The basic unit of capacitor is Farad (F), however, this unit is too large and rarely used in the field labeling.

??Other unit relationships are as follows.
??1F=1000mF
??1mF=1000μF
??1μF = 1000nF
??1nF = 1000pF

??The deviation between the actual capacitance and the nominal capacitance of the capacitor is called the error, and the deviation range allowed is called the precision.
??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%)
??General capacitors commonly used Ⅰ, Ⅱ, Ⅲ level, electrolytic capacitor ware with Ⅳ, Ⅴ, Ⅵ level, according to the use of the selection.

??Rated voltage
??The highest DC voltage RMS value that can be continuously added to the capacitor under the minimum ambient temperature and rated ambient temperature, usually marked directly on the capacitor case, if the working voltage exceeds the capacitor's withstand voltage, the capacitor will break down and cause irreparable permanent damage.

??Insulation resistance
??The ratio between the DC voltage applied to the capacitor and the leakage current generated is called insulation resistance.
??When the capacitance is small, it mainly depends on the surface state of the capacitor, and when the capacity is 〉0.1uf, it mainly depends on the performance of the dielectric, and the larger the insulation resistance is, the better it is.
??Time constant of capacitance: The time constant is introduced to properly evaluate the insulation condition of large capacity capacitor, and he is equal to the product of insulation resistance and capacity of capacitor.

??Loss
??The energy consumed by a capacitor due to heat in a unit time under the action of electric field is called loss. Each type of capacitor has its allowable loss value in a certain frequency range. The loss of capacitor is mainly caused by dielectric loss, conductance loss and resistance of all metal parts of capacitor.
??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 alternating electric field, the loss of capacitor is not only related to leakage conduction, but also related to the periodic polarization establishment process.

??Frequency Characteristics
??As the frequency rises, the general capacitor capacity shows a decreasing pattern.

??Structural characteristics of aluminum electrolytic capacitors.
??The aluminum case and the rubber cap are hermetically sealed to form an electrolytic capacitor. Compared with other types of capacitors, the structure of aluminum electrolytic capacitors shows the following obvious characteristics.
??(1) The working medium of aluminum electrolytic capacitor is a thin layer of aluminum trioxide (Al2O3) generated on the surface of aluminum foil by anodic oxidation, and this oxide dielectric layer and the anode of the capacitor are combined into a complete system, which are interdependent and cannot be independent of each other; the electrodes and dielectric of what we usually call capacitors are independent of each other.
??(2) The anode of aluminum electrolytic capacitor is the aluminum foil with Al2O3 dielectric layer on the surface, and the cathode is not the negative foil we are used to, but the electrolyte of the capacitor.
??(3) The negative foil plays the role of electrical lead in the electrolytic capacitor, because the electrolyte as the cathode of the electrolytic capacitor cannot be directly connected to the external circuit, but must form an electrical path through another metal electrode and other parts of the circuit.
??(4) The anode aluminum foil and cathode aluminum foil of aluminum electrolytic capacitors are usually corroded aluminum foil, and the actual surface area is much larger than its apparent surface area, which is one of the reasons why aluminum electrolytic capacitors usually have large electric capacity. Due to the use of aluminum foil with numerous microscopic etched holes, liquid electrolyte is usually required to utilize its actual electrode area more effectively.
??(5) Since the dielectric oxide film of aluminum electrolytic capacitors is obtained by anodic oxidation and its thickness is proportional to the voltage applied to the anodic oxidation, the thickness of the dielectric layer of aluminum electrolytic capacitors can be controlled artificially and precisely from the principle point of view.