DIY capacitance meter for non-polar capacitors. Homemade measuring instruments

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

One farad has a huge capacitance, so microfarads (µF) or picofarads (pF) are usually used. 1F = 106 µF = 109 nF = 1012 pF. In practice, values ​​ranging 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 depends not on the magnitude of the voltage, but on the rate of change of the latter. For this reason, measuring capacitance requires special circuit solutions based on the characteristics of the capacitor.

Designations on capacitors

The easiest way to determine the capacitance value is by the markings on the capacitor body.

Electrolytic (oxide) polar capacitor with a capacity of 22000 µF, designed for a nominal voltage of 50 V DC. There is a designation WV - operating voltage. The marking of a non-polar capacitor must indicate the possibility of operation in high voltage alternating current circuits (220 VAC).

Film capacitor with a capacity of 330000 pF (0.33 µF). The value in this case is determined by the last digit of a three-digit number, indicating the number of zeros. The following 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; the vast majority of capacitors are marked this way. The manufacturer can come up with his own unique designations, which are not always possible to decipher. This especially applies to the color code of domestic products. It is impossible to recognize the capacity by erased markings; in such a situation, you 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, it will discharge as shown in the graph.

The product RC is called the time constant of the circuit. When R is in ohms and C is in farads, the product RC corresponds to seconds. For a capacitance of 1 μF and a resistance of 1 kOhm, the time constant is 1 ms, if the capacitor was charged to a voltage of 1 V, when a 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 a time of 5 RC, the capacitor will be charged or discharged by 99%. At 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 simple devices.

Measuring circuit

To determine the capacitance of an unknown capacitor, you should include it in a circuit consisting of a resistor and a power source. The input voltage is selected slightly lower than the rated voltage of the capacitor; if it is unknown, 10–12 volts will be sufficient. 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 kiloohms. The voltage across the capacitor is monitored with a voltmeter. Time is counted from the moment the power is turned on - when charging and turning off, if the discharge is controlled. Having known resistance and time values, the capacitance is calculated using the formula t = RC.

It is more convenient to count the discharge time of the capacitor and mark the values ​​at 90% or 95% of the initial voltage; in this case, the calculation is carried out using 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 capacitances, using a 50 Hz transformer and calculating capacitance, gives an unpredictable error.

Measuring instruments

The most accessible 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 reading error does not exceed 1% and is proportional to the capacity. To check, just 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 it is often found with different measurement limits and methods of connecting the capacitor. To determine more detailed characteristics of the capacitor (loss tangent and others), other devices are used, designed for a specific task, often stationary devices.

The measurement circuit mainly implements the bridge method. 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 store, a novice radio amateur can make a simple device or an attachment for a multimeter. 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 used as a generator.

The frequency of rectangular pulses is set by selecting resistors R1–R8 and capacitors C1, C2 using switch SA1 and is equal to: 25 kHz, 2.5 kHz, 250 Hz, 25Hz - corresponding to switch positions 1, 2, 3 and 4–8. The capacitor Cx is charged at a pulse repetition rate through the diode VD1, to a fixed voltage. The discharge occurs during a pause through resistances R10, R12–R15. At this time, a pulse is formed with a duration depending on the capacitance Cx (the larger the capacitance, the longer the pulse). After passing through the integrating circuit R11 C3, a voltage appears at the output corresponding to the pulse length and proportional to the value of the capacitance Cx. A multimeter (X 1) is connected here to measure voltage at a limit of 200 mV. The positions of 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 structure must be done with a 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. Selected capacitors are connected one by one to X1. First of all, the subranges of 20 pF–20 nF are adjusted; for this, the corresponding trimming resistors R1, R3, R5, R7 are used to 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 with resistors R12–R15.

