Simple amateur radio devices for measuring inductance. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In a foreign amateur radio magazine [1], two schemes of devices for measuring inductance were published. Given that since 1991 this magazine has not been delivered to the CIS through the Soyuzpechat system, and the schemes are easy to repeat, it is advisable to briefly familiarize the readers of the magazine with them. I am sure that the schemes are of practical interest to radio amateurs.

In many cases of practical activity of radio amateurs, it is interesting for them, and in some cases it is necessary, to measure the inductance of inductors or similar radio components that they would like to use in their designs. In the overwhelming majority of cases, simple industrial devices for these purposes are not available, while complex and, accordingly, expensive devices are inaccessible to a wide range of radio amateurs. In both cases, the inductance is usually measured indirectly. It is converted into an "equivalent" DC voltage, as is done in the circuit of Fig. 1, or into a frequency-dependent pulse voltage - fig. 3.

Rice. 1 (click to enlarge)

On the element IC2-A, the master circuit generator is made (Fig. 1). As IC2, a CD4584 chip was used, containing six Schmitt triggers. This microcircuit is found on the radio market, but, alas, it is not very common with us at present. If there are difficulties with its acquisition, then it is advisable to try to use the domestic 1564TL2 microcircuit or the imported 54NS14. K561TL1 microcircuits (1561TL1, 564TL1) are very common, but they are less "capacious" in terms of the number of Schmitt triggers in one package - there are only four of them. You will have to use two cases of these microcircuits.

The inputs and outputs of IC2-B-IC2-D are paralleled. This is done to power the output of the master oscillator, since it is loaded with a low-resistance inductance Lk and resistor R2.

The measured inductance is connected to contacts 1-2 of terminal block K3. Through the resistor RZ, the voltage from the inductor Lk is fed to the input of a pair of inverters IC2-E and IC2-F. The output of the last of these inverters is connected to the integrating circuit R4C2. This circuit smooths out the ripple of the output voltage of IC2-F, so that on pins 1-2 of the output block K2 we get almost a DC voltage. Any high-resistance voltmeter is connected to this block (K2), for example, a DT830-B amateur radio tester.

The 9 V voltage supplying the entire device is supplied to the K1 block. Then it is stabilized at 5 V by IC1 type 78L05. In practice, you can use other types of stabilizers that have a slightly higher output voltage, for example 7806 or 7808.

The authors of the article [1] considered it expedient to slightly increase the potential of the lower plate of the capacitor C2 according to the scheme relative to the circuit case, bringing it closer to the potential of the upper plate of the capacitor C2. For this, a potentiometer R2 and a voltage divider R5R6 are used.

Now a few words about the parameters of the inductance meter. The device is designed to measure inductance in the range from 200 µH to 5 mH. In the case when a radio amateur needs to measure an inductance that is slightly different from the agreed range, this possibility, of course, exists. It is enough to have in your stock several inductors with pre-measured parameters. For example, having an inductance of 200 µH, you can connect tested inductances up to 200 µH in series with it and measure the total inductance. Then, subtracting 200 μH from the obtained measurement result, we find out the value of the unknown small inductance.

If the expected value of the measured inductance is assumed to be more than 5 mH, then during measurements it is necessary to connect a calibration inductor in parallel with the tested one, for example, with a value of 5 mH. The measurement result will be less than 5 mH, and it will be necessary to calculate the value of the checked inductance from it.

It is known that the total inductance of two inductors connected in series or in parallel changes in the same way as when connecting resistors. This principle of "expanding" the measurement range of the described inductance meter can and should be used in practice.

Potentiometer P1, when adjusting the device, achieves a reading of 500 mV of the DMM tester, if a pre-measured and selected inductance of 5 mH is connected to the short circuit block.

If a 1mH inductance is connected to the instrument, the DMM will display 100mV.

Potentiometer P2 sets the output voltage of the device, measured by DMM, at 0 V, if terminals 1-2 of K3 are closed.

On fig. 2 shows a drawing of the printed circuit board of the device and the location of parts on it.

Fig. 2

In the event that a radio amateur cannot purchase a CD4584 chip or experiment with replacing this chip, it is advisable for him to perform an inductance meter circuit according to fig. 3.

Fig. 3

To work with this circuit, you will need a frequency meter - a frequency meter. This device is not so scarce, since many radio amateurs were previously fond of manufacturing combined devices based on electronic clocks. As a rarity, I have a combined device - a clock / frequency meter / pulse counter / frequency meter of the input signal of the radio receiver at the local oscillator frequency. And the size of the "combine" does not exceed two packs of cigarettes! True, without taking into account the power source.

In the scheme of Fig. 3, an astable multivibrator is made on the IC1 chip of the NE555 type. The scheme is extremely simple. The range of measured inductances is from 500 µH to 10 mH.

The input supply voltage can be, for example, 9 ... 12 V. It is stabilized by IC2 type 78L05 at 5 V.

The measured inductance Lk is connected to terminals 1-2 K1. The larger the inductance value, the lower the frequency of IC1 generation. If a 500 µH inductance is connected, then the oscillator frequency must be set by adjusting P1 to 200 kHz. It should be borne in mind that for generation frequencies of more than 200 kHz, the linearity (accuracy) of the device operation deteriorates.

If a measured inductance is connected to the device, then its value is calculated by the formula:

L = 200 kHz/f (measurements) x 500 µH.

So, for example, if the frequency meter showed a frequency of 27 kHz when connected to an unknown inductance circuit, then its calculated value will be as follows:

L = 200 kHz / 27 kHz x 500 uH = 3,704 mH.

The average measurement error in the indicated range of inductances does not exceed 4% with a qualitative adjustment of the circuit.

On fig. 4 shows a drawing of the printed circuit board of the device and the location of the radio components on it.

Fig. 4

Literature

1. Pripravek pro mereni indukcnosti // Amaterske RADIO. - 2008. - No. 7. - S.15-16.

Author: E.L. Yakovlev, Uzhgorod

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