Simple high-precision constant-temperature bath based on logarithmic operation

Abstract

The application discloses a simple and high-precision constant-temperature tank based on logarithmic operation, wherein a detecting element and a heating element are R1 and R2, R1, R2, R3 and R4 constitute a measuring bridge, P1 is used for adjusting the ratio of R3 and R4, the voltage of two opposite diagonal points A-B of the bridge enters operational amplifiers A1 and A2, A1 and A2 are in-phase outputs, and two diodes are connected to two opposite-phase terminals for correcting the nonlinearity of the temperature coefficient of the heating element, so that A1 and A2 each constitute a logarithmic amplifier with the diodes, the outputs of A1 and A2 control transistors TR3 and TR4 respectively, and then control the adjusting tubes TR1 and TR2 of a constant-current voltage stabilizer, the voltage Eo between the emitters of TR1 and TR2 is always kept unchanged, the current io passing through the bridge is also supplied by a constant-current source, and the constant-temperature tank dynamically maintains constant temperature.

Description

Technical Field

[0001] This invention relates to a thermostatic bath design technology, particularly a method that utilizes resistive materials with significantly different temperature coefficients as heaters, while also using these materials as temperature sensing elements. A logarithmic operational amplifier is used for negative feedback control, thereby achieving high-precision thermostatic control. A thermostatic control device constructed using this novel structure maintains a temperature error within 0.05℃ during a week of continuous operation. Background Technology

[0002] A thermostatic bath is a laboratory device used to maintain low-temperature or ambient-temperature conditions. It is widely used in chemical, biological, and physical laboratories, as well as in the pharmaceutical, chemical, food, and metallurgical industries. By combining it with rotary evaporators, vacuum freeze dryers, circulating water vacuum pumps, and magnetic stirrers, it can perform multifunctional chemical reactions and store pharmaceuticals.

[0003] The constant temperature principle of the constant temperature bath mainly relies on the constant temperature controller to control the thermal balance. When the temperature of the constant temperature bath drops due to heat loss to the outside, the constant temperature controller will drive the electric heater in the constant temperature bath to work. After heating to the required temperature, the heating will stop, so that the temperature remains constant. This control method is achieved through a feedback mechanism to ensure temperature stability.

[0004] Furthermore, the constant temperature bath uses stirring to propel the working medium from top to bottom within the mixing zone. After heat exchange with the heater, the medium reaches a suitable temperature. Then, the stirrer vigorously agitates the medium, ensuring thorough mixing before it flows out from the bottom and is guided upwards into the working zone. This reduces heat exchange with the external environment and ensures uniform temperature within the working zone. This circulating flow and temperature control system uses temperature-sensing elements to measure temperature signals and adjust output pulse signals to drive the heater, thus controlling the bath temperature to operate at the set level. Of course, this control principle is also often used in the constant temperature control of room equipment in daily life.

[0005] As described above, a high-precision constant temperature bath typically has separate heating and temperature sensing components, and its structure is generally very complex and its total price is quite expensive.

[0006] One approach is to use resistive materials with significantly different temperature coefficients as heaters, and simultaneously use these materials as temperature sensing elements. By utilizing the temperature and voltage changes of these sensing elements for feedback, high-precision constant temperature control can be easily achieved.

[0007] This method overcomes control and transmission response errors, achieving long-term high-precision constant temperature control, and the entire control structure is relatively simple.

[0008] Using this novel structure to construct a constant temperature control device, the temperature error is within 0.05℃ during a week of continuous operation. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a constant temperature bath device that is simple in structure, low in cost, reliable in use, and capable of maintaining a constant temperature.