When choosing a power source, it should be taken into account 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 sufficient. The measurement error, if all conditions are met, can be about 5%; in the first and last subranges, due to the influence of the capacitance of the structure 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 pulsed, 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 almost any voltmeter with a high input impedance, which is calibrated for it. The power source must 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 layout

Video on the topic

One of the most common reasons for the failure of electronic equipment or the deterioration of its parameters is a change in the properties of electrolytic capacitors. Sometimes, when repairing equipment (especially those manufactured in the former USSR) made using certain types of electrolytic capacitors (for example, K50-...), in order to restore the functionality of the device, they resort to complete or partial replacement of old electrolytic capacitors. All this has to be done due to the fact that the properties of the materials included in the electrolytic (precisely electrolytic, since the composition uses an electrolyte) capacitor change over time under electrical, atmospheric, and thermal influences. And thus the most important characteristics of capacitors, such as capacitance and leakage current, also change (the capacitor “dries out” and its capacity increases, often even by more than 50% of the original, and the leakage current increases, i.e. internal resistance, shunting the capacitor decreases), which naturally leads to a change in characteristics, and in the worst case, to a complete failure of the equipment.

The meter has the following qualitative and quantitative characteristics:

1) capacitance measurement on 8 subranges:

• 0 ... 3 µF;
• 0 ... 10 µF;
• 0 ... 30 µF;
• 0 ... 100 µF;
• 0 ... 300 µF;
• 0 ... 1000 µF;
• 0 ... 3000 µF;
• 0 ... 10000 µF.

2) assessment of the capacitor leakage current using the LED indicator;
3) the ability to accurately measure when changing the supply voltage and ambient temperature (built-in calibration of the meter);
4) supply voltage 5-15 V;
5) determination of the polarity of electrolytic (polar) capacitors;
6) current consumption in static mode............ no more than 6 mA;
7) capacitance measurement time .................................... no more than 1 s;
8) current consumption during capacitance measurement increases with each subrange,
But................................................. ................................ no more than 150 mA on the last subrange.

The essence of the device is to measure the voltage at the output of the differentiating circuit, Fig. 1.

Voltage across the resistor: Ur = i*R,
where i is the total current through the circuit, R is the charging resistance;

Because the circuit is differentiating, then its current is: i = C*(dUc/dt),
where C is the charging capacitance of the circuit, but the capacitor will be charged linearly through the current source, i.e. stabilized current: i = С*const,
This means the voltage across the resistance (output for this circuit): Ur = i*R = C*R*const - is directly proportional to the capacitance of the capacitor being charged, which means that by measuring the voltage on the resistor with a voltmeter, we measure on a certain scale the capacitance of the capacitor under study.

The diagram is shown in Fig. 2.
In the initial position, the test capacitor Cx (or calibration C1 with toggle switch SA2 on) is discharged through R1. The measuring capacitor, on which (not on the subject directly) the voltage proportional to the capacitance of the subject Cx is measured, is discharged through the contacts SA1.2. When the SA1 button is pressed, the test subject Cx (C1) is charged through the resistors R2 ... R11 corresponding to the sub-range (switch SA3). In this case, the charging current Cx (C1) passes through the LED VD1, whose brightness allows us to judge the leakage current (resistance shunting the capacitor) at the end of the capacitor charge. Simultaneously with Cx (C1), through a stabilized current source VT1, VT2, R14, R15, the measuring (known to be good and with a low leakage current) capacitor C2 is charged. VD2, VD3 are used to prevent the discharge of the measuring capacitor through the supply voltage source and current stabilizer, respectively. After charging Cx (C1) to a level determined by R12, R13 (in this case to a level of approximately half the voltage of the power source), comparator DA1 turns off the current source, the charge of C2 synchronous with Cx (C1) stops and the voltage from it is proportional to the capacitance of the test Cx (C1) is indicated by microammeter PA1 (two scales with values ​​that are multiples of 3 and 10, although it can be adjusted to any scale) through voltage follower DA2 with high input impedance, which also ensures long-term charge retention on C2.

