High-precision small-current AC digital clamp meter

Abstract

The utility model relates to a high-precision small current AC digital clamp meter, including the current sampling circuit for the design of high-precision measurement small current, still including the clamp of being equipped with in the front end of cover shell, the clamp front end is equipped with the clamp head current transformer and alternating voltage detection pole piece respectively, realizes the accurate collection to AC small circuit, the cavity of cover shell is equipped with the circuit board, the first casing of cover shell is fastened on the second casing through the first fastening bolt, the side of cover shell is connected and is equipped with the trigger of controllable clamp open and close of stretching, the power module is embedded on the first casing, the surface of second casing outer wall from top to bottom with circuit board is provided with non -contact alternating voltage detection signal indicating lamp, rotatable gear switch, a plurality of panel control button, LCD display screen, two input terminals and respectively with the circuit board of being equipped in the cavity of cover shell electricity is connected.

Description

Technical Field

[0001] This utility model belongs to the field of electrical safety testing tools, and specifically relates to a high-precision, low-current AC digital clamp meter. Background Technology

[0002] When measuring current with a regular ammeter, the circuit must be disconnected and the machine stopped before the ammeter can be connected for measurement, which is cumbersome, and sometimes this is not allowed for normally operating motors. In such cases, a clamp-on ammeter is much more convenient, allowing current measurement without disconnecting the circuit. Therefore, clamp-on ammeters are widely used for current measurement. However, due to the limitation of the clamp's current sensor sensitivity, clamp-on ammeters are generally only suitable for measuring large currents; their accuracy becomes very poor when measuring smaller currents.

[0003] Existing technology, such as the utility model patent with patent number "CN215678528U", discloses a clamp-on ammeter capable of measuring weak currents. Its circuit includes a function switching circuit, a power supply circuit, a lighting circuit, a current sampling circuit, and an alarm backlight circuit. The current sampling circuit uses only a single capacitor C3, and compensates for frequency errors through impedance matching using R9 and R14. However, the compensation range is limited, and the large testing error obviously cannot meet normal testing requirements. The reason for this is that the voltage signal induced by the current transformer in the clamp head of the AC clamp meter is proportional to the frequency (…). Then, the high-frequency signal is easily attenuated due to the increase of inductive reactance (2πfL). In actual testing, the voltage value of the sampled signal entering the IC and the magnitude of the input test current or the frequency characteristics will show a very obvious nonlinear relationship. It can be seen that large error and poor frequency adaptability are common characteristics of existing technology products. Utility Model Content

[0004] To address the problems existing in the prior art, this utility model proposes a high-precision, low-current AC digital clamp meter.

[0005] The technical solution of this utility model is as follows: A high-precision, low-current AC digital clamp meter has an IC1 multimeter integrated chip on its internal circuit board. The IC1 multimeter integrated chip is electrically connected to a function switching circuit, a power supply circuit, a current sampling circuit, and an alarm backlight circuit. The power supply circuit is electrically connected to the alarm backlight circuit.

[0006] Furthermore, in the current sampling circuit, one end of capacitor C3 is connected to one end of capacitor C14 and one end of the clamp head current transformer, and the other end of capacitor C3 is connected to the other end of capacitor C14 and one end of resistor R12 and resistor R14. The other ends of the clamp head current transformer and resistor R12 are grounded. The other end of resistor R14 is connected to one end of resistor R13 and switch K5, and the other end of switch K5 is connected to port 11 of IC1 multimeter integrated chip. The other end of resistor R13 is connected to analog ground.

[0007] Compared with the prior art, the present invention has the following beneficial effects: Compared with traditional AC digital clamp meters, this invention optimizes the frequency response characteristics of small current sampling by improving the current sampling circuit, ensuring the accuracy of small current measurement and effectively avoiding nonlinear errors caused by frequency changes. Attached Figure Description

[0008] Figure 1 This is a schematic diagram of the structure of this utility model; Figure 2 This is an exploded view of the structure of this utility model; Figure 3 This is a circuit block diagram of the present invention; Figure 4 This invention relates to the principle of the function switching circuit of the present invention. Figure 5 This is the circuit schematic diagram of this utility model.

