A device for measuring the capacitance of capacitors. Digital capacitance meter Homemade capacitance meter

30.07.202327.05.2017

In this article, we will give the most complete instructions that will allow you to make a capacitor capacitance meter with your own hands, without the help of qualified craftsmen.

Unfortunately, equipment often fails. The reason is most often the same - the appearance of an electrolytic capacitor. All radio amateurs are familiar with the so-called "drying", which appears due to a violation of the tightness of the device case. The reactance increases due to the decrease in the nominal capacitance.

Further, during operation, electrochemical reactions begin to occur, they destroy the joints of the leads. As a result, the contacts are broken, forming a contact resistance, which is calculated, sometimes in tens of ohms. The same will happen when a resistor is connected to the working capacitor. The presence of this very series resistance will negatively affect the operation of the electronic device, the entire operation of the capacitors will be distorted in the circuit.

Due to the strong influence of resistance in the range of three to five ohms, switching power supplies become unusable, because expensive transistors and microcircuits burn out in them. If the parts were checked during assembly of the device, and no errors were made during installation, then there will be no problems with its adjustment.

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Scheme, principle of operation, device

This circuit is used with an operational amplifier. The device, which we are going to make with our own hands, will allow us to measure the capacitance of capacitors in the range from a couple of picofarads to one microfarad.

Let's take a look at the diagram below.:

Subranges. The aggregate has 6 "subranges", their high bounds are 10, 100; 1000 pF, as well as 0.01, 0.1 and 1 microfarad. The capacitance is measured on the measuring grid of the microammeter.
Purpose. The basis of the device is the measurement of alternating current, it passes through the capacitor, which must be investigated.
On the amplifier DA 1 is a pulse generator. The oscillations of their repetition are subject to the capacitance C 1-C 6 of the capacitors, as well as the position of the toggle switch of the "tuning" resistor R 5. The frequency will be variable from 100 Hz to 200 kHz. To the tuning resistor R 1 we determine the commensurate model of oscillations at the output of the generator.
The diodes indicated in the diagram, like D 3 and D 6, resistors (adjusted) R 7-R 11, microammeter RA 1, make up the AC meter itself. Inside the microammeter, the resistance must be no more than 3 kOhm, so that the measurement error does not exceed ten percent in the range up to 10 pF.
Trimmer resistors R 7 - R 11 are connected to other sub-ranges in parallel R A 1. The desired measuring sub-range is adjusted using the toggle switch S A 1. One category of contacts switches capacitors (frequency-setting) C 1 and C 6 in the generator, the second switches resistors in the indicator.
In order for the device to receive energy, it needs a 2-polar stabilized source (voltage from 8 to 15 V). The frequency-setting capacitor may have a 20% difference in ratings, but they themselves must have high temporal and temperature stability.

Of course, for an ordinary person who is not versed in physics, all this may seem complicated, but you must understand that in order to make a capacitor capacitance meter with your own hands, you need to have certain knowledge and skills. Next, let's talk about how to set up the device.

Setting up the measuring instrument

To make the correct adjustment, follow the instructions:

First, the symmetry of the oscillations is achieved using the resistor R 1. The “slider” of the resistor R 5 is in the middle.
The next step is to connect a 10 pF reference capacitor to the terminals marked with cx. With the help of resistor R 5, the arrow of the microammeter is moved to the corresponding scale of the capacitance of the reference capacitor.
Next, the waveform at the output of the generator is checked. Calibration is carried out on all subranges, resistors R 7 and R 11 are used here.

The mechanism of the device may be different. Dimension parameters depend on the type of microammeter. There are no special features when working with the device.

Creating different meter models

AVR series model

You can make such a meter based on a variable transistor. Here is the instruction:

We select a contactor;
We measure the output voltage;
negative resistance in the capacitance meter is not more than 45 ohms;
If the conductivity is 40 microns, then the overload will be 4 Amps;
To improve measurement accuracy, comparators must be used;
There is also an opinion that it is better to use only open filters, since they are not afraid of impulse noise in case of heavy workload;
It is also recommended to use pole stabilizers, but only grid comparators are not suitable for modifying the device;

Before turning on the capacitor capacitance meter, you need to measure the resistance, which should be about 40 ohms for well-made devices. But the indicator may differ, depending on the frequency of modification.

PIC16F628A based module can be adjustable type;

It is better not to install high conductivity filters;

Before we start soldering, we need to check the output voltage;

If the resistance is too high, then we change the transistor;

We use comparators to overcome impulse noise;

Additionally, we use conductive stabilizers;

The display can be text, which is the easiest and most convenient. You need to put them through the channel ports;

Next, using the tester, we configure the modification;

If the capacitance indicators of the capacitors are too high, then we change transistors with low conductivity.

