Low self-heating constant temperature difference control circuit and control method for compensating process error of thermal flowmeter

Abstract

The invention discloses a low-self-heating constant-temperature-difference control circuit and method for compensating process errors of a thermal flowmeter. The low-self-heating constant-temperature-difference control circuit comprises a heating resistor, an environment temperature resistor, a control resistor, a starting resistor, a first potentiometer, a second potentiometer, first to sixth resistors, a triode, a first operational amplifier, a second operational amplifier, a third operational amplifier and a power source. The third operational amplifier controls a Wheatstone bridge composed of the first resistor, the second resistor, the heating resistor and the environment temperature resistor as main bodies to be in a balanced state through negative feedback regulation; the third resistor, the fourth resistor, the fifth resistor and the sixth resistor reduce the heating power of the environment temperature resistor through voltage division, resistance values of the first potentiometer and the second potentiometer are adjustable, and errors of resistance values and temperature coefficients generated in the machining process of the heating resistor and the environment temperature resistor can be compensated. According to the invention, the overheating temperature of the heating resistor can be more accurately controlled to be constant, so that the measurement precision is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of flow sensors, and in particular to a low self-heating constant temperature difference control circuit and a control method for compensating for process errors of a thermal flow meter. Background Art

[0002] A complete thermal flow meter system includes a sensor chip and a corresponding interface circuit. When working, the sensor senses the changes in the state of the external fluid and responds, and the interface circuit controls the sensor to work in a suitable working state, suppresses the influence of various interference factors headed by temperature fluctuations, and achieves accurate readings, thereby making the best use of the sensor's strengths and avoiding its weaknesses.

[0003] Constant bias and constant temperature difference are the two most commonly used working modes of thermal flowmeters. The interface circuit required for the constant bias mode is relatively simple, but it is difficult to compensate for the drift of ambient temperature through hardware circuits. In complex working environments, it often needs to be used in conjunction with software compensation. In contrast, the constant temperature difference working circuit is based on a bridge structure. It controls the difference between the average temperature of the sensor sensitive area and the ambient temperature through negative feedback regulation to remain unchanged. It can adapt to various complex working environments and has the advantages of high accuracy and fast response speed. It is the most promising working mode.

[0004] The classic single-bridge constant temperature difference circuit has a simple structure and is widely used, but in actual use there is often a certain constant temperature error. This is because the circuit structure requires that the voltage across the ambient temperature resistor and the heating resistor is basically equal, so a certain heating power is applied to the ambient temperature resistor, causing the ambient temperature resistor to be heated and unable to accurately reflect the ambient temperature. This is called the self-heating effect of the ambient temperature resistor. In addition, process errors are difficult to avoid during the sensor's tape-out process. The resistance values ​​and temperature coefficients on different sensor chips in the same batch can differ by more than 5%, which will also introduce non-ideal factors in the interface circuit. The above two problems are difficult to solve with the classic single-bridge constant temperature difference circuit. Summary of the invention

[0005] The purpose of the present invention is to provide a low self-heating constant temperature difference control circuit and control method for compensating the process error of a thermal flow meter. By optimizing the design of the circuit, the self-heating effect of the ambient temperature resistor is effectively reduced, and the process errors in the resistance value and temperature coefficient of the heating resistor and the ambient temperature resistor can be compensated, thereby achieving high-precision constant temperature difference control.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A low self-heating constant temperature difference control circuit for compensating process errors of a thermal flow meter, comprising:

[0008] A heating resistor, an ambient temperature resistor, a control resistor, a starting resistor, a first potentiometer, a second potentiometer, first to sixth resistors, a transistor, a first operational amplifier, a second operational amplifier, a third operational amplifier and a power supply.

[0009] One end of the heating resistor is respectively connected to one end of the first resistor, one end of the first potentiometer and the positive input end of the first operational amplifier, and the other end of the heating resistor is grounded.

[0010] The negative input terminal of the first operational amplifier is respectively connected to the output terminal of the first operational amplifier and one end of the sixth resistor, the other end of the sixth resistor is respectively connected to one end of the fifth resistor and the negative input terminal of the third operational amplifier, and the other end of the fifth resistor is grounded.

