Device for detecting liquid level

Abstract

The utility model provides a device for detecting a liquid level. The device comprises a water sump, a positive electrode plate, a negative electrode plate and a sampling circuit, wherein the bottom of the water sump is provided with a first inward-convex groove and a second inward-convex groove, the positive electrode plate is located in the first inward-convex groove, and the negative electrode plate is located in the second inward-convex groove. As gaps are reserved between the first inward-convex groove and the side wall of the cavity as well as between the second inward-convex groove and the side wall of the cavity, when the interior of the water bin is waterproof, liquid can flow to the position between the positive electrode plate and the negative electrode plate, and the liquid does not make contact with any one of the positive electrode plate and the negative electrode plate. In addition, the positive electrode plate and the negative electrode plate are arranged in parallel, and the positive electrode plate and the negative electrode plate form a parallel counter electrode plate capacitor. On the basis, the sampling circuit is electrically connected with the positive electrode plate, the sampling circuit is electrically connected with the negative electrode plate, and the sampling circuit collects the resonant frequency of the electrode plates and calculates the liquid level value through the resonant frequency.

Description

TECHNICAL FIELD

[0001] The utility model relates to the field of liquid level detection, in particular to a device for detecting liquid level. BACKGROUND

[0002] At present, in the prior art, the sensor for measuring the liquid level of a small semi-closed sump is easily contaminated by liquid and scaled because it is in contact with the water surface. SUMMARY

[0003] The utility model aims at least to solve one of the technical problems existing in the prior art or related art.

[0004] To this end, the first aspect of the utility model provides a device for detecting liquid level.

[0005] Therefore, the first aspect of the utility model provides a device for detecting liquid level, which comprises a sump, a cavity in the sump, a first inner convex groove and a second inner convex groove at the bottom of the sump, and a gap between the first inner convex groove and the second inner convex groove and the side wall of the cavity; a positive electrode plate located in the first inner convex groove; a negative electrode plate connected to the sump and located in the second inner convex groove; a sampling circuit connected to the sump and located at the bottom of the outer side of the sump, the sampling circuit being electrically connected to the positive electrode plate and the negative electrode plate; and wherein the positive electrode plate and the negative electrode plate are arranged in parallel.

[0006] In addition, the device for detecting liquid level in the above technical solution provided by the utility model can also have the following additional technical features:

[0007] In some technical solutions of the utility model, the side of the positive electrode plate facing the negative electrode plate is provided with a first copper foil region exposed, and the side of the positive electrode plate away from the negative electrode plate is provided with a shielding layer; wherein the outer circle of the first copper foil region is provided with a shielding line.

[0008] In some technical solutions of the utility model, the side of the negative electrode plate facing the positive electrode plate is provided with a second copper foil region exposed; wherein the area of the second copper foil region is greater than the area of the first copper foil region.

[0009] In some technical solutions of the utility model, a plurality of installation limiting grooves are provided on the sump, a plurality of first protruding structures are provided on the bottom of the positive electrode plate and the bottom of the negative electrode plate, and a plurality of second protruding structures are provided on the top of the negative electrode plate; wherein the second protruding structures are adapted to the installation limiting grooves, the second protruding structures are connected to the installation limiting grooves, and the first protruding structures are provided with solder pads.

[0010] Optionally, the sump is made of plastic material.

[0011] In some technical solutions of the utility model, optionally, the sampling circuit comprises: a first operational amplifier, a positive input end of the first operational amplifier is electrically connected with the controller, a negative input end of the first operational amplifier is electrically connected with the output end of the first operational amplifier; a first capacitor, a first end of the first capacitor is electrically connected with the output end of the first operational amplifier, a second end of the first capacitor is electrically connected with the positive electrode plate; an inductor, a first end of the inductor is electrically connected with the second end of the first capacitor, a second end of the inductor is grounded; a second operational amplifier, a positive input end of the second operational amplifier is electrically connected with the second end of the first capacitor; a diode, an anode of the diode is electrically connected with the output end of the second operational amplifier, a cathode of the diode is electrically connected with the negative input end of the second operational amplifier; a second capacitor, a first end of the second capacitor is electrically connected with the cathode of the diode, a second end of the second capacitor is grounded; a third operational amplifier, a positive input end of the third operational amplifier is connected with the cathode of the diode, a negative input end of the third operational amplifier is electrically connected with the output end of the third operational amplifier; an ADC sampling circuit, the ADC sampling circuit is electrically connected with the output end of the third operational amplifier; wherein the negative electrode plate is connected with the ground electrode.

