 is connected in parallel with both ends of resistor R4, pin 10 of XTR101AP is connected to the connection between resistor R3 and B end of thermocouple in the cold junction temperature compensation circuit. Connection, pin 11 of XTR101AP is connected to the connection between the positive electrode of the temperature sensor AD590 and the resistor R1 in the cold junction temperature compensation circuit.

The expansion circuit includes a transistor Q1 and a resistor R6; wherein, one end of the resistor R6 is connected to the connection between the 8 feet of the XTR101AP in the XTR101AP conversion circuit and the cathode of the diode D1 in the anti-reverse circuit, and the other end of the resistor R6 is connected to the transistor Q1. Collector, the base of transistor Q1 is connected to pin 12 of XTR101AP in the XTR101AP conversion circuit, and the emitter of transistor Q1 is connected to pin 9 of XTR101AP in the XTR101AP conversion circuit.

The anti-reverse connection circuit includes a diode D1, a capacitor C3, a positive terminal of the output terminal and a negative terminal of the output terminal; wherein the connection between the cathode of the diode D1 and the 8-pin of the XTR101AP in the XTR101AP conversion circuit is connected to the resistor R6 in the current expansion circuit, and the diode The connection between the anode of D1 and the positive terminal of the output terminal is connected to one end of the capacitor C3, and the other end of the capacitor C3 is connected to the XTR101AP in the XTR101AP conversion circuit.

The beneficial effects of the utility model are as follows: the utility model adopts the high-precision and low-drift 4-20mA two-wire transmitter special chip XTR101AP, and innovatively adds the high-precision current-type temperature sensor AD590 to design the precision compensation circuit of the cold end of the thermocouple, It forms a unique high-precision precision cold junction compensation two-wire thermocouple temperature transmitter with excellent performance.

Description of drawings

Fig. 1 is the circuit schematic diagram of the present utility model;

Figure 2 is a schematic diagram of the "cold junction temperature compensation circuit + TR101AP conversion circuit" and its equivalent circuit of the present invention.

Detailed ways

Embodiment 1: As shown in Figure 1-2, a two-wire thermocouple temperature transmitter with cold junction temperature compensation includes a cold junction temperature compensation circuit and an XTR101AP conversion circuit; wherein, the A terminal of the thermocouple is converted with the XTR101AP Circuit connection, the B end of the thermocouple is connected with the cold junction temperature compensation circuit and the XTR101AP conversion circuit, and the XTR101AP conversion circuit is also connected with the cold junction temperature compensation circuit.

Further, it may further include: a current expansion circuit and an anti-reverse connection circuit, the current expansion circuit is connected to the XTR101AP conversion circuit and the anti-reverse connection circuit, and the anti-reverse connection circuit is also connected to the XTR101AP conversion circuit and an external circuit. Specifically, the cold junction temperature compensation circuit realizes the cold junction temperature compensation of the thermocouple; the XTR101AP conversion circuit realizes the conversion of the input voltage e _IN into the output current _IO ; the current expansion circuit is used to shunt part of the current of the internal transistor of the XTR101AP to Reduce the power consumption, internal heat and temperature effects of XTR101AP, thereby improving the accuracy and stability of XTR101AP; the anti-reverse circuit is used to prevent damage to the transmitter when the polarity of the external circuit power supply is reversed.

Further, the cold junction temperature compensation circuit can be set to include a resistor R1, a resistor R2, a resistor R3 and a temperature sensor AD590; wherein, the connection between one end of the resistor R3 and the B end of the thermocouple is connected to pin 10 of the XTR101AP in the XTR101AP conversion circuit. , the connection between the other end of the resistor R3 and one end of the resistor R2 is connected to the negative electrode of the temperature sensor AD590, one end of the resistor R4 in the XTR101AP conversion circuit, the connection between the positive electrode of the temperature sensor AD590 and the one end of the resistor R1 is connected to the 11 of the XTR101AP in the XTR101AP conversion circuit The other end of the resistor R1 and the other end of the resistor R2 are connected to pin 3 of the XTR101AP in the XTR101AP conversion circuit.