[0011] One end of the third resistor is connected to one end of the fourth resistor and the positive input terminal of the second operational amplifier respectively, and the other end of the third resistor is grounded.

[0012] The negative input terminal of the second operational amplifier is respectively connected to the output terminal of the second operational amplifier and one end of the second resistor, the other end of the second resistor is respectively connected to one end of the control resistor, one end of the second potentiometer and the positive input terminal of the third operational amplifier, the other end of the control resistor is respectively connected to one end of the ambient temperature resistor and the other end of the second potentiometer, and the other end of the ambient temperature resistor is respectively connected to the other end of the heating resistor and grounded.

[0013] The output end of the third operational amplifier is connected to the base of the transistor, the collector of the transistor is respectively connected to one end of the starting resistor and the power supply, and the emitter of the transistor is respectively connected to the other end of the starting resistor, the other end of the fourth resistor, the other end of the first resistor and the other end of the first potentiometer.

[0014] Furthermore, the temperature coefficients of the heating resistor and the ambient temperature resistor vary within a range of ±10%.

[0015] Further, the resistance ratio of the heating resistor to the ambient temperature resistor varies within a range of ±10%.

[0016] Furthermore, the first to sixth resistors are fixed resistors having no temperature coefficient.

[0017] Furthermore, the first potentiometer and the second potentiometer are low temperature drift potentiometers.

[0018] Furthermore, the power supply voltage is 5V.

[0019] Furthermore, the resistance of the heating resistor satisfies:

[0020] R H =R H0 (1+α H ΔT H )

[0021] Among them, R H is the heating resistance, R H0 is the resistance of the heating resistor at 0°C, α H is the temperature coefficient of the heating resistor, ΔT H is the average temperature of the heating resistor in Celsius.

[0022] Furthermore, the resistance of the ambient temperature resistor satisfies:

[0023] R S =R S0 (1+α S ΔT S )

[0024] Among them, R S is the ambient temperature resistance, R S0 is the resistance value of the ambient temperature resistor at 0°C, α S is the temperature coefficient of the ambient temperature resistance, ΔT S is the average Celsius temperature of the ambient temperature resistor.

[0025] Furthermore, an analog-to-digital converter is included; the two differential inputs of the analog-to-digital converter are heating resistors R H and the voltage across the first resistor R1.

[0026] Furthermore, the present invention also proposes a control method for a low self-heating constant temperature difference control circuit for compensating for process errors of a thermal flow meter, wherein a first resistor, a second resistor, a heating resistor and an ambient temperature resistor form a Wheatstone bridge.

[0027] The third resistor and the fourth resistor of the right arm of the Wheatstone bridge are divided by voltage and then buffered by the second operational amplifier, thereby reducing the bias voltage across the ambient temperature resistor and suppressing the self-heating effect of the ambient temperature resistor; one end of the heating resistor of the left arm of the Wheatstone bridge is buffered by the first operational amplifier and then divided by voltage by the fifth resistor and the sixth resistor, thereby completing the matching of the two arms of the bridge and controlling the overheat temperature of the heating resistor to be stable.

[0028] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0029] The circuit proposed by the present invention reduces the self-heating power of the ambient temperature resistor through the voltage division of the third and fourth resistors, so that it can more accurately reflect the ambient temperature; through the fifth and sixth resistors, the responses of the resistors on both sides of the bridge to the changes in the ambient temperature are consistent; through the first and second potentiometers, the process errors in the resistance value and temperature coefficient of the heating resistor and the ambient temperature resistor are compensated; and high-precision constant temperature difference control of the thermal flow meter is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a circuit overall structure diagram of the first embodiment of the present invention.

[0031] Figure 2 It is a circuit overall structure diagram of the second embodiment of the present invention.

[0032] Figure 3 It is a structural diagram of a MEMS thermal flow sensor used in the test of the first embodiment of the present invention.

[0033] Figure 4 It is a schematic diagram of a classic single-bridge constant temperature difference circuit.

