How to Measure Capacitance with a Pointer Multimeter
1. Test speakers, earphones, and dynamic microphones: use the R×1Ω gear, connect any test lead to one end, and the other test lead to touch the other end. It will make a crisp "da" sound under normal conditions. If there is no sound, the coil is broken. If the sound is small and sharp, there is a problem with the ring rubbing, and it cannot be used.
2. Capacitance measurement: use the resistance file, select the appropriate range according to the capacitance capacity, and pay attention to the black test lead of the electrolytic capacitor should be connected to the positive pole of the capacitor when measuring. ①. Estimate the size of the capacitor of the microwave method: it can be judged according to the maximum amplitude of the pointer swing by experience or referring to the standard capacitor of the same capacity. The referenced capacitors do not need to have the same withstand voltage value, as long as the capacity is the same. For example, a 100μF/250V capacitor can be used as a reference to estimate a 100μF/25V capacitor. As long as the maximum swing of their pointers is the same, it can be concluded that the capacity is the same. ②. Estimate the capacitance of picofarad capacitors: R×10kΩ should be used, but only capacitance above 1000pF can be measured. For a capacitance of 1000pF or slightly larger, as long as the hands of the watch swing slightly, the capacity can be considered sufficient. ③. To measure whether the capacitor is leaking: for a capacitor above 1,000 microfarads, you can first use the R×10Ω file to quickly charge it, and initially estimate the capacitor capacity, and then change to the R×1kΩ file to continue measuring for a while. At this time, the pointer does not It should return, but stop at or very close to ∞, otherwise there will be leakage.
For some timing or oscillating capacitors below tens of microfarads (such as the oscillating capacitors of color TV switching power supplies), the requirements for their leakage characteristics are very high, as long as there is a slight leakage, they cannot be used. At this time, they can be charged at the R×1kΩ level. Then use the R×10kΩ file to continue the measurement, and the hands should stop at ∞ and should not return.
3. Test the quality of diodes, triodes, and Zener tubes on the road: because in actual circuits, the bias resistance of triodes or the surrounding resistance of diodes and Zener tubes are generally relatively large, mostly in hundreds or thousands of ohms. , we can use the R×10Ω or R×1Ω file of the multimeter to measure the quality of the PN junction on the road. When measuring on the road, use the R×10Ω file to measure the PN junction should have obvious forward and reverse characteristics (if the difference between the forward and reverse resistance is not obvious, you can use the R×1Ω file to measure), generally the forward resistance is at R The hands should indicate about 200Ω when measuring in the ×10Ω range, and about 30Ω when measuring in the R×1Ω range (there may be slight differences depending on the phenotype). If the measurement result shows that the forward resistance is too large or the reverse resistance is too small, it means that there is a problem with the PN junction, and there is also a problem with the tube. This method is particularly effective for maintenance, and can find out bad pipes very quickly, and even detect pipes that have not completely broken but whose characteristics have deteriorated. For example, when you use a small resistance file to measure the forward resistance of a certain PN junction is too large, if you solder it down and use a commonly used R×1kΩ file to measure it, it may still be normal. In fact, the characteristics of this tube have deteriorated. Not working or unstable anymore.
4. Measuring resistance: It is important to select a good range. When the pointer indicates 1/3 to 2/3 of the full scale, the measurement accuracy is the highest and the reading is the most accurate. It should be noted that when using the R×10k resistance file to measure a large resistance of megohm level, do not pinch your fingers at both ends of the resistance, so that the resistance of the human body will make the measurement result smaller.
In the process of repairing home appliances, failures caused by capacitor leakage or capacity changes are common and the failure phenomena are different. General pointer multimeters and some digital multimeters cannot measure capacitance, especially those small capacitances, which cause great inconvenience to maintenance. Here, I will introduce several measurement methods of small-capacity capacitance for your 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 type to combine into a Darlington form, as shown in Figure 1. Connect the measured capacitor to the c-e junction of the triode (if it is a polarized capacitor, connect the positive pole of the capacitor to the c pole of the triode), and then use the multimeter R&TImes; 10k gear, connect the black test lead to the c pole, and the red pen to the e pole , observe the instantaneous swing of the hands. According to this method, use several normal (high-precision) capacitors with known capacities to test repeatedly, record the instantaneous maximum swing amplitude of the hands each time, and perform processing calculations to calculate the capacitance value that each small grid on the dial should represent. For future reference. When measuring the capacitance, the quality of the capacitance can be judged by comparing the swing amplitude of the measured capacitance gauge with the reference amplitude.
Method 2: Find a capacitor with known capacity with high precision (over 250V withstand voltage) and a transformer with adjustable auto-coupling output voltage, as shown in Figure 3. Cn is the known capacitance, and Cx is the capacitance to be tested. After connecting the wires and electrifying, measure the respective partial voltages on Cx and Cn. However, it should be noted that the output voltage after power transformation 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, you can directly connect two series capacitors to the 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 you only need to estimate the capacitor capacity, you can connect the capacitor according to Figure 4, connect the measured capacitor to a test lead of the multimeter in series, and then turn the multimeter to the voltage block (250V) to measure the AC voltage In this way, use multiple known capacitances to test, and remember the swing range of the hands, which can provide a basis for estimating the capacitance in the future (Note: This method is only limited to non-polar capacitances). Method 4: Measurement of electrolytic capacitors. Due to the polarity problem of the electrolytic capacitor, a half-wave rectifier diode can be connected as shown in Figure 5, and the output voltage of the autotransformer can be appropriately selected according to the withstand voltage of the measured capacitor. Co is an electrolytic capacitor with a known capacity, and cx is the capacitance to be measured. After wiring and measuring according to the diagram, the Cx capacity can be calculated according to the formula Co/Cx=U0/Uv.