[0012] The device for detecting liquid level provided by the utility model detects liquid level by using non-contact method, the sensor surface is less affected by liquid pollution and scaling, and has long service life;

[0013] The mechanical limiting and fixing parallel bipolar plate is accurate to install and simple and convenient to calibrate, and has high consistency;

[0014] The shielding structure is added, so that the overall sensor is less affected by external interference and has strong anti-interference capability.

[0015] The capacitance value is calculated after the resonance frequency of the LC resonance circuit is measured, so that the precision is high and the accuracy is good.

[0016] The additional aspects and advantages of the utility model will be partly given in the following description, partly will become obvious from the following description, or be understood through the practice of the utility model. BRIEF DESCRIPTION OF DRAWINGS

[0017] The above and / or additional aspects and advantages of the utility model will become obvious and easy to understand from the description of embodiments in connection with the following drawings, in which:

[0018] Fig. 1 A schematic view of the device for detecting liquid level according to one embodiment of the utility model is shown;

[0019] Fig. 2 A schematic diagram of the sampling circuit according to one embodiment of the utility model is shown.

[0020] wherein, Figs. 1-2 the correspondence between the reference signs and the component names in the drawings is as follows:

[0021] 1, water tank; 2, positive electrode plate; 3, negative electrode plate; 4, sampling circuit. DETAILED DESCRIPTION

[0022] In order to enable the above-mentioned purposes, features and advantages of the present application to be more clearly understood, the present application will be described in further detail below with reference to the drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0023] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be practiced in other ways different from those described herein, and therefore, the scope of protection of the present application is not limited by the specific embodiments disclosed below.

[0024] Reference will now be made to the following Figs. 1-2 describing a device for detecting a liquid level according to some embodiments of the present application.

[0025] In one embodiment of the present application, as Fig. 1 shown, a device for detecting a liquid level is proposed, comprising: a water tank 1, the water tank 1 is provided with a cavity, the bottom of the water tank 1 is provided with a first inner convex groove and a second inner convex groove, the first inner convex groove and the second inner convex groove are both provided with a gap with the side wall of the cavity; a positive electrode plate 2, the positive electrode plate 2 is located in the first inner convex groove; a negative electrode plate 3, the negative electrode plate 3 is connected with the water tank 1 and is located in the second inner convex groove; a sampling circuit 4, the sampling circuit 4 is connected with the water tank 1 and is located at the bottom outside the water tank 1, the sampling circuit 4 is electrically connected with the positive electrode plate 2, and the sampling circuit 4 is electrically connected with the negative electrode plate 3; wherein, the positive electrode plate 2 and the negative electrode plate 3 are arranged in parallel.

[0026] The present application proposes a device for detecting a liquid level, comprising a water tank 1, a positive electrode plate 2, a negative electrode plate 3 and a sampling circuit 4.

[0027] The bottom of the water tank 1 is provided with a first inner convex groove and a second inner convex groove, the positive electrode plate 2 is located in the first inner convex groove, and the negative electrode plate 3 is located in the second inner convex groove. Because the first inner convex groove and the second inner convex groove both have a gap with the side wall of the cavity, when the water tank 1 is waterproof, the water tank 1 can flow between the positive electrode plate 2 and the negative electrode plate 3, and the liquid does not contact any one of the positive electrode plate 2 or the negative electrode plate 3. Moreover, the positive electrode plate 2 and the negative electrode plate 3 are arranged in parallel, and the parallel electrode plate capacitor is composed of the positive electrode plate 2 and the negative electrode plate 3.

[0028] On this basis, the sampling circuit 4 is electrically connected with the positive electrode plate 2, the sampling circuit 4 is electrically connected with the negative electrode plate 3, and the sampling circuit 4 collects the resonant frequency of the electrode plate and calculates the liquid level value through the resonant frequency. The liquid level is detected by using a non-contact method, the sensor surface is less affected by liquid pollution and scaling, and the service life is long.