Further, the XTR101AP conversion circuit can be set to include XTR101AP, resistor Rs, resistor R4, capacitor C1 and capacitor C2; wherein, pin 3 of XTR101AP is connected to resistor R1 in the cold junction temperature compensation circuit and resistor R2 in the cold junction temperature compensation circuit. One end of capacitor C1 is grounded, the other end of capacitor C1 is connected to the connection between pin 4 of XTR101AP and end A of thermocouple, both ends of resistor Rs are connected to pins 5 and 6 of XTR101AP respectively, and one end of resistor R4 is connected to the cold The connection between resistor R3 and resistor R2 in the terminal temperature compensation circuit, pin 7 of XTR101AP is connected to the negative terminal of the output terminal and the other end of resistor R4 in the anti-reverse connection circuit, capacitor C2 is connected in parallel with both ends of resistor R4, and pin 8 of XTR101AP are respectively It is connected to the cathode of the resistor R6 in the expansion circuit and the diode D1 in the anti-reverse connection circuit. The 9th pin of XTR101AP is connected to the emitter of the transistor Q1 in the expansion circuit. The 10th pin of XTR101AP is connected to the cold junction temperature compensation circuit. Resistor R3 and The connection of the B terminal of the thermocouple, the 11 pin of XTR101AP is connected to the connection between the positive pole of the temperature sensor AD590 and the resistor R1 in the cold junction temperature compensation circuit, and the 12 pin of XTR101AP is connected to the base of the transistor Q1 in the current expansion circuit.

Further, the current spreading circuit can be set to include a transistor Q1 and a resistor R6; wherein, one end of the resistor R6 is connected to the connection between the 8 feet of the XTR101AP in the XTR101AP conversion circuit and the cathode of the diode D1 in the anti-reverse connection circuit, and the other end of the resistor R6 is connected. One end is connected to the collector of transistor Q1, the base of transistor Q1 is connected to pin 12 of XTR101AP in the XTR101AP conversion circuit, and the emitter of transistor Q1 is connected to pin 9 of XTR101AP in the XTR101AP conversion circuit.

Further, the anti-reverse connection circuit can be set to include a diode D1, a capacitor C3, a positive terminal of the output terminal and a negative terminal of the output terminal; wherein, the cathode of the diode D1 and the connection of the 8 feet of the XTR101AP in the XTR101AP conversion circuit are connected to the current expansion circuit. The middle resistor R6, the anode of diode D1 and the positive terminal of the output terminal are connected to one end of the capacitor C3, and the other end of the capacitor C3 is connected to the 7 pin of the XTR101AP in the XTR101AP conversion circuit and the negative terminal of the output terminal. Connect with external circuit.

Further, the external circuit can be set to include a power supply V _PS and a load R _L , the positive terminal of the power supply V _PS is connected to the positive terminal of the output terminal in the anti-reverse connection circuit, the negative terminal of the power supply V _PS is grounded and connected to one end of the load R _L , and the load The other end of R _L is connected to the negative terminal of the output terminal in the anti-reverse connection circuit.

Since the output current of the high-precision current-type temperature sensor AD590 is proportional to the absolute temperature (1uA/K), the utility model utilizes the accurate cold junction temperature current shunt function of the AD590 to make the compensation voltage change linearly with the temperature, which overcomes the problem that the diode is used as the cold junction. During temperature compensation, it compensates for the defect that the voltage varies nonlinearly with temperature, so as to achieve accurate cold junction temperature compensation. The utility model innovatively uses the temperature sensor AD590 to design the thermocouple cold end compensation circuit, has high temperature compensation accuracy, greatly improves the temperature measurement accuracy of the transmitter, has excellent performance, low cost, and simple and easy circuit implementation.

The working principle of the present utility model:

The utility model is designed based on the working principle of the XTR101AP. According to the working principle of the XTR101AP, the following points can be known: (1) The applied voltage difference between the input pin 3 and the pin 4 of the XTR101AP is e _IN = e ₂ - e ₁ After voltage amplification, a current _IO of 4-20mA flows in the output loop between XTR101AP output pin 7 and pin 8 (through R _L , V _PS and D ₁ ); (2) applied to the XTR101AP input pin The voltage e1 on ₃ will appear on pin 5 of the XTR101AP, similarly the voltage _e2 on XTR101AP input pin 4 will appear on pin 6 of the XTR101AP, so the current I in the range setting resistor R _{S S} =(e ₂ -e ₁ )/R _S =e _IN /R _S ; (3) _IO = 4mA when e _IN = 0V; according to the upper limit of e _IN , by selecting R _S correctly, the IO of _IO can be The upper limit corresponds to 20mA; R _S is calculated according to the variation range of the output current I _O 16mA and the full scale range of the input voltage e _IN , namely: (0.016+40/R _S )×(e _IN full scale)=16mA; (4) Since _IO is unipolar, e ₂ must remain greater than e ₁ ; that is, e _IN ≥ 0. In order to make _IO not exceed 20mA at the upper limit of the range, when R _S = ∞, e _IN must be kept less than 1V, and when R _S decreases, e _IN must be reduced proportionally.

