Homemade device for measuring small capacitors. Do-it-yourself capacitor capacitance meter. Description and configuration of the device. Modifications with a single-junction expander

Capacitor - element electrical circuit consisting of conducting 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. The capacitance is in farads, the voltage is in volts, and the charge is in coulombs. When a current of 1 A flows through a 1 F capacitor, the voltage changes by 1 V in 1 s.

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

Capacitor designations

The easiest way to determine the value of the capacitance is from the markings applied to the capacitor case.

Electrolytic (oxide) polar capacitor, with a capacity of 22,000 μF, rated for a nominal voltage of 50 V direct current... 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 μF). 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.

Capacities up to 100 pF are marked, with rare exceptions, with a corresponding number. This is enough to obtain data about the product, as the overwhelming number of capacitors are marked in this way. 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 shown 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 RC product is called the time constant of the circuit. With R in ohms and C in farads, the RC product is seconds. For a capacitance of 1 μF and a resistance of 1 kΩ, 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 a time t ≥ RC. In practice, the following rule applies: in 5 RC time, 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 capacity of an unknown capacitor, you should include it in a circuit from a resistor and a power source. The input voltage is selected slightly lower rated voltage capacitor, if it is unknown, 10-12 volts will be enough. A stopwatch is also needed. To exclude the influence of the internal resistance of the power supply on the parameters of the circuit, 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 with a voltmeter. The time is counted from the moment the power is turned on - when charging and when it is turned off, if the discharge is controlled. Having known values of resistance and time, the capacity is calculated by 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 according to the formulas 2.2t = 2.2RC and 3t = 3RC. In this way, you can find out the capacity electrolytic capacitors with an accuracy determined by the measurement errors of time, voltage and resistance. Its application for ceramic and other small capacities, using a 50 Hz transformer, calculating the capacitive resistance - gives an unpredictable error.

Measuring instruments

The most affordable method for measuring capacity is a widespread multimeter with this capability.

In most cases, similar 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, it is enough to insert the capacitor leads into the designated sockets and read the readings, the whole process takes a minimum of time. This function is not present in all multimeter models, but it is often found with different measurement ranges and capacitor connection methods. To determine more detailed characteristics capacitor (loss tangent and others), other devices designed for a specific task are used, are often stationary devices.

The measurement circuit mainly implements the bridge method. They are used limitedly in special professional areas and are not widespread.

Homemade S - meter

Not taking into account various exotic solutions, such as a ballistic galvanometer and bridge circuits with a resistance box, making a simple device or an attachment to a multimeter is within the power of a novice radio amateur. The widespread 555 series chip is quite suitable for this purpose. 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 the choice of resistors R1 – R8 and capacitors C1, C2 by switch SA1 and is equal to: 25 kHz, 2.5 kHz, 250 Hz, 25Hz - correspondingly to switch positions 1, 2, 3 and 4–8. The capacitor Cx is charged with a pulse repetition rate through the diode VD1, to a fixed voltage. The discharge occurs during a pause through the resistances R10, R12 – R15. At this time, a pulse is formed with a duration dependent on the capacitance Cx (more capacitance - longer pulse). After passing the integrating circuit R11 C3, a voltage appears at the output that corresponds to the pulse length and is proportional to the value of the capacitance Cx. This is where the (X 1) multimeter is connected to measure the voltage at the limit of 200 mV. Switch positions SA1 (starting with the first) correspond to the limits: 20 pF, 200 pF, 2 nF, 20 nF, 0.2 μF, 2 μF, 20 μF, 200 μF.

The adjustment of the structure must be done with the device, which will be used in the future. Capacitors for adjustment must be selected with a capacity equal to the measurement sub-ranges and as accurately as possible, the error will depend on this. Selected capacitors are alternately connected 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 readings of the multimeter, you may have to slightly change the ratings of the series-connected resistances. On other subranges (0.2 µF – 200 µF), calibration is performed with resistors R12 – R15.

When choosing a power source, it should be borne in mind that the pulse amplitude directly depends on its stability. Integrated stabilizers of the 78xx series are quite applicable here. The circuit consumes no more than 20-30 milliamperes 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%, on 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 pulse, film capacitors, with a low leakage current. Microcircuit - any of the 555 series (LM555, NE555 and others), Russian counterpart - KR1006VI1. The meter can be almost any voltmeter with a high input impedance, for which the calibration has been carried out. The power supply 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.

Option printed circuit board and arrangement of components

How does the meter work?

The standard version includes a module with an expander. The data is displayed. Some modifications operate on the basis of a relay transistor. It is capable of operating at different frequencies. However, it should be noted that this modification is not suitable for many types of capacitors.