[0029] Specifically, after detecting the liquid level, the empty water tank 1 and the full water tank 1 are mapped to the zero liquid level and the maximum liquid level by the “two-point calibration” method to improve the liquid level detection accuracy. First, ensure that there is no measured object, and perform one-point calibration operation. The single-channel frequency is collected and averaged multiple times to save as a zero point. Secondly, ensure that there is a measured object, and perform two-point calibration operation. The single-channel frequency is collected and averaged multiple times to save as a full point. Finally, through the transformation of the frequency between the two points, the corresponding normalized data is calculated to judge the liquid level change. The frequency normalized value SF, combined with the preset gear, confirms the current liquid level information, and the calculation formula is as follows: . Wherein, is the frequency value when the empty measurement is performed; is the frequency value when the full-scale liquid level is measured; is the current measured frequency real-time value; is the normalized value.

[0030] It should be understood that the first inner convex groove (the second inner convex groove) refers to the groove formed by the inward recess of the bottom of the water tank 1. In other words, the inner convex groove is placed from the outside to the groove, and the bottom of the outside is placed into the first inner convex groove (the second inner convex groove).

[0031] It should be noted that the parallel electrode plate capacitor is a common type of capacitor, which is composed of two parallel conductive plates, and a layer of insulating material (dielectric) is placed between the two plates. The capacitance of the parallel plate capacitor, that is, its ability to store electric charge, mainly depends on the area between the plates, the plate spacing, and the type of dielectric material. When the parallel electrode plate capacitor replaces the dielectric between the two electrode plates of the parallel plate capacitor from air to liquid, the capacitance of the capacitor will increase significantly. This is because the relative permittivity of the liquid is much larger than that of the air.

[0032] For example, taking water as the liquid, when all the air between the electrode plates is converted into water, the relative dielectric constant of air is about 1.0006, and the relative dielectric constant of water is about 80 (at 20°C), according to the formula of capacitance, the dielectric constant increases from 1.0006 to about 80, and thus the capacitance increases by about 80 times.

[0033] However, during the gradual increase of the liquid level, the parallel plate capacitor is a non-uniform dielectric distribution, in this case, the overall capacitance of the capacitor will be affected by the different dielectric constants of each part, the whole parallel plate capacitor can be regarded as two capacitors in parallel, one of which is a water dielectric, and the other is air, the capacitance of each part is calculated respectively, and then superimposed in parallel.

[0034] In actual calculation, because the relative dielectric constant of water is much larger than that of air, it dominates the total capacitance, so when the water is from nothing to something, the change rate of the capacitance is large, and with the increase of the water level, the capacitance value of the water dielectric also increases, the change rate decreases and basically remains unchanged.

[0035] Further, in some embodiments of the utility model, the one side of the positive electrode plate 2 facing the negative electrode plate 3 is provided with a bare first copper foil area, and the one side of the positive electrode plate 2 away from the negative electrode plate 3 is provided with a shielding layer; wherein the outer circle of the first copper electrode area is provided with a shielding line.

[0036] In this embodiment, the one side of the positive electrode plate 2 away from the negative electrode plate 3 is provided with a grid copper as a shielding layer, which converges the electric field lines emitted backward and shields the interference behind.

[0037] The one side of the positive electrode plate 2 facing the negative electrode plate 3 is provided with a bare copper foil, which is a first copper foil area, because the electric field lines of the parallel plate capacitor will diverge to the surrounding at the edge, so the outer circle of the first copper foil area is provided with a shielding line to converge the divergent electric field.

[0038] Through the positive electrode plate 2, the edge effect and electric field interference are reduced, and the accuracy and stability of the liquid level measurement are improved.

[0039] Specifically, the positive electrode plate 2 is composed of a double-layer PCB circuit board, and the plate thickness is 1.6 mm.

[0040] Further, in some embodiments of the utility model, the one side of the negative electrode plate 3 facing the positive electrode plate 2 is provided with a bare second copper foil area; wherein the area of the second copper foil area is larger than the area of the first copper foil area.

[0041] In this embodiment, the negative electrode plate 3 is provided with a bare second copper foil region on the side facing the positive electrode plate 2, and the area of the second copper foil region is larger than that of the first copper foil region, thereby ensuring that all the electric field lines of the positive plate, including the slightly divergent electric field lines at the edges, can be accepted, and the electric field lines are concentrated as much as possible and point to the negative electrode plate 3, thereby enhancing the overall anti-interference capability.

