⑴ Principle for capacitor selection consideration

Adequate margin should be left and not used reluctantly, otherwise unnecessary damage may occur. The following points should be considered:

① The type of capacitor should be selected according to the circuit requirements;

② Reasonably determine the capacitance and allowable deviation of capacitors;

③ The operating voltage of the selected capacitor should meet the circuit requirements;

④ Prioritize the use of capacitors with high insulation resistance, low dielectric loss, and low leakage current;

⑤ Capacitors should be selected based on their working environment.

⑵ Conventional selection of capacitors in circuits

① Selection of capacitor types under different circuit conditions

For low-frequency circuits and DC circuits with low requirements, paper capacitors or low-frequency ceramic capacitors can generally be used. In high-frequency circuits, when high electrical performance is required, mica capacitors, high-frequency ceramic capacitors, or through-hole ceramic capacitors can be selected. Plastic film capacitors can be used in high demand mid frequency and low-frequency circuits. Aluminum electrolytic capacitors are generally used in power filtering and decoupling circuits. For circuits that require high reliability and stability, mica capacitors, paint film capacitors, or tantalum electrolytic capacitors should be selected. For high-voltage circuits, high-voltage ceramic capacitors or other types of high-voltage capacitors should be selected. For tuning circuits, variable capacitors and fine-tuning capacitors should be selected.

② Selection of capacitor capacity under different circuit conditions

In low-frequency coupling and decoupling circuits, the capacitance requirements for capacitors are generally not very strict, as long as a slightly larger capacitance is selected according to the calculated value. In timing circuits, oscillation circuits, and tone control circuits, the capacitance requirements for capacitors are relatively strict. Therefore, the nominal value of the selected capacitance should be as consistent or close as possible to the calculated capacitance value, and capacitors with high accuracy should be selected as much as possible. In some special circuits, the capacitance of capacitors is often required to be very precise. In this case, high-precision capacitors with allowable deviations within the range of ± 0.1% to ± 0.5% should be selected.

③ Selection of capacitors for occasions with high voltage requirements

In general, the rated voltage of capacitors should be (1.2-1.3) times the actual operating voltage. For circuits with high working environment temperature or poor stability, the rated voltage of capacitors should be considered for derating use. The rated voltage of a capacitor generally refers to the DC voltage. If it is to be used in AC circuits, it should be selected according to the characteristics and specifications of the capacitor; If it is to be used in pulsating circuits, it should be selected based on the total sum of AC and DC components not exceeding the rated voltage of the capacitor.

④ Selection of capacitors for occasions with high environmental requirements

(a) Capacitors used under high temperature conditions should choose capacitors with high operating temperatures.

(b) For circuits operating in humid environments, sealed capacitors with good moisture resistance should be selected.

(c) Capacitors used under low temperature conditions should be cold resistant. This is particularly important for electrolytic capacitors, as ordinary electrolytic capacitors can cause the electrolyte to freeze and fail under low temperature conditions.

(d) When selecting capacitors, the requirements of the installation site should be considered. There are many shapes of capacitors, and the shape and pin size of capacitors should be selected according to the actual situation.

⑤ The general selection of common circuit capacitors (but considering that the performance parameters of the capacitor are closely related to the conditions of the usage environment)

(a) High frequency bypass: ceramic capacitors, mica capacitors, glass film capacitors, polyester capacitors.

(b) Low frequency bypass: paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polyester capacitors.

(c) Filtering: Aluminum electrolytic capacitors, paper capacitors, composite paper capacitors, liquid tantalum capacitors.

(d) Tuning: Ceramic capacitors, mica capacitors, glass film capacitors, polystyrene capacitors.

(e) High frequency coupling: ceramic capacitors, mica capacitors, polystyrene capacitors.

(f) Low frequency coupling: paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polyester capacitors, solid tantalum capacitors.

When welding, reduce the use of highly reactive and acidic fluxes, use non cleaning fluxes, and pay attention to welding conditions.