A practical circuit for measuring capacitance using a pointer multimeter
If you want to use a pointer multimeter to judge the quality of a 400 microfarad capacitor, you can use the appropriate resistance range of the multimeter to measure the two pins of the capacitor. You can know 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 is damaged by breakdown, the pointer will point to the minimum value. If the pointer stays at a certain value in the middle, it means that the capacitor has a certain leakage. If the questioner wants to use an ordinary pointer multimeter to measure the capacitance, he can use the LM358 operational amplifier and connect it to the circuit shown in the figure below to measure the capacity of the 400 microfarad capacitor.
A practical circuit for measuring capacitance using a pointer multimeter.
Low power consumption dual operational amplifier LM358, here an internal operational amplifier is used to connect to an inverting proportional amplifier. The closed-loop amplification factor of this circuit is - (R/Xc), where Xc is the capacitive reactance of the measured capacitor. We know that the capacitive reactance of the capacitor Xc=1/2πfC. If the frequency f is 50Hz and C is 400 microfarads, the capacitive reactance Xc=7.96Ω.
When measuring the capacity of a 400 microfarad capacitor, we can find a 6V power transformer, divide the 6V AC voltage output by the secondary resistor into a 100mV/50Hz sine wave as the signal source, and connect the 400 microfarad capacitor to The position of C in the figure is so that the output voltage Vout of the operational amplifier is measured with the AC voltage range of the pointer multimeter, and then the capacity of the measured capacitor can be known through simple calculation. Assume that Vin is a stable 100mV and the measured Vout=1000mV. According to the above formula, the capacitive reactance Xc of the capacitor=7.96Ω. Then according to the calculation formula of the capacitive reactance, the capacity of the capacitor can be known.
Most of today's digital multimeters use the above method to measure capacitance. Since the frequency, amplitude and resistance value of the sine wave signal source in this circuit are different from the value of the capacitance range of the digital multimeter, the capacitance cannot be read directly. Generally, the signal source used by digital multimeters is a 40mV/400Hz sine wave signal, and the value of R is also different from the above resistance value. Therefore, the output voltage of the amplifier is the same as the capacitance under test and can be read directly. The power supply voltage of the above measurement circuit can be ±5V voltage.
