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Capacitors have five important characteristic parameters to understand: nominal capacitance, allowable deviation, rated voltage, insulation resistance, and loss.
1. Nominal capacitance and tolerance
The nominal capacitance is the capacitance that is marked on the capacitor.
The deviation between the actual capacitance of the capacitor and the nominal capacitance is called the error, and the allowable range of deviation is called accuracy.
Correspondence between accuracy level and allowable error: 00(01)-±1%, 0(02)-±2%, I-±5%, II-±10%, III-±20%, IV-(+20%-10%), V-(+50%-20%), VI-(+50%-30%).
Generally, capacitors are commonly used in Class I, II, and III, and electrolytic capacitors are in Class IV, V, and VI,depending on the application.
2. Rated voltage
The maximum DC voltage RMS that can be continuously applied to the capacitor at the lowest ambient temperature and the rated ambient temperature is generally directly marked on the capacitor casing.If the operating voltage exceeds the capacitor's withstand voltage, the capacitor breaks down, causing irreparable permanent damage.
3. Insulation resistance
The DC voltage is applied to the capacitor and generates a leakage current. The ratio between the two is called the insulation resistance.
When the capacitance is small, the insulation resistance mainly depends on the surface state of the capacitor. When the capacity is > 0.1 uf, the insulation resistance depends mainly on the properties of the medium.
The time constant of the capacitor: a time constant is introduced for proper evaluation of the insulation of the bulk capacitor, which is equal to the product of the insulation resistance of the capacitor and the capacity.
Under the action of an electric field, the energy consumed by heat in a unit of time is called loss. All types of capacitors specify their allowable values for loss over a range of frequencies. The loss of capacitance is mainly caused by dielectric loss,conductance loss and the resistance of all metal parts of the capacitor.
Under the action of the DC electric field, the loss of the capacitor exists in the form of leakage conduction loss, and the loss is generally small. Under the action of alternating electric field, the loss of capacitance is not only related to the leakage conductance, but also related to the periodic polarization establishment process.
5. Frequency characteristics
As the frequency increases, the capacitance of a general capacitor exhibits a decreasing law.
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