Detection, function and characteristics of capacitors

1. Capacitance detection

Today's digital multimeters have a range for measuring capacitance, and the operation is simple and convenient. Needless to say, today I will focus on detecting capacitance with an analog multimeter.

1. Determination of leakage of fixed capacity capacitors

(1) Use the multimeter ohm switch R×10k range, and connect the test leads and the two poles of the capacitor in parallel, as shown in the figure.

Capacitor leakage judgment

(2) Reading of capacitor leakage resistance data, as shown in the figure. The larger this value is, the better. The larger it is, the better the insulation of the capacitor is. Generally, it should be several hundred to several thousand megaohms.

Capacitor leakage judgment

2. Preliminary estimation of capacitance

(1) When touching both ends of the capacitor with a test lead, the test needle should jump first and then gradually recover.

(2) Replace the test leads and measure again. The hands jump again, but they should jump farther and then gradually recover.

(3) The above steps (1) and (2) are the situations when the capacitor is charging and discharging. The greater the capacity of the capacitor, the greater the jump of the meter head, and the slower the recovery speed of the pointer, indicating that the charging and discharging time of the capacitor is longer. From this, the relative size of the capacitor capacity can be initially judged.

3. Determining the polarity of electrolytic capacitors

The polarity of the electrolytic capacitor can be determined based on the phenomenon that the leakage current is small when the electrolytic capacitor is connected in the forward direction and the leakage current is large when the electrolytic capacitor is connected in the reverse direction, as shown in the figure.

Judging the polarity of electrolytic capacitors

2. The role of capacitor

1.Bypass

Usually at the input end of the signal. A storage device that provides energy to local devices and can even out the signal. To reduce impedance, place the bypass capacitor as close to the load pin as possible.

2. Decoupling

Generally at the power input end and signal output end. Decoupling is also called decoupling. Use the interference of the output signal as a filter object to prevent the interference signal from returning to the power supply. Generally when designing a circuit, the power pin of the chip must be connected to a 0.1uF capacitor to ground. This can prevent the chip's own interference from affecting the purity of the power supply. If necessary, you can also use a 1uF or 10uF capacitor together with 0.1uF and connect it between the power pin and ground.

3. Filtering

Remove signals within a certain frequency band from the total signal. Generally, a large capacitor will be connected in parallel with a small capacitor. At this time, the large capacitor will pass low frequencies and the small capacitor will pass high frequencies. The best difference between the large and small capacitors is more than 10 times.

4. Energy storage

Capacitors store electric field energy.

5. Coupling

Used to connect the output of the previous stage and the input of the subsequent stage for signal transmission.

6. Oscillation/synchronization

Such as LC and RC oscillators.

7. Timing

3. Characteristics of capacitor

Capacitors have two major characteristics, one is to pass AC resistance DC, and the other is that the voltage cannot change suddenly. The functions of the above capacitors all rely on these two characteristics. To facilitate understanding, we compare the capacitor to a pool. The water in the pool cannot be filled up at once, nor can it be drained out at once. Therefore, the water in the pool cannot mutate. At most, it changes faster. This is one reason why the voltage across the capacitor cannot mutate, and it also reflects the capacitance. of charging and discharging.