Selection of capacitors of the same capacity. Scheme, description

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Selection of capacitors of the same capacity. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Often, to work in the range of electrical oscillations of sound frequencies, it is necessary to select the same capacitors. These are various balanced or bridge circuits, and identical (in terms of electrical parameters) channels of stereo or quad amplifiers ...

In the simplest version, the selection of capacities can be carried out by connecting the "common" wires of the avometer (tester) and the sound generator to each other. Then the investigated capacitor should be connected between the "phase" wires. The avometer must be prepared for measuring alternating voltage in the range of 0,1-1 V. The output parameters of the ZG should be set so that the readings of the dial indicator fall on 0,4-0,7 V (in the named section, the scale is more linear, and the accuracy measurements above).

When everything is prepared, by replacing the capacitors in the measuring circuit and tracking the readings of the avometer (and they should be identical with equal capacitances), the equality of the capacitances of the tested capacitors or their difference is determined: with a smaller capacitance of the capacitor, the deviation of the instrument arrow will be smaller and vice versa. In the event of a break in the capacitor, the avometer will not show any voltage, and in the event of a short circuit (or a strong difference in capacitance upwards), the arrow of the device will go off scale.

Pointer avometers, as a rule, have an extended area of work in the frequency plan at the lower limits and are applicable for such measurements. The accuracy of the selection of identical capacities will be affected by the stability of the frequency and output voltage of the sound generator, the accuracy of setting the arrow of the avometer to the risks of one of its scales (and it is not at all necessary that the AC voltage scale be used for this). You can try to use a digital multimeter as an indicator (avometer). If it turns out that it is capable of operating at low AC voltages and elevated (relative to "industrial" 50 Hz) frequencies, then the accuracy of capacitance selection with it will even increase.

Accuracy in the selection of capacitors of the same capacity is guaranteed by the identity of the corresponding readings of the avometer and sound generator

The swaying of the arrow of the device may indicate instability of the output voltage of the sound generator or modulation of its signal by voltages of infra-low frequencies, both in amplitude and in frequency. The manifestation of "flickering" of the controlled capacitance, that is, its chaotic change in time, is also not excluded. Of course, the use of such capacitors in critical circuits is not recommended.

If it really "presses" and it will be necessary under the same "hardware conditions" to more accurately measure the capacitance, then it can be carried out using reference capacitors and a calibration table of the avometer scale. And for changing the measurement ranges, it turns out to be quite acceptable here "every decade" switching the frequency of the signal produced by the sound generator.

The stability requirements for Fout and Uout of the generator remain valid. The capacitance measurement limits depend on the parameters of the sound generator itself (output voltage and frequency), as well as on the parasitic capacitances of the connecting wires and are possible at frequencies up to 15-30 kHz.

Author: V.Besedin

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