A simple circuit for measuring lc. Compact multifunctional device - L, C, ESR meter, probe-signal generator

This project is a simple LC meter based on the popular cheap PIC16F682A microcontroller. It is similar to another one recently published here. Typically, such features are difficult to find in low-cost commercial digital multimeters. And if some can still measure capacitance, then inductance definitely cannot. This means you will have to assemble such a device with your own hands, especially since there is nothing complicated in the circuit. It uses a PIC controller and all the necessary board files and HEX files for programming the microcontroller are available at the link.

Here is the circuit diagram of the LC meter

Choke at 82uH. Total consumption (with backlight) 30 mA. Resistor R11 limits the backlight and must be sized according to the actual current consumption of the LCD module.

The meter requires a 9V battery. Therefore, a 78L05 voltage stabilizer is used here. An automatic sleep mode for the circuit has also been added. The time in operating mode corresponds to the value of capacitor C10 at 680nF. This time in this case is 10 minutes. MOSFET Q2 can be replaced with BS170.

During the setup process, the next goal was to keep the current consumption as low as possible. By increasing the value of R11 to 1.2 kΩ, which controls the backlight, the total device current was reduced to 12 mA. It was possible to reduce it even more, but visibility suffers greatly.

The result of the assembled device

These photos show the LC meter in action. On the first there is a 1nF/1% capacitor, and on the second there is a 22uH/10% inductor. The device is very sensitive - when we install the probes, there is already 3-5 pF on the display, but this is eliminated when calibrating with a button. Of course, you can buy a ready-made meter with similar functions, but its design is so simple that it’s not at all a problem to solder it yourself.

Discuss the article LC METER

We consider a circuit for measuring capacitance of capacitors and inductance of coils, made with only five transistors and, despite its simplicity and accessibility, allows one to determine the capacitance and inductance of coils with acceptable accuracy over a wide range. There are four sub-ranges for capacitors and as many as five sub-ranges for coils. After a fairly simple calibration procedure, using two trimmers, the maximum error will be about 3%, which, you see, is not bad at all for an amateur radio homemade product.

I propose to solder this simple LC meter circuit with your own hands. The basis of the amateur radio homemade product is a generator made on VT1, VT2 and radio components of the harness. Its operating frequency is determined by the parameters of the LC oscillatory circuit, which consists of an unknown capacitance of the capacitor Cx and a parallel-connected coil L1, in the mode of determining the unknown capacitance - contacts X1 and X2 must be closed, and in the mode of measuring inductance Lx, it is connected in series with the coil L1 and parallel connected capacitor C1.

By connecting an unknown element to the LC meter, the generator begins to operate at a certain frequency, which is recorded by a very simple frequency meter assembled on transistors VT3 and VT4. The frequency value is then converted to direct current, which deflects the microammeter needle.

Inductance meter circuit assembly. It is recommended to keep the connecting wires as short as possible to connect unknown elements. After completing the general assembly process, it is necessary to calibrate the structure in all ranges.

Calibration is carried out by selecting the resistances of trimming resistors R12 and R15 when connecting to the measuring terminals of radioelements with previously known values. Since in one range the value of the trimming resistors will be one, and in another it will be different, it is necessary to determine something average for all ranges, and the measurement error should not exceed 3%.

This fairly accurate LC meter is built on a PIC16F628A microcontroller. The design of the LC meter is based on a frequency meter with an LC oscillator, the frequency of which changes depending on the measured values ​​of inductance or capacitance, and is calculated as a result. Frequency accuracy reaches 1 Hz.

Relay RL1 is necessary to select L or C measurement mode. The counter works based on mathematical equations. For both unknowns L And C, Equations 1 and 2 are general.

Calibration

When the power is turned on, the device is automatically calibrated. The initial operating mode is inductance. Wait a couple of minutes for the device circuits to warm up, then press the “zero” toggle switch to recalibrate. The display should show the values ind = 0.00. Now connect the test inductance value, such as 10uH or 100uH. The LC meter should display an accurate reading. There are jumpers to configure the counter Jp1~Jp4.