Low precision devices

You can make a low-precision ESR meter for capacitor capacitance with your own hands using an adapter module. However, the expander is used primarily. It is more expedient to select contacts for it with two semiconductors. With an output voltage of 5 V, the current should be no more than 2 A. Filters are used to protect the meter from failures. Tuning should be carried out at a frequency of 50 Hz. In this case, the tester should show a resistance of no more than 50 ohms. Some have problems with the conductivity of the cathode. In this case, replace the module.

Description of high precision models

When making a capacitor capacitance meter with your own hands, the accuracy calculation should be made based on a linear expander. The overload rating of the modification depends on the conductivity of the module. Many experts advise choosing a dipole transistor for the model. First of all, it is able to work without heat loss. It is also worth noting that the presented elements rarely overheat. The contactor for the meter can be used with low conductivity.

To make a simple, accurate do-it-yourself capacitor capacitance meter, you should take care of a thyristor. The specified element must operate at a voltage of at least 5 V. With a conductivity of 30 microns, the overload in such devices, as a rule, does not exceed 3 A. Filters are of different types. They should be installed behind the transistor. It is also worth noting that the display can only be connected via wired ports. 3W batteries are suitable for charging the meter.

How to make an AVR series model?

You can make a DIY AVR capacitor capacitance meter only on the basis of a variable transistor. First of all, a contactor is selected for modification. To set up the model, you should immediately measure the output voltage. The negative resistance of the meters should not exceed 45 ohms. With a conductivity of 40 microns, the overload in the devices is 4 A. Comparators are used to ensure maximum measurement accuracy.

Some experts recommend selecting only open filters. They are not afraid of impulse noise even under heavy workload. Pole stabilizers have been in great demand lately. Only grid comparators are not suitable for modification. Before turning on the device, a resistance measurement is made. For high-quality models, this parameter is approximately 40 ohms. However, in this case, much depends on the frequency of modification.

Setting up and assembling a model based on PIC16F628A

Making a capacitor capacitance meter with your own hands on the PIC16F628A is quite problematic. First of all, an open transceiver is selected for assembly. The module is allowed to be used of the regulated type. Some experts advise against installing high conductivity filters. The output voltage is checked before soldering the module.

If the resistance is high, it is recommended to replace the transistor. Comparators are used to overcome impulse noise. You can also use conductive stabilizers. Displays are often of the text type. They should be installed via channel ports. The modification is configured using the tester. With overestimated capacitance parameters, it is worth replacing low-conductivity transistors.

Model for electrolytic capacitors

If necessary, you can make a do-it-yourself meter for the capacity of electrolytic capacitors. Shop models of this type stand out for their low conductivity. Many modifications are made on contactor modules and operate at a voltage of no more than 40 V. Their protection system is of the RK class.

It is also worth noting that the meters of this type differ in reduced frequency. Their filters are used only of the transient type, they are able to effectively cope with impulse noise, as well as harmonic oscillations. If we talk about the disadvantages of modifications, then it is important to note that they have a small throughput... They show poor results in high humidity conditions. Experts also point out incompatibility with wired contactors. The devices cannot be used on an alternating current circuit.

Modifications for field capacitors

Devices for field capacitors are distinguished by their reduced sensitivity. Many models are capable of operating from straight-line contactors. The devices are most often used of the transitional type. In order to make the modification with your own hands, you need to use an adjustable transistor. Filters are installed in sequential order. To test the meter, small capacitors are used first. In this case, a negative resistance is recorded by the tester. If the deviation is more than 15%, it is necessary to check the operability of the transistor. Output voltage it should not exceed 15 V.

2V devices

At 2 V, a do-it-yourself capacitor capacitance meter is quite simple. First of all, experts recommend preparing an open transistor with low conductivity. It is also important to find a good modulator for it. Comparators are usually used with low sensitivity. The protection system for many models is used by the KR series on mesh-type filters. Wave stabilizers are used to overcome impulse fluctuations. It is also worth noting that the assembly of the modification involves the use of an expander for three contacts. To tune the model, use a contact tester, and the resistance indicator should not be lower than 50 ohms.

3 V modifications

When folding the capacitor capacitance meter with your own hands, you can use an adapter with an expander. It is more expedient to select a transistor of a linear type. On average, the conductivity of the meter should be 4 microns. It is also important to secure the contactor before installing the filters. Many modifications also include transceivers. However, these elements are not capable of working with field capacitors. Their limiting capacitance parameter is 4 pF. The protection system for the models is of the RK class.

