2021-03-12
566

1. Capacitance in the circuit is generally represented by "C" plus a number (for example, C13 represents the capacitance numbered 13). Capacitance is made up of two pieces of metal film close to each other and separated by insulating material. The main characteristic of capacitance is to separate direct current from alternating current.

The size of the capacitance is the size of the stored electric energy. The blocking effect of the capacitance on the AC signal is called capacitive reactance, which is related to the frequency and capacitance of the AC signal.

Capacitive reactance XC = 1 / 2 π F C (f represents the frequency of AC signal, C represents the capacitance) the types of capacitors commonly used in telephone include electrolytic capacitor, ceramic chip capacitor, chip capacitor, single stone capacitor, tantalum capacitor and polyester capacitor.

2. Identification method: the identification method of capacitance is basically the same as that of resistance, which is divided into three methods: direct standard method, color standard method and digital standard method. The basic unit of capacitance is expressed in farad (f), and other units are: millifahrenheit (MF), micro Fahrenheit (UF), nano Fahrenheit (NF) and Pico Fahrenheit (PF). Among them: 1 farad = 103mf = 106mf = 109nf = 1012pf

The capacitance with large capacity is directly indicated on the capacitance, such as 10 UF / 16V

Capacitance with small capacity is indicated by letters or numbers

Letter representation: 1m = 1000 UF 1P2 = 1.2pF 1n = 1000pf

Digital representation: generally, three digits are used to represent the capacity, the first two digits are significant digits, and the third digit is multiplier.

3. Capacitance error table

Symbol F G J K L M

The allowable error is ± 1% ± 2% ± 5% ± 10% ± 15% ± 20%

Rated voltage

The effective value of the maximum DC voltage that can be continuously applied to the capacitor at the lowest ambient temperature and rated ambient temperature is usually directly marked on the capacitor shell. If the working voltage exceeds the withstand voltage of the capacitor, the capacitor will break down and cause irreparable permanent damage.

insulation resistance

DC voltage is applied to the capacitor and produces leakage current. The ratio of the two is called insulation resistance.

For ceramic capacitors and film capacitors, the greater the insulation resistance, the better. For aluminum electrolytic capacitors, the smaller the insulation resistance, the better. Time constant of capacitance: the time constant is introduced to properly evaluate the insulation condition of large capacity capacitance, which is equal to the product of insulation resistance and capacity of capacitance.

loss

Under the action of electric field, the energy consumed in unit time due to heating is called loss. All kinds of capacitors have their allowable loss values in a certain frequency range. The loss of capacitors is mainly caused by dielectric loss, conductivity loss and resistance of all metal parts of capacitors.

Under the action of DC electric field, the loss of capacitor exists in the form of leakage conduction loss, which is generally small. Under the action of AC electric field, the loss of capacitor is not only related to leakage conduction, but also related to the process of periodic polarization establishment.

frequency characteristic

With the increase of frequency, the capacitance of general capacitor decreases.

temperature coefficient

In a certain temperature range, the relative change value of capacitance is calculated when the temperature changes by 1 ℃. The smaller the temperature coefficient, the better.

Common formula

In the formula of parallel plate capacitor, C = ε s / 4 π KD