Measuring capacitance with a pointer multimeter

The capacitance can be detected by using a pointer multimeter. The basis is that the resistance block of the multimeter is equivalent to a DC power supply with internal resistance, which can charge the capacitor. Over time, the voltage across the capacitor gradually increases, and the charging current gradually decreases until it reaches zero.

Operation steps

1. Select the appropriate gear for the resistance block. Generally, for a capacity below 0.01uF, select the x10k gear; for 1~10uF, select the x1k gear; for above 47uF, select the x100 gear or x10 gear.

2. Each time you measure, use a wire to short-circuit the capacitor, and then discharge it before the next test.

3. Electrolytic capacitors have polarity, and the positive electrode should have a higher potential than the negative electrode when used. Since the black test pen is connected to the positive electrode of the battery in the meter, the black test pen is connected to the positive electrode of the electrolytic capacitor, and the red test pen is connected to the negative electrode of the capacitor. At this time, the leakage of the electrolytic capacitor is small, and the reverse connection will result in a large leakage. The performance of a good capacitor is that the pointer deflects one down during the test, and then gradually returns to the mechanical zero (that is, infinite resistance) position. The amount of pointer deflection is related to the capacitance and the resistance position. The larger the capacitance, the larger the deflection. In practice, we should pay attention to the rules and accumulate data. The method of adjusting the mechanical zero of the meter is to use a flat-head screwdriver to align the mechanical zero adjustment notch on the meter head when the test leads are neither short-circuited nor measuring any device, and rotate it left and right to make the meter needle point to zero. The performance of a capacitor that has lost its capacity is that the detection pointer does not deflect. It does not need to be discharged. The pointer does not deflect after quickly exchanging the test leads. The performance of a capacitor that has lost part of its capacity is that compared with the standard capacitor, the pointer deflects out of place. It can be judged based on experience or reference to the standard capacitor of the same capacity according to the maximum amplitude of the pointer swing. The reference capacitor does not need to have the same withstand voltage value, as long as the capacity is the same. For example, to estimate a 100uF/250V capacitor, a 100uF/25V capacitor can be used as a reference first. As long as the maximum amplitude of their pointer swings is the same, it can be determined that the capacity is the same. The performance of a leakage capacitor is that the pointer cannot return to the mechanical zero (that is, infinite resistance) position. It should be noted that electrolytic capacitors have leakage, large or small. Low-voltage capacitors have large leakage, while high-voltage capacitors have small leakage. Use x10k to measure large leakage, and use the range below x1k to measure small leakage to determine whether the capacitor is leaking. For capacitors above 1000uF, you can first use the Rx10 range to quickly charge it and preliminarily estimate the capacitance, then change to the Rx1k range to continue measuring for a while. At this time, the pointer should not return, but should stop at or very close to infinity, otherwise there may be leakage. For some capacitors below tens of microfarads, after charging at the Rx1k range, switch to the Rx10k range to continue measuring. Similarly, the needle should stop at infinity and not return. In addition to electrolytic capacitors, ceramic, polyester, metallized paper, and monolithic capacitors have a withstand voltage greater than 40V. When tested with a multimeter, no matter which range, a good capacitor should not leak. When measuring small-capacity capacitors with a multimeter, you can use the amplification effect of a low-power silicon NPN transistor . The method is shown in Figure 1 (f). Use the resistor Rx1k block, connect the black test lead to the collector, and the red test lead to the emitter. Touch the small capacitor to the collector, and the pointer should deflect. The principle is that when the capacitor is charged, the charging current injects the base current into the base. This current is amplified by the transistor, and the pointer deflection is more obvious.

(c) in the attached figure is an aluminum electrolytic capacitor structure, which has strict requirements on the polarity of use. From the formula in the figure, it can be seen that the capacity of a flat plate capacitor is proportional to the dielectric constant of the medium. It is proportional to the relative area of ​​the plates and inversely proportional to the distance between the plates. The positive electrode is aluminum foil. In order to expand the area, the inner surface of the aluminum foil is corroded into unevenness. The medium is an insulating material aluminum oxide, which is very thin. The negative electrode is an electrolyte, and the aluminum foil on the right acts as a negative electrode lead. When used correctly, the positive electrode is connected to a high potential and the negative electrode is connected to a low potential. Under the action of direct current, the electrolyte can decompose oxygen atoms and generate aluminum oxide with the positive aluminum foil to maintain insulation. When used incorrectly, the positive electrode is connected to a low potential and the negative electrode is connected to a high potential. Under the action of direct current, the electrolyte will corrode the aluminum oxide and damage the insulation. In the case of leakage, it will cause heat generation or even bursting. Therefore, you must pay attention to the polarity when using it, and often power it on to age it if it is not used for a long time.