The inductance meter project presented below is very easy to replicate and consists of a minimum of radio components. Inductance measurement ranges: - 10nG - 1000nG; 1 µG - 1000 µG; 1mG - 100mG. Capacitance measurement ranges:- 0.1pF - 1000pF - 1nF - 900nF

The measuring device supports auto-calibration when power is turned on, eliminating the possibility of human error during manual calibration. Absolutely, you can recalibrate the meter at any time by simply pressing the reset button. The device has automatic selection of the measurement range.

There is no need to use any precision or expensive radio components in the design of the device. The only thing is that you need to have one “external” capacity, the nominal value of which is known with great accuracy. Two capacitors with a capacity of 1000 pF should be of normal quality, it is advisable to use polystyrene, and two capacitors of 10 µF should be tantalum.

Quartz must be taken exactly at 4,000 MHz. Every 1% frequency mismatch will result in a 2% measurement error. Relay with low coil current, because The microcontroller is not capable of providing a current higher than 30 mA. Don’t forget to place a diode in parallel with the relay coil to suppress reverse current and eliminate bounce.

Printed circuit board and microcontroller firmware from the link above.

SOURCE: Radio magazine No. 7 2004

In the practice of a radio amateur, measuring the parameters of the radio elements used is the first fundamental step in achieving the goals set when creating a radio engineering or electronic complex. Without knowing the properties of “elementary bricks,” it is very difficult to say what properties a house built from them will have. This article offers the reader a description of a simple measuring device that every radio amateur should have in his laboratory.

The operating principle of the proposed LC meter is based on measuring the energy accumulated in the electric field of the capacitor and the magnetic field of the coil. For the first time in relation to amateur design, this method was described in, and in subsequent years, with minor modifications, it was widely used in many designs of inductance and capacitance meters. The use of a microcontroller and an LCD indicator in this design made it possible to create a simple, small-sized, cheap and easy-to-use device with a fairly high measurement accuracy. When working with the device, you do not need to manipulate any controls; you just need to connect the element being measured and read the readings from the indicator.

Specifications

Range of measured capacitance............0.1pF...5μF
Range of measured inductance........0.1 µH...5 H
Error of the measured value, no more than, %.........±3
Supply voltage, V......7.5...9
Current consumption, mA, no more...........................15
Automatic range selection
Software zero correction
Dimensions, mm............140x40x30

The schematic diagram of the device is shown in rice. 1

The rectangular-shaped exciting voltage signal from pin 6 (PB1) of the microcontroller DD1, through the three lower buffer elements DD2 in the circuit, is supplied to the measuring part of the device. During a high voltage level, the measured capacitor Cx is charged through resistor R9 and diode VD6, and during a low voltage level, it is discharged through R9 and VD5. The average discharge current, proportional to the value of the measured capacitance, is converted by the device into voltage using the operational amplifier DA1. Capacitors C5 and C7 smooth out its ripples. Resistor R14 is used to accurately zero the op-amp.

When measuring inductance during a high level, the current in the coil increases to a value determined by resistor R10, and during a low level, the current created by the self-inductance emf of the measured coil also enters the input of the DA1 microcircuit through VD4 and R11.

Thus, at a constant supply voltage and signal frequency, the voltage at the op-amp output is directly proportional to the values ​​of the measured capacitance or inductance. But this is only true if the capacitor is fully charged during half the period of the exciting voltage and also fully discharged during the other half. The same goes for the inductor. The current in it must have time to increase to the maximum value and drop to zero. These conditions can be ensured by appropriate selection of resistors R9-R11 and the frequency of the exciting voltage.

A voltage proportional to the parameter value of the element being measured is supplied from the op-amp output through filter R6C2 to the built-in ten-bit ADC of microcontroller DD1. Capacitor C1 is a filter of the internal reference voltage source of the ADC.

The three top elements in the circuit, DD2, as well as VD1, VD2, C4, C11, are used to generate the -5 V voltage required for operation of the op-amp

The device displays the measurement result on a ten-digit seven-segment LCD HG1 (KO-4V, serially produced by Telesystems in Zelenograd). A similar indicator is used in PANAPHONE phones.