Settings

When setting, the position of the calibration variable resistor R17 is fixed in some position (for example, in the middle). By connecting reference capacitors with precisely known capacitance values ​​in the appropriate range, resistors R2, R4, R6-R11 calibrate the meter - such a charge current is selected so that the reference capacitance values ​​correspond to certain values ​​on the selected scale.

In my circuit, the exact values ​​of the charging resistances at a supply voltage of 9 V were:

After calibration, one of the reference capacitors becomes calibration capacitor C1. Now, when the supply voltage changes (changes in ambient temperature, for example, when a ready-made, debugged device is heavily cooled in the cold, the capacitance readings turn out to be underestimated by 5 percent) or simply to control the accuracy of measurements, just connect C1 with the SA2 toggle switch and, by pressing SA1, use the calibration resistor R17 to adjusting PA1 to the selected value of capacitance C1.

Design

Before starting to manufacture the device, it is necessary to select a microammeter with a suitable scale(s), dimensions and current of maximum needle deflection, but the current can be any (of the order of tens, hundreds of microamps) due to the ability to configure and calibrate the device. I used an EA0630 microammeter with In = 150 µA, accuracy class 1.5 and two scales 0 ... 10 and 0 ... 30.

The board was designed taking into account the fact that it will be mounted directly on the microammeter using nuts on its terminals, Fig. 3. This solution ensures both mechanical and electrical integrity of the structure. The device is placed in a housing of suitable dimensions, sufficient to accommodate also (except for the microammeter and board):

SA1 - KM2-1 button of two small-sized switches;
- SA2 - small-sized toggle switch MT-1;
- SA3 - small-sized biscuit switch with 12 positions PG2-5-12P1NV;
- R17 - SP3-9a - VD1 - any, I used one of the KIPkh-xx series, red in color;
- 9-volt Corundum battery with dimensions 26.5 x 17.5 x 48.5 mm (excluding the length of the contacts).

SA1, SA2, SA3, R17, VD1 are fixed on the top cover (panel) of the device and are located above the board (the battery is strengthened using a wire frame directly on the board), but are connected to the board with wires, and all other radio elements of the circuit are located on the board (and under microammeter directly too) and are connected by printed wiring. I did not provide a separate power switch (and it would not have fit into the selected case), combining it with the wires for connecting the test capacitor Cx in the SG5 type connector. The “female” XS1 connector has a plastic case for installation on a printed circuit board (it is installed in the corner of the board), and the “male” XP1 is connected through a hole in the end of the device body. When connecting the male connector, its contacts 2-3 turn on the power to the device. It would be a good idea to attach a connector (block) of some design in parallel to the Cx wires to connect individual sealed capacitors.

Working with the device

When working with the device, you need to be careful with the polarity of connecting electrolytic (polar) capacitors. For any connection polarity, the indicator shows the same capacitance value of the capacitor, but if the connection polarity is incorrect, i.e. “+” of the capacitor to the “-” of the device, LED VD1 indicates a large leakage current (after charging the capacitor, the LED continues to light brightly), while with the correct polarity of the connection, the LED flashes and gradually goes out, demonstrating a decrease in the charging current to a very small value, almost completely extinction (should be observed for 5-7 seconds), provided that the capacitor under test has a low leakage current. Non-polar, non-electrolytic capacitors have a very low leakage current, as can be seen from the very fast and complete extinguishing of the LED. But if the leakage current is large (the resistance shunting the capacitor is small), i.e. the capacitor is old and “leaking”, then the glow of the LED is visible already at Rleakage = 100 kOhm, and with lower shunt resistances the LED lights up even brighter.
Thus, it is possible to determine the polarity of electrolytic capacitors by the glow of the LED: when connected, when the leakage current is less (the LED is less bright), the polarity of the capacitor corresponds to the polarity of the device.

Important note!

For greater accuracy of readings, any measurement should be repeated at least 2 times, because for the first time, part of the charge current goes to create the oxide layer of the capacitor, i.e. Capacity readings are slightly underestimated.