The schematic diagram of the circuit is shown in Figure 1. If the thermocouple cold junction compensation is not performed (ie: connect the A end of the thermocouple directly to pin 4 of XTR101AP, and the B end of the thermocouple directly to pin 3 of XTR101AP), but:

e _IN =e _(t-t0) , instead of e _IN =e _t , where t is the temperature at the measuring junction of the thermocouple and t ₀ is the temperature at the cold junction of the thermocouple. Therefore, there is a large error between the output value of the transmitter and the temperature t detected by the thermocouple, and the error will change or even increase with the change of the cold junction temperature t ₀ . In order to improve the conversion accuracy of the transmitter, it is necessary to realize the compensation of the cold junction of the thermocouple. The utility model designs a unique and innovative cold junction compensation circuit in the application circuit of the XTR101AP.

The left half of Figure 2 shows the cold junction temperature compensation + XTR101AP conversion circuit of the utility model. The cold junction temperature compensation circuit is composed of resistors R1, R2, R3 and temperature sensor AD590, using pins 10 and 11 of XTR101AP. The output constant current source forms the cold junction compensation voltage. AD590 is a two-wire current-type integrated circuit temperature sensor. Its output current is proportional to the absolute temperature. When the temperature increases by 1K, the output current increases by 1μA. The output corresponding to the K temperature scale is 1uA/K, and the output for the Celsius temperature scale is 273.2+1uA/ °C. It is used to detect the temperature t ₀ of the cold junction of the thermocouple and convert it into a current I _t0 =I ₁ = 273.2+t ₀ (uA).

The right half of Fig. 2 shows the equivalent voltage loop of the cold junction temperature compensation circuit of the present invention. For this voltage loop: e _IN +e _R2 -e _R3 -e _(t-t0) =0, where: e _IN is the input voltage of XTR101AP; e _t is the thermoelectric potential when the cold end of the thermocouple is 0°C and the temperature at the measuring end is t; e _(t-t0) is the thermoelectricity when the cold end of the thermocouple is t ₀ and the temperature at the measuring end is t potential; e _R2 and e _R3 are the voltages formed on the resistor R2 and the resistor R3.

According to the thermoelectric potential principle of thermocouple: e _(t-t0) = e _t – e _t0 ;

Therefore e _IN =e _R3 -e _R2 +e _t -e _t0 =e _t +(e _R3 -e _R2 -e _t0 );

Therefore, when the circuit completely (precisely) compensates the temperature of the cold junction of the thermocouple, e _IN = e _t , that is, e _R3 – e _R2 – e _t0 = 0;

It can be obtained from this: there should be a thermocouple cold junction potential e _t0 =e _R3 –e _R2 ;

Since e _R3 = 1 mA×R3, e _R2 =I ₂ ×R2=(1–I ₁ )×R2=(1000–273.2–t ₀ )×10 ⁻³ mA×R2;

It can be seen that as long as R2 and R3 are adjusted to appropriate values, it can be achieved:

e _t0 =e _R3 -e _R2 ; that is, e _IN = e _t (e _IN is independent of the cold junction temperature, fully compensated for the cold junction temperature)

Therefore, the temperature compensation voltage e R2 can be accurately compensated for the temperature t _{0 of the cold junction of the thermocouple, and the temperature measurement of the temperature t 0 of} the cold junction of the thermocouple can be eliminated by using the characteristic that the temperature compensation voltage e _R2 changes linearly with the temperature generated by the accurate cold junction temperature and current shunt of the AD590. impact, to achieve full compensation.

Other technical notes:

1. For applications requiring moderate precision, the XTR101AP can work very efficiently by simply utilizing its internal drive transistors. For applications with higher requirements (high gain and high precision), an external NPN transistor Q1 can be added, which is connected in parallel with the internal transistor of XTR101AP, the purpose is to shunt part of the current of the internal transistor of XTR101AP, reduce the heat generation inside the XTR101AP package, reduce Small temperature effect, improve accuracy.

2. External circuit power supply V _PS range: DC 12V ~ 40V, typical application 24VDC.

3. Two-wire output load R _L : when V _PS =24V, R _L =0~600Ω; when V _PS =40V, R _L =0~1400Ω.

4. The function of the diode D1 is to protect the polarity of the power supply from being reversed.

5. V _PS and R _L are the power supply and output load of the external circuit, and are connected according to the standard connection method of the two-wire transmitter.

6. R2, R3 and Rs are adjustable manganin wire resistances, and the resistance value is determined by the thermocouple type (division) and range (the parameters marked in the figure are K-type thermocouples and the upper limit of the temperature range is 800 °C as an example).

