Attachment to the multimeter for measuring the capacitance of capacitors. Scheme, description

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Attachment to the multimeter for measuring the capacitance of capacitors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Digital multimeters are increasingly common in the amateur radio lab. The simplest of them are relatively inexpensive and have acceptable characteristics. Having made simple prefixes for such a multimeter, you can expand its functionality. Description of one of these attachments for measuring the capacitance of capacitors, the author offers to the attention of readers.

Using a simple attachment to a digital multimeter, you can measure the capacitance of capacitors in the range of 2 pF ... 200 uF. It is assembled on two microcircuits, one of which is an integral timer.

The attachment diagram is shown in fig. 1. The principle of its operation is based on the periodic charging of the measured capacitor to a fixed voltage and its subsequent discharge through a reference resistor. On the DA2 chip, a rectangular pulse generator is assembled, the frequency of which is set by selecting one of the current-setting resistors R1-R8 and capacitors C3, C4 by switch SA1; using the SA1.3 section, the reference resistors R12-R15 are switched. The amplitude of the generator pulses on the DA2 chip is supported by the integrated voltage regulator on DA1.

Attachment to the multimeter for measuring the capacitance of capacitors

The attachment works as follows. After connecting the tested capacitor Cx to the XS3 sockets, at the moment a voltage pulse appears at the DA2 output, it is quickly charged through the VD2 diode. During a pause, the capacitor is discharged through the reference resistance, and in this case a pulse is formed, the duration of which is proportional to the capacitance of the capacitor Cx. These pulses are fed to the integrating circuit R11C5, at the output of which a voltage is generated that is proportional to the duration of these pulses and, accordingly, the capacitance of the measured capacitor. A multimeter is connected to the output of this circuit in the voltage measurement mode at the limit of 200 mV.

The generator generates pulses with a repetition rate of approximately 25 kHz (position 1 of the SA1 switch, subrange 20 pF); 2,5 kHz (position 2, 200 pF); 250 Hz (position 3, 2000 pF) and 25 Hz (positions 4-8, subranges 0,02-200 µF). To increase efficiency, the power supply to the attachment is supplied through the SB1 button only for the duration of the measurement. This allows you to power the device from an autonomous source, for example, batteries "Krona", "Korund", "Nika" 7D-0,125. The maximum current consumed by the attachment when measuring the capacitance of polar capacitors in the 200 μF subrange is 25 ... 30 mA. On the sub-range of 20 μF, it decreases by about one and a half times, and on the rest it is 10 ... 12 mA. Diode VD1 protects the prefix from applying reverse polarity voltage.

Most of the parts of the console are placed on a printed circuit board with dimensions of 32 (24 mm) made of one-sided foil fiberglass, a sketch of which is shown in Fig. 2, the arrangement of elements is in Fig. 3. The board is placed in a metal or plastic case. A switch, a button, and also sockets and connectors.The rest of the parts are mounted either on sockets or on the switch and button by surface mounting.

Attachment to the multimeter for measuring the capacitance of capacitors

Parts can be used in the device: DA2 - M1006VI1 (but you will have to adjust the printed circuit board), diodes - any impulse, polar capacitors C1, C2 - groups K50, K52, K53, C3 - K73, C4 - KM, K10-17. Trimmer resistors - SP3-19 or similar, constant - MLT, S2-33. Button SB1 with self-return (non-latching) of any type, for example KM, switch - PG2 or similar in three directions and at least eight positions. Connector sockets X1, X2, X4, X5 - any suitable for connecting cords, half of the socket for the microcircuit was used as the XS3 connector.

The set-top box is set up in conjunction with the multimeter with which it is supposed to be used. Reference capacitors will be required, the capacitance of which was previously measured with an accuracy of no worse than 1 ... 2%. For each sub-range, such a capacitor is needed with a capacitance corresponding to the limit value or somewhat less. After checking the correct installation and performance of the attachment, its adjustment begins with a subrange of 20 pF. For this, a reference capacitor is connected and a trimmer resistor R1 achieves multimeter readings (at the measurement limit of 200 mV) corresponding to the capacitance of the capacitor. A similar procedure is carried out on the 200 pF subrange, but this time using the resistor R3. The prefix is also calibrated in the next subrange of 2000 pF with resistor R5, and in the subrange of 0,02 μF with resistor R7. If the change in the resistance of the tuning resistors is not enough to obtain a calibration, you will have to change the resistance of the corresponding fixed resistor (R2, R4, R6, R8). After calibration at the specified measurement limits, the trimming resistor sliders can no longer be moved.

On subranges with limits from 0,2 μF to 200 μF, the calibration of the set-top box is carried out by selecting resistors R12-R15, respectively, they are placed directly on the SA1 switch. In this case, the resistors R12-R15 may have to be made up of at least two connected in series.

If the tuning is carried out carefully using capacitors whose capacitance is measured with the above accuracy, then the error in measuring the set-top box together with a good multimeter will be no more than 5%, with the exception of the first and eighth subranges. In the first subrange, when measuring capacitors with a capacity of less than 5 pF, the error increases to 20 ... 30% due to the influence of the mounting capacitance and the VD2 diode, but this error can be easily taken into account. In the last sub-range, due to the influence of the output resistance of the DA2 microcircuit, the error also increases to 20 ... 30%, but it can also be taken into account.

Author: I. Nechaev, Kursk

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