CN214585714U

CN214585714U - Capacitance measuring device

Info

Publication number: CN214585714U
Application number: CN202120706565.7U
Authority: CN
Inventors: 崔建国; 宁永香; 崔燚
Original assignee: Shanxi Institute of Technology
Current assignee: Shanxi Institute of Technology
Priority date: 2021-04-08
Filing date: 2021-04-08
Publication date: 2021-11-02
Anticipated expiration: 2031-04-08

Abstract

The utility model discloses a capacitance measuring device, which comprises a counter gating signal circuit, a non-repeatable triggered monostable oscillator circuit, a reference oscillator circuit, a driving circuit, a modulation circuit and a counter circuit; the counter gating signal circuit is composed of a counter circuit, the non-repeatable triggering monostable oscillator circuit is composed of an integrated circuit IC1 and a peripheral circuit, gating pulse signals generated by the counter enter a pin 5 of an IC1, a pin 6 of an output end of the IC1 is connected with a pin 12 of a NAND gate N4 of the modulation circuit, the reference oscillator circuit is composed of NAND gates N1, N2, a capacitor C1, a resistor R2 and a crystal, reference oscillation pulses output by the reference oscillator circuit are connected with a pin 13 of an N4 through the NAND gate N3 of the driving circuit, and a pin 11 of an output end of the N4 is connected with the counter circuit through a cable.

Description

Capacitance measuring device

Technical Field

The utility model relates to a measure the technique of the equipment of electric capacity, especially a principle that electric capacity was surveyed to imitative singlechip utilizes a can not repeatability monostable trigger by RC circuit control's transient steady state time to gate the pulse of another multivibrator, utilizes the counter to count the pulse, and the pulse width of single pulse is equivalent to an electric capacity constant, finally reachs the capacity of the electric capacity that awaits measuring.

Background

In the design, production, maintenance and other processes of electronic products, the capacitor is used very frequently and plays an important role, so that the link of capacitance measurement is also indispensable. Generally, the capacitance is measured by the following four methods.

1) The experimental principle of measuring capacitance by using capacitor discharge is as follows: after the capacitor is charged, the charged quantity Q and the voltage U between two polar plates and the capacitance C satisfy the relation that Q is CU, U can be measured by a direct current voltmeter, and Q can be measured by the discharge of the capacitor. The capacitor is discharged through a high resistor, the discharge current is reduced along with the voltage drop between two polar plates of the capacitor, the discharge current value at different moments is measured until I is 0, a curve of the change of the discharge current I along with the time is made, and the area under the curve is equal to the charged capacity of the capacitor. The capacitance value of the capacitor can be obtained from C ═ Q/U.

2) Capacitance was measured using discharge time ratio: the measuring principle is that the measured capacitor and the reference capacitor are connected to the same resistor to form an RC network. By measuring the ratio of the discharge time of the two capacitors, the capacitance of the measured capacitor can be determined. The measurement range is from pF to several tens of nF, and has great advantages in terms of suppression of parasitic capacitance and temperature stability.

3) Measuring a time constant RC by using a single chip microcomputer to measure pulses, and then calculating a capacitance: the measuring principle is that a capacitor to be measured and a resistor are connected in series to form an RC network, then an oscillator can be started by utilizing the time constant, the waveform of an oscillation signal is adjusted, then the pulse value is counted, and the possible period is T = A₀The value of C can be calculated through the period, the value can be measured by a single chip microcomputer, theoretically, the measured value can be more than n, and the value is greatly more than that described above.

4) The more classical measurement method can use the balance principle of an alternating current bridge to measure capacitance or inductance.

The principle of measuring the capacitance by a single chip microcomputer can be imitated, the transient steady state time of a non-repeatable monostable trigger controlled by an RC circuit is utilized to gate the pulse of another multivibrator, the pulse is counted by a counter, the pulse width of a single pulse is equivalent to a constant of the capacitance, and finally the capacity of the capacitance to be measured is obtained.

Disclosure of Invention

The utility model aims to solve the technical problem that an equipment that simple structure, low in cost, use reliably, can be quick, conveniently measure capacitance capacity is provided.

In order to achieve the above object, the present invention provides a capacitance measuring device, which includes a counter gating signal circuit, a non-repetitive triggerable monostable oscillator circuit, a reference oscillator circuit, a driving circuit, a modulation circuit, and a counter circuit; the counter gating signal circuit is composed of the counter circuit, the non-repeatable triggering monostable oscillator circuit is composed of an integrated circuit IC1 and peripheral elements, an output gating pulse signal of the counter gating signal circuit is connected with an input end of the non-repeatable triggering monostable oscillator circuit, namely a pin 5 of an IC1, a pin 6 of an IC1 is connected with a pin 12 of a NAND gate N4 of the modulation circuit, the reference oscillator circuit is composed of NAND gates N1, N2, a capacitor C1, a resistor R1, an R2 and a 27M crystal, the reference oscillation pulse signal output by the reference oscillator circuit is buffered and amplified by a NAND gate N3 of the driving circuit, an output end of the N3 is connected with a pin 13 of an NAND gate N4 of the modulation circuit, and an output end of the N4 is connected with the counter circuit through a cable.