[0042] Specifically, the negative electrode plate 3 is composed of a single-layer PCB circuit board, the plate thickness is 1.0 mm, and the overall copper area is slightly larger than that of the positive electrode plate 2, so as to ensure that all the electric field lines of the positive plate, including the slightly divergent electric field lines at the edges, can be accepted. The upper and lower edges are respectively provided with two protrusions for limiting the entire electrode plate, and the lower protrusion is provided with a solder pad for connecting the GND electrode copper to the B end of the LC resonant circuit.

[0043] In this way, the two electrode plates form a parallel-plate capacitor, wherein the shielding line and the back shielding layer surrounding the positive electrode plate 2 ensure that the electric field lines are concentrated as much as possible and point to the GND electrode plate, one of the two electrodes is the GND electrode, and the overall anti-interference capability is further enhanced.

[0044] Further, in some embodiments of the utility model, the water tank 1 is provided with a plurality of mounting limiting grooves, the bottom of the positive electrode plate 2 and the bottom of the negative electrode plate 3 are both provided with a plurality of first protruding structures, and the top of the negative electrode plate 3 is provided with a plurality of second protruding structures; wherein the second protruding structure is matched with the mounting limiting groove, the second protruding structure is connected with the mounting limiting groove, and the first protruding structure is provided with a solder pad.

[0045] In this embodiment, the water tank 1 is provided with a plurality of mounting limiting grooves, the top of the negative electrode plate 3 is provided with a plurality of second protruding structures, and the second protruding structure is matched with the mounting limiting groove, so that the negative electrode plate 3 is connected with the water tank 1 through the second protruding structure and the mounting limiting groove.

[0046] Moreover, the bottom of the positive electrode plate 2 and the bottom of the negative electrode plate 3 are both provided with a plurality of first protruding structures, and the first protruding structure is provided with a solder pad, so as to facilitate the connection of the positive electrode plate 2 and the negative electrode plate 3 with the sampling circuit 4.

[0047] Further, in some embodiments of the utility model, the water tank 1 is made of plastic.

[0048] In this embodiment, the material of the water tank 1 is plastic. Because the dielectric constant of plastic is usually about 2-4, which is relatively small compared with air and greatly different from liquid, and the wall thickness of the water tank 1 is 2 mm, the capacitive influence of this part can be ignored when the electrode plate is tightly attached to the outer wall of the water tank 1. Therefore, the current system places the electrode plate outside the water tank 1 in a non-contact state with the liquid, which can reduce the influence of liquid pollution and fouling on the inter-plate capacitance of the parallel-plate capacitor.

[0049] Further, in some embodiments of the utility model, as shown in Fig. 2 As shown, the sampling circuit 4 comprises: a first operational amplifier, the positive input end of the first operational amplifier is electrically connected with the controller, and the negative input end of the first operational amplifier is electrically connected with the output end of the first operational amplifier; a first capacitor, the first end of the first capacitor is electrically connected with the output end of the first operational amplifier, and the second end of the first capacitor is electrically connected with the positive electrode plate 2; an inductor, the first end of the inductor is electrically connected with the second end of the first capacitor, and the second end of the inductor is grounded; a second operational amplifier, the positive input end of the second operational amplifier is electrically connected with the second end of the first capacitor; a diode, the anode of the diode is electrically connected with the output end of the second operational amplifier, and the cathode of the diode is electrically connected with the negative input end of the second operational amplifier; a second capacitor, the first end of the second capacitor is electrically connected with the cathode of the diode, and the second end of the second capacitor is grounded; a third operational amplifier, the positive input end of the third operational amplifier is connected with the cathode of the diode, and the negative input end of the third operational amplifier is electrically connected with the output end of the third operational amplifier; an ADC sampling circuit, the ADC sampling circuit is electrically connected with the output end of the third operational amplifier; wherein the negative electrode plate 3 is connected with the ground.

[0050] In the embodiment, the sampling circuit 4 comprises a first operational amplifier, a first resistor, a first capacitor, an inductor, a second operational amplifier, a diode, a second resistor, a second capacitor, a third operational amplifier, a third resistor and an ADC sampling circuit.

