Capacitance meter on a logic chip. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Below is a capacitance meter that is simple in terms of circuit design, which even a novice radio amateur can make. Nevertheless, the device has a measurement accuracy of about 4%, which is quite acceptable for many amateur radio work.
The device has four sub-ranges determined by the position of the SA1 switch. In position 1, capacitors with a capacitance of up to 50 pF are measured, 2 - up to 500 pF, 3 - up to 5000 pF and 4 - up to 0,05 μF. On the elements DA1.1 and DA1.2, a generator of rectangular pulses is made.
The generation frequency depends on the capacitance of the capacitor and the resistance of the resistor included in the feedback circuit by the switch. It is largest in switch position 1 and smallest in position 4. Logic elements DD1.3 and DD1.4 are connected in parallel. This provides a sufficiently large charging current of the measured capacitor Cx, which is connected to the sockets XS1 and XS2.
Increasing the charging current reduces the effect of the leakage resistance of the capacitor Cx on the indicator readings. The indication is carried out by a pointer measuring head with a total deflection current of 1 mA and a frame resistance of 240 Ohm. An important condition for the accuracy of work is the selection of diodes VD1 and VD2 - they must be high-frequency and have the same parameters.
The capacitance meter is adjusted separately for each subrange. The range switch is set to the appropriate position. A capacitor with a precisely known capacitance is connected to the sockets. By adjusting one of the tuning resistors R1 - R 4 (of the corresponding subrange), the desired reading is achieved on the indicator scale.
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