[0009] The reference numerals in the figure are as follows: 1. IC1 multimeter integrated chip; 2. Input terminal; 3. Function switching switch circuit; 4. Power supply circuit; 5. Clamp head current transformer; 6. Current sampling circuit; 7. Panel control button; 8. LCD display; 9. Alarm backlight circuit; 10. Clamp; 11. Non-contact AC voltage detection signal indicator; 12. Trigger; 13. Cover; 20. Rotatable gear switch; 23. AC voltage detection electrode; 101. First clamp head; 102. Second clamp head; 301. Circuit board; 302. First housing; 303. Second housing; 304. Support base; 305. Power module; 306. Cavity; 311. Second fastening bolt; 312. First fastening bolt. Detailed Implementation

[0010] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present utility model.

[0011] It should be understood that the step numbers used in the text are for ease of description only and are not intended to limit the order in which the steps are performed.

[0012] It should be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the scope of the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0013] The terms “comprising” and “including” indicate the presence of the described feature, whole, step, operation, element and / or component, but do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or collections thereof.

[0014] The term “and / or” refers to any combination of one or more of the associated listed items, as well as all possible combinations, and includes these combinations.

[0015] like Figure 3 As shown, this embodiment provides a high-precision low-current AC digital clamp meter. An IC1 multimeter integrated chip 1 is provided on the internal circuit board 301. The IC1 multimeter integrated chip 1 is electrically connected to the function switching circuit 3, the power supply circuit 4, the current sampling circuit 6, and the alarm backlight circuit 9. The power supply circuit 4 is electrically connected to the alarm backlight circuit 9.

[0016] For a preferred embodiment, see Figure 4 The current sampling circuit 6 includes resistors R12, R13, R14, capacitors C13 and C14, and a clamp current transformer L1. One end of capacitor C3 is connected to one end of capacitor C14 and one end of clamp current transformer L1. The other end of capacitor C3 is connected to the other end of capacitor C14 and one end of resistors R12 and R14. The other ends of clamp current transformer L1 and resistor R12 are grounded. The other end of resistor R14 is connected to one end of resistor R13 and switch K5. The other end of switch K5 is connected to port 11 of IC1 multimeter integrated chip. The other end of resistor R13 is connected to analog ground. In the circuit diagram, L1, (C3∥C14)∥R12, R13 and R14 constitute the current sampling circuit.

[0017] Specifically, C3 and C14 are connected in parallel (C3∥C14) to form part of an RC filter network, which, together with inductor L1 (the equivalent inductance of the clamp current transformer), forms an LC parallel resonant circuit. Its core function is to optimize the frequency response characteristics of the current sampling, ensuring the accuracy of small current measurements. (Note: The resistance value of R12 is normally only a few ohms, and can be considered equivalent to the internal resistance of the signal source when analyzing the circuit.) It should be noted that the specific function of C3 and C14 connected in parallel is as follows: 1. Frequency compensation to widen the frequency response range: Because the voltage signal induced by the clamp current transformer is proportional to the frequency (E = nΔΦ / Δdt), high-frequency signals are easily attenuated due to the increase of inductive reactance (2πfL). This causes the amplitude or frequency of the sampling input signal entering the IC to exhibit a significant nonlinear relationship.

[0018] The capacitive reactance of C3∥C14 ( The value decreases as the frequency increases, forming a parallel resonance with the inductor, at a specific frequency point (resonance frequency). This cancels out the inductive reactance, making the circuit purely resistive, avoiding high-frequency signal attenuation, and improving the frequency stability of small current measurements.

[0019] 2. Suppress harmonic interference and improve signal purity: Parallel capacitors can filter out high-frequency harmonic noise in the current signal, and RC filtering reduces the impact of spurious signals on the sampled voltage. To prevent interference and ensure the accuracy of the signal input to the IC1 integrated chip.