[0010] To achieve the above objectives, this invention provides a simple, high-precision constant temperature bath based on logarithmic operations. It includes a measurement bridge circuit, two logarithmic amplifier circuits, two pre-amplification circuits, two regulating transistor circuits controlling constant current and voltage regulators, a +15V power supply, and a -15V power supply. Heating elements R1 and R2, resistor R3, potentiometer P1, and resistor R4 constitute the measurement bridge circuit, with heating elements R1 and R2 forming one arm and resistors R3, P1, and R4 forming the other arm. Operational amplifier A1, diode D1, resistor R5, R7, and capacitor C2 constitute the first logarithmic amplifier circuit. In the logarithmic amplifier circuit, high-frequency distortion compensation capacitor C2 is connected between pins 8 and 6 of operational amplifier A1. Operational amplifier A2, diode D2, resistors R6 and R8, and capacitor C3 constitute the second logarithmic amplifier circuit in the two logarithmic amplifier circuits. High-frequency distortion compensation capacitor C3 is connected between pins 8 and 6 of operational amplifier A2. The connection point of heating elements R1 and R2 is connected to the non-inverting input terminal of operational amplifier A1 through resistor R7. The connection point of heating elements R1 and R2 is also connected to the non-inverting input terminal of operational amplifier A2 through resistor R8. The sliding end of potentiometer P1 is connected to the inverting input terminal of operational amplifier A1 through resistor R5. The inverting input terminal is connected to the output terminal of operational amplifier A1 via a forward diode D1; the inverting input terminal of operational amplifier A2 is connected to ground via resistor R6, and the inverting input terminal of operational amplifier A2 is connected to the output terminal of operational amplifier A2 via a forward diode D2; transistor TR3 constitutes the first pre-amplifier circuit of the two pre-amplifier circuits, the output terminal of operational amplifier A1 is connected to the base of transistor TR3 via a current-limiting resistor R9, the base of transistor TR3 is connected to ground via a high-frequency filter capacitor C4, and the emitter of transistor TR3 is connected to ground via a resistor R12; transistor TR4 constitutes the second pre-amplifier circuit of the two pre-amplifier circuits, and the output terminal of operational amplifier A2... The base of transistor TR4 is connected via current-limiting resistor R10. The base of transistor TR4 is connected to the working ground via high-frequency filter capacitor C5. The emitter of transistor TR4 is connected to the working ground via resistor R13. Transistor TR1 and resistor R11 constitute the first regulating transistor circuit of the two regulating transistor circuits for controlling constant current voltage sources. Resistor R11 is connected between the collector and base of transistor TR1. Transistor TR2 and resistor R14 constitute the second regulating transistor circuit of the two regulating transistor circuits for controlling constant current voltage sources. Resistor R14 is connected between the collector and base of transistor TR2.The collector of transistor TR3 is connected to the base of transistor TR1, and the collector of transistor TR4 is connected to the base of transistor TR2. The +15V power supply is connected to point C of the measuring bridge circuit through the CE terminal of transistor TR1, and the -15V power supply is connected to point D of the measuring bridge circuit through the CE terminal of transistor TR2. The voltage E0 between the emitters of transistor TR1 and transistor TR2 remains constant, and the current flows through the measuring bridge circuit. i O It is also powered by a constant current source.

[0011] In the measuring bridge circuit, the emitter of transistor TR1 is connected to the emitter of transistor TR2 in sequence through heating elements R1 and R2. The emitter of transistor TR1 is also connected to the emitter of transistor TR2 in sequence through resistor R3, potentiometer P1, and resistor R4. Heating elements R1 and R2 are both made of positive temperature coefficient (PTC) capacitors, but their temperature coefficients differ; R2 has a higher temperature coefficient than R1. Resistors R3 and R4 have the same temperature coefficient, and R3*R4 must be much larger than R1*R2. This ensures that the current flowing through the measuring bridge circuit... i O The main flow passes through heating elements R1 and R2 to improve control sensitivity. Adjusting the sliding end of potentiometer P1 allows for the selection of the constant temperature bath temperature.

[0012] In the two pre-amplification circuits, transistor TR3 is an NPN transistor and transistor TR4 is a PNP transistor.

[0013] The two regulating transistor circuits that control the constant current and voltage regulators are described above. Transistor TR1 is an NPN transistor, and transistor TR2 is a PNP transistor. Attached Figure Description

[0014] Appendix Figure 1 Appendix Figure 2 Appendix Figure 3 This document is provided to provide a further understanding of the invention and forms part of this application. Figure 1 This describes the temperature control principle of a constant temperature bath; (attached) Figure 2 This is the practical control circuit principle of a constant temperature bath; attached. Figure 3 It is a logarithmic circuit using diodes. Detailed Implementation

[0015] The embodiments of the present invention are further described below with reference to the accompanying drawings.

[0016] Constant temperature control principle block diagram The simplified principle of this constant temperature bath is as follows: Figure 1As shown, resistors R1, R2, R3, and R4 form a bridge circuit, where R1 and R2 are heating elements and also serve to detect voltage within the bridge.

[0017] If the resistance values ​​of the sensing elements R1 and R2 change due to temperature variations, an error correction voltage change ΔE will be generated at points A and B on the bridge diagonal. i The error voltage ΔE i The signal is sent to an operational amplifier for amplification, and the output current is controlled to regulate the current flowing through R1 and R2. i The temperature of the thermostat also changes, thereby controlling the heat generated by the heating elements R1 and R2. Through this feedback, the temperature of the thermostat bath remains constant.