To increase accuracy, the device has nine measurement subranges. The frequency of the exciting voltage in the first subband is 800 kHz. At this frequency, capacitors with a capacitance of up to approximately 90 pF and coils with inductance up to 90 μH are measured. At each subsequent subrange, the frequency is reduced by 4 times, and the measurement limit is accordingly expanded by the same amount. In the ninth subband, the frequency is 12 Hz, which ensures the measurement of capacitors with a capacity of up to 5 μF and coils with an inductance of up to 5 H. The device selects the required subrange automatically, and after turning on the power, the measurement begins from the ninth subrange. During the switching process, the subband number is displayed on the indicator, which allows you to determine at what frequency the measurement is being performed.

After selecting the desired subrange, the measurement result in pF or μH is displayed on the indicator. For ease of reading, tenths of pF (μH) and units of μF (H) are separated by an empty space, and the result is rounded to three significant figures.

The HL1 red LED is used as a 1.5 V stabistor to power the indicator. Button SB1 is used for software zero correction, which helps compensate for the capacitance and inductance of the terminals and switch SA1. This switch can be eliminated by installing separate terminals for connecting the measured inductance and capacitance, but this is less convenient to use. Resistor R7 is designed to quickly discharge capacitors C9 and C10 when the power is turned off. Without it, re-activation, ensuring correct operation of the indicator, is possible no earlier than after 10 s, which is somewhat inconvenient during operation.

All parts of the device, except for the SA1 switch, are mounted on a single-sided printed circuit board, which is shown in rice. 2.

The HG1 indicator and the SB1 button are installed on the mounting side and displayed on the front panel. The length of the wires to the switch SA1 and the input terminals should not exceed 2...3 cm. Diodes VD3-VD6 are high-frequency with a low voltage drop, you can use D311, D18, D20. Trimmer resistors R11, R12, R14 are small-sized type SPZ-19. Replacing R11 with a wirewound resistor is undesirable, as it will lead to a decrease in measurement accuracy. The 140UD1208 microcircuit can be replaced with any other op-amp that has a zero-setting circuit and is capable of operating from a voltage of ±5 V, and the K561LN2 can be replaced with any CMOS microcircuit of the 1561, 1554, 74NS, 74AC series, containing six inverters, for example, 74NS14. The use of TTL series 155, 555, 1533, etc. is undesirable. The ATtinyl 5L microcontroller from ATMEL has no analogue and replacing it with another type, for example the popular AT90S2313, is impossible without adjusting the program.

The capacitance rating of capacitors C4, C5, C11 should not be reduced. Switch SA1 should be small in size and with minimal capacitance between the pins.

When programming the microcontroller, all FUSE bits should be left at default: BODLEVEL=0, BODEN=1, SPIEN=0, RSTDISBL=1, CKSEL1 ...0=00. The calibration byte must be written to the low byte of the program at address $000F. This will ensure precise setting of the clock frequency of 1.6 MHz and, accordingly, the frequency of the driving voltage for the measuring circuit on the first range of 800 kHz. In the ATtinyl 5L copy that the author had, the calibration byte is equal to $8B. The microcontroller firmware codes can be downloaded to the ftp server of the Radio magazine (see. ), or .

For setup, it is necessary to select several coils and capacitors with parameter values ​​in the measuring range of the device and having a minimum deviation tolerance according to the nominal value. If possible, their exact values ​​should be measured using an industrial LC meter. These will be your "model" elements. Considering that the meter scale is linear, in principle, one capacitor and one coil are sufficient. But it is better to control the entire range. Normalized chokes of the DM and DP types are well suited as model coils.

Having configured the device in capacitance measurement mode, you should move SA1 to the bottom position according to the diagram, close the input jacks and press SB1. After zero correction, connect a reference coil to the input and use resistor R11 to set the required readings. The price of the least significant digit is 0.1 μH. In this case, you should pay attention that the resistance of R11 is at least 800 Ohms, otherwise you should reduce the resistance of resistor R10. If R11 is greater than 1 kOhm, R10 must be increased, i.e. R10 and R11 must be close in nominal value. This setting ensures approximately the same time constant for “charging” and “discharging” the coil and, accordingly, a minimum measurement error.