[0020] By adopting the above technical solution, the parallel capacitor network composed of C3||C14 effectively suppresses high-frequency harmonic noise in the current signal, playing a crucial RC filtering role and reducing the impact of stray interference on the quality of the sampled voltage signal. This greatly improves the purity of the signal input to the multimeter integrated chip IC1. Meanwhile, R12, as a critical load resistor, ensures that the secondary winding of the transformer has sufficient load capacity and stable output characteristics, while the fine-tuning function of the R13 / R14 voltage divider network precisely controls the amplitude of the sampled signal sent to IC1. These two resistors work together to provide a stable and accurate input reference for subsequent circuits, thereby achieving higher precision measurement of small current signals.

[0021] 3. Optimize linearity and reduce measurement error: Under ideal resonance conditions (inductive reactance = capacitive reactance), R12 (normally taken as 1~10Ω mainly to ensure sufficient load capacity of the clamp head induction coil) can be equivalent to the internal resistance of the signal source, and the output circuit impedance can be determined as a pure resistance. R14 acts as a voltage divider, and the sampling voltage on R13 is finely adjusted by adjusting the value of R14 to make the sampling voltage and the measured current linearly related, avoiding nonlinear errors caused by frequency changes (compared to the existing products with a 2A range error of 4%+30 digits, the error of this device is significantly reduced).

[0022] C3 can be selected as 47~470μF, and C14 (multiple capacitors can be used in parallel if necessary) can be matched with a parallel value according to the actual resonant frequency requirements so that f_0 (resonant frequency) covers the target measurement frequency range.

[0023] By adopting the above technical solution, the parallel capacitors C3 and C14 (C3∥C14) on the secondary side of the clamp current transformer L1, together with L1, form an LC parallel resonant circuit, making the circuit exhibit pure resistivity near the target resonant frequency. The low resistance value R12, originally used as a damping / load resistor, can be equivalent to the internal resistance of the signal source, avoiding nonlinear errors caused by changes in the inductive or capacitive reactance of the circuit due to frequency variations. The design of the voltage divider structure of R14 and R13 allows for optimization of the sampling voltage range by finely adjusting the resistance value of R14, further ensuring a better linear relationship between the measured current and its sampling voltage over a wide frequency range, and significantly reducing the nonlinear errors common in small current measurements. Furthermore, the capacitive reactance of C3||C14 decreases with increasing frequency, compensating for the high-frequency signal attenuation caused by the inductive reactance of current transformer L1 (which increases with increasing frequency). They resonate at a specific resonant frequency (f_0), canceling out the inductive reactance and significantly widening the frequency response range of the entire current sampling circuit. This enables the device to stably and accurately measure small current signals at higher frequencies (e.g., up to 400Hz), where traditional products would experience significantly increased errors. By selecting the capacitance values ​​of C3 (47~470μF) and C14 to set the resonant point (f_0) within the target frequency band, this device maintains a low error better than ±4.5%+15 digits at multiple frequency points such as 50Hz / 60Hz / 400Hz, meeting the requirements for wide-bandwidth, high-precision small current measurement.

[0024] See Figure 1 , Figure 2As shown, a high-precision low-current AC digital clamp meter of this embodiment includes a clamp 10 disposed at the front end of a cover 13. The front end of the clamp 10 is respectively provided with a clamp head current transformer 5 and an AC voltage detection electrode 23. The cover 13 includes a first housing 302, which is fastened to a second housing 303 by a first fastening bolt 312. A circuit board 301 is disposed in the cavity 306 of the cover 13. A trigger 12 that can control the opening and closing of the clamp 10 is stretchedly connected to one side of the cover 13. The trigger 12 is stretchedly connected to the clamp 10 by a spring built into the cavity 306. Pressing the trigger 12 can drive the clamp 10 to open. The clamp 10 includes a first clamp head 101 and a second clamp head 102. The front end of the first clamp head 101 is provided with a clamp head current transformer 5, and the front end of the second clamp head 102 is provided with an AC voltage detection electrode 23. The front ends of the first clamp head 101 and the second clamp head 102 can be fitted together.

[0025] A power module 305 is embedded in the first housing 302, and a support base 304 is vertically provided on the inner wall of the second housing 303. The second fastening bolt 311 fastens the circuit board 301 to the support base 304.

[0026] The outer surface of the second housing 303, from top to bottom, is provided with a non-contact AC voltage detection signal indicator light 11, a rotary gear switch 20, several panel control buttons 7, an LCD display screen 8, and two input terminals 2, which are electrically connected to the circuit board 301.

