LC meter attachment for digital voltmeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The digital measuring instrument in the amateur radio laboratory is no longer a rarity. However, it is not often possible for them to measure the parameters of capacitors and inductors, even if it is a multimeter. The simple prefix described here is intended for use in conjunction with multimeters or digital voltmeters (for example, M-830V, M-832 and the like) that do not have a mode for measuring the parameters of reactive elements.

To measure the capacitance and inductance using a simple attachment, the principle described in detail in A. Stepanov's article "Simple LC-meter" in "Radio" No. 3 for 1982 was used. The proposed meter is somewhat simplified (instead of an oscillator with a quartz resonator and a decade frequency divider multivibrator with a switchable generation frequency), but it allows measuring the capacitance within 2 pF ... 1 μF and the inductance 2 μH ... 1 H with sufficient accuracy for practice. In addition, it generates a rectangular voltage with fixed frequencies of 1 MHz, 100 kHz, 10 kHz, 1 kHz, 100 Hz and adjustable amplitude from 0 to 5 V, which expands the scope of the device.

The master oscillator of the meter (Fig. 1) is made on the elements of the DD1 (CMOS) microcircuit, the frequency at its output is changed using the SA1 switch within 1 MHz - 100 Hz by connecting capacitors C1-C5. From the generator, the signal is fed to an electronic key assembled on a transistor VT1. Switch SA2 select the measurement mode "L" or "C". In the position of the switch shown in the diagram, the attachment measures the inductance. The measured inductor is connected to sockets X4, X5, the capacitor - to X4, X6, and the voltmeter - to sockets X7, XXNUMX.

(click to enlarge)

During operation, the voltmeter is set to the DC voltage measurement mode with an upper limit of 1 - 2V. It should be noted that at the output of the set-top box, the voltage varies within 0 ... 1 V. On the sockets X1, X2 in the capacitance measurement mode (switch SA2 - in position "C") there is an adjustable rectangular voltage. Its amplitude can be smoothly changed by a variable resistor R4.

The set-top box is powered by a GB1 battery with a voltage of 9 V ("Corundum" or similar) through a stabilizer on a VT2 transistor and a VD3 zener diode.

The K561LA7 microcircuit can be replaced with K561LE5 or K561LA9 (excluding DD1.4), transistors VT1 and VT2 can be replaced with any low-power silicon of the corresponding structure, we will replace the VD3 zener diode with KS156A, KS168A. Diodes VD1, VD2 - any point germanium, for example, D2, D9, D18. Switches are desirable to use miniature.

The case of the device is home-made or ready-made of suitable sizes. Mounting parts (Fig. 2) in the case - hinged on switches, resistor R4 and sockets. The appearance option is shown in the figure. XZ-X5 connectors are self-made, made of sheet brass or copper with a thickness of 0,1 ... 0,2 mm, their design is clear from fig. 3. To connect a capacitor or coil, it is necessary to insert the leads of the part all the way into the wedge-shaped gap of the plates; this achieves fast and reliable fixation of the findings.

The adjustment of the device is carried out using a frequency meter and an oscilloscope. Switch SA1 is moved to the upper position according to the scheme and by selecting capacitor C1 and resistor R1 a frequency of 1 MHz is achieved at the output of the generator. Then the switch is sequentially transferred to subsequent positions and by selecting capacitors C2 - C5, the generation frequencies are set to 100 kHz, 10 kHz, 1 kHz and 100 Hz. Next, the oscilloscope is connected to the collector of the transistor VT1, the SA2 switch is in the capacitance measurement position. By selecting the resistor R3, the oscillation form is achieved, close to the meander on all ranges. Then the SA1 switch is again set to the upper position according to the diagram, a digital or analog voltmeter is connected to the sockets X6, X7, and an exemplary capacitor with a capacity of 3 pF is connected to the sockets X4, X100. By adjusting the resistor R7, a voltmeter reading of 1 V is achieved. Then the SA2 switch is switched to the inductance measurement mode and an exemplary coil with an inductance of 4 μH is connected to the sockets X5, X100, the voltmeter readings are also set to 6 V with resistor R1.

This completes the setup of the device. On the remaining ranges, the accuracy of the readings depends only on the accuracy of the selection of capacitors C2-C5.

It is better to start setting up the generator with a frequency of 100 Hz, which is set by selecting the resistor R1, the capacitor C5 is not selected. It should be remembered that capacitors C3 - C5 must be paper or, better, metal-film (K71, K73, K77, K78). With limited possibilities in the selection of capacitors, you can use the switching section SA1.2 resistors R1 and their selection, and the number of capacitors must be reduced to two (C1, C3). The resistance values ​​of the resistors will be in this case: 4,7: 47; 470 k0m.

Author:I. Potachin, Fokino, Bryansk region; Publication: N. Bolshakov, rf.atnn.ru

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