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A device for measuring the capacitance of capacitors. DIY capacitor capacitance meter

Almost two years ago I bought a digital capacitance meter, I took, one might say, the first thing that came across. I was so tired of the inability of the Mastech MY62 multimeter to measure the capacitance of capacitors more than 20 microfarads, and it did not measure correctly less than 100 picofarads. I liked two factors in the SM-7115A:

Measures the entire required range
Compactness and convenience

Paid 750 rubles. He sincerely believed that he was not worth the money, and the price was "inflated" due to the complete absence of competitive products. The country of origin is, of course, China. He was afraid that he would “fake”, more than that he was sure of this - but in vain.

The capacitance meter and wires to it were packed in polyethylene, each in its own sheath and enclosed in a box of thick cardboard, the free space was filled with foam. Also included in the box were instructions for English language. Overall dimensions of the device 135 x 72 x 36 mm, weight 180 grams. The body color is black, the front panel with a lilac tint. It has a liquid crystal display, nine measurement ranges, two power off positions, a zero adjustment knob, 15 centimeter, different color(red - black) wires, with which the measured capacitor is connected to the device, end with crocodile clips, and the sockets on the body of the device for connecting them are marked with a color designation of the corresponding polarity, it is additionally possible to measure without them (which increases accuracy) , for which there are two oblong sockets, which are signed with the symbol of the measured capacitor. A 9 volt battery is used, there is a function for automatically indicating its discharge. Three-digit liquid crystal indicator +1 decimal place, the measurement range declared by the manufacturer is from 0.1 pF to 20,000 μF, with the possibility of adjustment on the measurement range from 0 to 200 pF, to set zero, within +/- 20 pF, one measurement time 2-3 seconds.

Table of permissible errors in measurements, individually by ranges. Provided by the manufacturer.

There is an integrated stand on the back half of the case. It makes it possible to place the meter more compactly at the workplace and improves the visibility of the liquid crystal indicator.

The battery compartment is made completely autonomously, to change the battery, it is enough to move its cover to the side. Convenience from the category of inconspicuous, when it is.

In order to take off back cover case, it is enough to unscrew one self-tapping screw. The heaviest component printed circuit board- 500 mA fuse.

The basis of the work measuring device the method of double integration is posited. It is assembled on logical counters HEF4518BT - 2 pcs, key HEF4066BT, decimal counter with decoder HCF4017 and smd transistors: J6 - 4 pcs, M6 - 2 pcs.

Having unscrewed six more screws, you can see the other side of the printed circuit board. The variable resistor with which the setting is made to "0" is so that it can be easily replaced if necessary. On the left are the pins for connecting the measured capacitor, those above are for direct connection (without wires).

The device is set to the zero reference point not immediately, but the set reading holds. With the wires disconnected, this is much easier to do.

For a visual demonstration of the difference in measurement accuracy with different measurement methods (with and without wires), I took small capacitors with factory markings - 8.2 pF

Video review of the device

Without wires With wires
#1 8 pF 7.3 pF
#2 7.6 pF 8.3 pF
#3 8.1pF 9.3pF

Everything is clear, unambiguously without wires, measurements will be more accurate, although the discrepancy is practically within 1 pF. I also repeatedly measured the capacitors on the boards - the measurement readings of the serviceable ones are quite adequate according to the nominal value indicated on them. If not to be a very big nitpick, then it is quite possible to say that the quality factor of the measurement of the device is quite high.

Disadvantages of the device

setting to zero is not done immediately,
the petals of the contacts, for measuring without wires, have no elasticity, after unclenching they do not return to their original position,
The meter is not equipped with a calibration container.

conclusions

In general, I am satisfied with the device. It measures well, it is compact (it easily fits in a pocket), so on the radio market I take not what they give, but what I need. I plan, as there will be time, to finalize: to replace the potentiometer and contacts for direct measurement. His scheme, or something similar, can be searched in the section. He told “everything as it is”, and you already decide for yourself whether it is worth replenishing your home laboratory with such a device. Author - Babay.

DIY ESR meter. There is a wide list of equipment breakdowns, the cause of which is just electrolytic. The main factor in the failure 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 capacitance.

