Wide-temperature-range high-precision detection circuit for measuring temperature of heating device based on NTC (Negative Temperature Coefficient)

Abstract

The utility model discloses a wide-temperature-range high-precision detection circuit for measuring the temperature of a heating device based on an NTC (Negative Temperature Coefficient), and belongs to the technical field of dehumidifiers. A measurement temperature range of the detection circuit is divided into two measurement sub-temperature ranges which correspond to two gears of the detection circuit; the circuit comprises a bridge unit, a low-pass filtering unit, a buffer amplification unit and a differential amplification unit which are electrically connected in sequence, the four resistors of the bridge unit are respectively a first bridge circuit, a second bridge circuit, a third bridge circuit and a fourth bridge circuit; according to the fourth bridge circuit, an NTC unit is connected with a fourth resistor R4 in series and then connected with a fifth resistor R5 in parallel through a first analog switch U1. The fourth bridge circuit is controlled by the first analog switch U1 to switch two resistance modes, and the two resistance modes correspond to the two measurement sub-temperature ranges respectively.

Description

Technical Field

[0001] The utility model relates to the technical field of dehumidifiers, and in particular to a high-precision detection circuit with a wide temperature range for measuring the temperature of a heating device based on an NTC. Background Art

[0002] In the prior art, a three-wire platinum resistor is generally used for testing the wide temperature range of -30 to 200°C for heating devices, which is relatively expensive. The NTC temperature sensor is a thermistor whose resistance value drops rapidly as the temperature rises. Utilizing this characteristic, the NTC thermistor can be used to determine the corresponding temperature by measuring its resistance value, thereby achieving the purpose of detecting and controlling the temperature. The prior art uses the voltage divider method to measure the NTC resistance, but this method has a large measurement error over a wide temperature range, and has poor anti-interference and consistency. Taking the NTC with a B value of 3950K as an example, the resistance of the NTC is 771.62KΩ at -30°C, and the resistance of the NTC is 0.271KΩ at 200°C, with a resistance difference of 2847 times. Use such as Figure 1 The conventional voltage division method is used to measure the NTC resistance value. R17 and C7 form a low-pass filter, and P2 is the NTC interface. VFOUT = P2 / (R16+P2)*VCC. When R16 is 20K, VCC is connected to an LDO with an output of 3.3V, and the LDO output accuracy error is ±2% (voltage is between 3.234V and 3.366V), the corresponding conditions of temperature, NTC resistance value and VFOUT are as follows:

[0003] Table 1. Correspondence between temperature, NTC resistance and VFOUT in existing technology

[0004]

[0005] It can be seen that for the same NTC, the temperature error measured when VCC fluctuates will reach about 10°C, and the product consistency and anti-interference performance are poor. Utility Model Content

[0006] In view of the shortcomings of the prior art, the technical problem to be solved by the utility model is to provide a wide temperature range and high-precision detection circuit based on NTC for measuring the temperature of a heating device, aiming to measure the temperature through an NTC thermistor and reduce costs; and in the circuit, through a bridge unit, a single-power supply buffer amplifier unit with variable gain, and a single-power supply differential amplifier unit, not only a wide temperature range measurement is achieved, but also the consistency and anti-interference ability of the measurement are improved.

[0007] To achieve the above object, the utility model provides a wide temperature range high-precision detection circuit for measuring the temperature of a heating device based on NTC, wherein the measurement temperature range of the detection circuit is divided into two measurement sub-temperature ranges corresponding to the two gears of the detection circuit; the circuit comprises: a bridge unit, a low-pass filter unit, a buffer amplifier unit, and a differential amplifier unit electrically connected in sequence;

[0008] The four resistors of the bridge unit are: a first resistor R1 of the first bridge, a first resistor and a second resistor R2 of the second bridge, a third resistor R3 of the third bridge, and a fourth bridge; wherein the first resistor R1 and the second resistor R2 are set to the input positive potential side, and the third resistor R3 and the fourth bridge are set to the input negative potential side; and the connection node between the first resistor R1 and the fourth bridge is a positive output end, and the connection node between the second resistor R2 and the third resistor R3 is a negative output end;

[0009] The fourth bridge circuit includes: an NTC unit and a fourth resistor R4 are connected in series and then connected in parallel with a fifth resistor R5 through a first analog switch U1; the fourth bridge circuit is controlled by the first analog switch U1 to switch between two resistance modes, which correspond to the two measurement sub-temperature ranges respectively;

[0010] The low-pass filter unit is used to perform low-pass filtering on the positive phase output and the negative phase output of the bridge unit respectively;

[0011] The buffer amplifier unit includes a second analog switch U2, and the second analog switch U2 is used to connect a sixth resistor or a seventh resistor with different resistance values ​​to the buffer amplifier unit to adjust the amplification gain of the buffer amplifier unit.

