Capacitor selection and differentiation

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The characteristics of capacitors are mainly DC through AC (in the circuit, blocking DC current, through the action of AC current), often used for interpole coupling, filtering, decoupling, bypass and signal tuning.

1. In the power supply circuit, the rectifier circuit turns the AC into pulsating DC, and after the rectifier circuit, a larger capacity electrolytic capacitor is connected, and its charge-discharge characteristics are used to make the pulsating DC voltage after rectification become a relatively stable DC voltage.

In practice, in order to prevent the power supply voltage of each part of the circuit from changing due to load changes, electrolytic capacitors of tens to hundreds of microfarads are generally connected to the output end of the power supply and the power input end of the load. Since large-capacity electrolytic capacitors generally have a certain inductance, they cannot effectively filter out high-frequency and pulse interference signals, so a capacitor with a capacity of 0.001--0.lpF is connected in parallel at both ends to filter out high-frequency and pulse interference.

2. Coupling effect: In the process of transmission and amplification of low-frequency signals, in order to prevent the static working points of the front and rear two-stage circuits from affecting each other, capacitor coupling is often used. In order to prevent excessive loss of low-frequency components in the signal, electrolytic capacitors with larger capacity are generally used.

The main indicator of capacitance

1. The nominal capacity of the capacitor - the capacitance value indicated on the capacitor.

2. The allowable error of the capacitor

(1) There is always a certain deviation between the actual capacity of the capacitor and the nominal capacity, which is called the error. Because this error is within the allowable range specified by the national standard, it is called the allowable error. The allowable error is divided into 5 levels: ±1% (00 level), ±2% (0 level), ±5% (I.), ±10% (II.), ±20% (III.).

(2) When the value on the capacitor is tens, hundreds, and thousands, the unit is pF. For example, 3300 means 3300pF.

3. The rated working voltage of the capacitor

(1) The DC voltage value of the capacitor that works for a long time without causing any damage to the dielectric electrical properties, also known as withstand voltage. When the capacitor is operating, the maximum value of the actual applied voltage cannot exceed the rated operating voltage. If an AC voltage is added across the capacitor, the maximum AC voltage applied must not exceed its rated operating voltage.

(2) Some capacitors use three digits to indicate the nominal capacity, where the first two digits represent the significant digits of the capacitance, and the last digit word indicates how many zeros are added after the significant digits, and the unit is also pF.

4. Quality parameters of capacitors:

(1) Voltage: DC rated withstand voltage value

(2) Peak voltage: the peak of DC voltage that can be withheld

(3) Capacity: the capacitance of the electrolytic capacitor

(4) Deviation: the positive and negative deviation value of the capacity

(5) Leakage current: leakage current under rated withstand voltage

(6) Ripple current: The allowable ripple current at a frequency of 120Hz at a certain temperature (such as 40°C) is equivalent to the internal resistance of the electrolytic capacitor

(7) Temperature: working temperature range (such as -55~+105°C)