In the non-repeatable triggering monostable oscillator circuit, the

pins

3, 4 and 7 of the integrated circuit IC1 are short-circuited and connected to the working ground, the +5V power supply is connected with the pin 11 of the IC1 through a resistor R3 and a C-E electrode of a transistor T1 in sequence, and the +5V power supply is connected with the potentiometer R in sequence_x2Resistance R_x1And the capacitor Cx is connected with a pin 10 of the IC 1.

In the reference oscillator circuit, pins 1 and 2 of a NAND gate N1 are short-circuited,

pins

4 and 5 of a NAND gate N2 are short-circuited, a resistor R1 is bridged between the input end and the output end of the NAND gate N1, the resistor R2 is bridged between the input end and the output end of a NAND gate N2, the output end of the NAND gate N1 is connected with the input end of a NAND gate N2 through a 27M crystal, the output end of the NAND gate N2 is connected with the input end of the NAND gate N1 through a capacitor C1, and the output end of the NAND gate N2 is simultaneously connected with the input end of the NAND gate N3.

The pins 9 and 10 of the NAND gate N3 of the driving circuit are short-circuited.

Drawings

Fig. 1, 2, 3, 4, and 5 are included to provide a further understanding of the present invention and form a part of the present application, and fig. 1 is a schematic diagram of a series capacitance comparison bridge; FIG. 2 is a schematic diagram of a parallel capacitance comparison bridge; FIG. 3 is a schematic diagram of the monostable flip-flop 74121; FIG. 4 is a waveform illustrating the operating characteristics of the monostable flip-flop 74LS 121; fig. 5 is a schematic diagram of a capacitance-to-frequency converter based on a monostable flip-flop.

Detailed Description

Measuring capacitance with an AC bridge

There are two common ac bridge circuits for measuring capacitance, one is a series capacitance comparison bridge for measuring low-loss capacitance, as shown in fig. 1; the other is a parallel comparison capacitor bridge for measuring high loss capacitance, as shown in fig. 2, and the working principle of the two is briefly described below.

Series capacitance comparison bridge (measuring low loss capacitance)

FIG. 1 shows a series capacitance comparison bridge, C₄Standard capacitance (negligible losses), C_XFor the capacitance to be measured, R₂、R₃、R₄For non-inductive resistance, the bridge is balanced under the conditions of

The real part and the imaginary part are respectively equal

Loss factor

Get C₄、R₄For adjustable parameters, fix R₂、R₃Can realize respective reading, is easy to adjust balance, and requires R if the bridge is used for measuring high-loss capacitance₄Large, resulting in a much reduced bridge sensitivity.

1.2 parallel capacitance comparison bridge (high loss capacitance)

FIG. 2 shows a parallel capacitance comparison bridge, C₄Is a standard capacitor, R₂、R₃And R₄For non-inductive resistance, the balance condition is

Loss factor

Both of the above-mentioned bridges have the following characteristics: firstly, two groups of capacitors are compared, so that the method is visual and convenient; second, two sets of capacitors

There is no magnetic field coupling and the interference is small.

Through the actual measurement, the experimental error of measuring the electric capacity with the alternating current bridge is bigger, it is inaccurate to survey at all, this is because the characteristic of electric capacity is to lead to the alternating current and hinder direct current, if adopt the alternating current bridge, the electric capacity must pass through after the two-stage test of voltage test and capacitive reactance test, the coulomb law of rethread electric capacity calculates, wherein the tolerance receives the influence of alternating current waveform, receives the influence of one-way full-bridge, receives the influence of load (alternating current must connect the load, otherwise can appear two kinds of phenomena, one kind is that the electric capacity is overheated to damage, one kind is that the electric capacity is charged unidirectionally, but this kind of one-way charging is difficult to calculate through the full rate).

Therefore, this bridge measurement method has a problem, not an experimental error.

2 capacitance-frequency converter based on monostable trigger

It is possible to try to generate a pulse train with a simple circuit, the (average) frequency of which is proportional to the value of the capacitor to be measured.

The method is characterized in that the pulses of a reference oscillator are modulated by the transient output pulses of a non-repeatable triggering monostable trigger, the number of pulses which are allowed to pass through is proportional to the transient time which is proportional to the capacity of C of an RC timer, therefore, the number of pulses during the transient time can be directly added to a counter, and the counter indicates the capacitance value (unit: 10 pF), namely, the capacitance C = n 10pF, wherein n is the number of pulses counted by the counter.