RadioHobby 5"2000

List of radioelements

Homemade measuring instruments

V. VASILIEV, Naberezhnye Chelny
Radio, 1998, No. 4

Anyone who repairs household or industrial radio equipment knows that serviceability of capacitors comfortable check without dismantling them. However, many capacitor capacitance meters do not provide this capability. True, one similar design was described in. It has a small measurement range and a non-linear countdown scale, which reduces accuracy. When designing a new meter, the problem of creating a device with a wide range, linear scale and direct reading was solved, so that it could be used as a laboratory one. In addition, the device must be diagnostic, i.e., capable of testing capacitors shunted by p-n junctions of semiconductor devices and resistor resistances.

Device diagram

The principle of operation of the device is as follows. A triangular voltage is applied to the input of the differentiator, in which the capacitor being tested is used as a differentiating one. In this case, its output produces a square wave with an amplitude proportional to the capacitance of this capacitor. Next, the detector selects the amplitude value of the meander and outputs a constant voltage to the measuring head.

The amplitude of the measuring voltage on the probes of the device is approximately 50 mV, which is not enough to open p-n junctions of semiconductor devices, so they do not have their shunting effect.

The device has two switches. Limit switch "Scale" with five positions: 10 µF, 1 µF, 0.1 µF, 0.01 µF, 1000 pF. The "Multiplier" switch (X1000, x10O, x10, X1) changes the measurement frequency. Thus, the device has eight capacitance measurement subranges from 10,000 μF to 1000 pF, which is practically sufficient in most cases.

The triangular oscillation generator is assembled on op-amp chips DA1.1, DA1.2, DA1.4 (Fig. 1). One of them, DA1.1, operates in comparator mode and generates a rectangular signal, which is fed to the input of the integrator DA1.2. The integrator converts rectangular oscillations into triangular ones. The generator frequency is determined by elements R4, C1 - C4. In the feedback circuit of the generator there is an inverter based on op-amp DA1.4, which provides self-oscillating mode. Switch SA1 can be used to set one of the measurement frequencies (multiplier): 1 Hz (X1000), 10Hz (x10O), 10 Hz (x10), 1 kHz (X1).

Op-amp DA2.1 is a voltage follower, at its output is a triangular signal with an amplitude of about 50 mV, which is used to create a measuring current through the capacitor Cx being tested.

Since the capacitance of the capacitor is measured in the board, there may be residual voltage on it, therefore, to prevent damage to the meter, two back-to-back bridge diodes VD1 are connected parallel to its probes.

Op-amp DA2.2 works as a differentiator and acts as a current-voltage converter. Its output voltage:

Uout=(Rl2...R16)·IBX=(Rl2...Rl6)Cx-dU/dt.

For example, when measuring a capacitance of 100 μF at a frequency of 100 Hz, it turns out: Iin=Cx dU/dt=100-100MB/5MC = 2MA, Uout= R16 lBX= 1 kOhm mA= 2 V.

Elements R11, C5 - C9 are necessary for stable operation of the differentiator. Capacitors eliminate oscillatory processes at the meander fronts, which make it impossible to accurately measure its amplitude. As a result, the output of DA2.2 produces a meander with smooth edges and an amplitude proportional to the measured capacitance. Resistor R11 also limits the input current when the probes are shorted or when the capacitor is broken. For the input circuit of the meter the following inequality must be satisfied:

(3...5)CxR1<1/(2f).

If this inequality is not satisfied, then in half the period the current IBX does not reach the steady-state value, and the meander does not reach the corresponding amplitude, and an error in the measurement occurs. For example, in the meter described in, when measuring a 1000 µF capacitance at a frequency of 1 Hz, the time constant is determined as

Cx R25 = 10OO uF - 910 Ohm = 0.91 s.

Half of the oscillation period T/2 is only 0.5 s, so on this scale the measurements will be noticeably nonlinear.