7. R4 is an input bias resistor, which flows through a 2mA current to form an input bias voltage of 2V; capacitor C2 is a decoupling (filtering) capacitor for the bias voltage.

The specific embodiments of the present utility model have been described in detail above in conjunction with the accompanying drawings, but the present utility model is not limited to the above-mentioned embodiments, and within the scope of knowledge possessed by those of ordinary skill in the art, the utility model can also be used without departing from the purpose of the present utility model. Various changes are made under the premise.

Claims (6)

1. a two-wire thermocouple temperature transmitter with cold junction temperature compensation is characterized in that: comprise cold junction temperature compensation circuit, XTR101AP conversion circuit; wherein, the A end of the thermocouple is connected with the XTR101AP conversion circuit, and the thermocouple The B terminal is connected with the cold terminal temperature compensation circuit and the XTR101AP conversion circuit, and the XTR101AP conversion circuit is also connected with the cold terminal temperature compensation circuit. 2. The two-wire thermocouple temperature transmitter with cold junction temperature compensation according to claim 1, it is characterized in that: also comprises: expansion circuit, anti-reverse connection circuit, expansion circuit and XTR101AP conversion circuit, anti-reverse circuit The connection circuit is connected, and the anti-reverse connection circuit is also connected with the XTR101AP conversion circuit. 3. The two-wire thermocouple temperature transmitter with cold junction temperature compensation according to claim 1, wherein the cold junction temperature compensation circuit comprises a resistor R1, a resistor R2, a resistor R3 and a temperature sensor AD590; wherein , the connection between one end of the resistor R3 and the B end of the thermocouple is connected to pin 10 of XTR101AP in the XTR101AP conversion circuit, the other end of the resistor R3 and the connection of one end of the resistor R2 are connected to the negative electrode of the temperature sensor AD590, and the resistor R4 in the XTR101AP conversion circuit One end, the connection between the positive pole of the temperature sensor AD590 and one end of the resistor R1 is connected to pin 11 of the XTR101AP in the XTR101AP conversion circuit, and the other end of the resistor R1 and the other end of the resistor R2 are connected to the XTR101AP conversion circuit. 4. The two-wire thermocouple temperature transmitter with cold junction temperature compensation according to claim 1, wherein the XTR101AP conversion circuit comprises XTR101AP, resistance Rs, resistance R4, capacitance C1 and capacitance C2; wherein, Pin 3 of XTR101AP is connected to the connection between resistor R1 in the cold junction temperature compensation circuit and resistor R2 in the cold junction temperature compensation circuit, one end of capacitor C1 is grounded, and the other end of capacitor C1 is connected to the connection between pin 4 of XTR101AP and end A of thermocouple , the two ends of the resistor Rs are respectively connected to the 5th pin and 6th pin of the XTR101AP, one end of the resistor R4 is connected to the connection between the resistor R3 and the resistor R2 in the cold junction temperature compensation circuit, the 7th pin of the XTR101AP is connected to the other end of the resistor R4, and the capacitor C2 Connect the two ends of resistor R4 in parallel, pin 10 of XTR101AP is connected to the connection between resistor R3 and the B end of the thermocouple in the cold junction temperature compensation circuit, pin 11 of XTR101AP is connected to the anode and the resistance of the temperature sensor AD590 in the cold junction temperature compensation circuit connection to R1. 5. The two-wire thermocouple temperature transmitter with cold junction temperature compensation according to claim 2, wherein the current expansion circuit comprises a transistor Q1 and a resistor R6; wherein, one end of the resistor R6 is connected to the XTR101AP conversion In the circuit, pin 8 of XTR101AP is connected to the cathode of diode D1 in the anti-reverse connection circuit. The other end of resistor R6 is connected to the collector of transistor Q1. The base of transistor Q1 is connected to pin 12 of XTR101AP in the XTR101AP conversion circuit. The emitter is connected to pin 9 of XTR101AP in the XTR101AP conversion circuit. 6. The two-wire thermocouple temperature transmitter with cold junction temperature compensation according to claim 2, wherein the anti-reverse connection circuit comprises a diode D1, a capacitor C3, an output positive terminal and an output negative terminal; The connection between the cathode of diode D1 and pin 8 of XTR101AP in the XTR101AP conversion circuit is connected to resistor R6 in the current expansion circuit, the connection between the anode of diode D1 and the positive terminal of the output terminal is connected to one end of capacitor C3, and the other end of capacitor C3 is connected to XTR101AP In the conversion circuit, the 7-pin of the XTR101AP is connected to the negative terminal of the output terminal, and the positive terminal of the output terminal and the negative terminal of the output terminal are connected to the external circuit.

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