[0018] Traditional constant temperature baths have separate heaters and detectors, while this device combines them into one. This not only reduces the system's response delay and temperature fluctuations, but also achieves high precision, simple structure, and smooth response characteristics.

[0019] Both R1 and R2 are made of positive temperature coefficient (PTC) wire. Specifically, R1 is made of manganin wire with a temperature coefficient of a1 = 0.0001, and R2 is made of copper wire with a2 = 0.004. They are wound together to form the heating element of the thermostatic bath, and also form two arms of a temperature sensing bridge. The other two arms of the bridge are made of two external resistors R3 and R4 with the same temperature coefficient.

[0020] The external resistors R3 and R4 must satisfy the condition R3*R4>>R1*R2, so that the current through the bridge circuit is... i O The main flow passes through R1 and R2 to improve control sensitivity. Adjusting the ratio of R3 and R4 allows for the selection of the thermostatic bath temperature.

[0021] When the ratio of R3 / R4 is constant, the voltage E obtained from the two diagonal points of the bridge is applied to the two non-inverting input terminals of the high-input-impedance operational amplifier. The magnitude of this input voltage controls the entire feedback circuit, thereby controlling the current of the output constant current source. i O This is also a certainty, thus achieving the purpose of pre-selecting the temperature.

[0022] In actual circuits, R3 and R4 are adjusted using potentiometers so that the temperature of the thermostatic bath can be selected within a wide range.

[0023] The actual control circuit of the constant temperature bath The actual control circuit of the constant temperature bath is as follows: Figure 2As shown, the circuit consists of a measurement bridge circuit, two logarithmic amplifier circuits, two pre-amplifier circuits, two regulating transistor circuits controlling the constant current voltage source, and a power supply section.

[0024] Logarithmic amplifier circuit Almost all temperature measurements require the use of temperature sensors, and the quality of the sensor characteristics is crucial to the performance of the entire system. In this paper, the heating and temperature detection elements are both provided by PTC (positive temperature coefficient) thermistors R1 and R2.

[0025] As is well known, thermistors are made of semiconductor materials and are sensitive to temperature. Thermistors have many characteristics. First, thermistors are small in size and have low thermal inertia, thus requiring signal conditioning circuits. Second, thermistors have a high nominal resistance, so the resistance of the connecting wires is negligible in comparison. Third, thermistors have extremely high accuracy and good interchangeability, making them easy to use. Fourth, the disadvantage of thermistors is that their resistance changes non-linearly with temperature, which increases the complexity of temperature signal display, processing, or control.

[0026] To ensure a certain level of measurement accuracy and enable wider application in actual industrial production, nonlinear compensation of the circuit is required. The simplest compensation method is to utilize the exponential law of the PN junction's current-voltage characteristic by connecting diodes or transistors to the feedback and input circuits of the integrated operational amplifier, respectively. Figure 3 As shown, logarithmic operations can be performed.

[0027] Logarithmic circuits can achieve linear transformations through piecewise linear approximation circuits, feedback amplifier circuits, and integrated circuits. Figure 3 To enable the diodes to conduct in a logarithmic circuit, the input voltage is... u I It should be greater than "0".

[0028] Measurement bridge circuit Figure 1 The design of this measuring bridge has a unique structure, differing from traditional constant temperature baths. Detailed circuit diagrams and parameters are as follows: Figure 2 As shown, the detection and heating elements of the circuit are PTC resistors R1 and R2, which, together with resistors R3 and R4, form a measuring bridge. Potentiometer P1 is used to adjust the ratio of R3 and R4. The voltages at the two diagonal points A and B of this measuring bridge are applied to the input terminals of two operational amplifiers A1 and A2, as shown. Figure 2 As shown.

[0029] Both op-amps A1 and A2 are non-inverting outputs, and each op-amp has a diode D1 and a diode D2 connected between its inverting input and its output. The purpose of this connection is to correct the non-linear changes in the positive temperature coefficient (PTC) resistors R1 and R2. Therefore, both op-amps A1 and A2 are essentially a logarithmic amplifier circuit.

[0030] The operational amplifiers A1 and A2 are model AD797. The AD797 datasheet clearly states that this can be resolved by adding a 50pF capacitor between pins 6 and 8. Figure 2 Capacitors C2 and C3 are used to eliminate internal high-frequency distortion. Adding these capacitors helps improve performance.