In addition, during operation, electrochemical reactions take place in it, which corrode the connection points of the leads with the plates. The contact worsens, 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 placed inside it. This resistance is also called "equivalent series resistance" or ESR.

The existence of series resistance adversely affects performance electronic devices, distorting the operation of capacitors in the circuit. An extremely strong influence is exerted by an increased ESR (of the order of 3 ... 5 Ohms) 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 various capacities, depending on the voltage for which they are rated.

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 ohms. Measuring the drop of 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 logical elements DD1.1 and DD1.2. The oscillator frequency is determined by the RC circuit on the elements R1 and C1.

Element DD1.3 has been introduced for harmonization. To increase the power of pulses from the generator, elements DD1.4 ... DD1.6 are introduced into the circuit. Next, the signal passes through a voltage divider on resistors R2 and R3 and enters the investigated capacitor Cx. The AC voltage measurement unit contains diodes VD1 and VD2 and a multimeter, as a voltage meter, for example, M838. The multimeter must be put into measurement mode constant voltage. Adjustment of the ESR meter is carried out 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 components of the ESR meter are located on which you can make yourself. 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 long at the end of which is a needle. 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.

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 to achieve 1mV on the multimeter. Then, instead of 1 ohm, connect the next resistor (5 ohms) 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 \u200b\u200bwill be obtained, from which it will be possible to determine reactance.

This article provides an elementary circuit of a capacitance meter on a logic chip. Such a classical and elementary circuit solution can be reproduced quickly and simply. Therefore, this article will be useful to a novice radio amateur who decided to assemble an elementary capacitor capacitance meter.

The operation of the capacitance meter circuit:

Figure No. 1 - Capacitance meter circuit

List of elements of the capacitance meter:

R1- R4 - 47 kΩ

R5 - 1.1 kOhm

C3 - 1500 pF

C4 - 12000 pF

C5 -0.1uF

C meas. - the capacitor whose capacitance you want to measure

SA1 - button switch

DA1 - K155LA3 or SN7400

VD1-VD2 - KD509 or equivalent 1N903A

PA1 - Pointer indicator head (total deflection current 1 mA, frame resistance 240 Ohm)

XS1-XS2 - alligator connectors

This version of the capacitor capacitance meter has four ranges that can be selected with the SA1 switch. For example, in position "1" you can measure capacitors with a capacitance of 50 pF, in position "2" - up to 500 pF, in position "3" - up to 5000 pF, in position "4" - up to 0.05 microfarads.

The elements of the DA1 chip provide sufficient current to charge the measured capacitor (C meas.). It is especially important for measurement accuracy to adequately select diodes VD1-VD2, they must have the same (most similar) characteristics.

Setting up the capacitance meter circuit:

Setting up such a circuit is quite simple, you need to connect C rev. with known characteristics (with a known capacitance). Select the required measurement range with the SA1 switch and turn the trim resistor knob until you reach the desired reading on the indicator head PA1 (I recommend calibrating it according to your readings, this can be done by disassembling the indicator head and gluing a new scale with new inscriptions)

This is an ESR meter (EPS) + capacitor capacitance meter.

The device measures the ESR (equivalent series resistance) of a capacitor and its capacitance by measuring the charging time direct current. The controlled zener diode TL431 and a p-n-p transistor act as a current source.

Capacitance measures within 1 - 150,000 microfarads, ESR - up to 10 ohms.

The whole structure was successfully borrowed from the pro-radio site, where Oleg Gints (aka GO and the author of the structure) posted his work for everyone to see. This design was repeated more than a dozen, or even a hundred times, tested and approved by the people. With proper assembly, it remains only to set the correction factors for capacitance and resistance.

The device is assembled on a PIC16F876A microcontroller, a common WH-1602 type LCD display based on the HD44780 and loose powder. The controller can be replaced with PIC16F873 - at the end of the article there are firmware for both models.

The capacitance and ESR of capacitors of about 1000 microfarads is measured in a fraction of a second. It also measures low resistance with great accuracy. That is, you can use it when you need to make a shunt for the ammeter :)

It also measures capacitance in-circuit very well. Only if there is inductance - it can lie. In this case, solder the element.