[0027] The circuit board 301 includes IC1 multimeter integrated chip 1, which is electrically connected to the function switching switch circuit 3, the power supply circuit 4, the current sampling circuit 6, and the alarm backlight circuit 9. The power supply circuit 4 is electrically connected to the alarm backlight circuit 9.

[0028] Input terminal 2 is electrically connected to IC1 multimeter integrated chip 1 through function switching circuit 3. Input terminal 2 INPUT is the positive polarity connection terminal for measuring all physical quantities except for measuring current or detecting AC voltage through clamps. Input terminal 2 COM is the negative polarity connection terminal for measuring all physical quantities and is also the analog ground terminal of the multimeter.

[0029] The non-contact AC voltage detection signal indicator 11 is connected to the IC1 multimeter integrated chip 1 through the alarm backlight circuit 9. The function switching switch circuit 3 is electrically connected to the rotary range switch 20. The power supply circuit 4 is connected to the power module 305. The clamp head current transformer 5 and the AC voltage detection electrode 23 are respectively connected to the IC1 multimeter integrated chip 1 through the function switching switch circuit 3 and the current sampling circuit 6. The panel control button 7 and the LCD screen are respectively connected to the IC1 multimeter integrated chip 1.

[0030] See Figure 4The function switching circuit 3 includes a complex array of gold finger contact switches (K1, K2, K5, K6, K7). The input terminal 2 INPUT is connected to one end of R25, and the other end of R25 is connected to the 7th port of IC1 multimeter integrated chip 1. The input terminal 2 INPUT is also connected to one end of the thermistor PTC1 and resistor R87. The other end of the thermistor PTC1 is connected to one end of switch K1. The other end of switch K1 is connected to the emitter of NPN transistor Q5 and one end of resistor R86. The base and collector of NPN transistor Q5 are simultaneously connected to the base and collector of NPN transistor Q4. The emitter of NPN transistor Q4 is connected to analog ground. The other end of resistor R86 is connected to the 8th port of IC1 multimeter integrated chip 1. The other end of switch K2 is connected to the 11th port of IC1 multimeter integrated chip 1.

[0031] For voltage measurement, only power switch K7 needs to be connected. R25 is a voltage divider resistor. The 7th port of IC1 multimeter integrated chip 1 is the analog signal input port. For non-contact AC voltage detection, switch K6 needs to be connected. For current measurement through clamps, switch K5, resistor, and diode need to be connected. For continuity and capacitance measurement, switches K2 and K1 need to be connected simultaneously. For temperature measurement, switch K1 needs to be connected. PTC1, Q4, and Q5 are input terminals 2. In addition to voltage measurement, they are also protective devices for misoperation (inputting a voltage signal) measurement.

[0032] The AC voltage detection electrode 23 NCV is connected to one end of resistors R3 and R7 via wires. The other end of resistor R7 is connected to analog ground, and the other end of resistor R3 is connected to the other end of switch K6.

[0033] Experimental test results for EM4316 show that, under a 6A range, the error at 50Hz / 60Hz / 400Hz frequencies is ≤4.5%+15, while the product manual's normal specifications leave a large margin. The figure also shows the accuracy indicators of another product's manual, which are continuously being improved, further verifying the optimization effect of C3∥C14 on broadband measurement accuracy.

[0034] The IC1 multimeter integrated chip 1 consists of an ADC (analog-to-digital converter) and a DSP (digital processor). The second port of the IC1 multimeter integrated chip 1 is connected to the positive power supply VDD, the third port of the IC1 multimeter integrated chip 1 is connected to the negative power supply V-, the seventh port A1 of the IC1 multimeter integrated chip 1, the eighth port A2 and the eleventh port A5 of the IC1 multimeter integrated chip 1 are analog signal input ports, and the twelfth port of the IC1 multimeter integrated chip 1 is connected to the ground wire.

[0035] The power supply circuit 4 includes a 3V power supply battery. The positive terminal of the 3V power supply battery is connected to one end of K7, and the other end of K7 outputs VDD as the system power supply.