[0031] The output signal of logarithmic amplifier A1 provides the base operating current to preamplifier transistor TR3 through resistor R9. Resistor R9 is the base current limiting resistor for TR3. Because TR3 has a low base-emitter impedance, if the output of op-amp A1 is directly connected to transistor TR3, the large base drive current may damage op-amp A1. Similarly, the output signal of logarithmic amplifier A2 is connected to the base current limiting resistor R9 through resistor R9. 10 Provides base operating current for the pre-amplified transistor TR4.

[0032] The base-to-ground capacitors C4 and C5 of the preamplifier transistors TR3 and TR4 are bypass capacitors used for high-frequency filtering and to improve anti-interference capability. When high-frequency AC noise interference occurs, these two capacitors can short-circuit it to ground, thereby eliminating the interference. Therefore, TR3 and TR4 operate in DC amplifier mode.

[0033] The outputs of pre-amplifier transistors TR3 and TR4 control the regulating transistors TR1 and TR2 of the constant current voltage regulator, respectively. The advantage of this circuit structure is that the voltage E0 between the emitters of the two regulating transistors TR1 and TR2 remains constant, while the current through the bridge... i O It is also powered by a constant current source.

[0034] This is easy to understand. PTC thermistors have different temperature coefficients. The higher the temperature coefficient of the thermistor, the stronger its sensitivity to temperature changes; that is, the greater the change in resistance with temperature. When the temperature inside the tank exceeds the predetermined value, because R2 has a higher temperature coefficient, it is more sensitive to temperature changes. The resistance of R2 increases first, the voltage at point A increases, the output voltage of the logarithmic amplifier composed of operational amplifier A1 increases, the conduction of preamplifier transistor TR3 increases, the collector voltage of TR3 decreases, the conduction of regulating transistor TR1 decreases, and the current... i O The voltage at point A will decrease slightly; for a logarithmic amplifier composed of operational amplifier A2, the control process is similar, therefore the load current is determined by the regulating transistors TR1 and TR2. iO aisle.

[0035] Therefore, in general, regarding current i O Although there are slight fluctuations throughout the control process, it can be considered a constant value.

[0036] Furthermore, based on the above analysis, it can be considered that PTC resistor R2 can be regarded as a sensor element of a traditional bridge tester, PTC resistor R1 can be regarded as a heating element, and of course R2 is also a heating element.

[0037] The structure of the constant temperature bath and its high precision research The constant temperature bath used in actual applications is cylindrical. This cylinder is made of a material with good thermal conductivity. Heating coils are wound around the outer surface of this cylinder, which also serve as temperature sensing coils R1 and R2. The coils are coated with a silicon mixture, i.e., refractory material, and the outermost layer is wrapped with heat insulation material.

[0038] Design data: At room temperature (25℃), the resistance values ​​of each arm of the bridge are R1=41.5Ω; R2=35.2Ω; R3=47kΩ; R4=3kΩ. The measured temperature inside the insulation tank is 59.75℃. If an operational amplifier is used for amplification and control, the temperature sensitivity can be greatly improved, that is... ℃ This thermostatic bath can guarantee high-precision control over a long period of time; that is, its temperature error is within 0.05℃ during a week of continuous operation.

[0039] The specific working process of this device is as follows: when the ambient temperature is 28℃, the ratio of R3 and R4 is determined by adjusting potentiometer P1, and the required temperature is finally determined. The temperature can be raised to the specified value in just 5 minutes after the device is turned on. The initial current during the rise is 150mA, and then it enters high-precision control.

[0040] The design concept of this paper is quite novel. It changes the traditional constant temperature device where the heating device and the sensor are separate parts. Instead, it uses resistive materials with significantly different temperature coefficients as heaters, and at the same time, uses them as temperature sensing elements. Then, it uses the temperature and voltage changes of the sensing elements for feedback, thereby obtaining high-precision constant temperature control.

[0041] However, as can be seen from the above description, this design is generally suitable for low-voltage, small-scale constant temperature devices, and not very suitable for large-scale constant temperature equipment directly powered by industrial high-voltage electricity. However, its application range is still wide, such as household constant temperature devices, breeding insulation devices, and university teaching experimental projects.

