## DTXXX digital multimeter with capacitance measurement circuit

This is DTXXX digital million used, commonly used measurement of capacitance of the circuit, the basic principle is to use capacitor capacitance. 400Hz sinusoidal process is the measured capacitance Cx acts on both ends, using the resulting exchange of reactance Xc capacitance and voltage to achieve the conversion, and then by measuring the AC voltage to obtain the capacity of Cx.

Measurement is divided into five files (of course, you may also need to increase or decrease), respectively 2nF, 20nF, 200nF, 2uF, 20uF, measurement accuracy is 2.5%, the resolution used by your AC voltage measurement tool decision. Circuit is divided into four parts.

First look at the first part of the A4 as the center is a Wien bridge circuit. When R8 = R14, C3 = C4 when the frequency is:

This is a positive feedback circuit, U01 via R14, C4 back to the A4-phase side, but when the frequency is equal to the frequency, A4-phase input voltage is only Uo1 Dacron 1 / 3, which also requires A4 magnification must be greater than 3, that is, 1 + R13/R123, that R132R12.However, this factor can not be too large, otherwise the sine wave may be deformed.

Look at the second part, this part of the A3 as the center, relatively simple arithmetic circuit for a reverse ratio, but also have a dampening effect, but it should be noted that, here is the Uo1 scaled down, the ratio of (R10 + VR1) / R11. Adjust the proportion of VR1 can change size.

The third part is the capacitors – AC voltage converter circuit (C / ACV), the circuit A1 as the center, the second part of the operation as a reverse ratio, but here the input resistance of the capacitance Cx. Different range, the feedback resistor is also different. The following table corresponding to the different range for different Rf values. Can be seen that the maximum product of measured capacitance and Rf equal.

RC is the same result, that no matter what the scale, the maximum range of magnification is equal to about 5.03. So Uo3 = 5.03Uo2. Is a constant for Uo2, so every TV output voltage range is equal, which ensures accuracy of different amounts of said the same. Can also be seen. For the same range, the output voltage is proportional to capacitance Cx.

The fourth section to A2 as the center for the active bandpass filter, the center frequency of 400Hz, to filter out other frequency interference. Finally, the amplitude of the output voltage Uo4 is proportional with the measured capacitance Cx 400Hz AC sine voltage. Instrument used to measure sine wave, the capacitance can be measured.

It should be noted that the measurement results, is proportional to the column capacitance, so you have to the public in accordance with your measurements to adjust the ratio to get a direct reading.