How to choose a capacitor

1. The right choice of capacitors

(1) The type of capacitor should be selected according to the circuit requirements. For less demanding low-frequency circuits and DC circuits, paper dielectric capacitors or low-frequency ceramic dielectric capacitors are generally available. In high-frequency circuits, when high electrical performance is required, mica capacitors, high-frequency ceramic dielectric capacitors or through-hole ceramic dielectric capacitors can be used. Plastic film capacitors can be used in high-frequency and low-frequency circuits that require higher quality. In power supply filtering and decoupling circuits, aluminum electrolytic capacitors are generally available. For circuits requiring high reliability and high stability, mica capacitors, paint film capacitors or tantalum electrolytic capacitors should be used. For high-voltage circuits, high-voltage ceramic capacitors or other types of high-voltage capacitors should be used. For tuning circuits, variable capacitors and trimmer capacitors should be used.

(2) Reasonably determine the capacitance and allowable deviation of the capacitor. In low-frequency coupling and decoupling circuits, the capacitance requirements of capacitors are generally not too strict, as long as a slightly larger capacitance is selected according to the calculated value. In circuits such as timing circuits, oscillating loops, and tone control, the capacitance of capacitors is relatively strict. Therefore, the nominal value of capacitance should be as close as possible to the calculated capacitance value or as close as possible, and capacitors with high accuracy should be selected as much as possible. . In some special circuits, the capacitance of the capacitor is often required to be very accurate. At this time, a high-precision capacitor with a tolerance of ± 0.1% to ± 0.5% should be selected.

(3) The working voltage of the selected capacitor should meet the circuit requirements. Under normal circumstances, the rated voltage of the capacitor should be 1.2 to 1.3 times the actual working voltage. For the circuit with higher working environment temperature or poor stability, the rated voltage of the capacitor should be derated, leaving a larger margin. If the operating voltage in the circuit where the capacitor is located is higher than the rated voltage of the capacitor, the capacitor is often prone to breakdown, making the entire circuit inoperable. The rated voltage of a capacitor generally refers to a DC voltage. If it is used in an AC circuit, it should be selected according to the characteristics and specifications of the capacitor; if it is used in a pulsating circuit, it should be selected according to the sum of the AC and DC components not exceeding the rated voltage of the capacitor.

(4) Capacitors with large insulation resistance, low dielectric loss and low leakage current are preferred.

(5) The capacitor should be selected according to the working environment of the capacitor. The performance parameters of capacitors are closely related to the conditions of the use environment, so you should pay attention to when choosing capacitors: ① capacitors used under high temperature conditions should use capacitors with high operating temperatures; ② circuits working in humid environments should use good humidity resistance Sealed capacitors; ③ For capacitors used under low temperature conditions, cold-resistant capacitors should be used, which is particularly important for electrolytic capacitors, because ordinary electrolytic capacitors will freeze the electrolyte under low temperature conditions and fail.

(6) The requirements of the installation site should be considered when selecting capacitors.

There are many shapes of capacitors, and the shape and pin size of the capacitor should be selected according to the actual situation. For example, as a capacitor for high-frequency bypass, it is best to use a through-type capacitor, which not only facilitates installation, but also doubles as a binding post.

2. How to use capacitors and precautions

(1) Before the capacitor is used, the quality of the capacitor should be checked to prevent non-compliant capacitors from being installed in the circuit.

(2) When designing the component installation, the capacitor should be kept away from the heat source, otherwise the capacitor temperature will be too high and premature aging. When installing small-capacity capacitors and high-frequency circuit capacitors, a bracket should be used to hold up the capacitors to reduce the influence of distributed capacitance on the circuit.

(3) When installing the electrolytic capacitor into the circuit, it must be noted that its polarity cannot be reversed, otherwise it will cause a large increase in leakage current, causing the capacitor to quickly heat up and be damaged.

(4) The welding time of the capacitor is not too long, because the welding temperature for too long will be transmitted to the internal medium of the capacitor through the electrode pin, so that the performance of the medium will change.

(5) When the electrolytic capacitor needs to be used after long-term storage, the rated voltage cannot be directly added, otherwise there is a danger of explosion. The correct use method is: first add a smaller working voltage, and then gradually increase the voltage until the rated voltage and maintain it at this voltage for a not too long time, and then put it into use.

(6) When installing the capacitor in the circuit, the mark of the capacitor should be installed in a position that is easy to observe for verification and maintenance.

(7) When the capacitors are used in parallel, the total capacitance is equal to the sum of each capacity, but it should be noted that the working voltage after the capacitors are connected in parallel cannot exceed the lowest rated voltage.

(8) The series connection of capacitors can increase the withstand voltage. If two capacitors with the same capacity are connected in series, the total withstand voltage can be doubled; if two capacitors with different capacities are connected in series, the capacitor with a smaller capacitance will bear a higher voltage than a capacitor with a larger capacity.

(9) Polarized electrolytic capacitors are not allowed to be used under negative pressure. If this requirement is exceeded, non-polar electrolytic capacitors should be used or the negative poles of two capacitors of the same specification should be connected. The two positive poles should be connected to the circuit respectively. At this time, the actual capacitance is the equivalent capacitance after two capacitors are connected in series.

(10) When the electrolytic capacitor is used as a filter or bypass in a wider frequency band, in order to change the high-frequency characteristics, a small-capacity capacitor can be connected in parallel with the electrolytic capacitor, which can function as a bypass electrolytic capacitor.

(11) In high frequency circuits above 500MHz, leadless capacitors should be used. If a leaded capacitor is used, the lead wire should be as short as possible.

(12) When several large-capacity capacitors are used in series for filtering or bypassing, the leakage current of the capacitor will affect the voltage distribution, which may cause a capacitor to break down. At this time, a resistor with a resistance value less than the insulation resistance of the capacitor can be connected in parallel at both ends of each capacitor to ensure that the voltage division of each capacitor is uniform. The resistance of the resistor is generally between 100kΩ ~ lMΩ.

(13) When using variable capacitors, the degree of tightness when rotating the shaft should be moderate. Do not use capacitors that are too tight or loose. In addition, capacitors with bumps or short circuits should not be used.

(14) When using fine-tuning capacitors, pay attention to the tightness of the fine-tuning mechanism. The capacity of capacitors that are too loosely adjusted will not be stable, and capacitors that are too tightly adjusted are prone to damage during adjustment.

3. Judgment of capacitor quality

Insulation resistance test Insulation resistance test is generally the following methods:

The multimeter method is suitable for measuring the insulation resistance of non-polar capacitors. During the measurement, put the multimeter's electrical barrier in the 10kΩ block, and connect the two test leads to the two pins of the capacitor. For the capacitor with a small capacitance, the measured resistance value should be infinity or close to infinity; for larger capacity Of the capacitor, the pointer of the multimeter will first swing in a clockwise direction, and then it will quickly point to infinity. If the measured resistance value is less than 1MΩ, it means that the capacitor leakage is serious or the medium is damaged, and the capacitor cannot be used.

When testing, be careful not to hold the two leads of the capacitor by hand, otherwise the body resistance will affect the test results.