Options and Notes:
1. The pressure
(1) In the circuit, the actual voltage of the capacitor shall not exceed its voltage withstand value.
(2) In the filter circuit, the voltage withstand value of the capacitor shall not be less than 1.42 times the effective value of the AC voltage. (3) When using polar capacitors such as electrolytic capacitors, pay attention not to connect the positive and negative poles in reverse. (4) The voltage resistance of linear power capacitor is generally retained by 40%.
2. The capacitance
(1) Large filter capacitors should be selected, generally hundreds to thousands of micromethods should be adopted.
(2) In order to achieve better filtering effect, a large and a small capacitor can be used, with a capacity difference of at least two orders of magnitude.
(3) When there is no corresponding capacity in the nominal series, it can be obtained by parallel or series capacitance.
(4) Under normal circumstances, when the fixed capacitor is damaged, the same value, the same voltage capacitor should be replaced.
(5) The general circuit allows the same capacity of large voltage capacitor to replace the small voltage capacitor, and vice versa.
6) Polar electrolytic capacitors shall not be used to replace polar capacitors, nor shall polar capacitors be used to replace polar capacitors.
(7) The coupling capacitance of audio circuit is generally 0.1~LPF.
Type 3.
Electrolytic capacitors are used in power filter and low frequency decoupling circuits.
(2) Porcelain dielectric vessel and mica capacitor are used for high frequency and high voltage circuits.
(3) In the resonant circuit, can choose mica capacitor, ceramic capacitor and organic film capacitor.
(4) When dc is isolated, paper capacitor, polyester capacitor, mica capacitor, electrolytic capacitor, ceramic capacitor and so on can be selected.
4. Allow for deviations.
The allowable deviation of capacitors used in oscillating or delay circuits should be as small as possible (generally less than 5%), and in other cases slightly larger (generally 10%~20%).
5. Check capacitor before use - Check polarity capacitor.
For polar capacitors with large capacity, a pointer multimeter can be used to measure whether the charge and discharge performance is normal, so as to judge the quality of the capacitor.
6. Basic properties of capacitors.
Charging and discharging are the basic properties of capacitors.
(1) charge
When the capacitor is connected to a power supply (or two potential differences), the capacitor will be charged (i.e. the capacitor has the same amount of charge on both plates). Charging direction: positive pole of power supply (or high potential point) → plate connected to positive pole or high potential point → another plate → negative pole of power supply (or low potential point).
When charging, the voltage at both ends of the capacitor increases with the charging progress. When the resistance value of power supply and charging circuit is fixed, the two ends of the capacitor are increased according to the index, and the charging current is reduced according to the index.
(2) the discharge
The capacitor will discharge when connected to the load. Discharge direction: capacitor plate with positive charge → load → another plate, until the charge on the plate is zero, the discharge ends.
When discharging, the voltage at both ends of the capacitor decreases with discharge, and the voltage and discharge current at both ends of the capacitor decreases exponentially.