4V models

Collecting the capacitor capacitance meter with your own hands is allowed only on linear transistors. Also, the model will require a high-quality expander and adapter. If you believe the experts, then it is more expedient to use filters of a transitional type. If we consider market modifications, then they can use two expanders. Models work at a frequency of no more than 45 Hz. At the same time, their sensitivity often changes.

If you assemble a simple meter, then the contactor can be used without a triode. He has low conductivity, but he is able to work under heavy workload. It is also worth noting that the modification should include several pole filters that will pay attention to harmonic vibrations.

Modifications with a single-junction expander

It is quite simple to make a capacitor capacitance meter with your own hands on the basis of a single-junction expander. First of all, it is recommended to select a module with low conductivity for modification. In this case, the sensitivity parameter should be no more than 4 mV. Some models have a serious conductivity problem. Transistors are used, as a rule, of the wave type. When using grid filters, the thyristor heats up quickly.

To avoid such problems, it is recommended to install two filters at once on mesh adapters. At the end of the work, all that remains is to solder the comparator. To increase the efficiency of the modification, channel stabilizers are installed. It is also worth noting that there are devices with variable contactors. They are capable of operating at a frequency of no more than 50 Hz.

Models based on double-junction expanders: assembly and adjustment

It is quite simple to fold a digital capacitor capacitance meter on two-junction expanders with your own hands. However, only adjustable transistors are suitable for the normal operation of the modifications. It is also worth noting that when assembling, you need to select pulse comparators.

Display for device will fit string type. In this case, the port is allowed to be used for three channels. Low sensitivity filters are used to solve circuit distortion problems. It is also worth noting that modifications must be assembled on diode stabilizers. The model is tuned with a negative resistance of 55 ohms.

Technicians who repair radio equipment are most often faced with capacitor breakdown or with a decrease in capacitance. To find out whether a part is working or not, you need to measure the capacitance of the capacitor. There are various devices for this.

Condenser design and characteristics

The capacitor contains two metal plates, between which a dielectric is placed. For the dielectric, air, plastic, mica, cardboard, and ceramic materials are used.

In more modern parts, instead of metal, foil is used, which is rolled into rolls. Thus, with a smaller capacitor, its capacity can be increased.

Capacitors are classified by dielectric material, mounting method, plate shape, etc. By polarity, they are divided into:

electrolytic, or oxide, with polarity;
non-polar.

Electrolytic capacitor elements require mandatory observance of polarity when switched on. An oxide layer formed on a tantalum (aluminum) anode serves as a dielectric in them. The cathode is an electrolyte in the form of a liquid or gel. The measurement of the capacitance of a capacitor of this type must be carried out taking into account the marking of the poles of the part.

The main property of a capacitor is accumulation electric charge due to which it is widely used in various filters. With it, you can transfer the signal between the amplification stages, separate the high and low frequencies etc.

Capacitor parameters:

Capacity. The ability to accumulate charge, depending on the area of the plates, the distance between them, the nature of the material used as an electrolyte. Measured in farads;
Rated voltage. Shows at what voltage a long and stable operation of the element is possible. If the parameter is exceeded, breakdown may occur.

Possible capacitor malfunctions

There are several types of capacitor faults that affect the operation of the electrical circuit:

complete breakdown (short circuit between plates);
violation of external tightness from mechanical damage;
decrease in capacity;
an increase in internal resistance;
a decrease in voltage at which a reversible breakdown of the element occurs.

In most cases, parts fail due to prolonged operation under overheating conditions. It is always important to ensure the optimal temperature regime for the operation of the equipment.

How to check the condition of a capacitor

At the first stage, it is necessary to make a visual inspection of the part for the presence of mechanical damage, deformation of the case, and discoloration. In electrolytic cells, this swelling in the upper part, which may be small, but noticeable in comparison with serviceable counterparts. The part often looks normal on the outside. Then, to check it, you will need special devices:

a multimeter in which the capacitance measurement function is implemented;
special capacitor meter;
LC meter;
ESR device.

Using a multimeter, it is sometimes difficult to infer a fault because the capacitance of a damaged capacitor element is reduced by very small amounts. With the help of LC meters or special instruments, its value can be determined more accurately. ESR devices are used to measure the capacitance of electrolytic capacitors. Moreover, measurements are made without soldering out the parts from the circuit.

If there is no special device, then capacitive measurements of non-polar elements can be made with a multimeter that measures resistance. At the same time, they are soldered from the board.

On the scale of the multimeter, set the limit "200 kOhm". The scale limit varies depending on the nominal capacitive value;
Discharge the evaporated capacitor elements, as a residual charge may exist. The discharge is produced by short-circuiting their terminals;
Connect the probes of the device to the capacitor terminals and observe its readings. Try not to touch the contact part of the probes with your hands.

