Measurement of inductance with a combined instrument. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Combined instrument [1], modified in accordance with [2, 3], lacks one important function - the measurement of inductance. Meanwhile, inductors are perhaps the only home-made elements that radio amateurs have to deal with in their practice, either winding them on their own or using those made by someone. And if the parameters of factory-made radio elements are indicated on their cases or in the documentation, then the only way to obtain information about the inductance of a homemade coil is to measure it. Therefore, during the next refinement of the device, the author introduced the inductance measurement mode into it.

The chosen method for measuring inductance is as follows. Measuring coil L_x forms with the capacitor C, whose capacitance is precisely known, a parallel oscillatory circuit. This circuit is part of the generator of electrical oscillations, setting their frequency F. This frequency is measured by a frequency meter and the measured value is determined.

formula inductance

L_x = 25330/(C F²).

If the frequency is measured in megahertz and the capacitance is in picofarads, the inductance will be obtained in microhenries.

In order to reduce the cost of modernizing the combined instrument, the main condition for the practical implementation of this method in it, the author has set non-interference in the existing hardware. The device has a frequency counter mode, there is a microcontroller that can successfully perform the necessary calculations. Only the generator is missing, which is advisable to be made in the form of an external set-top box connected to the device through the connector already on it.

Radio amateurs often use similar prefixes to frequency meters to measure capacitance and inductance. At the same time, to simplify the calculations, an exemplary capacitance of 25330 pF is often chosen. In this case, the above formula takes the form

L_x = 1/F².

Examples of the use of such prefixes are given in [4, 5]. In the case under consideration, it is not necessary to use a capacitor of just such a capacity, since the microcontroller of the device is able to perform the calculation at any value.

The schematic diagram of the attachment is shown in fig. 1. It is similar to that used in [5], and small differences are associated with the use of parts of other types. The output signal of the set-top box is a sequence of rectangular pulses with an amplitude of about 3 V, following with a frequency equal to the resonant frequency of the measuring oscillatory circuit L_xC 1. The purpose of the circuit elements and the operation of the device are described in [4, 5] and therefore are not considered here.

Rice. 1. Schematic diagram of the set-top box (click to enlarge)

The attachment is connected to socket XS1 of the combined instrument panel with a three-wire flat cable. The refinement of the device itself was reduced to a change in the program of its microcontroller, which now, in addition to the previously available functions, provides for receiving the set-top box signal, processing it and displaying the value of the measured inductance on the LCD.

Main Specifications

• Measured inductance, μH ....... 8... 999000
• Measurement error, %, no more: from 8 μH to 15 μH ....... 5
• 15 µH to 20 mH .......2,5
• 20 mH to 150 mH.......5
• 150 mH to 999 mH.......20
• Inductance readout resolution, μH: from 8 to 999 μH .......0,1
• 1 to 999 mH.......10
• Supply voltage, V ....... 5
• Consumption current, mA....... 8

The interval of permissible values ​​of the measured inductance is limited by software. In principle, it is also possible to measure outside this interval, especially in the direction of large values, but the error increases significantly there.

The set-top box uses only surface-mounted components, which made it possible to place them on a 22x65 mm printed circuit board, the drawing of which is shown in fig. 2. Resistors and capacitors - size 1206.

Rice. 2. Printed circuit board

It is unacceptable to use a capacitor with a nominal capacity that differs from that indicated in the diagram as C1 (included in the measuring oscillatory circuit), since this can lead to program malfunctions. But there is no need to select its capacity with great accuracy. The selection is replaced by software calibration of the device. However, it is desirable to install a capacitor with a minimum TKE here, for example, with an NPO dielectric.

Place the finished board in a case of suitable dimensions. To connect the measured inductance, it is convenient to use a two-pin spring clip for acoustic systems.

In the combined instrument panel itself, it is necessary to carry out the modification described in [3], if it has not been done earlier. After it, +2 V voltage should be present on pin 1 of the XS5 connector. The codes from the attached file Osc-L-_2_04.hex should be loaded into the FLASH memory of the microcontroller.

After connecting the set-top box and supplying power, the main menu will appear on the LCD of the device (Fig. 3). To enter the inductance measurement mode, press the "GN" key twice. The first will put the device into generator mode, and the second into inductance measurement mode. The name of the mode will be displayed at the top of the LCD screen, and a hint will be displayed in its bottom line, from which it follows that key 2 must be pressed to perform calibration, and key D to measure inductance.

Rice. 3. Main menu

Calibration is required before using the instrument for the first time. In the future, it should be carried out only after the repair of the device or attachment, as well as in case of doubt about the correctness of the measurement results.

A few words about the content of calibration. To calculate the inductance according to the formula considered at the beginning of the article, it is necessary to know the exact value of the capacitance of the oscillatory circuit. But in addition to the capacitance of the capacitor C1, it also includes other components - the parasitic capacitances of other components and the mounting capacitance. At the first start of the program, the true value of the loop capacitance is unknown to the program and it operates with the nominal value of the capacitance of the capacitor C1 22000 pF. The task of calibration is to calculate the true capacitance of the attachment's oscillatory circuit in order to use this value in the future during measurements.

To do this, you need to connect to the console as L_x coil of precisely known inductance L_arr. By measuring the frequency of the signal generated by the attachment with such a coil, calculate the true capacitance of the oscillatory circuit using the formula

C = 25330/(L_arr.F²)

The program writes the value of this capacitance obtained during the calibration to the EEPROM of the microcontroller and then uses it to calculate the inductance. The accuracy of the calibration, and hence the subsequent measurements, depends on the accuracy of the value of the reference inductance. Therefore, it is necessary to know it with an error of no more than 1 ... 2%, for example, by measuring it with a verified instrument of the appropriate accuracy class.

When calibration starts, a message is displayed on the screen (Fig. 4) with a suggestion to connect a reference inductance to the attachment, enter its value and perform calibration, or refuse it. It is recommended to choose a reference inductance within the limits indicated on the screen, since in this case the measurement error is minimal. If you make a mistake while entering a value, you can press the # key to enter it again.

Rice. 4. Message when starting calibration

Having completed the calibration, the device automatically measures the reference inductance and displays its value on the screen (Fig. 5). If you refuse to calibrate, the measurement of the reference inductance will also be performed, but with an uncalibrated device with an unreliable result.

Rice. 5. The value of the measured inductance on the device screen

To measure an unknown inductance, you need to connect it to the attachment and press the D key of the device. If you try to measure the inductance, the value of which is outside the allowable limits for the device, a message will be displayed on the screen about the refusal to measure for this reason.

Exit the inductance measurement mode by pressing one of the OS, LA or GN keys, which transfer the device to the corresponding operating modes.

Modified microcontroller program: ftp://ftp.radio.ru/pub/2017/01/osc-L-2-04.zip.

Literature

1. Savchenko A. Combined instrument based on ATxmega microcontroller. - Radio, 2014, No. 4, p. 18-22; No. 5 p. 22-25.
2. Savchenko A. Improvement of the combined instrument based on the ATxmega microcontroller. - Radio, 2015, No. 3, p. 29-34.
3. Savchenko A. New modes in the combined measuring instrument. - Radio, 2015, No. 9, p. 17-19.
4. Belenetsky S. Prefix for measuring inductance in the practice of a radio amateur. - Radio, 2005, No. 5, p. 26-28.
5. Zorin S., Koroleva I. Radio amateur frequency meter. - Radio, 2002, No. 6, p. 28, 29; No. 7, p. 39, 40.

Author: A. Savchenko

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