[0012] In a specific implementation, the first resistor R1 , the second resistor R2 , the third resistor R3 , and the fourth resistor R4 have the same resistance.

[0013] In a specific embodiment, the low-pass filter unit includes a first low-pass filter and a second low-pass filter; the first low-pass filter includes a first RC circuit connected in parallel to the positive phase output; the second low-pass filter includes a second RC circuit connected in parallel to the negative phase output.

[0014] In a specific implementation, the buffer amplifier unit includes: a first operational amplifier, a second operational amplifier;

[0015] The non-inverting input terminal of the first operational amplifier is electrically connected to the positive output terminal of the bridge unit via the low-pass filter unit; the output terminal of the first operational amplifier is electrically connected to the inverting input terminal of the first operational amplifier via the eighth resistor R8;

[0016] The positive input terminal of the second operational amplifier is electrically connected to the negative output terminal of the bridge unit via the low-pass filter unit; the output terminal of the second operational amplifier is electrically connected to the inverting input terminal of the second operational amplifier via the ninth resistor R9;

[0017] The inverting input terminal of the first operational amplifier and the inverting input terminal of the second operational amplifier are electrically connected to the tenth resistor R10 and the eleventh resistor R11 via the second module switch U2; the second module switch U2 is used to switch the gain of the buffer amplifier unit.

[0018] In a specific embodiment, the differential amplification unit includes: a third operational amplifier and a twelfth resistor R12; the twelfth resistor R12 is electrically connected between the output terminal of the third operational amplifier and the inverting input terminal of the third operational amplifier; the positive input terminal of the third operational amplifier is electrically connected to the output terminal of the first operational amplifier, and the negative input terminal of the third operational amplifier is electrically connected to the output terminal of the second operational amplifier.

[0019] In a specific embodiment, it is characterized in that the measurement temperature range of the detection circuit is: [-30°C, 250°C], and the two measurement sub-temperature ranges are: [-30°C, 0°C] and [0°C, 250°C] respectively.

[0020] The beneficial effects of the utility model are as follows: the present invention uses NTC thermistors to measure temperature, which reduces the cost. The circuit not only realizes wide temperature range measurement through the bridge unit, the gain-variable single-power buffer amplifier unit, and the single-power differential amplifier unit, but also improves the consistency and anti-interference ability of the measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a circuit schematic diagram based on NTC detection circuit in the prior art;

[0022] Figure 2 The utility model discloses a high-precision detection circuit with a wide temperature range and based on NTC for measuring the temperature of a heating device in a specific embodiment of the utility model. DETAILED DESCRIPTION

[0023] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0024] The present invention provides a high-precision detection circuit with a wide temperature range based on NTC to measure the temperature of a heating device. Figure 2As shown, the measurement temperature domain of the detection circuit is divided into two measurement sub-temperature domains corresponding to the two gears of the detection circuit; the circuit comprises: a bridge unit 100, a low-pass filter unit, a buffer amplifier unit 200, and a differential amplifier unit 300 electrically connected in sequence;

[0025] The four resistors of the bridge unit 100 are: a first resistor R1 of the first bridge, a first resistor and a second resistor R2 of the second bridge, a third resistor R3 of the third bridge, and a fourth bridge; wherein the first resistor R1 and the second resistor R2 are set to the input positive potential side, and the third resistor R3 and the fourth bridge are set to the input negative potential side; and the connection node between the first resistor R1 and the fourth bridge is a positive output end, and the connection node between the second resistor R2 and the third resistor R3 is a negative output end;

[0026] The fourth bridge circuit includes: an NTC unit and a fourth resistor R4 are connected in series and then connected in parallel with a fifth resistor R5 through a first analog switch U1; the fourth bridge circuit is controlled by the first analog switch U1 to switch between two resistance modes, which correspond to the two measurement sub-temperature ranges respectively;

[0027] The low-pass filter unit is used to perform low-pass filtering on the positive phase output and the negative phase output of the bridge unit 100 respectively;

[0028] The buffer amplifier unit 200 includes a second analog switch U2 , and the second analog switch U2 is used to connect a sixth resistor or a seventh resistor with different resistance values ​​to the buffer amplifier unit 200 to adjust the amplification gain of the buffer amplifier unit 200 .