The synchronous detector consists of a switch on a field-effect transistor VT1, a key control unit on an op-amp DA1.3 and a storage capacitor C10. Op-amp DA1.2 outputs a control signal to switch VT1 during the positive half-wave of the meander, when its amplitude is set. Capacitor C10 stores the constant voltage generated by the detector.

From capacitor C10, the voltage, which carries information about the value of capacitance Cx, is supplied through repeater DA2.3 to microammeter RA1. Capacitors C11, C12 are smoothing. The voltage is removed from the variable calibration resistor R22 to a digital voltmeter with a measurement limit of 2 V.

The power supply (Fig. 2) produces bipolar voltages ±9 V. The reference voltages are formed by thermally stable zener diodes VD5, VD6. Resistors R25, R26 set the required output voltage. Structurally, the power source is combined with the measuring part of the device on a common circuit board.

The device uses variable resistors of the SPZ-22 type (R21, R22, R25, R26). Fixed resistors R12 - R16 - type C2-36 or C2-14 with a permissible deviation of ±1%. Resistance R16 is obtained by connecting several selected resistors in series. The resistances of resistors R12 - R16 can be used in other types, but they must be selected using a digital ohmmeter (multimeter). The remaining fixed resistors are any with a dissipation power of 0.125 W. Capacitor C10 - K53-1A, capacitors C11 - C16 - K50-16. Capacitors C1, C2 - K73-17 or other metal film, SZ, C4 - KM-5, KM-6 or other ceramic with TKE no worse than M750, they must also be selected with an error of no more than 1%. The remaining capacitors are any.

Switches SA1, SA2 - P2G-3 5P2N. In the design, it is permissible to use a CVD transistor (VT1) with the letter indices A, B, C, G, I. Transistors VT2, VT3 voltage stabilizers can be replaced by other low-power silicon transistors of the corresponding structure. Instead of the K1401UD4 op-amp, you can use the K1401UD2A, but then at the “1000 pF” limit, an error may occur due to the bias of the differentiator input created by the input current DA2.2 on R16.

Power transformer T1 has an overall power of 1 W. It is permissible to use a transformer with two 12 V secondary windings, but then two rectifier bridges are required.

To configure and debug the device, you will need an oscilloscope. It is a good idea to have a frequency meter to check the frequencies of the triangle oscillator. Model capacitors will also be needed.

The device begins to be configured by setting the voltages +9 V and -9 V using resistors R25, R26. After this, the operation of the triangular oscillation generator is checked (oscillograms 1, 2, 3, 4 in Fig. 3). If you have a frequency meter, measure the frequency of the generator at different positions of switch SA1. It is acceptable if the frequencies differ from the values ​​1 Hz, 10 Hz, 100 Hz, 1 kHz, but among themselves they must differ exactly 10 times, since the correctness of the instrument readings on different scales depends on this. If the generator frequencies are not a multiple of ten, then the required accuracy (with an error of 1%) is achieved by selecting capacitors connected in parallel with capacitors C1 - C4. If the capacitances of capacitors C1 - C4 are selected with the required accuracy, you can do without measuring frequencies.

Next, check the operation of op-amp DA1.3 (oscillograms 5, 6). After this, set the measurement limit to “10 µF”, the multiplier to the “x1” position and connect a standard capacitor with a capacity of 10 µF. The output of the differentiator should be rectangular, but with prolonged, smoothed fronts, oscillations with an amplitude of about 2 V (oscillogram 7). Resistor R21 sets the instrument readings - the needle deflects to full scale. A digital voltmeter (at a limit of 2 V) is connected to sockets XS3, XS4 and resistor R22 is used to set the reading to 1000 mV. If capacitors C1 - C4 and resistors R12 - R16 are precisely selected, then the instrument readings will be multiples on other scales, which can be checked using standard capacitors.

