Expansion of capacitance measurement function of digital multimeter
Precautions
1) Read the instruction manual before use to understand the use method and precautions of the instrument.
2) The instrument should not be placed in places with high temperature, humidity, sewage, dust, air or chemical gas with high hydrochloric acid and alkali components.
3) Pay attention to the polarity when installing the battery or external power supply, and do not reverse the connection. The battery should be removed if it is not used for a long time, so as not to damage the instrument due to leakage.
4) Do not disassemble the microphone to prevent throwing, and place it properly when not in use.
5) Do not disassemble the instrument without authorization. If the instrument is not normal, it can be sent to the repair unit or the factory for overhaul.
1. Online measurement of capacitance
According to the properties of differential and integral circuits, capacitance measurement can be converted into voltage measurement.
The core part of the circuit CX/V adopts a simple active RC inverse differential and integral circuit. The Wen oscillator generates a fixed frequency AC signal Vr, which excites the CX/V conversion circuit to obtain an AC voltage V0 (V1) proportional to CX. After being filtered by a second-order bandpass filter to remove clutter outside the fixed frequency, it is then subjected to AC/DC to obtain a DC output voltage V proportional to CX. When the AC signal Vr excites the CX/V circuit, the output voltage of the inverter integrator
That is, the measured capacitance CX is directly proportional to the output voltage C0, thus achieving CX → V conversion. In order to match the basic capacitance level with the 2V level of the digital multimeter, the oscillation frequency of the Wentz oscillator is selected as 400Hz, the effective voltage value is 1V, R1 is taken as 20k Ω, and C1 is taken as 0.1 μ F. R2 varies from 200 Ω -2k Ω -20k Ω -200k Ω -200k Ω -2M Ω, corresponding to a measurement capacitance range of 20 μ F-2 μ F-200nF-20nF-2nF.
2. Measure small capacitance
The typical range of a three and a half digit multimeter for measuring capacitance is 2000pF to 20 μ F, and it is powerless to measure small capacitors below 1pF. According to the capacitive impedance method and using high-frequency signals, the measurement of small capacitors can be achieved. The measurement circuit diagram is shown in Figure 2. CX is the measured capacitance, and Rf is the feedback resistance of the reverse phase terminal. When the input frequency of a sine signal Vi is f, the impedance presented on CX and the gain of the operational amplifier are: when A and Rf are constant, the frequency of the sine signal f is inversely proportional to the measured capacitance CX. To measure smaller capacitance, use high-frequency signals for measurement.
