Capacitance measurement circuit b

is a capacitance measuring instrument, and b is the circuit of a direct-reading capacitance measuring instrument. This measuring instrument can measure capacitance from several picofarads to 0.1 microfarads
, and the measuring range is divided into five levels: 25pF, 100pF, 100opF,
0.011 and 0.lvF. Since the measurement principle is
linear. It can be read directly using the ammeter scale. G
charging, as indicated by the solid line 7F in Figure 5-21b.
    When VT4 is turned on, the measured capacitor e' and
diode VI) are discharged. When VT4 is turned on and off once, the capacitor under test
is charged and discharged once. The capacitance being measured is large, and
a large amount of electricity passes through the ammeter each time it is charged. in addition. VT4
switches more times per second, and
more electricity passes through the ammeter per second. The amount of electricity passing through the ammeter per second
is the current flowing through the ammeter. Therefore,
the current through the ammeter is proportional to the measured capacitance and the switching
frequency of VT4. If the current through the ammeter is required
to be the same. Then when measuring small capacitors,
the switching frequency of VT4 must be relatively high.
    Transistors VT1, VT2, VT3, etc. form a complementary
regenerative pair multivibrator. VT3 is used to increase
the oscillation frequency of the circuit. When analyzing the circuit, it can be considered
that the collector-emitter of VT3 is short-circuited. Once the power is turned
on, the power supply charges cj through Yan, Feng and RP, as
shown by the solid line in Figure 5-21c. At the beginning moment. The voltage drop across Ci is equal to zero, and the voltage drop of the hurricane is very small, which cannot cause VT1 to conduct. Since
VT1 is in the cut-off state, the voltage drop on R1 is also very small, causing VT2 to also cut off. Point A outputs high potential. As C charges, the voltage on G
gradually increases, positive on the right and negative on the left. When it reaches a certain value, VT1 is turned on. The voltage drop on Ri increases, causing VT2 to conduct. Point A outputs low potential
. At this time, the power supply reverse charges c] through the emitter, base, Rz, and collector and emitter of VT2, as
shown by the dotted line in Figure 5-21c. With the reverse charging of c1, the voltage on cl gradually increases, positive on the left and negative on the right. When it reaches a certain value, VT1 is cut off. VT2 is also cut
off, point A outputs high potential, and C- returns to the charging process. If this continues, high potential and low potential will appear alternately at point A, which is used to control
the on and off of VT4. Connect different oscillation capacitors C. ~c5: "You can change the oscillation frequency, that is, change the VT4 switching frequency, to
adapt to the needs of measuring different capacitances.
    C7-Cii is the calibration capacitor. Before measuring the measured capacitance, use the calibration capacitor to calibrate the ammeter. RP is used It is used for frequency fine-tuning.
Adjust it to make the ammeter fully deflected during calibration. SB is a double-pole five-throw switch, composed of SB1 and SB2, for range selection. Turn SB, and the oscillation capacitance and the calibration
capacitance correspond to each other.
    For example, if you want to measure a few For a capacitor of 100 picofarads, first turn S2 to the "calibration" position. Turn SB1 to position 3, adjust RP to make the ammeter pointer
fully deflected. Then connect the capacitor under test to the red and black terminals, and then turn S2 Go to the "measurement point". At this time, if the ammeter is 100yA
and the needle falls at 56FcA, then the capacitance to be measured is 560pF. If the capacitance range of the capacitance to be measured is not clear, first use the maximum range to measure and
wait to determine the range. Then use the appropriate range to measure. Before measurement, it is best to use the ohm setting of a meter to determine whether the capacitor under test has a short circuit . Do not use this meter to measure capacitance that has been short-circuited, otherwise the ammeter
will be easily damaged .