Measuring the capacitance of a capacitor soldered into a board with other elements is usually quite accurate within the range of 0.1 - 10,000 uF, except when the capacitor is shunted by a low-resistance resistive circuit. Since its equivalent resistance depends on the frequency Xc = 1/ωС, to reduce the shunting effect of other elements of the device it is necessary to increase the measurement frequency with a decrease in the capacitance of the measured capacitors. If, when measuring capacitors with a capacity of 10,000 μF, 1000 μF, 100 μF, 10 μF, frequencies of 1 Hz, 10 Hz, 100 Hz, 1 kHz are used, respectively, then the shunting effect of the resistors will affect the reading of the device with a parallel connected resistor with a resistance of 300 Ohms (an error of about 4%) or less. When measuring capacitors with a capacity of 0.1 and 1 μF at a frequency of 1 kHz, an error of 4% will be due to the influence of a parallel-connected resistor with a resistance of 30 and 3 kOhm, respectively.

At the limits of 0.01 μF and 1000 pF, it is advisable to check the capacitors with the shunt circuits turned off, since the measuring current is small (2 μA, 200 nA). It is worth recalling, however, that the reliability of small capacitors is noticeably higher due to their design and higher permissible voltage.

Sometimes, for example, when measuring some capacitors with an oxide dielectric (K50-6, etc.) with a capacity from 1 µF to 10 µF at a frequency of 1 kHz, an error appears, apparently associated with the capacitor’s own inductance and losses in its dielectric ; The instrument readings are lower. Therefore, it may be advisable to carry out measurements at a lower frequency (for example, in our case at a frequency of 100 Hz), although in this case the shunting properties of parallel resistors will be reflected already at a higher resistance.

LITERATURE
1. Kuchin S. Device for measuring capacitance. - Radio. 1993, ╧ 6, pp. 21 - 23.
2. Bolgov A. Tester of oxide capacitors. - Radio, 1989, ╧ 6, p. 44.

DIY ESR meter. There is a wide list of equipment breakdowns, the cause of which is precisely electrolytic. The main factor in the malfunction of electrolytic capacitors is “drying out,” familiar to all radio amateurs, which occurs due to poor sealing of the housing. In this case, its capacitive or, in other words, reactance increases as a result of a decrease in its nominal capacity.

In addition, during operation, electrochemical reactions take place in it, which corrode the connection points between the leads and the plates. The contact deteriorates, eventually forming “contact resistance”, sometimes reaching several tens of ohms. This is exactly the same if a resistor is connected in series to a working capacitor, and moreover, this resistor is placed inside it. This resistance is also called “equivalent series resistance” or ESR.

The existence of series resistance negatively affects the operation of electronic devices by distorting the operation of capacitors in the circuit. Increased ESR (about 3...5 Ohms) has an extremely strong impact on performance, leading to the burning of expensive microcircuits and transistors.

The table below shows the average ESR values ​​(in milliohms) for new capacitors of various capacities depending on the voltage for which they are designed.

It is no secret that reactance decreases with increasing frequency. For example, at a frequency of 100 kHz and a capacitance of 10 μF, the capacitive component will be no more than 0.2 Ohm. When measuring the drop in alternating voltage having a frequency of 100 kHz and higher, we can assume that with an error in the region of 10...20%, the result of the measurement will be the active resistance of the capacitor. Therefore, it is not at all difficult to assemble.

Description of ESR meter for capacitors

The pulse generator with a frequency of 120 kHz is assembled using logic elements DD1.1 and DD1.2. The generator frequency is determined by the RC circuit on elements R1 and C1.

For coordination, element DD1.3 was introduced. To increase the power of pulses from the generator, elements DD1.4...DD1.6 were introduced into the circuit. Next, the signal passes through the voltage divider across resistors R2 and R3 and goes to the capacitor Cx under study. The alternating voltage measurement unit contains diodes VD1 and VD2 and a multimeter as a voltage meter, for example, M838. The multimeter must be switched to DC voltage measurement mode. The ESR meter is adjusted by changing the R2 value.

The DD1 - K561LN2 microcircuit can be replaced with K1561LN2. Diodes VD1 and VD2 are germanium, it is possible to use D9, GD507, D18.