The focus of the design is a non-retriably triggerable monostable flip-flop circuit, which will first be briefly described.

Non-retriggerable monostable oscillator

This non-retriggerable monostable uses a monostable flip-flop 74LS121, the outline of which is shown in figure 3.

The input of the 74LS121 adopts a Schmitt trigger input structure, so that the anti-interference capability is stronger no matter the pin 3 (A)₁）、4（A₂) (both falling edge triggered inputs) or pin 5 (in-phase schmitt trigger, i.e. rising edge input, which is also valid for slowly varying edges), the rising edge threshold U of 74LS121 is known by looking up the manual_IT+Minimum 2v, falling edge threshold minimum 0.8 v.

The design connects the 3 and 4 pins of the 74LS121 to ground, and the 5 pin (B terminal) is connected with the strobe pulse of the frequency meter.

The chip pin 10 (C)_ext) And pin 11 (R)_ext/C_ext) To an external capacitor (C)₁) Pin 9 (R)_int) Pin 14 (V) in general_CC+ 5V) connection, if pin 11 is an external timing resistor terminal, it will bePin 9 is open-circuited, and an external resistor R is connected₁₁Connected between pin 11 and pin 14, the monostable flip-flop outputs a transient steady state pulse width t from pin 6 (Q terminal)_w=R₁₁*C₄Will allow the narrow pulses of the multivibrator to pass during transient periods and eventually reach the counter or frequency meter.

Therefore, the level of the output terminal 6 pin (Q terminal) of the monostable flip-flop 74LS121 depends on the rising edge trigger signal of the 5 pin and is influenced by the timer of the flip-flop itself, and the strobe signal of the 5 pin becomes a modulation pulse synchronized with the strobe signal after passing through the monostable flip-flop.

FIG. 4 illustrates the operating waveforms of the monostable flip-flop 74LS121, as can be seen during a transient steady state (i.e., t)_WInner) has no effect on the transient steady state time when triggering again, and therefore, the output pulse width t_WDoes not change, it depends only on R_extAnd C_extRegardless of the trigger pulse, therefore, 74LS121 is a non-repeatable monostable flip-flop.

Capacitance-frequency converter principle based on monostable trigger

The electrical principle of the capacitance-frequency converter based on the monostable trigger is shown in fig. 5, and circuits are seen in the figure and comprise a counter gating signal circuit, a non-repeatable triggering monostable oscillator circuit, a reference oscillator circuit, a driving circuit, a modulation circuit and a counter circuit.

Non-retriggerable monostable oscillator circuit composed of integrated circuit IC₁And a peripheral resistor-capacitor unit, as for the transistor T₁Action of (A), T₁Operating in common collector amplification mode, thereby applying to the IC₁For pin 11, transistor T₁Can be regarded as a resistor with a large resistance value, and can accelerate the speed of inverting the 11 pins to 0 level.

Pulse width of monostable flip-flop, i.e. transient steady state period t_WCan be calculated from the following formula:

wherein C is_XFor the capacitance to be measured, R_XComprising a resistor R_X1And a potentiometer R_X2If R is_XIs a fixed value, obviously a transient steady state period t_WAnd the capacitor C to be measured_XIs in direct proportion.

Non-retriggerable monostable oscillator circuit IC₁Triggered by strobes of counters, i.e. IC₁The 5-pin signal of (1) is acted on by the strobe of the counter.

The reference oscillator circuit is composed of NAND gate N₁、N₂Quartz crystal, resistor R₁、R₂Capacitor C₁Forming a crystal oscillator composed of NAND gate, a resistor R₁、R₂Respectively connected across NAND gates N₁And NAND gate N₂The input and output terminals of the NAND gate N have negative feedback function, so that the circuit can work in a linear region well₃C in the circuit, which has buffering, isolating and driving functions and outputs oscillation signal₁The circuit is used for frequency fine adjustment and has the characteristics of easy starting and stable frequency.

The reference oscillator uses a standard 27MHz (typical control band) triple overtone crystal oscillator, but in this circuit the crystal oscillates at a base frequency around 9 MHz.

Monostable flip-flop IC₁After being triggered by high level of selected pulse, IC ₁10 pins of the IC are instantly changed into '0', IC₁The output 6 pins of the voltage transformer become '1' based on the capacitance C to be measured_xThe terminal capacitor of (1) can not change suddenly, the level of the 11 pins is changed into '0', and 5V power supply is carried out through the resistor R_XTo the capacitor C to be measured_XCharging, after full charging, T₁To become "1" T₁And (4) conducting, changing the pin 11 to high level, and inverting the pin 6 of the output to be 0 again, so that the transient steady-state time is ended.

