A practical circuit for measuring capacitance using an analog multimeter
To use a pointer multimeter to judge the quality of a 400 microfarad capacitor, you can measure the two pins of the capacitor with the appropriate resistance range of the multimeter, and determine the quality of the capacitor based on the amplitude of the pointer swing. Generally, a good capacitor pointer can point to the maximum value (or close to the maximum value). If the capacitor breaks down and is damaged, the pointer points to the minimum value. If the pointer stays at a certain value in the middle, it indicates that the capacitor has some leakage. If the questioner wants to measure the capacitance of the capacitor with a regular pointer multimeter, they can use the LM358 operational amplifier to connect the circuit as shown in the following figure to measure the capacitance of a 400 microfarads capacitor.
A practical circuit for measuring capacitance capacity using a pointer multimeter.
A is a low-power dual operational amplifier LM358, which uses an internal operational amplifier connected to form an inverting proportional amplifier. The closed-loop amplification factor of this circuit is - (R/Xc), where Xc is the capacitance impedance of the measured capacitor. We know that the capacitance impedance Xc of a capacitor is 1/2 π fC. If the frequency f is 50Hz and C is 400 microfarads, the capacitance impedance Xc is 7.96 Ω.
When measuring the capacity of a 400 microfarad capacitor, we can use a 6V power transformer to divide the 6V AC voltage output from its secondary into a 100mV/50Hz sine wave through a resistor as the signal source. Connect the 400 microfarad capacitor to position C in the diagram, and use a pointer multimeter to measure the output voltage Vout of the operational amplifier in the AC voltage range. Then, through simple calculations, we can determine the capacity of the measured capacitor. Assuming Vin is a stable 100mV and the measured Vout is 1000mV, according to the above formula, the capacitance impedance Xc of the capacitor is 7.96 Ω, and then the capacitance of the capacitor can be calculated based on the capacitance impedance formula.
Most current digital multimeters use the above method to measure capacitance. Due to the difference in frequency, amplitude, and resistance R values between the sine wave signal source in this circuit and the capacitance range of the digital multimeter, it is not possible to directly read the capacitance. The signal source used for a typical digital multimeter is a 40mV/400Hz sine wave signal, and the value of R is also different from the resistance value mentioned above. Therefore, the output voltage of the amplifier is the same as the measured capacitance and can be read directly. The power supply voltage for the above measurement circuit can be selected as ± 5V voltage.