The radio components of the ESR meter are located on, which you can make yourself. Structurally, the device is made in the same housing with the battery. Probe X1 is made in the form of an awl and attached to the body of the device, probe X2 is a wire no more than 10 cm in length with a needle at the end. Capacitors can be checked directly on the board; there is no need to unsolder them, which makes it much easier to find a faulty capacitor during repairs.

Device setup

1, 5, 10, 15, 25, 30, 40, 60, 70 and 80 ohms.

It is necessary to connect a 1 Ohm resistor to the probes X1 and X2 and rotate R2 until the multimeter reads 1 mV. Then, instead of 1 Ohm, connect the next resistor (5 Ohms) and, without changing R2, record the multimeter reading. Do the same with the remaining resistances. The result is a table of values ​​from which the reactance can be determined.

When repairing or radio designing, you often have to deal with such an element as a capacitor. Its main characteristic is capacity. Due to the characteristics of the device and operating modes, failure of electrolytes becomes one of the main causes of malfunctions of radio equipment. To determine the capacity of an element, various testing devices are used. They are easy to buy in a store, or you can make them yourself.

Physical definition of a capacitor

A capacitor is an electrical element that serves to store charge or energy. Structurally, the radio element consists of two plates made of conductive material, between which there is a dielectric layer. The conductive plates are called plates. They are not connected to each other by a common contact, but each has its own terminal.

Capacitors have a multilayer appearance, in which a dielectric layer alternates with layers of plates. They are a cylinder or parallelepiped with rounded corners. The main parameter of an electrical element is capacitance, the unit of measurement of which is the farad (F, Ф). On diagrams and in literature, a radio component is designated by the Latin letter C. After the symbol, the serial number on the diagram and the value of the nominal capacity are indicated.

Since one farad is a fairly large value, the actual values ​​of the capacitor capacitance are much lower. Therefore, when recording It is customary to use conditional abbreviations:

• P - picofarad (pF, pF);
• N - nanofarad (nF, nF);
• M - microfarad (mF, µF).

Principle of operation

The operating principle of the radio component depends on the type of electrical network. When connected to the terminals of the plates of a direct current source, charge carriers fall on the conductive plates of the capacitor, where they accumulate. At the same time, a potential difference appears at the terminals of the plates. Its value increases until it reaches a value equal to the current source. As soon as this value is leveled out, charge stops accumulating on the plates and the electrical circuit is broken.

In an alternating current network, a capacitor represents a resistance. Its value is related to the frequency of the current: the higher it is, the lower the resistance and vice versa. When a radio element is exposed to alternating current, a charge accumulates. Over time, the charge current decreases and disappears completely. During this process, charges of different signs are concentrated on the plates of the device.

The dielectric placed between them prevents their movement. At the moment of half-wave change, the capacitor is discharged through the load connected to its terminals. A discharge current occurs, that is, the energy accumulated by the radio element begins to flow into the electrical circuit.

Capacitors are used in almost any electronic circuit. They serve as filter elements to convert current ripples and cut off various frequencies. In addition, they compensate for reactive power.

Characteristics and types

Measuring the parameters of capacitors involves finding the values ​​of their characteristics. But among them, the most important is capacity, which is usually measured. This value indicates the amount of charge that a radio element can accumulate. In physics, electrical capacity is a value equal to the ratio of the charge on any plate to the potential difference between them.

In this case, the capacitance of the capacitor depends on the area of ​​the plates of the element and the thickness of the dielectric. In addition to capacity, a radio device is also characterized by polarity and the value of internal resistance. Using special instruments, these quantities can also be measured. The resistance of the device affects the self-discharge of the element. Besides, The main characteristics of the capacitor include:

Capacitors are classified according to different criteria, but first of all they are divided according to the type of dielectric. It can be gaseous, liquid and solid. Most often, glass, mica, ceramics, paper and synthetic films are used. Besides, capacitors vary in their ability to change the capacitance value and can be:

Also, depending on the purpose, capacitors are of general and special purpose. The first type of devices are low-voltage, and the second type are pulsed, starting, etc. But regardless of the type and purpose, the principle of measuring their parameters is identical.