You can learn more about how to make a capacitor capacitance meter with your own hands from the video below.

Video instructions

Electrolytic capacitors, due to a decrease in capacitance or a significant leakage current, are often the cause of radio equipment malfunction. The capacitance meter, the circuit of which we will consider today, allows us to determine the feasibility of further use of the capacitor, which was supposedly the cause of the malfunction. Together with a multi-limit avometer (at the limit of 5 V) or a separate measuring head (100 μA), this tester can measure capacitances from 10 to 10,000 μF, as well as determine the degree of leakage of capacitors.

See also diagram

The tester is based on the principle of controlling the residual charge on the poles of the capacitor, which was charged with a current of a certain value for a certain time. For example, a capacitance of 1 F, which received a charge with a current of 1 A for 1 s, will have a potential difference on the plates equal to 1 V.

The almost constant charge current of the tested capacitor C is provided by a current generator assembled on the transistor V5. On the first capacitance range, you can measure up to 100 microfarads (capacitor charge current 10 microamps), on the second - up to 1000 microfarads (100 microamperes) and on the third - up to 10,000 microfarads (1 mA). The charge time Cx is chosen to be 5 s and is counted either automatically using a time relay or using a stopwatch.

Capacitor capacitance meter circuit and necessary details

As for the radio components, you will need:

4 diodes (V1–V4) - SAY12.
Transistor (V5) - SF136C.
2 bipolar transistors (V6, V7) - KT326B.
Capacitor (C1) - 0.022 uF.
Electrolytic capacitor (C2) - 100 uF.
6 resistors - R1 1 kOhm; R3 56 kΩ; R5, R10 4.7 kΩ; R7 470 Ohm; R9 4.7 ohm.
4 tuning resistors - R2 50 kOhm; R4 2.5 kΩ; R6 250 Ohm; R8 500 Ohm.
Microammeter (U).
3 position switch (S1).
Double switch with 3 positions (S2).
Power supply 9V.
Terminal clamp.

Do-it-yourself capacitor capacitance meter installation sequence

Before starting the measurement in the “discharge” switch position S2, with the potentiometer R8 we set the balance of the bridge formed by the base-emitter junctions of transistors V6 and V7, resistors R8, R9, R10 and diodes V3, V4 used as a low-voltage reference voltage source.
Then switch S1 select the expected capacitance measurement range. If the capacitor is not marked or has lost part of the capacitance, we start measurements in the first range.
Before measuring, the switch of the type of work S2 is set to the “Discharge” position, in this case the connected capacitance Cx is immediately discharged through the resistor R9.
In the “Charge” position, hold the switch S2 for 5 s, and then move it to the “Countdown” position and immediately count the measurement result.

The capacitance value (in microfarads) is inversely proportional to the voltage divisions (V) printed on the instrument scale and is determined by the formula C \u003d A / U, where A is a constant equal to 50, 500, 5000, respectively, for the first, second and third measurement ranges. If the capacitor is faulty and has a large leakage current, the meter needle will quickly return to zero on the scale. The magnitude of the leakage current is not determined.

See also diagrams and photos

Setting up the tester is simple and comes down mainly to setting the previously indicated charge currents with the potentiometers R2, R4, R6 using the microammeter included in the Cx terminals.

Note! In the capacitance meter, diodes KD202B and transistor KT340V can be used. An additional resistor should be connected in series with the microammeter to obtain a range of 5 V on the full scale, or use an avometer connected to the appropriate measurement limit.

Video about assembling a do-it-yourself capacitor capacitance meter:

DIY capacitor capacitance meter- below is a diagram and a description of how, without much effort, you can independently make a device for testing the capacitance of capacitors. Such a device can be very useful when buying containers in the electronic market. With its help, a low-quality or defective element of the accumulation of an electric charge is detected without problems. The schematic diagram of this ESR, as most electronics engineers usually call it, is nothing complicated and even a novice radio amateur can assemble such an apparatus.

Moreover, the capacitor capacitance meter does not imply a long time and large financial costs for its assembly; it takes literally two to three hours to manufacture a probe of equivalent series resistance. Also, it is not necessary to run to the radio store - for sure, any radio amateur will have unused parts suitable for this design. All you need to repeat this circuit is a multimeter of almost any model, it is only desirable that it be digital and with a dozen parts. There is no need to make any alterations or modernization of the digital tester, all that needs to be done with it is to solder the leads of the parts to the necessary sites on its board.

