# A capacitance measuring circuit

Posted on Mar 28, 2007

Capacitive measuring instrument, b is the direct reading instrument measuring the capacitance of the circuit. The measuring instrument is capable of measuring a few picofarads

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to 0.1 microfarads electrical capacity, range into 25pF, lOOpF, lOOOpF, 0. Oll and 0.lvF fifth gear. Since the measurement principle is linear. Can directly meter readings scale. G charging, as shown by the solid line 5-21b 7F. When VT4 conduction, when the measured capacitance e and diode VI) discharge. VT4 switch once, the measured electrical capacitance to charge and discharge time. Measured capacitance large, each charge by the ammeter when more power. Also. VT4 second clock switch multiple times per second by electric ammeter amount more. Meter per second through the power, it is a current through the ammeter. Therefore, ammeter and current proportional to the measured capacitance, and switching VT4 proportional to the frequency. If the current through the ammeter requirements as large. So, when measuring small capacitance, VT4 the switching frequency will be higher. Transistor VT1, VT2, VT3 and the like complementary to renewable multivibrator. VT3 which is to improve the oscillation frequency of the circuit, when the circuit analysis can be considered as the collector emitter VT3 very short-circuited. A power supply connected to the power supply input by Yan, wind, RP to cj charge, as shown by the solid line in Figure 5-21c. At the moment began. Ci zero voltage drop across, the hurricane voltage drop is small, can not make VT1 conduction. By the VT1 is in the OFF state, the voltage drop on R1 is small, so that VT2 also closed. A high point of the output potential. With C, charging, G on the voltage gradually increased, the left and right positive negative, so to a certain value after VT1 conduction. Ri voltage drop on the increases, the Tuen VT2 conduction. A low point output bits. At this time, the power supply through VT1 emitter, base, Rz, VT2 collector emitter, c] to reverse charging, 5-21c is shown in dashed lines. With anti-c1 charging voltage cl is gradually increased, the left and right positive negative, and then to make a certain value VT1 deadline. VT2 also cut dead, A fork point output high potential, C- returned to the charging process. So go on, A point constantly alternating high potential and low potential for controlling VT4s turned on and off. Connect different oscillation capacitance C. ~ C5: You can change the oscillation frequency, the switching frequency is changed VT4, to adapt to the measurement of different capacity needs. C7-Cii is calibrated before measuring capacitance measured capacitance, first with the calibration capacitor to calibrate meter .RP use. in frequency tuning, calibration adjustment which enables full meter deflection is .SB pole five throw switch from SB1, SB2 composition, make range selection Turn the SB, the oscillation capacitance and calibration Pseudocapacitive corresponding to one or two. for example, to measure a few capacitance hundred picofarads, first S2 to position calibration at .SB1 to 3, adjust RP, ammeter pointer to full deflection., then the measured capacitance connected to the red, the black terminal, and then dial the S2 to Survey Office. at this time, if the meter is lOOyA, the line and the needle drops 56FcA place, then the measured capacitance is 560pF. If the measured capacitance range is not clear, first with the largest measuring range, etc. judged range and then with a suitable measuring range. before measuring, it is best to use one hundred multimeter ohm file, determine whether there is a short circuit current measured capacitance like. Do not use this instrument to measure short-circuited capacitor, doing so can damage the meter.

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