[0042] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A simple, high-precision constant temperature bath based on logarithmic operations, characterized in that, The constant temperature bath includes a measuring bridge circuit, two logarithmic amplifier circuits, two pre-amplifier circuits, two regulating transistor circuits controlling constant current and voltage regulators, a +15V power supply, and a -15V power supply. Heating element R1, heating element R2, resistor R3, potentiometer P1, and resistor R4 constitute the measuring bridge circuit, with heating element R1 and heating element R2 forming one arm, and resistor R3, potentiometer P1, and resistor R4 forming the other arm. Operational amplifier A1, diode D1, resistor R5, resistor R7, and capacitor C2 constitute the first logarithmic amplifier circuit in the two logarithmic amplifier circuits. High-frequency distortion compensation capacitor C2 is connected to the... Between pins 8 and 6 of amplifier A1, operational amplifier A2, diode D2, resistors R6 and R8, and capacitor C3 constitute the second logarithmic amplifier circuit in the two logarithmic amplifier circuits. High-frequency distortion compensation capacitor C3 is connected between pins 8 and 6 of operational amplifier A2. The connection point of heating elements R1 and R2 is connected to the non-inverting input of operational amplifier A1 through resistor R7. The connection point of heating elements R1 and R2 is also connected to the non-inverting input of operational amplifier A2 through resistor R8. The sliding contact of potentiometer P1 is connected to the inverting input of operational amplifier A1 through resistor R5. The inverting input of operational amplifier A1 is connected to the forward-biased diode D1. Connect the output terminal of operational amplifier A1; the inverting input terminal of operational amplifier A2 is connected to the working ground through resistor R6, and the inverting input terminal of operational amplifier A2 is connected to the output terminal of operational amplifier A2 through forward diode D2; transistor TR3 constitutes the first pre-amplification circuit of the two pre-amplification circuits, the output terminal of operational amplifier A1 is connected to the base of transistor TR3 through current-limiting resistor R9, the base of transistor TR3 is connected to the working ground through high-frequency filter capacitor C4, and the emitter of transistor TR3 is connected to the working ground through resistor R12; transistor TR4 constitutes the second pre-amplification circuit of the two pre-amplification circuits, and the output terminal of operational amplifier A2 is connected to the base of transistor TR3 through current-limiting resistor R12. 10 connects to the base of transistor TR4, which is connected to the working ground via high-frequency filter capacitor C5. The emitter of transistor TR4 is connected to the working ground via resistor R13. Transistor TR1 and resistor R11 constitute the first regulating transistor circuit of the two regulating transistor circuits for controlling constant current voltage sources. Resistor R11 is connected between the collector and base of transistor TR1. Transistor TR2 and resistor R14 constitute the second regulating transistor circuit of the two regulating transistor circuits for controlling constant current voltage sources. Resistor R14 is connected between the collector and base of transistor TR2.The collector of transistor TR3 is connected to the base of transistor TR1, and the collector of transistor TR4 is connected to the base of transistor TR2. The +15V power supply is connected to point C of the measuring bridge circuit through the CE terminal of transistor TR1, and the -15V power supply is connected to point D of the measuring bridge circuit through the CE terminal of transistor TR2. The voltage E0 between the emitters of transistor TR1 and transistor TR2 remains constant, and the current flows through the measuring bridge circuit. i O It is also powered by a constant current source.

2. The simple, high-precision constant temperature bath based on logarithmic operations according to claim 1, characterized in that, In the aforementioned measuring bridge circuit, the emitter of transistor TR1 is connected to the emitter of transistor TR2 in sequence through heating elements R1 and R2. Simultaneously, the emitter of transistor TR1 is connected to the emitter of transistor TR2 in sequence through resistor R3, potentiometer P1, and resistor R4. Heating elements R1 and R2 are both made of positive temperature coefficient (PTC) capacitors, but their temperature coefficients differ; the temperature coefficient of heating element R2 is higher than that of heating element R1. Resistors R3 and R4 have the same temperature coefficient, and R3*R4 must be much larger than R1*R2. This ensures that the current flowing through the measuring bridge circuit... i O The main flow passes through heating elements R1 and R2 to improve control sensitivity. Adjusting the sliding end of potentiometer P1 allows for the selection of the constant temperature bath temperature.

3. The simple, high-precision constant temperature bath based on logarithmic operations according to claim 1, characterized in that, In the two pre-amplification circuits, transistor TR3 is an NPN transistor and transistor TR4 is a PNP transistor.

4. The simple, high-precision constant temperature bath based on logarithmic operations according to claim 1, characterized in that, The two regulating transistor circuits that control the constant current and voltage regulators are described above. Transistor TR1 is an NPN transistor, and transistor TR2 is a PNP transistor.

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