An error of no worse than ±2...3% when measuring capacitors can be achieved without difficulty, but when measuring coils, everything is somewhat more complicated. The inductance of the coil largely depends on a number of accompanying conditions - the active resistance of the winding, losses in the magnetic circuits due to eddy currents, hysteresis, the magnetic permeability of ferromagnets nonlinearly depends on the magnetic field strength, etc. When measuring, coils are exposed to various external fields, and all real ferromagnets have quite high value of residual induction. The processes occurring during magnetization of magnetic materials are described in more detail in. As a result of the influence of all these factors, the readings of the device when measuring the inductance of some coils may not coincide with the readings of an industrial device measuring complex resistance at a fixed frequency. But do not rush to criticize this device and its author. You just have to take into account the peculiarities of the measurement principle. For coils without a magnetic core, for open magnetic cores and for ferromagnetic magnetic cores with a gap, the measurement accuracy is quite satisfactory if the active resistance of the coil does not exceed 20...30 Ohms. This means that the inductance of all coils and chokes of high-frequency devices, transformers for switching power supplies, etc. can be measured very accurately.

But when measuring the inductance of small-sized coils with a large number of turns of thin wire and a closed magnetic circuit without a gap (especially from transformer steel), there will be a large error. But in a real device, the operating conditions of the coil may not correspond to the ideal that is ensured when measuring complex resistance. For example, the inductance of the winding of one of the transformers available to the author, measured with an industrial LC meter, turned out to be about 3 H. When a DC bias current of only 5 mA was applied, the readings became about 450 mH, i.e., the inductance decreased by 7 times! But in real working devices, the current through the coils almost always has a constant component. The described meter showed the inductance of the winding of this transformer to be 1.5 H. And it remains to be seen which figure will be closer to real working conditions.

All of the above is true to one degree or another for all amateur LC meters without exception. It’s just that their authors are modestly silent about it. Not least for this reason, the function of measuring capacitance is found in many models of inexpensive multimeters, while only expensive and complex professional devices can measure inductance. In amateur conditions, it is very difficult to make a good and accurate complex resistance meter; it is easier to purchase an industrial one if you really need it. If this is impossible for one reason or another, I think the proposed design can serve as a good compromise with an optimal ratio of price, quality and ease of use.

LITERATURE

1. Stepanov A. Simple LC meter. - Radio, 1982, ╧ 3, p. 47, 48.
2. Semenov B. Power electronics. - M.: SOLON-R, 2001.

Although I have a professional automatic bridge E7-8, it is too bulky and heavy - 35 kg!

Therefore, I wanted to try my hand at making a simple LC meter on a microcontroller. The simplest (but with claims to good quality of work) circuit was found on an outdated, but fairly affordable microcontroller 16F84A, LM311N and LCD indicator type 1601.

A 90x65 mm printed circuit board version of this LC meter from YL2GL (I did not install jumper J3 on the board (there is no need for it) - the backlight of the LCD indicator 1601, if it has one, is constantly on!):

View of some of the parts for which the printed circuit board is designed:

One of the options for the LC meter printed circuit board made using the LUT method:

Four versions of the firmware file in *.hex format for programming the PIC 16F84A are placed in the File Catalog of the site (the third version of the firmware is recommended, as the version with auto-calibration of the device...):

Programming the PIC 16F84A can be done using a simple JDM programmer connected to the COM1 port of the computer (you must remember that the JDM programmer works well with older computers, but with the newest ones - dual-core and all types of laptops, notebooks, it may not work, since they are forced to limit the current on the COM port contacts. Therefore, look for a computer that will work with the JDM programmer without problems, or make the programmer according to a different scheme - with external power supply):

and ICprog programs.

Taking into account the purchase of LCD indicator 1601 for:

I would like to note from the device diagram that you need to pay attention to the presence or absence of a 10...12 Ohm resistor installed on the LCD indicator board 1601 in the backlight circuit. If it is missing, it must be soldered in series with the backlight, otherwise you can simply burn it out when installing jumper J3!