[0034] Figure 5 It is a schematic diagram of the constant temperature difference control effect of the constant temperature difference control circuit in the first embodiment of the present invention.

[0035] Figure 6 It is a schematic diagram of the effect of suppressing the self-heating effect of the ambient temperature resistor of the constant temperature difference control circuit in the first embodiment of the present invention. DETAILED DESCRIPTION

[0036] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0037] It should be understood that when used in this specification and the appended claims, the terms "include" and "comprises" indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0038] It should also be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include plural forms.

[0039] To achieve the above object, the present invention proposes a low self-heating constant temperature difference control circuit for compensating the process error of a thermal flow meter, such as Figure 1 As shown, it is the first embodiment of the present invention, which specifically includes:

[0040] Heating resistor R H , Ambient temperature resistance R S , control resistance R T , starting resistance R B 、The first potentiometer R p1 , the second potentiometer R p2, first to sixth resistors R1 to R6, transistor Q1, first operational amplifier A1, second operational amplifier A2, third operational amplifier A3 and power supply VCC. The circuit can reduce the self-heating effect of the ambient temperature resistor and compensate for the process error of the resistance and temperature coefficient of the heating resistor and the ambient temperature resistor.

[0041] The third operational amplifier A3 controls the first resistor R1, the second resistor R2, and the heating resistor R H and ambient temperature resistance R S The Wheatstone bridge formed by the main body is in a balanced state; the third to sixth resistors R3 to R6 reduce the heating power of the ambient temperature resistor by voltage division, thereby suppressing the self-heating effect of the ambient temperature resistor; the first and second potentiometers R p1 , R p2 The resistance value is adjustable to compensate the heating resistor R H Resistance R S The error of resistance and temperature coefficient caused by processing.

[0042] Heating resistor R H One end of the first resistor R1 and the first potentiometer R p1 One end is connected to the positive input terminal of the first operational amplifier A1, and the heating resistor R H The negative input terminal of the first operational amplifier A1 is connected to the output terminal of the first operational amplifier A1 and one end of the sixth resistor R6 respectively, the other end of the sixth resistor R6 is connected to one end of the fifth resistor R5 and the negative input terminal of the third operational amplifier A3 respectively, and the other end of the fifth resistor R5 is grounded.

[0043] One end of the third resistor R3 is connected to one end of the fourth resistor R4 and the positive input end of the second operational amplifier A2, respectively, and the other end of the third resistor R3 is grounded. The negative input end of the second operational amplifier A2 is connected to the output end of the second operational amplifier A2 and one end of the second resistor R2, respectively, and the other end of the second resistor R2 is connected to the control resistor R T One end of the second potentiometer R p2 One end is connected to the positive input terminal of the third operational amplifier A3, and the control resistor R T The other end is connected to the ambient temperature resistor R S One end of the second potentiometer R p2 The other end is connected to the ambient temperature resistor R S The other end is connected to the heating resistor R H The output end of the third operational amplifier A3 is connected to the base of the transistor Q1, and the collector of the transistor Q1 is connected to the start-up resistor R B One end of the transistor Q1 is connected to the power supply VCC, and the emitter of the transistor Q1 is connected to the starting resistor RB The other end of the fourth resistor R4, the other end of the first resistor R1 and the first potentiometer R p1 The other end of the connection.

[0044] Heating resistor R H and ambient temperature resistance R S The temperature coefficient of the heating resistor R H Resistance R S The resistance ratio varies within the range of ±10%.

[0045] The first to sixth resistors R1 to R6 are fixed resistors having no temperature coefficient.

[0046] The first potentiometer R p1 and the second potentiometer R p2 It is a low temperature drift potentiometer.

[0047] The voltage of the power supply VCC is 5V.

[0048] like Figure 2 As shown in the figure, it is the second embodiment of the present invention. The second embodiment adds an analog-to-digital converter ADC1 on the basis of the first embodiment. The two differential inputs of the analog-to-digital conversion circuit are respectively the heating resistor R H and the voltage across the first resistor R1.