[0036] The alarm backlight circuit 9 includes a resistor R33. The 22nd port of IC1 multimeter integrated chip 1 is connected to one end of resistor R33. The other end of resistor R33 is connected to the base of NPN transistor Q2. The collector of NPN transistor Q2 is connected to one end of buzzer BUZZ. The other end of buzzer BUZZ is connected to resistor R1. The other end of resistor R1 is connected to the positive terminal of VDD power supply. The emitter of NPN transistor Q2 is connected to the negative power supply V-. The 17th port of IC1 multimeter integrated chip 1 is connected to one end of resistor R2. The other end of resistor R2 is connected to the anode of NCV (non-contact AC voltage) indicator light. The cathode of NCV is connected to V-. The 16th port of IC1 multimeter integrated chip 1 is connected to one end of resistor R66. The other end of resistor R66 is connected to the anode of BLED (backlight) indicator light. The cathode of BLED is connected to V-.

[0037] The panel control buttons 7 include a function selection button SEL button. One end of the function selection button SEL button is connected to port 14 of IC1 multimeter integrated chip 1, and the other end is connected to the negative power supply V-. When pressed briefly, it is used for function selection, and when pressed for relative value function, it is used for relative value function. The maximum or minimum value measurement MAX / MIN button is connected to port 13 of IC1 multimeter integrated chip 1, and the other end is connected to the negative power supply V-. When pressed briefly, it is used for maximum or minimum value function switching. The reading hold and backlight function button HOLD / BL is connected to port 15, and the other end is connected to the negative power supply V-. When pressed briefly, it is used for reading hold function, and when pressed for backlight function, it is used to turn the backlight on or off.

[0038] The working principle of this utility model: A high-precision, low-current AC digital clamp meter is designed based on a physically compensated LC resonant circuit and a precision resistor voltage divider network. When the wire passes through the clamp head 10, the AC signal induced by the clamp head current transformer 5 is input to a compensation circuit composed of inductor L1 and parallel capacitors (C3 / / C14). By setting the resonant frequency to cover the 50-400Hz frequency band, the inductive reactance and capacitive reactance cancel each other out, thereby eliminating high-frequency attenuation. The circuit is purely resistive within the target frequency band and suppresses harmonic interference. The optimized signal undergoes a current-to-voltage linear conversion through the resistor voltage divider network. R12 (1–10Ω) serves as the load resistance of the transformer to stabilize the signal source. R14 adjusts the voltage division ratio to fine-tune the voltage range of the input to the A5 pin of the multimeter chip IC1. R13 is connected in series and grounded to form a sampling voltage U. Finally, the IC1 chip performs true RMS calculation.

[0039] In this application embodiment, "at least one" refers to one or more, and "more than one" refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent the existence of A alone, A and B simultaneously, or B alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one of the following" and similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, at least one of a, b, and c can represent: a, b, c, a and b, a and c, b and c, or a and b and c, where a, b, and c can be single or multiple.

[0040] The above description is merely an embodiment of this utility model and does not limit the patent scope of this utility model. Any equivalent structural or procedural transformations made based on the description and drawings of this utility model, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this utility model.

Claims

1. A high-precision, low-current AC digital clamp meter, characterized in that: An IC1 multimeter integrated chip (1) is provided on the internal circuit board (301). The IC1 multimeter integrated chip (1) is electrically connected to the function switching switch circuit (3), the power supply circuit (4), the current sampling circuit (6), and the alarm backlight circuit (9). The power supply circuit (4) is electrically connected to the alarm backlight circuit (9).

2. The high-precision low-current AC digital clamp meter according to claim 1, characterized in that: In the current sampling circuit (6), one end of capacitor C3 is connected to one end of capacitor C14 and one end of clamp current transformer (5). The other end of capacitor C3 is connected to the other end of capacitor C14 and one end of resistor R12 and resistor R14. The other ends of clamp current transformer (5) and resistor R12 are grounded. The other end of resistor R14 is connected to one end of resistor R13 and switch K5. The other end of switch K5 is connected to port 11 of IC1 multimeter integrated chip (1). The other end of resistor R13 is connected to analog ground.

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