Capacitor selection and matters needing attention

1. Withstand pressure

(1) In the circuit, the capacitor to withstand the actual voltage should not exceed its withstand voltage value.

(2) In the filter circuit, the voltage of the capacitor should not be less than 1.42 times of the effective value of the AC voltage. (3) When using polar capacitors such as electrolytic capacitors, pay attention to the positive and negative terminals do not reverse. (4) For capacitor voltage resistance of linear power supply, generally consider reserving 40%.

2. Electrical capacity

(1) The filter capacitance should be larger, which should generally be above several hundred to several thousand micromethods.

(2) In order to achieve a better filtering effect, a large and a small capacitor with a capacity difference of at least two orders of magnitude can be used

(3) When there is no corresponding capacity in the nominal series, it can be obtained by means of parallel or series capacitors.

(4) Under normal circumstances, the fixed value capacitor is damaged, should be replaced with the same value, the same voltage withstand capacitor.

(5) The general circuit allows the replacement of a small voltage capacitor with a large capacitance of the same capacity, and vice versa.

(6) A non-polar capacitor cannot be replaced by a polar electrolytic capacitor, nor can a non-polar capacitor be replaced by a polar capacitor.

(7) audio circuit coupling capacitance generally desirable 0.1~lpF.

Step 3: Type

(1) In the power filter and low-frequency decoupling circuit, electrolytic capacitors should be selected.

(2) In high frequency circuit and high voltage circuit, porcelain dielectric vessel and mica capacitor should be selected.

(3) In resonant circuit, mica capacitor, ceramic capacitor and organic film capacitor can be selected.

(4) For DC separation, can choose paper capacitor, polyester capacitor, mica capacitor, electrolytic capacitor and ceramic capacitor.

4. Allow deviations

For capacitors used in oscillating or delay circuits, the allowable deviation should be as small as possible (generally less than 5%). In other cases, the allowable deviation can be slightly larger (generally 10%~20%).

5. Inspection of capacitors before use -- inspection of polar capacitors

For polar capacitors with large capacity, a pointer multimeter can be used to measure whether the charging and discharging performance is normal, so as to judge whether the capacitor is good or bad.

3. Basic properties of capacitors

The basic properties of a capacitor are charge and discharge.

(1) Charging

When the capacitor is connected to the power supply (or two points with potential difference), it will charge the capacitor (that is, the two plates of the capacitor have equal dissimilar charge). The charging direction is: the positive electrode of the power supply (or high potential point)→ the plate connected to the positive or high potential point → the other plate → the negative electrode of the power supply (or low potential point).

When charging, the voltage at both ends of the capacitor increases with the progress of charging. When the resistance value of the power supply and the charging loop is fixed, the capacitor ends increase according to the exponential law, and the charging current decreases according to the exponential law.

(2) discharge

When the capacitor is connected to the load, the capacitor will discharge. The discharge direction is: the positive charge of the capacitor electrode → the load → the other plate, until the charge on the plate is zero, the discharge is over.

Current direction Current direction

When discharging, the voltage at both ends of the capacitor decreases with the discharge process. When the resistance value of the discharge loop is fixed, the voltage at both ends of the capacitor and the discharge current decrease exponentially.