[0049] The principle of the circuit proposed by the present invention is:

[0050] The increase of fluid velocity makes the heating resistor R H The convection heat dissipation power increases, resulting in the heating resistor R H The temperature drops, causing a voltage difference between the positive and negative inputs of the third operational amplifier A3, resulting in an increase in the output of the third operational amplifier A3, and re-heating the heating resistor R H Heated to the equilibrium temperature; therefore, when the fluid flow rate is different, the heating resistance R H The corresponding heating power is also different. By measuring the heating resistance R H The fluid flow rate can be measured by the heating power. If the ambient temperature changes during this process, the ambient temperature resistance R S This change will be sensed and fed back to the positive input of the third operational amplifier A3. The third operational amplifier A3 will adjust the circuit through negative feedback to keep the circuit balanced again, thereby achieving a constant temperature difference.

[0051] In this process, in order to make the ambient temperature resistor R S Accurately reflects the ambient temperature, and reduces the voltage across the ambient temperature resistor Rs during operation through the voltage division of the third resistor R3 and the fourth resistor R4, thereby suppressing the ambient temperature resistor R STo compensate for the self-heating effect of different sensor chips, the heating resistor R H Resistance R S The circuit controls the resistance R T 、The first potentiometer R p1 , the second potentiometer R p2 The equivalent resistance and temperature coefficient between the positive input terminal of the third operational amplifier A3 and the ground, and the equivalent resistance between the positive input terminal of the first operational amplifier A1 and the top of the bridge are adjusted in series and parallel form, so that the responses of the two arms of the bridge to the ambient temperature match.

[0052] In order to conduct system-level simulation verification of the circuit proposed in this invention, it is necessary to establish a macro model of the thermal flow meter that is coupled with multiple physical fields and can be simulated in coordination with the electronic circuit. Therefore, a method for constructing a MEMS device macro model based on physical field analysis is adopted, that is, the heating resistor R is constructed using the energy analysis method. H Resistance R S The macro model is used, and the op amp, instrumentation amplifier, transistor and other devices all use the officially provided SPICE models.

[0053] The thermal flow meter used in the present invention is a MEMS thermal flow meter, such as Figure 3 As shown, its working principle is governed by King's Law. The semi-empirical formula shows that the heat dissipation power of the sensor includes the power P dissipated by the solid heat conduction of the substrate cond And the convection heat dissipation power P of the heating resistor and air conv , the specific expression is:

[0054]

[0055] Among them, k s is the thermal conductivity of the substrate, D is the thickness of the substrate, L is the length of the sensor chip, and ΔT is the heating resistance R H The superheat temperature, k f is the thermal conductivity of the gas, α is the thermal diffusivity of the gas, γ is the dynamic viscosity of the gas, and U is the flow rate of the fluid.

[0056] When the thermal flowmeter is in thermal equilibrium, the heating resistor R H The heating power is equal to the heat dissipation power. The relationship between the output power and the fluid flow rate is solved. The specific expression is:

[0057]

[0058] Among them, R H0 is the resistance of the heating resistor at 0°C, α H is the temperature coefficient of the heating resistor, ΔT His the average Celsius temperature of the heating resistor, V H is the voltage across the heating resistor.

[0059] To reduce the ambient temperature resistor R S The voltage at both ends suppresses its self-heating effect. The resistance expression of the third to sixth resistors R3 to R6 is:

[0060]

[0061] Where n is the resistor voltage divider ratio coefficient.

[0062] When n is 3, the ambient temperature resistance R S The self-heating power is reduced to that of the classic single-bridge constant temperature difference circuit. This reduces its self-heating temperature by 95%.

[0063] If the heating resistor R H Resistance R S The temperature coefficients of the first and second potentiometers R p1 , R p2 Adjust to 0.5 times the maximum resistance value. At this time, the overheat temperature is controlled by the resistor R T The specific expression is:

[0064]

[0065] Among them, R P2M The second potentiometer R p2 The maximum value, R S0 is the resistance value of the ambient temperature resistor at 0°C, α S is the temperature coefficient of the resistance at ambient temperature.

