LC meter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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LC meter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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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. In this article, the reader is offered a description of a simple measuring device that every radio amateur should have in the laboratory.

The principle of operation 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 an amateur design, this method was described in [1], and in subsequent years, with minor changes, 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, just connect the measured element and read the readings from the indicator.

Technical specifications

Measured capacitance range......0,1pF...5mkF
Measured inductance range......0,1 μH...5 H
Error of the measured value, no more, %......±3
Supply voltage, V ...... 7,5 ... 9
Consumption current, mA, no more ...... 15
Automatic range selection
Software Zero
Dimensions, mm......140x40x30

The schematic diagram of the device is shown in fig. one

(click to enlarge)

The excitation voltage signal of a rectangular shape from pin 6 (PB1) of the microcontroller DD1 through the three lower buffer elements DD2 according to the scheme is fed to the measuring part of the device. During a high voltage level, the measured capacitor Cx is charged through a resistor R9 and a 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, the device converts using the operational amplifier DA1 into voltage. Capacitors C5 and C7 smooth out its ripples. Resistor R14 is used to accurately zero the op-amp.

When measuring the inductance during a high level, the current in the coil rises to the value determined by the resistor R10, and during a low level, the current created by the self-inductance EMF of the measured coil is also fed to the input of the DA4 microcircuit through VD11 and R1.

Thus, with a constant supply voltage and signal frequency, the voltage at the output of the op-amp is directly proportional to the values of the measured capacitance or inductance. But this is only true on the condition that the capacitor is fully charged during half the period of the excitation voltage and also completely discharged during the other half. The same is true for the inductor. The current in it should have time to grow to a maximum value and fall to zero. These conditions can be ensured by an appropriate choice of resistors R9-R11 and the frequency of the exciting voltage.

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

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

The instrument 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 improve accuracy, the device has nine measurement sub-ranges. The frequency of the excitation voltage in the first subband is 800 kHz. At this frequency, capacitors with a capacitance up to about 90 pF and coils with an inductance up to 90 μH are measured. At each subsequent subrange, the frequency is reduced by 4 times, respectively, the measurement limit is expanded by the same number of times. On the ninth subrange, the frequency is 12 Hz, which ensures the measurement of capacitors with a capacitance of up to 5 μF and coils with an inductance of up to 5 H. The device selects the desired subrange automatically, and after turning on the power, the measurement starts 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 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 character space, and the result is rounded to three significant figures.

The red HL1 LED is used as a 1,5 V stabistor to power the indicator. The SB1 button is used for software zero correction, which helps to 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 in operation. Resistor R7 is designed to quickly discharge capacitors C9 and C10 when the power is turned off. Without it, switching on again, which ensures the 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 fig. 2.

LC Meter

The HG1 indicator and the SB1 button are installed from the installation side and brought to the front panel. The length of the wires to the SA1 switch and the input terminals should not exceed 2 ... 3 cm. VD3-VD6 diodes are high-frequency with a low voltage drop, D311, D18, D20 can be used. Trimmer resistors R11, R12, R14 small-sized type SPZ-19. Replacing R11 with a wire resistor is undesirable, as it will lead to a decrease in measurement accuracy. The 140UD1208 chip can be replaced with some other op-amp that has a zero-setting circuit and can operate on a voltage of ±5 V, and the K561LN2 can be replaced with any CMOS chip 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 it is impossible to replace it with another type, for example, the popular AT90S2313, without adjusting the program.

The value of the capacitances of capacitors C4, C5, C11 should not be reduced. Switch SA1 should be small and with a minimum capacitance between the outputs.

When programming the microcontroller, all FUSE bits should be left at their defaults: 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 provide an accurate setting of the clock frequency of 1,6 MHz and, accordingly, the frequency of the excitation voltage for the measuring circuit on the first range of 800 kHz. In the copy of ATtinyl 5L that the author had, the calibration byte is $8B.

Microcontroller firmware codes

For adjustment, 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 at face value. If possible, their exact values should be measured with an industrial LC meter. These will be your "reference" elements. Considering that the scale of the meter is linear, in principle, one capacitor and one coil are sufficient. But it is better to control the entire range. Normalized chokes of types DM, DP are well suited as exemplary coils.

The adjustment begins with zeroing the DA1 chip, controlling the voltage at its output with a multimeter. This voltage should be set within 0 ... + 5 mV with resistor R14. The slider of the resistor R12 should be in the middle position, and it is advisable to disconnect the SA1 switch from the board to reduce the parasitic capacitance of the input. In this case, the indicator readings should be within 0...3. Then restore the connection SA1, press and release the button SB1. After 2 s the indicator should show 0...±1. After that, an exemplary capacitance is connected to the input terminals and, by rotating the R12 slider, the readings are set to correspond to the true value of the capacitance of the selected capacitor. The price of the least significant digit is 0,1 pF. Then it is necessary to check the entire range and, if necessary, to clarify the position of the R12 engine, trying to get an error no worse than 2 ... 3%. Zero adjustment is also acceptable if the readings at the end of the scale are slightly underestimated or overestimated. But after each change in the position of the R14 slider, the measured capacitor should be turned off and the zero setting button should be pressed.

Having set up the device in the capacitance measurement mode, you should move SA1 to the lower position according to the diagram, close the input jacks and press SB1. After zero correction to the input, connect the exemplary coil and set the required readings with resistor R11. The price of the least significant digit is 0,1 μH. In this case, you should pay attention that the resistance R11 is at least 800 ohms, otherwise you should reduce the resistance of the resistor R10. If R11 is greater than 1 kOhm, R10 must be increased, i.e. R10 and R11 must be close in value. This setting provides approximately the same time constant for "charging" and "discharging" the coil and, accordingly, the minimum measurement error.

An error 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, the losses in the magnetic circuits due to eddy currents, hysteresis, the magnetic permeability of ferromagnets depends nonlinearly on the magnetic field strength, etc. Coils during measurement are affected by various external fields, and all real ferromagnets have rather high value of residual induction. The processes occurring during the magnetization of magnetic materials are described in more detail in [2]. As a result 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 that measures the complex resistance at a fixed frequency. But do not rush to scold this device and its author. You just have to take into account the peculiarities of the principle of measurement. For coils without a magnetic core, for non-closed magnetic cores and for ferromagnetic cores with a gap, the measurement accuracy is quite satisfactory if the active resistance of the coil does not exceed 20 ... 30 Ohm. And 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 a thin wire and a closed magnetic circuit without a gap (especially from transformer steel), there will be a large error. But after all, in a real device, the operating conditions of the coil may not correspond to the ideal that is provided when measuring the complex resistance. For example, the winding inductance 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 a factor of 7! And 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 1,5 Gn. And it is still unknown which figure will be closer to real working conditions.

All of the above is true to some extent for all amateur LC meters without exception. It's just that their authors are modestly silent about it. Not least for this reason, the capacitance measurement function is available in many models of inexpensive multimeters, and 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 not possible 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

Stepanov A. Simple LC-meter. - Radio, 1982, No. 3, p. 47, 48.
Semenov B. Power electronics. - M.: SOLON-R, 2001.

Author: I. Khlyupin, Kirov

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