Capacitance measuring instrument circuit

This measuring instrument can measure capacitance from several pF to 0.1uF, and the range is divided into five levels: 25pF, 100pF, 1000pF, 0.01uF and 0.1uF. Since the measurement principle is linear, it can be read directly using the ammeter scale.

When VT4 is turned on, the measured capacitor Cx and diode VD discharge. When VT4 switches once, the measured capacitor is charged and discharged once. The capacitance being measured is large, and more 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 capacitance being measured, and proportional to the switching frequency of VT4. If the current passing through the ammeter is required to be the same, then the switching frequency of VT4 must be higher when measuring small capacitors.