The displayed resistance value will gradually increase, and then it will show "1", which means "infinity" on the digital device. In capacitors with small capacitance, the process of changing the resistance is accelerated so that it can not be fixed.

Important! A properly charged capacitor element has an "infinite" resistance.

If the part is faulty, immediately, without the previous increase, the values "1" will be visible, indicating a breakage inside the part, or "0" - an internal short circuit. A smooth increase in resistance is observed due to the charging of the part from the battery of the multimeter.

Old analog testers can also be used for capacitive measurements. In this case, observations are carried out for the movements of the arrow. It should immediately deflect to the right at a speed dependent on the capacitor, continuing its slow motion to the full scale. If it does not twitch or, deviating, stops, this indicates damage. The same is signaled by a sharp rush to the limit numbers.

Important! A multimeter can be used to test capacitor elements with a capacity of up to 0.25 μF. For smaller parameters, verification is carried out on LC meters.

Measurement of actual capacitive values

It is impossible to determine the quantitative capacitive values in the above-described way; one can only conclude that the capacitor element is in good condition. Instruments that measure the capacitance in farads immediately determine its deviation from the nominal parameter. A zero value indicates breakdown, a reduced value also signals that the part needs to be replaced.

Indirectly, the value of the capacitance can be judged by the rate of rise of resistance at the time of connection to the multimeter. The lower it is, the larger the capacity. You can calculate its approximate value by connecting serviceable capacitor elements with a predetermined capacitance and measuring the time in seconds for which the resistance reaches "infinity". The conclusion is made based on comparison with the tested capacitor element.

On the front panel of a multimeter designed for capacitive measurements, there are special CX input connectors, marked with "plus" and "minus". Instead of them, ordinary probes may be present. For measurement, the capacitor elements are inserted into these connectors with the obligatory observance of the polarity of the electrolytic parts. The marking is also present on the capacitors themselves. This is irrelevant for non-polar elements. The limiting value of the scale of the measured capacitance must be set based on the capacitor parameters.

Important! Before connecting to the device, it is necessary to remove the residual charge from the capacitor.

ESR measurement

ESR stands for Equivalent Series Resistance, a very important parameter for an electrolytic capacitor. When this resistance increases, the charging current decreases, causing the electrical circuit to malfunction. Moreover, the capacity measured by traditional methods may not go beyond the normal limits. The effect of the equivalent resistance is especially noticeable for parts with a capacity of more than 5 μF. For stable operation, the parameter should not exceed 1 Ohm.

When checking capacitor elements without soldering from the board, such a device gives more accurate results... Attempts to similarly measure the parameters of a part with a multimeter will not give a reliable picture. There are other elements near the capacitor: inductance, resistance, etc., which introduce a distorting effect. Usually, it is concluded that the capacitor element is in good working order using indirect measurements, or another one with identical characteristics is soldered in parallel to it. This is only possible in low voltage circuits.

Reducing the breakdown voltage of the capacitor

Masters-radio amateurs may encounter a case when all the characteristics of the capacitor are normal when measured with a multimeter, but when working in the circuit, there are signs of its breakdown. This occurs when the breakdown voltage drops below the nominal value. If the part is designed for a voltage of 25 V, and the breakdown occurs at 15 V, then when measured with a multimeter, a malfunction of the capacitor element will not be detected, since the breakdown is reversible.

To determine such a malfunction, it is necessary to use a constant current source with the ability to regulate the voltage level. By connecting a part to it and gradually increasing the applied voltage, it becomes clear that there is a damage, noticeable by a sharp increase in current until it is triggered protective shutdown SP.

Capacitor measurements can be made different ways... It is easy to find a faulty element with an ohmmeter, more accurate results are obtained when using LC meters and ESR instruments.

Video

DIY ESR meter... There is a wide range of equipment breakdowns, the cause of which is precisely electrolytic. The main factor in the malfunction of electrolytic capacitors is the "drying out" familiar to all radio amateurs, which occurs due to poor sealing of the case. 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 of the leads with the plates. The contact deteriorates, and as a result, a "contact resistance" is formed, sometimes reaching several tens of ohms. This is exactly the same if a resistor is connected in series to a working capacitor, and besides, this resistor is located inside it. This resistance is also called "equivalent series resistance" or ESR.

The existence of series resistance negatively affects the operation of electronic devices, distorting the operation of capacitors in the circuit. The increased ESR (about 3 ... 5 Ohm) has an extremely strong effect on performance, leading to the combustion of expensive microcircuits and transistors.

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

Material: ABS + Metal + Acrylic Lenses. LED lights...