[0051] Firstly, the positive input end of the first operational amplifier is electrically connected with the controller, and the controller inputs the sweep signal to the positive input end of the first operational amplifier.

[0052] Then, the first end of the first capacitor is electrically connected with the output end of the first operational amplifier, the second end of the first capacitor is electrically connected with the positive electrode plate 2, the first end of the inductor is electrically connected with the second end of the first capacitor, the second end of the inductor is connected with the ground, and the negative electrode plate 3 is connected with the ground. Actually, the first capacitor, the inductor, the positive electrode plate 2 and the negative electrode plate 3 form an LC resonance circuit. It should be noted that the second end of the first capacitor is connected with the positive electrode plate 2 and the inductor respectively, and the second end of the inductor and the negative electrode plate 3 are both connected with the ground, which actually forms a capacitor and an inductor in parallel by the positive electrode plate 2 and the negative electrode plate 3, and thus an LC resonance circuit is equivalent. Then, the LC resonance circuit resonates at a specific resonance frequency to obtain a sampling result.

[0053] Further, the positive input end of the second operational amplifier is electrically connected with the second end of the first capacitor, the anode of the diode is electrically connected with the output end of the second operational amplifier, the cathode of the diode is electrically connected with the negative input end of the second operational amplifier, the first end of the second resistor is connected with the cathode of the diode, the second end of the second resistor is connected with the ground, the first end of the second capacitor is electrically connected with the cathode of the diode, and the second end of the second capacitor is connected with the ground. The essence is that the peak value detection circuit is composed of the second operational amplifier, the diode, the second resistor and the second capacitor, and the sampling result is sent to the peak value detection circuit by the LC resonant circuit. The peak value detection circuit outputs to the ADC sampling circuit, and the ADC sampling circuit determines the current resonant frequency value according to the received different amplitudes, and then converts the resonant frequency into the current capacitance value according to the formula.

[0054] On this basis, in order to isolate the signal while ensuring that the power of the circuit is not affected, the utility model takes the first operational amplifier and the third operational amplifier as a follower, isolates the signals of the front and rear stages, and ensures that different parts are not interfered with each other. After determining the resonant frequency, the capacitance value of the current parallel plate capacitor is calculated according to the calculation formula of the LC resonant frequency, and then the liquid level value is indirectly measured through the mapping relationship between the capacitance value and the liquid level value.

[0055] And, because there is a leakage current and a parasitic capacitance in most actual phase voltage follower circuits, the first end of the first resistor is connected with the negative input end of the first operational amplifier, the second end of the first resistor is electrically connected with the output end of the first operational amplifier, the first end of the third resistor is connected with the negative input end of the third operational amplifier, and the second end of the third resistor is electrically connected with the output end of the third operational amplifier, so as to reduce the influence of these leakage currents and improve stability.

[0056] It can be understood that, for the convenience of understanding, the positive electrode plate 2 is located at P+ of Fig. 2 , and the negative electrode plate 3 is located at P- of Fig. 2 .

[0057] Specifically, the sampling circuit 4 is placed on the main control board, the back of the main control board is placed with the LC resonant circuit through screw connection, 1% precision C0G or NP0 capacitor and 2% precision wire inductance are used to obtain maximum consistency, and the connection line of the inductor and the positive and negative electrodes is as short and thick as possible to reduce the line parasitic capacitance.

[0058] In one specific embodiment, the working process of the utility model is as follows:

[0059] The positive electrode plate 2 and the negative electrode plate 3 are respectively placed into the first inner convex groove and the second inner convex groove, and the positive electrode plate 2 and the negative electrode plate 3 are respectively welded on the sampling circuit 4, and an appropriate amount of water is added into the water tank 1. The LC resonance circuit composed of the first capacitor, the inductor, the positive electrode plate 2 and the negative electrode plate 3 resonates at a specific resonance frequency, and the sampling result is obtained, and the LC resonance circuit sends the sampling result to the peak detection circuit composed of the second operational amplifier, the diode, the second resistor and the second capacitor. The peak detection circuit outputs to the ADC sampling circuit 4, and the ADC sampling circuit 4 determines the specific value of the current resonance frequency according to the received different amplitudes, and converts the resonance frequency into the current capacitance value according to the formula.