Schematic diagram of the ESR device:

The list of elements required for the assembly of the meter:

One of the main components of the device is a transformer, which should have a ratio of turns 11/1. Ferrite ring core M2000NM1-36 K10x6x3, which must first be wrapped with insulating material. Then wind the primary winding on it, arranging the turns according to the principle - turn to turn, while filling the entire circle. The secondary winding must also be carried out with a uniform distribution around the entire perimeter. The approximate number of turns in the primary winding for the K10x6x3 ring will be 60-90 turns, and the secondary should be eleven times less.

You can use almost any silicon diode with a reverse voltage of at least 40v, if you don’t really need super accuracy in measurements, then the KA220 is quite suitable. For a more accurate determination of the capacitance, you will have to put a diode with a small voltage drop in the direct connection option - Schottky. The protective suppressor diode D2 must be rated for reverse voltage from 28v to 38v. Low-power silicon p-n-p transistor: for example, KT361 or its equivalent.

Measure the EPS value in the voltage range of 20v. When the external meter connector is connected, the ESR add-on to the multimeter immediately enters the capacitance test operation mode. In this case, a reading of about 35v will be visually displayed on the device in the test range of 200v and 1000v (this depends on the use of a suppressor diode). In the case of a capacitance test at 20 volts, the reading will be displayed as “out of measurement limit”. When the connector of the external meter is disconnected, the EPS set-top box instantly switches to the mode of operation as an ordinary multimeter.

Conclusion

The principle of operation of the device - to start the device, you need to connect the adapter to the network, while the ESR meter turns on, when the ESR is turned off, the multimeter automatically switches to the standard functions. To calibrate the device, you need to select a constant resistor so that it matches the scale. For clarity, the picture is below:

When the probes are shorted, 0.00-0.01 will be displayed on the multimeter scale, this reading means the instrument's error in the measurement range up to 1 ohm.

A capacitor is an element of an electrical circuit consisting of conductive electrodes (plates) separated by a dielectric. Designed to use its electrical capacity. A capacitor with a capacity of C, to which a voltage U is applied, accumulates a charge Q on one side and - Q - on the other. Capacitance is in farads, voltage is in volts, charge is in coulombs. When a current of 1 A flows through a 1 F capacitor, the voltage changes by 1 V in 1 second.

One farad capacitance is huge, so microfarads (uF) or picofarads (pF) are usually used. 1F = 106 uF = 109 nF = 1012 pF. In practice, values from a few picofarads to tens of thousands of microfarads are used. The charging current of a capacitor is different from the current through a resistor. It does not depend on the magnitude of the voltage, but on the rate of change of the latter. For this reason, capacitance measurement requires special circuit solutions, in relation to the features of the capacitor.

Designations on capacitors

The easiest way to determine the value of the capacitance is by marking applied to the capacitor case.

Electrolytic (oxide) polar capacitor, 22000 uF, rated at 50 V DC. There is a designation WV - operating voltage. The marking of a non-polar capacitor must indicate the possibility of working in high voltage alternating current circuits (220 VAC).

Film capacitor with a capacity of 330,000 pF (0.33 uF). The value in this case is determined by the last digit of the three-digit number, indicating the number of zeros. Further, the letter indicates the permissible error, here - 5%. The third digit can be 8 or 9. Then the first two are multiplied by 0.01 or 0.1 respectively.

Capacitances up to 100 pF are marked, with rare exceptions, with the corresponding number. This is enough to obtain data about the product, this is how the vast majority of capacitors are marked. The manufacturer can come up with his own, unique designations, which are not always possible to decipher. This is especially true for the color code of domestic products. It is impossible to recognize the capacity by the erased marking, in such a situation one cannot do without measurements.

Calculations using electrical engineering formulas

The simplest RC circuit consists of a resistor and a capacitor connected in parallel.

After performing mathematical transformations (not given here), the properties of the circuit are determined, from which it follows that if a charged capacitor is connected to a resistor, then it will be discharged as shown in the graph.

The product RC is called the time constant of the circuit. With R in ohms and C in farads, the RC product corresponds to seconds. For a capacitance of 1 uF and a resistance of 1 kOhm, the time constant is 1 ms, if the capacitor was charged to a voltage of 1 V, when the resistor is connected, the current in the circuit will be 1 mA. When charging, the voltage across the capacitor will reach Vo in time t ≥ RC. In practice, the following rule applies: in the time of 5 RC, the capacitor will be charged or discharged by 99%. For other values, the voltage will change exponentially. At 2.2 RC it will be 90%, at 3 RC it will be 95%. This information is sufficient to calculate the capacity using the simplest devices.