It's 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. Measuring the fall alternating voltage having a frequency of 100 kHz and higher, it can be assumed 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

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

For agreement introduced element DD1.3. To increase the power of pulses from the generator, elements DD1.4… DD1.6 are introduced into the circuit. Then the signal passes through the voltage divider across the resistors R2 and R3 and goes to the investigated capacitor Cx. The AC voltage measuring unit contains diodes VD1 and VD2 and a multimeter, as a voltage meter, for example, M838. The multimeter must be set to DC voltage measurement mode. The ESR meter is adjusted by changing the value of R2.

Chip DD1 - K561LN2 can be changed to K1561LN2. Diodes VD1 and VD2 are germanium, it is possible to use D9, GD507, D18.

The radio parts of the ESR meter are located on, which can be made by hand. Structurally, the device is made in one housing with a battery. Probe X1 is made in the form of an awl and is attached to the body of the device, probe X2 is a wire no more than 10 cm in length at the end of which there is a needle. The capacitors can be checked directly on the board; it is not necessary to solder them, which greatly facilitates the search for a faulty capacitor during repair.

Setting up the device

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

It is necessary to connect a 1 Ohm resistor to the X1 and X2 probes and rotate R2 to achieve 1mV on the multimeter. Then, instead of 1 Ohm, connect the next resistor (5 Ohm) and without changing R2, record the reading of the multimeter. Do the same with the remaining resistances. As a result of this, a table of values will be obtained, from which it will be possible to determine the reactance.

With this capacitance meter, you can easily measure any capacitance from units of pF to hundreds of microfarads. There are several methods for measuring capacitance. This project uses an integration method.

The main advantage of using this method is that the measurement is based on the measurement of time, which can be done fairly accurately on the MC. This method is very suitable for a homemade capacitance meter, moreover, it is easy to implement on a microcontroller.

The principle of operation of the capacity meter

The phenomena that occur when the state of the circuit changes are called transients. This is one of the fundamental concepts of digital circuits. When the switch in Figure 1 is open, the capacitor charges through R and the voltage across it changes as shown in Figure 1b. The ratio determining the voltage across the capacitor has the form:

Values are expressed in SI units, t seconds, R ohms, C farads. The time it takes for the voltage across the capacitor to reach V C1 is roughly expressed by the following formula:

From this formula it follows that the time t1 is proportional to the capacitance of the capacitor. Therefore, the capacitance can be calculated from the charging time of the capacitor.

Scheme

To measure the charging time, a comparator and a microcontroller timer and a digital logic microcircuit are enough. It is quite reasonable to use the AT90S2313 microcontroller (modern analog - ATtiny2313). The comparator output is used as a flip-flop T C1. The threshold voltage is set with a resistor divider. The charging time is independent of the supply voltage. The charging time is determined by formula 2, therefore it does not depend on the supply voltage, since the ratio in the VC 1 / E formula is determined only by the divider factor. Of course, the supply voltage must be constant during the measurement.

Formula 2 expresses the charging time of a capacitor from 0 volts. However, it is difficult to work with a voltage close to zero due to the following reasons:

The voltage does not drop to 0 Volts. It takes time to fully discharge the capacitor. This will increase the times and measurements.

It takes time between startcharging and starting the timer. This will cause measurement error. This is not critical for AVR. it only takes one measure.

Leakage current at the analog input. According to the AVR datasheet, leakage current increases when the input voltage is close to zero volts.

To prevent these complications, two threshold voltages VC 1 (0.17 Vcc) and VC 2 (0.5 Vcc) are used. The surface of the printed circuit board must be clean to minimize leakage currents. The required supply voltage of the microcontroller is provided by a DC-DC converter powered by a 1.5VAA battery. Instead of a DC-DC converter, it is advisable to use 9 Vbattery and converter 78 L05, preferablyalsodo not turn offBODotherwise there may be problems with EEPROM.

Calibration

To calibrate the low range: Using the SW1 button. Then, connect pin # 1 and pin # 3 on the P1 connector, insert a 1nF capacitor and press SW1.

To calibrate the high range: Short pin # 4 and # 6 of P1, insert a 100nF capacitor and press SW1.

“E4” on power-up means that no calibration value was found in the EEPROM.

Usage

Automatic range detection

Charging starts through a 3.3M resistor. If the voltage across the capacitor does not reach 0.5 Vcc in less than 130 mS (> 57nF), the capacitor will discharge and new charge, but already through a 3.3kOhm resistor. If the voltage across the capacitor does not reach 0.5 Vcc in 1 second (> 440µF), the inscription “E2”. When the time is measured, the capacity is calculated and displayed. The last segment displays the measuring range (pF, nF, µF).