[0060] In the claims, the specification, and the drawings of the present application, the terms "a plurality" means two or more, unless otherwise expressly specified and limited by the context, that the terms "upper", "lower", and the like indicate the orientation or position relationship based on the orientation or position relationship shown in the drawings, only for the convenience of describing the present application and making the description process more simple, and not for indicating or implying that the device or element must have the described specific orientation, structure and operation, therefore these descriptions cannot be understood as the limitation of the present application; the terms "connection", "installation", "fixation" and the like should be understood in a broad sense, for example, "connection" can be fixed connection between multiple objects, can also be detachable connection between multiple objects, or integrally connected; can be direct connection between multiple objects, can also be indirect connection between multiple objects through intermediate medium. For ordinary skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances of the above data.

[0061] In the claims, the specification, and the drawings of the present application, the description of the terms "one embodiment", "some embodiments", "a specific embodiment" and the like means that the specific features, structures, materials or characteristics described in combination with the embodiment or example are included in at least one embodiment or example of the present application. In the claims, the specification and the drawings of the present application, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0062] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various changes and modifications. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A device for detecting a liquid level, characterized in that The device for detecting liquid level comprises a water tank (1), a positive electrode plate (2), a negative electrode plate (3), a sampling circuit (4), and a controller. The water tank (1) is internally provided with a cavity, and the bottom of the water tank (1) is provided with a first inner convex groove and a second inner convex groove, both of which leave a gap with the side wall of the cavity. The positive electrode plate (2) is located in the first inner convex groove. The negative electrode plate (3) is connected with the water tank (1) and located in the second inner convex groove. The sampling circuit (4) is connected with the water tank (1) and located at the bottom outside the water tank (1), and is connected with the positive electrode plate (2) and the negative electrode plate (3). The positive electrode plate (2) and the negative electrode plate (3) are arranged in parallel.

2. The device for detecting liquid level according to claim 1, wherein one side of the positive electrode plate (2) facing the negative electrode plate (3) is provided with a first copper foil region exposed, and the other side of the positive electrode plate (2) away from the negative electrode plate (3) is provided with a shielding layer. The outer ring of the first copper foil region is provided with a shielding line.

3. The device for detecting liquid level according to claim 2, wherein one side of the negative electrode plate (3) facing the positive electrode plate (2) is provided with a second copper foil region exposed. The area of the second copper foil region is greater than that of the first copper foil region.

4. The device for detecting liquid level according to claim 1, wherein the water tank (1) is provided with a plurality of mounting limiting grooves, the bottom of the positive electrode plate (2) and the bottom of the negative electrode plate (3) are each provided with a plurality of first protruding structures, and the top of the negative electrode plate (3) is provided with a plurality of second protruding structures. The second protruding structures are matched with and connected to the mounting limiting grooves, and the first protruding structures are provided with solder pads.

5. The device for detecting liquid level according to claim 1, wherein the water tank (1) is made of plastic. The sampling circuit (4) comprises a first operational amplifier, a first capacitor, an inductor, a second operational amplifier, a diode, and a second capacitor. The positive input end of the first operational amplifier is electrically connected with the controller, and the negative input end of the first operational amplifier is electrically connected with the output end of the first operational amplifier. The first end of the first capacitor is electrically connected with the output end of the first operational amplifier, and the second end of the first capacitor is electrically connected with the positive electrode plate (2). The first end of the inductor is electrically connected with the second end of the first capacitor, and the second end of the inductor is grounded.

6. The device for detecting a liquid level according to claim 3, characterized in that, The positive input end of the second operational amplifier is electrically connected with the second end of the first capacitor. The anode of the diode is electrically connected with the output end of the second operational amplifier, and the cathode of the diode is electrically connected with the negative input end of the second operational amplifier. The first end of the second capacitor is electrically connected with the cathode of the diode, and the second end of the second capacitor is grounded. ​ ​ ​ ​ A third operational amplifier, a positive input end of the third operational amplifier is connected with the cathode of the diode, and a negative input end of the third operational amplifier is electrically connected with an output end of the third operational amplifier; An ADC sampling circuit (4), which is electrically connected with the output end of the third operational amplifier; Wherein, the negative electrode plate (3) is connected with the ground.

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