The case, Z-42, chose the good old, reliable USB 2.0 port as a connector for connecting probes using a four-wire circuit.

Old, Soviet, dried up electrolytic capacitor.

And this is a non-working capacitor from the processor power circuit on the motherboard.

How does it work.

The capacitor is pre-discharged, a 10 mA current source is turned on, both inputs of the measuring amplifier are connected to Cx, a delay of about 3.6 µs is made to eliminate the effect of ringing in the wires. Simultaneously through the keys DD2.3 || DD2.4 charges capacitor C1, which actually remembers the highest voltage that was on Cx. The next step opens the keys DD2.3 || DD2.4 and turn off the current source. The inverting input of the remote control remains connected to Cx, on which, after turning off the current, the voltage drops by 10mA * ESR. That's actually all - then you can safely measure the voltage at the output of the remote control - there are two channels, one with KU \u003d 330 for a limit of 1 Ohm and KU \u003d 33 for 10 Ohms.

On the source forum, where the printed circuit board and firmware are posted, the signet was double-sided. On the one hand - all the tracks, on the other - a solid layer of earth and just holes for the components. I didn’t have such a textolite at the time of assembly, so I had to make the ground with wires. One way or another, this did not cause any particular difficulties and did not affect the performance and accuracy of the device in any way.

In the last picture - a current source, a negative voltage source and a power switch.

The board is simple, the setup is even easier.

The first inclusion - we check the presence of + 5V after 78L05 and -5V (4.7V) at the output of DA4 (ICL7660). By selecting R31 we achieve normal contrast on the indicator.
Turning on the device with the Set button pressed puts it into the mode of setting correction factors. There are only three of them - for channels 1 ohm, 10 ohm and for the capacitance. Changing the coefficients with the + and - buttons, writing to EEPROM and enumeration - with the same Set button.
There is also a debug mode - in this mode, the measured values are displayed on the indicator without processing - for the capacitance - the state of the timer (approximately 15 counts per 1 uF) and both ESR measurement channels (1 ADC step = 5V / 1024). Switching to debug mode - when the "+" button is pressed
And one more thing - setting zero. To do this, we close the input, press and hold the "+" button and with the help of R4 we achieve the minimum readings (but not zero!) Simultaneously on both channels. Without releasing the "+" button, press Set - the indicator will display a message about saving U0 in EEPROM.
Next, we measure exemplary resistances of 1 ohm (or less), 10 ohms and capacitance (which you trust), we determine the correction factors. We turn off the device, turn it on with the Set button pressed and set to-you according to the measurement results.
Board in three stages, top view:

Device diagram:

Here is a small list of FAQ, formed on the source forum.

Q. When a resistor of 0.22 Ohm is connected, it writes - 1 with a penny, when a resistor of 2.7 Ohm is connected, it writes ESR\u003e 12.044 Ohm.

A. There may be deviations, but within 5-10%, and here it is 5 times. It is necessary to check the analog part, the culprits can be in descending order of probability:

current source,
diff. amplifier
keys
Start with a power source. It should give out 10 (+/-0.5) mA, you can check it either in the dynamics with an oscilloscope, loading it with 10 ohms - there should be no more than 100 mV in the pulse. If you don’t want to catch needles - check in statics - remove the jumper (zero resistance) between RC0 and R3, the lower end of R3 to ground, and turn on the milliammeter between the VT1 collector and the ground (although it may interfere with VT2 - then when checking, it is better to disconnect the VT1 collector from scheme).

In fact, the solution was this: - "I blindly mixed up 102 and 201 - and instead of 1 kilo-ohm I rattled 200 ohms."

Q. Is it possible to replace TL082 with TL072?

A. There are no special requirements for the OU, except for field workers at the entrance, it should work with TL072.

Q. Why are there two input connectors on your signet: one is connected to transistor diodes, and the other is connected to DD2?

A. To compensate for the voltage drop on the wires, it is better to connect the element under test using a 4-wire circuit, therefore the connector is 4-pin, and the wires are combined together already on alligator clips.