[0029] In this embodiment, the fourth bridge is controlled by the first analog switch U1 to switch between two resistance modes, and corresponds to the two measurement sub-temperature domains respectively; for example, in this embodiment, a temperature measurement range of -30°C to 250°C can be achieved, and the analog switch U1 is used to divide the two ranges for measurement. When the temperature range is -30°C to 0°C, a control signal is input to the analog switch U1 to connect pin 1 with pin 4. At this time, the fourth resistor R4 is connected in series with the NTC and then in parallel with the fifth resistor R5. When the temperature range is 0°C to 250°C, a control signal can be input to the analog switch U1 to connect pin 3 with pin 4. At this time, the NTC is only connected in series with R3. Based on this, the switching of the two temperature measurement ranges is realized, so that different bridge access resistors are used in different measurements to improve the subsequent output accuracy. It is worth mentioning that the analog switch U1 of this circuit can provide an interface to the outside, and the specific control operation can be controlled by inputting "0" or "1" to its pin according to the model of the analog switch chip, and the control of the analog switch chip itself belongs to the prior art, which will not be repeated here.

[0030] In this embodiment, the first resistor R1 , the second resistor R2 , the third resistor R3 , and the fourth resistor R4 have the same resistance.

[0031] In this embodiment, the low-pass filter unit includes a first low-pass filter and a second low-pass filter; the first low-pass filter includes a first RC circuit connected in parallel to the positive phase output; the second low-pass filter includes a second RC circuit connected in parallel to the negative phase output.

[0032] In this embodiment, the buffer amplifier unit includes: a first operational amplifier and a second operational amplifier;

[0033] The non-inverting input terminal of the first operational amplifier is electrically connected to the positive output terminal of the bridge unit 100 via the low-pass filter unit; the output terminal of the first operational amplifier is electrically connected to the inverting input terminal of the first operational amplifier via the eighth resistor R8;

[0034] The non-inverting input terminal of the second operational amplifier is electrically connected to the negative output terminal of the bridge unit 100 via the low-pass filter unit; the output terminal of the second operational amplifier is electrically connected to the inverting input terminal of the second operational amplifier via the ninth resistor R9;

[0035] The inverting input terminal of the first operational amplifier and the inverting input terminal of the second operational amplifier are electrically connected to the tenth resistor R10 and the eleventh resistor R11 via the second module switch U2; the second module switch U2 is used to switch the gain of the buffer amplifier unit.

[0036] Schematically, an operational amplifier chip powered by a single power supply can be used, and the operational amplifier chip can include a first operational amplifier and a second operational amplifier; and the buffer amplifier unit 200 is formed by an analog switch U2, an eighth resistor R8, a ninth resistor R9, a tenth resistor R10, and an eleventh resistor R11; optionally, R8=R19=R10=15*R11. Different operational amplifier gains can be set according to the measurement temperature range; for example, when the temperature range is 0°C to -30°C and 101°C to 250°C, a control signal is input to the analog switch U2 to connect pin 1 with pin 4, and through the resistance switching, R10 changes to R11, and the amplification factor is changed by changing the resistance value, and the input voltage is amplified to improve the measurement accuracy. In this example, 2 gears of variable gain can be set, which are multiples 3 and 31 respectively.

[0037] It is worth mentioning that the analog switch U2 of this circuit can provide an interface to the outside. The specific control operation can be controlled by inputting "0" or "1" to its pin according to the model of the analog switch chip. The control of the analog switch chip itself belongs to the existing technology and will not be repeated here.

[0038] In this embodiment, the differential amplifier unit 300 includes: a third operational amplifier and a twelfth resistor R12; the twelfth resistor R12 is electrically connected between the output terminal of the third operational amplifier and the inverting input terminal of the third operational amplifier; the positive input terminal of the third operational amplifier is electrically connected to the output terminal of the first operational amplifier, and the negative input terminal of the third operational amplifier is electrically connected to the output terminal of the second operational amplifier.