There are two LC meter circuits, differing in the circuit for connecting the low-voltage relay winding. In the second circuit, the relay winding is connected to ground through a quenching resistor, and not to +5V:

PIC 16F84A firmware is given under the first version of the circuit, located at the beginning of the article. They can, of course, work with the latest version of the circuit, but a “-” sign will appear before the readings of the capacitance and inductance values.

After assembling the LC meter, the device starts up the first time it is turned on. For a single-line LCD indicator 1601, jumper J1 must be closed. For two-line, type 1602 - leave open. Use a 10K trimmer to adjust the contrast of the LCD display. The closer the resistor slider is to ground, the higher the contrast of the display.

After the first turn on, you need to check the generator frequency at the output of LM311N by closing jumper J2 with the L/C switch positioned at C.

The frequency on the LCD screen should be around 550 kHz.

Then, use a short jumper to connect the device sockets in mode L.

The device writes - Calibrating and after a second goes into measurement mode: L=0.00 mkH.

We take out the jumper, insert the measured reference inductance into the sockets and look at the readings of the device. If the value differs from what we measured on the reference device, then we select more precisely the inductance of 82 μH of the device.

Therefore, it is advisable to use a choke with the ability to adjust the inductance (ferrite frame with a tuning core).

Then we switch to the capacitance measurement mode C.

The LCD indicator will display C=x.x pF

Briefly press the SW1 button - calibration.

Although I have a professional automatic bridge E7-8, it is too bulky and heavy - 35 kg!

Therefore, I wanted to try my hand at making a simple LC meter on a microcontroller. The simplest (but with claims to good quality of work) circuit was found on an outdated, but fairly affordable microcontroller 16F84A, LM311N and LCD indicator type 1601.

A 90x65 mm printed circuit board version of this LC meter from YL2GL (I did not install jumper J3 on the board (there is no need for it) - the backlight of the LCD indicator 1601, if it has one, is constantly on!):

View of some of the parts for which the printed circuit board is designed:

One of the options for the LC meter printed circuit board made using the LUT method:

Four versions of the firmware file in *.hex format for programming the PIC 16F84A are placed in the site’s File Catalog (the third firmware version is recommended, as a version with auto-calibration of the device when turned on):

Programming the PIC 16F84A can be done using a simple JDM programmer connected to the COM1 port of the computer (you must remember that the JDM programmer works well with older computers, but with the newest ones - dual-core and all types of laptops, notebooks, it may not work, since they are forced to limit the current on the COM port contacts. Therefore, look for a computer that will work with the JDM programmer without problems, or make the programmer according to a different scheme - with external power supply):

and ICprog programs.

Taking into account the purchase of LCD indicator 1601 for:

I would like to note from the device diagram that you need to pay attention to the presence or absence of a 10...12 Ohm resistor installed on the LCD indicator board 1601 in the backlight circuit. If it is missing, it must be soldered in series with the backlight, otherwise you can simply burn it out when installing jumper J3!

There are two LC meter circuits, differing in the circuit for connecting the low-voltage relay winding. In the second circuit, the relay winding is connected to ground through a quenching resistor, and not to +5V:

PIC 16F84A firmware is given under the first version of the circuit, located at the beginning of the article. They can, of course, work with the latest version of the circuit, but a “-” sign will appear before the readings of the capacitance and inductance values.

After assembling the LC meter, the device starts up the first time it is turned on. For a single-line LCD indicator 1601, jumper J1 must be closed. For two-line, type 1602 - leave open. Use a 10K trimmer to adjust the contrast of the LCD display. The closer the resistor slider is to ground, the higher the contrast of the display.

After the first turn on, you need to check the generator frequency at the output of LM311N by closing jumper J2 with the L/C switch positioned at C.

The frequency on the LCD screen should be around 550 kHz.

The readings on the display will be without one zero - 55000.

If you have containers with a 1% spread indicated on them, then you can use them.

It is better to start setting up the device in the capacitance measurement mode - C.

Press the SW1 button - calibration.

The inscription Calibrating will appear briefly on the device screen and the readings on the screen will be reset to C=0.0 pF.

We insert a reference capacitance into the sockets and if the instrument readings differ from the required value, then select the capacitance in series with the contacts of the low-voltage relay, repeating the calibration of the device each time.