Q. At idle, the negative voltage is -4 Volts and is highly dependent on the type of capacitor between terminals 2 and 4 of the ICL 7660. With a conventional electrolyte, there was only -2 V.

A. After replacing with tantalum, torn from the 286 motherboard, it became -4 V.

Q. The WH-1602 indicator does not work or the indicator controller heats up.

A. The pinout of the WINSTAR WH-1602 indicator is incorrectly indicated in terms of power wiring, pins 1 and 2 are mixed up! On alldatasheet 1602L, which matches the pinout indicated by Winstar and on the diagram. I came across 1602D - here it has "tangled" 1 and 2 conclusions.

The inscription Cx ---- is displayed in the following cases:

When capacitance is measured, a timeout is triggered, i.e. during the allotted measurement time, the device did not wait for the switching of both comparators. This happens when measuring resistors, shorted probes, or when the measured capacitance is >150,000 uF, etc.
When the voltage measured at the DA2.2 output exceeds 0x300 (this is the ADC reading in hexadecimal code), the capacitance measurement procedure is not performed and Cx ---- is also displayed on the indicator.
With open probes (or R> 10 ohms) it should be so.

The ">" sign in the ESR line appears when the voltage at the output DA2.2 0x300 is exceeded (in ADC units)

Summing up: we poison the board, solder the elements without errors, flash the controller - and the device works.

After a couple of years, I decided to make the device autonomous. Based on the charger for smartphones, a step-up converter was made to 7 V output voltage. It would be possible immediately to 5 V, but since the board is fixed in the case with glue, I didn’t tear it off, and the voltage drop on KREN7805 of two Volts is a small loss :)

My new constructor looked like this:

A small scarf of the converter was "shod" in heat shrink, all the wires were unsoldered, we no longer need a crown connector. It’s just that the hole in the case doesn’t look very good, so we’ll leave it, but bite off the wires. There was no space left inside the case for the battery, so I glued the battery to the back of the device and attached legs to it so that it would not lie on the battery in working order.

On the front side, I cut out holes for the power button and the LED for indicating successful charging. Didn't do any indication of battery charge.

Then I decided that since such a booze went on, it would be nice to see the screen in the dark, in case of repairs by candlelight, if the lights were turned off, but I wanted to work :)

But this is after the more pontoon RLC-2 appeared. Read more about this device in this article.

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 the integration method.

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

The principle of operation of the capacitance 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 is charged through resistor R and the voltage across it will change as shown in Figure 1b. The ratio determining the voltage across the capacitor is:

Values are expressed in SI units, t seconds, R ohms, C farads. The time it takes for the voltage on the capacitor to reach the value V C1 is approximately 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 chip are enough. It is quite reasonable to use the AT90S2313 microcontroller (the modern analogue is ATtiny2313). The output of the comparator is used as a trigger T C1 . The threshold voltage is set by a resistor divider. The charging time does not depend on the supply voltage. The charging time is determined by formula 2, therefore it does not depend on the supply voltage. the ratio in the formula VC 1 /E is determined only by the divisor coefficient. Of course, during the measurement, the supply voltage must be constant.

Formula 2 expresses the charging time of the 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 time and measurement.

Required time between startcharging and starting the timer. This will cause measurement error. For AVR, this is not critical. it only takes one beat.

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

To prevent these difficulties, two threshold voltages VC 1 (0.17 Vcc) and VC 2 (0.5 Vcc) were used. The PCB surface must be clean to minimize leakage currents. The necessary supply voltage for 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 lower range: With the SW1 button. Next, connect pin #1 and pin #3 on connector P1, insert a 1nF capacitor, and press SW1.

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

The inscription “E4” when turned on means that the calibration value was not found in the EEPROM.

Usage

Automatic range detection

Charging starts through a 3.3M resistor. If the voltage on the capacitor does not reach 0.5 Vcc in less than 130 mS (>57nF), the capacitor is discharged and charged again, but through a 3.3kΩ resistor. If the voltage across the capacitor does not reach 0.5 Vcc for 1 second (>440µF), write “E2”. When the time is measured, the capacity is calculated and displayed. The last segment displays the measuring range (pF, nF, µF).