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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Attachment to the multimeter for measuring the capacitance of capacitors
Encyclopedia of radio electronics and electrical engineering / Measuring technology The operation of the attachment (Fig. 1) is based on a well-known principle - first, the capacitor Cx is charged to a stable voltage U, then it is discharged through a current meter. If such charge-discharge cycles are performed at a frequency E, the average current I through the meter will be I = UFCX. It is convenient to use the following dimensions in this formula: microamps, volts, hertz, microfarads.
The described prefix has five measurement limits - 2000 and 20000 pF, 0,2, 2 and 20 microfarads. The current meter is the M-832 multimeter, operating in the mode of a DC millivoltmeter with a limit of 200 mV, supplemented by shunts installed in the attachment. The recharge frequencies of the tested capacitor are chosen to be 5 kHz at the first measurement limit, 500 Hz at the next two and 50 Hz at the last. At a voltage to which the capacitor is charged, equal to 3 V, the current through the meter, corresponding to the maximum measurable capacitance and calculated according to the above formula, is 30 μA at the first two limits, 300 μA at the next two and 3 mA at the last. The prefix (Fig. 2) is connected to three sockets of the multimeter - to its inputs "VΩmA" and "COM" (Common), as well as to the "E PNP" socket for connecting the emitter of the pn-p transistor when measuring transistor parameters.
The generator that determines the recharge frequency of the tested capacitor is assembled on one inverting element - the Schmitt trigger DD1.1, and the switch that alternately connects the capacitor Cx to the plus of the power source and to the current meter - on the CMOS switches of the DD2 microcircuit. To reduce the resistance of public keys, both channels of the microcircuit are connected in parallel. At a low level at input 1 of the microcircuit, its conclusions 13 and 3 are connected to the outputs XO and Y0, respectively, the tested capacitor Cx is charged to a voltage of 3 V. When a positive polarity pulse arrives at this input, these conclusions are connected to the outputs X1 and Y1, the capacitor Cx is discharged through one of the shunts R6 - R9. To power the set-top box, an internal multimeter stabilizer with a voltage of about 3 V was used. It was removed from its "E PNP" and "COM" sockets. However, the keys of the K2KP561 chip used as DD1 at a supply voltage of 3 V pass signals well only with "digital" levels, that is, close to the supply voltage and the common wire. With a smoothly changing switching voltage near half the supply voltage, the resistance of the switch transistors increases rapidly and the capacitor Cx does not have time to recharge. To increase the supply voltage, a converter was introduced into the set-top box on the DA1 chip and capacitors C1 - C4, which generates a voltage of -3 V relative to the common wire. The operation of such a converter is described in the author's article "Voltage Converters on Switched Capacitors", published in "Radio", 2001, No. 12, p. 44, 45. The output voltage of the converter is added to the output voltage of the multimeter stabilizer and is used to power the DD1 and DD2 microcircuits. Resistors R1 - R3, switched by the switch section SA1.1, together with the capacitor C5 determine the frequency of the generator. The output capacitance of the keys, the mounting capacitance of the circuit connected in parallel with the capacitor being tested, the input capacitance of the multimeter increase the meter reading by about 40 pF. To exclude such a shift in readings, resistors R4 and R5 are introduced, by selecting which you can compensate for the error in readings. The prefix is assembled on a printed circuit board (Fig. 3) from one-sided foil fiberglass 1 mm thick.
Resistors MLT, S2-23, KIM (R5), capacitors K50-16 (C3, C4), imported analog K50-35 (C1), KM-6 (C2), K73-9 for a voltage of 100 V (C5) were used. You can use any other resistors and capacitors that are suitable in size, but the C5 capacitor must be metal film (K73 series) or paper, installation of ceramic capacitors is unacceptable due to their low temperature stability. Switch SA1 - PR2-5P2N, PG2-2-6P2N, PG2-9-6P2N, P2G-3-5P2N, P2G-3-6P2N, PGZ-5P2N or any other small-sized switch for the required number of positions and directions. Chips of the K561 series are interchangeable with similar KR1561 series, and the KR1168EP1 chip can be replaced with its imported analogue ICL7660 or ICL7660A. To simplify the connection of the set-top box to the multimeter sockets, two split pins with a diameter of 4 mm are fixed on the board with nuts from the plugs ("VΩmA" and "COM" circuits) and a brass pin with a diameter of 0,8 mm is soldered ("E PNP" circuit). The switch is mounted on a bracket made of brass 1 mm thick. The bracket is fixed on the board with a COM pin nut and an M2,5 screw with a nut, for which a corresponding hole is provided on the board. To connect the capacitor under test, two sockets are soldered to the board from the 2PM connector for pins with a diameter of 1 mm. These pins with perpendicularly soldered alligator clips can be inserted into them, which will allow you to connect measured capacitors of various sizes. The board is covered with a casing soldered from foil fiberglass and fixed to the board at the corners by soldering. The casing foil is connected to a common wire and acts as a screen. When manufacturing a board for the operation of a set-top box with a multimeter of a different type, the location of the contact pins should be clarified. In order to facilitate tuning, there are two seats on the board for each selection resistor. The relatively low-resistance shunt resistors R6 - R9 are made up of two connected in parallel, and the high-resistance shunts R1 - R5 are made up of two connected in series. Set up the attachment in the following order. First, all the elements are installed on the board, except for the resistors and the bracket with the switch. In the holes of the board, marked in Fig. 3 with the inscriptions "to SA1.1" and "to SA1.2", and into the left (according to Fig. 3) output of the resistor R3 and the lower R9 (common wire) are soldered along a piece of hard copper wire about 40 mm long. Between terminal 5 DD2 and a common wire (to the corresponding pair of wire segments), a resistor with a nominal value of 680 ohms and a tolerance of at least ± 10% is soldered. A capacitor with a capacity of 1 ... 