[0066] If there is a process error, the heating resistor R H Resistance R S If the temperature coefficients are inconsistent, the second potentiometer R p2 The overheat temperature is controlled by the resistance R T and the second potentiometer R p2 Joint decision, the specific expression is:

[0067]

[0068] If there is a process error, the heating resistor R H Resistance R S The resistance ratio If it deviates from the design value, the first potentiometer R needs to be adjusted p1 The resistance value of the bridge arm is set to make the resistance ratio of the upper and lower parts of each bridge arm equal. The specific expression is:

[0069]

[0070] If the ambient temperature changes, the positive and negative input voltages of the third operational amplifier A3 are:

[0071]

[0072] Among them, V + is the positive input voltage of the third operational amplifier A3, V - is the negative input voltage of the third operational amplifier A3, V CTD is the emitter voltage of transistor Q1.

[0073] Combining equations (3), (5), (6) and (7) we can obtain V + =V - , the circuit proposed by the present invention compensates for the heating resistor R H Resistance R S process error, suppressing the ambient temperature resistance R S The self-heating effect of the flow meter can be accurately controlled to work in the constant temperature difference mode.

[0074] In order to verify the effect of the circuit proposed by the present invention, the circuit of the present invention is compared with the circuit of the present invention in the simulation test. Figure 4 According to the user manual, at 0°C, the resistance of the heating resistor of the MEMS thermal flow meter is 30.8Ω, and the resistance of the ambient temperature resistor is 606Ω, and the temperature coefficients of both are 3740ppm / K; at this time, let The overheat temperature of the heating resistor can be set to 30°C. However, actual tests show that the resistance of the heating resistor of the MEMS thermal flow meter is 30.5Ω at 0°C, and the temperature coefficient α H is 3430ppm / K, the resistance of the ambient temperature resistor is 626Ω, and the temperature coefficient is α S It is 3480ppm / K.

[0075] Figure 5 This is a comparison of the constant temperature accuracy of the circuit proposed by the present invention and the classic single-bridge constant temperature difference circuit in the ambient temperature range of 0℃~40℃. The simulation test results show that the average overheating temperature of the heating resistor in the classic single-bridge constant temperature difference circuit is 38.72℃ due to process errors, and the average constant temperature error exceeds 29%; while the average overheating temperature of the heating resistor in the circuit proposed by the present invention is 29.85℃, the average constant temperature error is less than 0.5%, and the maximum constant temperature error is only 0.55℃, which effectively compensates for the impact caused by process errors.

[0076] Figure 6This is a comparison of the self-heating power on the ambient temperature resistor in the circuit proposed by the present invention and the classic single-bridge constant temperature difference circuit within the ambient temperature range of 0℃ to 40℃. The simulation test results show that the average self-heating power of the ambient temperature resistor in the classic single-bridge constant temperature difference circuit is as high as 4.2mW, while the average self-heating power of the ambient temperature resistor in the circuit proposed by the present invention is only 0.19mV, which is only 4.5% of that in the classic single-bridge constant temperature difference circuit, effectively suppressing the self-heating effect of the ambient temperature resistor.

[0077] The simulation test results show that the circuit proposed in the present invention effectively suppresses the heating resistance R H Resistance R S The process error in resistance and temperature coefficient is S The self-heating power at both ends is reduced by more than 95%, which greatly suppresses the self-heating effect, thereby achieving the goal of heating resistor R H Precise constant temperature differential control.