[0039] The differential amplifier unit 300 can be as follows Figure 2 As shown, the operational amplifier of the differential amplifier unit 300 is composed of multiple resistors and can be powered by a single power supply. Figure 2 In the differential amplifier, the resistors are the same and the differential gain is 1.

[0040] In this embodiment, the measurement temperature range of the detection circuit is: [-30°C, 250°C], and the two measurement sub-temperature ranges are: [-30°C, 0°C] and [0°C, 250°C] respectively.

[0041] The relevant input and output calculations are as follows:

[0042] 1. When the range is -30℃ to 0℃, the resistor R4 in the bridge unit is connected in series with the NTC and then in parallel with the resistor R5, and the series resistor in the buffer amplifier unit is R11.

[0043] The relationship between V2 and V1 output by the bridge unit is as follows:

[0044] Rn=(R4+P1)*R5 / (R4+P1+R5);

[0045] V2=VCC*Rn / (Rn+R1), V1=VCC*R3 / (R3+R2)=0.5VCC;

[0046] The relationship between V2OUT and V1OUT output by the buffer amplifier unit is as follows:

[0047] V2OUT-V1OUT=(V2-V1) / R11*(R8+R11+R9) where R8=R9=R10=15*R15, so V2OUT-V1OUT=(V2-V1)*(2*R8 / R11+1);

[0048] The relationship between the differential amplifier unit VOUT, V2OUT, and V1OUT is as follows:

[0049] V4=R13 / (R15+R13)*V2OUT,(V1OUT-V5) / R12=(V5-VOUT) / R14,V4=V5。So VOUT=[R13 / (R15+R13)]*[(R12+R14) / R12]*V2OUT-R14 / R12*V1OUT。

[0050] R12=R13=R14=R15, so VOUT=(R13 / R15)*(V2OUT-V1OUT)=V2OUT-V1OUT.

[0051] In summary, the relationship between VOUT and NTC resistor P1 is as follows:

[0052] VOUT=(V2-V1)*(2*R8 / R11+1)=[VCC*Rn / (Rn+R1)-0.5VCC]*(2*R8 / R11+1);

[0053] Among them, Rn=(R4+P1)*R5 / (R4+P1+R5).

[0054] 2. When the range is 1℃ to 100℃, R5 in the bridge unit is disconnected and does not work. The series resistor in the buffer amplifier unit is R10. At this time, the relationship between VOUT and NTC resistor P1 is as follows:

[0055] VOUT==[VCC*Rn / (Rn+R1)-0.5VCC]*(2*R8 / R10+1), where Rn=R4+P1.

[0056] 3. When the range is 101℃ to 250℃, R5 in the bridge unit is disconnected and does not work. The series resistor in the buffer amplifier unit is R11. At this time, the relationship between VOUT and NTC resistor P1 is as follows:

[0057] VOUT==[VCC*Rn / (Rn+R1)-0.5VCC]*(2*R8 / R11+1), where Rn=R4+P1.

[0058] By obtaining the VOUT value, the current NTC resistance can be calculated, and then the current temperature can be obtained through the RT table.

[0059] Further, taking R1=R2=R3=R4=22K, R5=24K, R8=R9=R10=150K, R11=10K, R12=R13=R14=R15=10K as an example, B is a 3950K NTC, the power supply VCC is 3.3V, and the accuracy error is ±2%, then the corresponding situation of temperature, NTC resistance and VFOUT is as follows:

[0060] Table 2. Correspondence between temperature, NTC resistance and VFOUT in this embodiment

[0061]

[0062]

[0063] It can be seen that when the power supply accuracy error of the utility model is ±2%, the measured temperature error is within plus or minus 1°C, which effectively ensures the accuracy of measurement and improves the anti-interference ability and product consistency.

[0064] It is worth mentioning that the above examples are only preferred examples; in practical applications, operational amplifiers can be deleted or added according to actual needs, and the gain amplification factor can be set according to actual needs.

[0065] It is worth mentioning that those skilled in the art can also make equivalent or expansive adjustments within the scope of existing technologies and common knowledge. For example, to improve measurement accuracy, the temperature range can be subdivided into multiple gears, and the parallel resistance of R5 can be added to the bridge unit; the gain resistance can also be added to the buffer amplifier circuit, thereby increasing the gain adjustment gear and improving accuracy.

[0066] This patent is not limited to the above-mentioned best implementation method. Anyone can derive other forms of NTC temperature measurement circuits under the inspiration of this patent. All equal changes and modifications made according to the scope of the patent application of this utility model should be covered by this patent.

