Two examples of troubleshooting of pointer multimeter

In the process of home appliance maintenance, faults caused by capacitor leakage or capacity change are common and the fault phenomena vary. General pointer multimeters and some digital multimeters cannot measure capacitors, especially those small capacitors, which causes great inconvenience to maintenance. Here, I will introduce several methods for measuring small-capacity capacitors for reference.

Method 1: Find a crystal triode with β≥250 (the penetration current is required to be small). If you can't find it for a while, you can use two triodes of the same model to form a Darlington form, as shown in Figure 1. Connect the capacitor to be measured to the ce junction of the triode (if it is a polarized capacitor, the positive pole of the capacitor is connected to the c pole of the triode), then use the multimeter R×10k block, connect the black test pen to the c pole, and the red pen to the e pole, as shown in Figure 2, and observe the instantaneous swing degree of the needle. According to this method, use several normal (high-precision) capacitors with known capacity to test repeatedly, record the instantaneous maximum swing amplitude of the needle each time, process and calculate, and calculate the capacitance value that each small grid on the dial should represent for future reference. When measuring the capacitor, the quality of the capacitor can be judged by comparing the swing amplitude of the measured capacitance needle with the reference amplitude.
Method 2: Find a capacitor with high-precision known capacity (withstand voltage of more than 250V) and a transformer with adjustable auto-coupling output voltage , as shown in Figure 3. Cn is the known capacitor, Cx is the capacitor to be tested, and after connecting the wires and powering on, measure the respective voltage divisions on Cx and Cn, but it should be noted that the output voltage after the power supply is transformed should not be greater than the withstand voltage of Cx. At this time, the capacity of Cx can be calculated according to the formula Uo/Ux=Co/Cx. If the withstand voltage of Cx is above 300V, the two series capacitors can be directly connected to a 220V AC power supply (Note: This method is only suitable for non-polar capacitors). Method 3: If the withstand voltage of the capacitor is above 400V and only the capacitance needs to be estimated, the wiring can be done according to Figure 4. After the capacitor to be tested is connected in series with a probe of the multimeter, the multimeter is set to the voltage range (250V) to measure the AC voltage. In this way, multiple known capacitors are tested and the swing amplitude of the needle is remembered. This can provide a basis for estimating the capacitance in the future (Note: This method is limited to non-polar capacitors). Method 4: Measurement of electrolytic capacitors. Because electrolytic capacitors have polarity problems, a half-wave rectifier diode can be connected according to Figure 5, and the output voltage of the autotransformer can be appropriately selected according to the withstand voltage of the capacitor to be tested . Co is an electrolytic capacitor of known capacity, and cx is the capacitor to be tested. After wiring and measuring according to the figure, the capacity of Cx can be calculated according to the formula Co/Cx=U0/Uv.