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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Attachment - capacitance meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur [an error occurred while processing this directive] Often, when establishing and repairing various equipment, a radio amateur needs to check capacitors. This happens especially often when repairing switching power supplies and line scanning TVs. A common capacitor malfunction is an open output. It is difficult to detect it using a conventional measuring device, turned on in the resistance measurement mode, especially if the capacitance of the capacitor under test is small (no more than 1 μF). By the way, the overwhelming majority of such capacitors in the mentioned blocks. Capacitors are checked with an ohmmeter as follows: by connecting an uncharged capacitor to the probes of the device, observe the deviation of the arrow. At the first moment, it deviates by a certain angle, since the capacitor charging current flows through the device from the built-in battery. This current decreases rapidly, and the pointer returns to the top of the scale, indicating infinite resistance. If the device continues to show a certain resistance, then the capacitor has a large leakage current. If the resistance is low, the capacitor is broken. If the arrow does not deviate at all, then either the output of the capacitor is broken, or its capacitance is too small. The arrow will not deviate even with a good capacitor, if it was previously charged to a voltage equal to the voltage of the device battery. The simplest device, the diagram of which is shown in Fig. 1.
The push-button switch without fixing SB1 closes the tested capacitor with its contacts, ensuring its complete discharge. When the button is pressed, the capacitor is connected to the device. In the author's version, the tests were carried out with the Ts4342 instrument switched on in the resistance measurement mode at the limit of x100 kOhm. According to the angle of the initial deviation of the arrow, one can judge the capacitance of the capacitor, an approximate calibration is carried out by connecting several serviceable capacitors of various (known) capacitance to the device in turn. At the same limit of the multimeter, it is also possible to determine the leakage current of the tested capacitor. The pushbutton switch can be structurally designed as an attachment to the multimeter by selecting the appropriate clamps or connectors. Oxide capacitors should be connected to the set-top box, observing the polarity and having previously determined which of the terminals of the multimeter has a positive voltage of the internal battery. This simplest device has a number of disadvantages: the deviation of the arrow is so short that it is not possible to accurately read the readings of the device, in addition, manually switching the button at different intervals gives different readings of the device. From the listed shortcomings, a more complex device is free, in which the discharge and charging of the tested capacitor are carried out automatically, with a constant period. The scheme of the improved set-top box is shown in fig. 2. It contains keys made on transistors VT1 and VT2, and a multivibrator assembled on transistors VT3 and VT4. Multivibrator capacitors C1 and C2 are selected with different capacities, so the duration of the positive polarity pulse on the collector of transistor VT3 is less than on the collector of transistor VT4. The first of these pulses opens the VT1 key, and the tested capacitor Cx is charged from the multimeter battery. A positive pulse taken from the VT4 collector opens the VT2 key, and the capacitor under test is discharged. The long duration of the "discharge" pulse ensures the complete discharge of the capacitor. Thus, alternately charging and discharging the capacitor ensures that a certain current is drawn from the measuring device, which is proportional to the pulse repetition rate and the capacitance of the capacitor under test.
This device is also made as an attachment to the multimeter and is equipped with connectors X1 and X2 for connecting to the device and any sockets or clamps XT1 and XT2 for connecting the tested capacitors. Mounting of the prefix can be arbitrary. The prefix is fixed on the input terminals of the device. It receives power from one built-in G1 element. With the values of the elements indicated in the diagram, the capacitance measurement range is from 1000 pF to 1 μF. The multimeter scale should be calibrated using a set of capacitors with a known capacitance, and a table of correspondence between the readings of the device and the capacitance of the capacitor should be compiled. The prefix is also useful in cases where it is necessary to determine the capacitance of a capacitor, on the body of which, for some reason, there is no marking. Author: V. Polovinkin, Zheleznogorsk, Kursk region
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