[0067] The preferred specific embodiments of the utility model are described in detail above. It should be understood that ordinary technicians in this field can make many modifications and changes based on the concept of the utility model without creative work. Therefore, all technical solutions that can be obtained by technicians in this technical field based on the concept of the utility model through logical analysis, reasoning or limited experiments on the basis of the existing technology should be within the scope of protection determined by the claims.

Claims

1. A high-precision detection circuit with a wide temperature range for measuring the temperature of a heating device based on NTC, characterized in that: The measurement temperature range of the detection circuit is divided into two measurement sub-temperature ranges corresponding to the two gears of the detection circuit; the circuit comprises: a bridge unit, a low-pass filter unit, a buffer amplifier unit, and a differential amplifier unit electrically connected in sequence; The four resistors of the bridge unit are: a first resistor R1 of the first bridge, a first resistor and a second resistor R2 of the second bridge, a third resistor R3 of the third bridge, and a fourth bridge; wherein the first resistor R1 and the second resistor R2 are set to the input positive potential side, and the third resistor R3 and the fourth bridge are set to the input negative potential side; and the connection node between the first resistor R1 and the fourth bridge is a positive output end, and the connection node between the second resistor R2 and the third resistor R3 is a negative output end; The fourth bridge circuit includes: an NTC unit and a fourth resistor R4 are connected in series and then connected in parallel with a fifth resistor R5 through a first analog switch U1; the fourth bridge circuit is controlled by the first analog switch U1 to switch between two resistance modes, which correspond to the two measurement sub-temperature ranges respectively; The low-pass filter unit is used to perform low-pass filtering on the positive phase output and the negative phase output of the bridge unit respectively; The buffer amplifier unit includes a second analog switch U2, and the second analog switch U2 is used to connect a sixth resistor or a seventh resistor with different resistance values ​​to the buffer amplifier unit to adjust the amplification gain of the buffer amplifier unit.

2. A high-precision detection circuit with a wide temperature range for measuring the temperature of a heating device based on NTC as claimed in claim 1, characterized in that: The first resistor R1 , the second resistor R2 , the third resistor R3 , and the fourth resistor R4 have the same resistance.

3. A high-precision detection circuit with a wide temperature range for measuring the temperature of a heating device based on NTC as claimed in claim 1, characterized in that: The low-pass filter unit includes a first low-pass filter and a second low-pass filter; the first low-pass filter includes a first RC circuit connected in parallel to a positive-phase output; the second low-pass filter includes a second RC circuit connected in parallel to an inverse-phase output.

4. A high-precision detection circuit with a wide temperature range for measuring the temperature of a heating device based on NTC as claimed in claim 1, characterized in that: The buffer amplifier unit includes: a first operational amplifier and a second operational amplifier; The non-inverting input terminal of the first operational amplifier is electrically connected to the positive output terminal of the bridge unit via the low-pass filter unit; the output terminal of the first operational amplifier is electrically connected to the inverting input terminal of the first operational amplifier via the eighth resistor R8; The positive input terminal of the second operational amplifier is electrically connected to the negative output terminal of the bridge unit via the low-pass filter unit; the output terminal of the second operational amplifier is electrically connected to the inverting input terminal of the second operational amplifier via the ninth resistor R9; The inverting input terminal of the first operational amplifier and the inverting input terminal of the second operational amplifier are electrically connected to the tenth resistor R10 and the eleventh resistor R11 via the second module switch U2; the second module switch U2 is used to switch the gain of the buffer amplifier unit.

5. A high-precision detection circuit with a wide temperature range for measuring the temperature of a heating device based on NTC as claimed in claim 4, characterized in that: The differential amplification unit includes: a third operational amplifier and a twelfth resistor R12; the twelfth resistor R12 is electrically connected between the output end of the third operational amplifier and the inverting input end of the third operational amplifier; the positive input end of the third operational amplifier is electrically connected to the output end of the first operational amplifier, and the negative input end of the third operational amplifier is electrically connected to the output end of the second operational amplifier.

6. A high-precision detection circuit with a wide temperature range for measuring the temperature of a heating device based on NTC as claimed in claim 1, characterized in that: The measurement temperature range of the detection circuit is: [-30°C, 250°C], and the two measurement sub-temperature ranges are: [-30°C, 0°C] and [0°C, 250°C] respectively.

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