Measurement scheme

To determine the capacitance of an unknown capacitor, you must include it in a circuit of a resistor and a power source. The input voltage is chosen slightly lower than the rated voltage of the capacitor, if it is unknown, 10-12 volts will be enough. You also need a stopwatch. To eliminate the influence of the internal resistance of the power source on the circuit parameters, a switch must be installed at the input.

The resistance is selected experimentally, more for the convenience of timing, in most cases within five to ten kilo-ohms. The voltage across the capacitor is monitored by a voltmeter. The time is counted from the moment the power is turned on - when charging and turning off, if the discharge is controlled. Having known values of resistance and time, the capacitance is calculated using the formula t \u003d RC.

It is more convenient to count the time of discharging the capacitor and note values of 90% or 95% of the initial voltage, in this case the calculation is carried out according to the formulas 2.2t = 2.2RC and 3t = 3RC. In this way, you can find out the capacitance of electrolytic capacitors with an accuracy determined by the measurement errors of time, voltage and resistance. Using it for ceramic and other small capacitance, using a 50 Hz transformer, calculating capacitance - gives an unpredictable error.

Measuring instruments

The most affordable method for measuring capacitance is a widely used multimeter with this capability.

In most cases, such devices have an upper measurement limit of tens of microfarads, which is sufficient for standard applications. The error of indications does not exceed 1% and is proportional to the capacitance. To check, it is enough to insert the capacitor leads into the intended sockets and read the readings, the whole process takes a minimum of time. This function is not present in all models of multimeters, but is often found with different measurement limits and ways to connect a capacitor. To determine more detailed characteristics of the capacitor (loss tangent and others), other devices designed for a specific task are used, which are often stationary devices.

In the measurement scheme, the bridge method is mainly implemented. They are used limitedly in special professional areas and are not widely used.

Homemade C - meter

Without taking into account various exotic solutions, such as a ballistic galvanometer and bridge circuits with a resistance box, it is possible to make a simple device or prefix to a multimeter according to the forces of a novice radio amateur. The widely used 555 series chip is quite suitable for these purposes. This is a real-time timer with a built-in digital comparator, in this case it is used as a generator.

The frequency of rectangular pulses is set by choosing resistors R1–R8 and capacitors C1, C2 by switch SA1 and equals: 25 kHz, 2.5 kHz, 250 Hz, 25Hz - respectively, switch positions 1, 2, 3 and 4–8. Capacitor Cx is charged with a pulse repetition rate through the diode VD1, up to a fixed voltage. The discharge occurs during a pause through the resistance R10, R12-R15. At this time, a pulse is formed with a duration dependent on the capacitance Cx (more capacitance - longer pulse). After passing through the integrating circuit R11 C3, a voltage appears at the output corresponding to the pulse length and proportional to the capacitance Cx. This is where the (X 1) multimeter is connected to measure the voltage at the limit of 200 mV. The positions of the switch SA1 (starting from the first) correspond to the limits: 20 pF, 200 pF, 2 nF, 20 nF, 0.2 μF, 2 μF, 20 μF, 200 μF.

Adjustment of the design must be done with the device that will be used in the future. Capacitors for adjustment must be selected with a capacity equal to the measurement subranges and as accurately as possible, the error will depend on this. The selected capacitors are connected in turn to X1. First of all, the subranges of 20 pF–20 nF are tuned, for this, the corresponding trimming resistors R1, R3, R5, R7 achieve the corresponding multimeter readings, you may have to slightly change the values of the series-connected resistances. On other subranges (0.2 μF–200 μF), calibration is carried out by resistors R12–R15.

When choosing a power source, it should be borne in mind that the amplitude of the pulses directly depends on its stability. Integrated stabilizers of the 78xx series are quite applicable here. The circuit consumes a current of no more than 20-30 milliamps and a filter capacitor with a capacity of 47-100 microfarads will be enough. The measurement error, subject to all conditions, can be about 5%, in the first and last subranges, due to the influence of the capacitance of the design itself and the output resistance of the timer, it increases to 20%. This must be taken into account when working at extreme limits.

Construction and details

R1, R5 6.8k R12 12k R10 100k C1 47nF

R2, R6 51k R13 1.2k R11 100k C2 470pF

R3, R7 68k R14 120 C3 0.47mkF

R4, R8 510k R15 13

Diode VD1 - any low-power pulse, film capacitors, with low leakage current. The microcircuit is any of the 555 series (LM555, NE555 and others), the Russian analogue is KR1006VI1. The meter can be virtually any high-impedance voltmeter that has been calibrated. The power source should have an output of 5-15 volts at a current of 0.1 A. Stabilizers with a fixed voltage are suitable: 7805, 7809, 7812, 78Lxx.

PCB Option and Component Location