Measuring instruments

To measure the parameters of capacitors, both specialized instruments and general-purpose instruments are used. Capacitance meters are divided into two types according to their type: digital and analog. Specialized devices can measure the capacitance of an element and its internal resistance. A simple tester usually diagnoses only a dielectric breakdown or a large leak. In addition, if the tester is multifunctional (multimeter), then it can also measure capacitance, but usually its measurement limit is low.

Therefore, as a capacitor tester can be used:

• ESR or RLC meter;
• multimeter;
• tester.

In this case, diagnostics of the element with a device belonging to the first type can be carried out without desoldering it from the circuit. If the second or third type is used, then the element or at least one of its terminals must be disconnected from it.

Using an ESR Meter

Measuring the ESR parameter is very important when testing a capacitor for performance. The fact is that almost all modern technology is pulsed, using high frequencies in its operation. If the equivalent resistance of the capacitor is high, then power is released on it, and this causes heating of the radio element, leading to its degradation.

Structurally, the specialized meter consists of a housing with a liquid crystal screen. A KRONA type battery is used as its power source. The device has two connectors of different colors to which probes are connected. A red probe is considered positive, and a black probe is considered negative. This is done so that polar capacitor measurements can be taken correctly.

Before measuring ESR resistance, the radio component must be discharged, otherwise the device may fail. To do this, the terminals of the capacitor are closed with a resistance of about one kilo-ohm for a short time.

Direct measurement occurs by connecting the terminals of the radio component to the probes of the device. In the case of an electrolytic capacitor, it is necessary to observe polarity, that is, connect plus to plus, and minus to minus. After this, the device turns on, and after some time the results of measuring the resistance and the capacitance of the element appear on its screen.

It should be noted that the bulk of such devices are manufactured in China. Their operation is based on the use of a microcontroller, the operation of which is controlled by a program. When measuring, the controller compares the signal passed through the radio element with the internal one and, based on the differences, produces data using a complex algorithm. Therefore, the measurement accuracy of such devices depends mainly on the quality of the components used in their manufacture.

When measuring capacitance, you can also use an immittance meter. It is similar in appearance to an ESR meter, but can additionally measure inductance. The principle of its operation is based on the passage of a test signal through the measured element and analysis of the obtained data.

Checking with a multimeter

A multimeter can measure almost all basic parameters, but the accuracy of these results will be lower than when using an ESR device. Measuring with a multimeter can be represented as follows:

If the tester displays the value OL or Overload, this means that the capacitance is too high to be measured with a multimeter or the capacitor is broken. When the result obtained is preceded by several zeros, the measurement limit must be lowered.

Application of the tester

If you don’t have a multimeter at hand that can measure capacitance, you can take measurements with improvised means. To do this, you will need a resistor, a power supply with a constant output signal level, and a device that measures voltage. It is better to consider the measurement technique using a specific example.

Let there be a capacitor whose capacity is unknown. To get to know her you will need to do the following:

This measurement algorithm cannot be called accurate, but it is quite capable of giving a general idea of ​​the capacity of the radio element.

If you have knowledge of amateur radio, you can assemble a device for measuring capacitance with your own hands. There are many circuit solutions of varying levels of complexity. Many of them are based on measuring the frequency and period of pulses in a circuit with a measured capacitor. Such circuits are complex, so it is easier to use measurements based on calculating reactance when passing pulses of a fixed frequency.

The circuit of such a device is based on a multivibrator, the operating frequency of which is determined by the capacitance and resistance of the resistor connected to terminals D1.1 and D1.2. Using switch S1, the measurement range is set, that is, the frequency changes. From the output of the multivibrator, pulses are sent to a power amplifier and then to a voltmeter.

The instrument is calibrated at each limit using a reference capacitor. The sensitivity is set by resistor R6.