[0078] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A low self-heating constant temperature difference control circuit for compensating process errors of a thermal flow meter, characterized in that: include: Heating resistor R H , Ambient temperature resistance R S , control resistance R T , starting resistance R B 、The first potentiometer R p1 , the second potentiometer R p2 , first to sixth resistors R1 to R6, transistor Q1, first operational amplifier A1, second operational amplifier A2, third operational amplifier A3 and power supply VCC; Heating resistor R H One end of the first resistor R1 and the first potentiometer R p1 One end is connected to the positive input terminal of the first operational amplifier A1, and the heating resistor R H The other end is grounded; The negative input terminal of the first operational amplifier A1 is respectively connected to the output terminal of the first operational amplifier A1 and one end of the sixth resistor R6, the other end of the sixth resistor R6 is respectively connected to one end of the fifth resistor R5 and the negative input terminal of the third operational amplifier A3, and the other end of the fifth resistor R5 is grounded; One end of the third resistor R3 is respectively connected to one end of the fourth resistor R4 and the positive input end of the second operational amplifier A2, and the other end of the third resistor R3 is grounded; The negative input terminal of the second operational amplifier A2 is connected to the output terminal of the second operational amplifier A2 and one end of the second resistor R2, respectively. The other end of the second resistor R2 is connected to the control resistor R T One end of the second potentiometer R p2 One end is connected to the positive input terminal of the third operational amplifier A3, and the control resistor R T The other end is connected to the ambient temperature resistor R S One end of the second potentiometer R p2 The other end is connected to the ambient temperature resistor R S The other end is connected to the heating resistor R H The other end is connected and grounded; The output terminal of the third operational amplifier A3 is connected to the base of the transistor Q1, and the collector of the transistor Q1 is connected to the starting resistor R B One end of the transistor Q1 is connected to the power supply VCC, and the emitter of the transistor Q1 is connected to the starting resistor R B The other end of the fourth resistor R4, the other end of the first resistor R1 and the first potentiometer R p1 The other end of the connection.

2. The low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1 is characterized in that: Heating resistor R H and ambient temperature resistance R S The temperature coefficient varies within ±10%.

3. The low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1, characterized in that: Heating resistor R H Resistance R S The resistance ratio varies within the range of ±10%.

4. The low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1, characterized in that: The first to sixth resistors R1 to R6 are fixed resistors having no temperature coefficient.

5. The low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1, characterized in that: The first potentiometer R p1 and the second potentiometer R p2 It is a low temperature drift potentiometer.

6. The low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1, characterized in that: The voltage of the power supply VCC is 5V.

7. The low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1, characterized in that: Heating resistor R H The resistance value satisfies: R H =R H0 (1+a H ΔT H ) Among them, R H0 is the resistance of the heating resistor at 0°C, α H is the temperature coefficient of the heating resistor, ΔT H is the average temperature of the heating resistor in Celsius.

8. The low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1, characterized in that: Ambient temperature resistance R S The resistance value satisfies: R S =R S0 (1+a S ΔT S ) Among them, R S0 is the resistance value of the ambient temperature resistor at 0°C, α S is the temperature coefficient of the ambient temperature resistance, ΔT S is the average Celsius temperature of the ambient temperature resistor.

9. The low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1, characterized in that: The analog-to-digital converter ADC1 is also included; the two differential inputs of the analog-to-digital converter ADC1 are respectively a heating resistor R H and the voltage across the first resistor R1.

10. The control method of the low self-heating constant temperature difference control circuit for compensating the process error of the thermal flow meter according to claim 1 is characterized in that: The first resistor R1, the second resistor R2, the heating resistor R H and ambient temperature resistance R S Construct a Wheatstone bridge; The third resistor R3 and the fourth resistor R4 of the right arm of the Wheatstone bridge divide the voltage and pass it through the second operational amplifier A2 for buffering, thereby reducing the ambient temperature resistor R S The bias voltage across the two ends suppresses the ambient temperature resistance R S The self-heating effect of the left arm of the Wheatstone bridge is H After one end is buffered by the first operational amplifier A1, the voltage is divided by the fifth resistor R5 and the sixth resistor R6 to complete the matching of the two arms of the bridge and control the heating resistor R H The superheat temperature remains stable; The resistance expressions of the third to sixth resistors R3 to R6 are: Where n is the resistance voltage division ratio coefficient; When the heating resistor R H Resistance R S When the temperature coefficient is consistent, the overheat temperature is controlled by the control resistor R T The specific expression is: Among them, R P2M The second potentiometer R p2 The maximum value, R S0 is the resistance value of the ambient temperature resistor at 0°C, α S is the temperature coefficient of the ambient temperature resistance, ΔT H is the average temperature of the heating resistor in Celsius.

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