IC₁During a transient steady state, a high level "1" is output, and a narrow pulse output from the reference oscillator is supplied to the IC₁High level modulation of pin 6 via nand gate N₄Inverting the output, i.e. polarity of reference pulse and N₄Of the output pulseThe counter starts counting once the IC is in reverse polarity₁The transient steady state of (1) is over, its 6 feet become "0", NAND gate N₄Locked, the output is constantly '1', and the counter finishes counting.

For the circuit to work properly, the period of the counter strobe must be longer than the monostable oscillator IC₁Is still longer, the maximum period of the monostable oscillator is about 20mS from the component values shown in figure 5.

Debugging and calibrating

In calibrating the circuit, a capacitor of precisely known capacitance is connected across the capacitor C to be measured_XAnd adjusting the preset potentiometer R_X2Until the indicated count equals the capacitance value, note that the capacitance value is in units of 10pF, e.g., C_XIs 10n capacitor, the counter shows 1000 instead of 10000, any silver-plated mica capacitor with 1% precision and capacity greater than 1000pF can be used for calibration, and the potentiometer R is used to make the circuit calibration not affected by temperature_X2A good quality multi-turn cermet trimming resistor should be used.

The reason for the capacitance unit of 10pF is mainly because the period of the reference oscillator is exactly equal to that of the monostable oscillator IC₁Is equivalent to the shortest period C corresponding to the shortest period_XThe value is about 10 pF.

During a number of actual tests, the conclusion is drawn: after modulation, i.e. NAND gate N₄Average frequency of output pulse train and capacitor C to be measured_XIs proportional to the capacitance C to be measured, so that the capacitance C to be measured is accurate_XShould have a value of at least 1000pF, i.e. IC₁Is passed through N during the shortest transient steady state period₄Must be greater than 100, when corresponding to C_XThe value is 1000 pF.

Therefore, using the component values shown in FIG. 5, the circuit can measure capacitances from 1000pF to 1uF, reducing R_X(including R)_X1And R_X2) The measurement range can also be extended to higher values.

The capacitance measuring device is similar to the principle of measuring capacitance by using a single chip microcomputer, and compared with the principle of measuring capacitance by using a bridge method, the capacitance measuring device has the advantages of high measurement precision, high cost performance, relatively high measuring range and popularization value.

Claims

1. A capacitance measuring device, characterized by: the capacitance measuring device comprises a counter gating signal circuit, a non-repeatable-triggering monostable oscillator circuit, a reference oscillator circuit, a driving circuit, a modulation circuit and a counter circuit; the counter gating signal circuit is composed of the counter circuit, the non-repeatable triggering monostable oscillator circuit is composed of an integrated circuit IC1 and peripheral elements, an output gating pulse signal of the counter gating signal circuit is connected with an input end of the non-repeatable triggering monostable oscillator circuit, namely a pin 5 of an IC1, a pin 6 of an IC1 is connected with a pin 12 of a NAND gate N4 of the modulation circuit, the reference oscillator circuit is composed of NAND gates N1, N2, a capacitor C1, a resistor R1, an R2 and a 27M crystal, the reference oscillation pulse signal output by the reference oscillator circuit is buffered and amplified by a NAND gate N3 of the driving circuit, an output end of the N3 is connected with a pin 13 of an NAND gate N4 of the modulation circuit, and an output end of the N4 is connected with the counter circuit through a cable.

2. A capacitance measuring device according to claim 1, wherein: in the non-repeatable triggering monostable oscillator circuit, the pins 3, 4 and 7 of the integrated circuit IC1 are short-circuited and connected to the working ground, the +5V power supply is connected with the pin 11 of the IC1 through a resistor R3 and a C-E electrode of a transistor T1 in sequence, and the +5V power supply is connected with the potentiometer R in sequence_x2Resistance R_x1And the capacitor Cx is connected with a pin 10 of the IC 1.

3. A capacitance measuring device according to claim 1, wherein: in the reference oscillator circuit, pins 1 and 2 of a NAND gate N1 are short-circuited, pins 4 and 5 of a NAND gate N2 are short-circuited, a resistor R1 is bridged between the input end and the output end of the NAND gate N1, the resistor R2 is bridged between the input end and the output end of a NAND gate N2, the output end of the NAND gate N1 is connected with the input end of a NAND gate N2 through a 27M crystal, the output end of the NAND gate N2 is connected with the input end of the NAND gate N1 through a capacitor C1, and the output end of the NAND gate N2 is simultaneously connected with the input end of the NAND gate N3.

4. A capacitance measuring device according to claim 1, wherein: the pins 9 and 10 of the NAND gate N3 of the driving circuit are short-circuited.