2 microfarads is included in sockets X1, X1,5, and between terminals 9 and 10 of the DD1 microcircuit (also to the corresponding segments), a constant resistor with a resistance of 1,5 MΩ is soldered in series with a variable 470 kΩ. For this tuning step, the accuracy of the capacitance of the capacitor is not important. Set the switch of the multimeter to the "200 mV" position and insert the attachment with the pins into the corresponding sockets of the multimeter. Measure the voltage at pins 14 and 7 of the DD1 chip with any voltmeter relative to the common wire (COM) - it should be +3 and -3 V, respectively. They are convinced of the presence of generation with a frequency of the order of 50 Hz using an oscilloscope connected in parallel with Cx, or, in its absence, by connecting any piezo emitter there. The readings of the multimeter should approximately correspond to the capacitance of the capacitor, but may change randomly within certain limits. By smoothly turning the shaft of the variable resistor, maximum stability of the multimeter readings is achieved (reading fluctuations within 0,5% of the measured value are allowed). In this case, the generator frequency should be equal to 50 Hz - it is advisable to check it with an oscilloscope or frequency meter. Input voltage ripples with this frequency (and multiples of it) are well suppressed by the analog-to-digital converter of the multimeter, and when deviated from it, they manifest themselves in the above-mentioned chaotic change in readings. The total resistance of the constant and variable resistors is measured and a constant of the same resistance is selected. If this is difficult to do, you can take a resistor of a slightly lower resistance, and turn on the variable in series with it. Repeat the adjustment for no change in readings, and measure the resistance of only the variable resistor. Replace the variable with a constant of the same resistance - high accuracy is not required here. By replacing the Cx capacitor with a precisely known capacitance of 1,5 ... 1,9 uF, they achieve the corresponding readings on the multimeter display by selecting the resistor R8. For convenience, you can take a resistor of a slightly larger resistance and connect a 22 kΩ variable in parallel with it. By measuring the resistance of the input part of the variable resistor, select the appropriate constant. Further, without changing the frequency of the generator and using a capacitor of known capacity of about 10 microfarads, the resistor R9 is selected similarly. Having soldered the selected resistor R8 and included a reference capacitor with a capacity of 0,15 ... 0,19 μF in the sockets, the resistor R2 is selected. In this case, the generator frequency should be about 500 Hz. Having retained such a generator frequency and a reference capacitor, a resistor R7 is selected. It should be borne in mind that the readings of the set-top box will be overestimated by about 40 pF, therefore, say, the reference capacitor of 0,015 uF should correspond to the readings of 1504. Remove the shift in the readings by selecting the resistor R5. Next, select a resistor R6 of the same resistance as R7. Having inserted a reference capacitor with a capacity of 1500 ... 1900 pF into the sockets, they select the resistor R3, and to eliminate the shift in readings, the resistor R4. If there is a digital frequency meter, you can first set the generator frequencies to 50, 500, 5000 Hz by selecting resistors R1, R2 and R3, respectively, and then select resistors R6 - R9 using the reference capacitors of the above capacitance. The selected resistors are soldered into the board, the switch is installed on the bracket and its outputs are connected to the board. With careful selection of resistors, the measurement accuracy at the first four limits will be no worse than 2%, at the limit of 20 μF, linearity is maintained up to 10 μF, and with a capacitance of 20 μF, the readings will be underestimated by about 8%. In the absence of a KR1168EP1 or ICL7660 microcircuit, it is advisable to power the -3 V circuit of the set-top box from the multimeter battery through a -6 V voltage stabilizer, which can be used as a KR1168EN6 or 79L06 microcircuit with any prefixes and suffixes (Fig. 4). To do this, install a small-sized socket on the multimeter case, connecting it to the negative terminal of the battery. The output "Input" of the DA2 chip must be provided with a flexible conductor with a plug, which is included in the additional socket of the multimeter.
The prefix can be used as a pulse generator with frequencies of 50, 500 and 5000 Hz and an amplitude of 3 V, removing them from the terminals intended for connecting the tested capacitor. It should be remembered that the output resistance of such a generator is not less than the resistance of the resistor R1.2 - R6 included in the SA9 section. If pulses are removed from pins 4 and 7 of DD1, their amplitude will be 6 V, and the output resistance will decrease. Author: S. Biryukov
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