Comprehensive Guide to Capacitor Detection and Selection

PublishTime： 2024-10-26 Article Source：XIANGRU ELECTRONICS Official Website

Firstly, the detection of capacitors

The main faults of capacitors are breakdown, short circuit, leakage, capacity reduction, deterioration, and damage.

Appearance inspection

The appearance should be intact and undamaged, with no cracks, dirt, or corrosion on the surface. The markings should be clear, and the lead out electrode should not be damaged; Adjustable capacitors should rotate flexibly, with no collision or rubbing between the moving and fixed plates, and the rotation between each pair should be synchronized.

Test leakage resistance

Use a multimeter in the ohm range (R × 100 or R × 1k) to connect the probe to the two leads of the capacitor. When it is first connected, the pointer of the meter head will swing and then gradually return to the point of R=∞, which is the phenomenon of capacitor charging and discharging (this phenomenon cannot be observed for capacitors below 0.1μF). The larger the swing of the pointer, the larger the capacity, and the value indicated by the stable pointer is the leakage resistance value. Its value is generally several hundred to several thousand megaohms, and the higher the resistance, the better the insulation performance of the capacitor. When detecting, if the pointer of the meter head points to or near the zero ohm point, it indicates an internal short circuit in the capacitor. If the pointer remains stationary and always points to R=∞, it indicates an internal open circuit or failure in the capacitor.

Capacitors above 5000pF can be distinguished by the highest resistance of a multimeter, while small capacity capacitors below 5000pF should be distinguished by specialized measuring instruments.

Polarity detection of electrolytic capacitors

The positive and negative polarities of electrolytic capacitors are not allowed to be connected incorrectly. When the polarity mark cannot be recognized, it can be detected and judged based on the characteristics of high leakage resistance when connected in the forward direction and low leakage resistance when connected in the reverse direction. Measure the leakage resistance value twice before and after swapping the probes. When the resistance value is higher, the black probe is in contact with the positive pole. (Because the black probe is connected to the positive terminal of the battery inside the meter)

Detection of contact or leakage of variable capacitors

Set the multimeter to the R × 10 position, place the two probes on the moving and fixed plates of the variable capacitor, and slowly rotate the moving plate. If the pointer on the meter remains stationary, there will be no contact or leakage; If the pointer of the meter head reaches 0 Ω when rotated to a certain angle, it indicates that there is a collision at this point. If the pointer of the meter head has a certain indication or slight swing, it indicates that there is a leakage phenomenon.

Secondly, the selection method of capacitors

Choose the appropriate model

According to circuit requirements, it is generally used for low-frequency coupling, bypass decoupling, etc. When the electrical performance requirements are low, paper capacitors, electrolytic capacitors, etc. can be used.

The coupling capacitor for the transistor low-frequency amplifier is an electrolytic capacitor with a temperature range of 1-22μF. The bypass capacitor is selected based on the operating frequency of the circuit. For example, in low-frequency circuits, electrolytic capacitors with a capacity between 10 and 220μF are used for the emitter bypass capacitor; In the intermediate frequency circuit, paper media with 0.01~0.1μF, metalized paper media, organic film capacitors, etc. can be selected; High-frequency ceramic capacitors should be selected in high-frequency circuits; If working at high temperatures is required, glass glazed capacitors should be selected. Electrolytic capacitors can be used in power filtering and decoupling circuits. Because in these usage scenarios, the performance requirements for capacitors are not high, as long as the volume is not large and the capacity is sufficient.

For variable capacitors, it should be determined whether to use single or multi-connected variable capacitors based on the number of capacitor tuning stages and then according to

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