A capacitor is an element that stores electric quantity and electric energy (electric potential energy). One conductor is surrounded by another conductor, or the electric field lines emitted by one conductor are all terminated in the conductor system of the other conductor, which is called a capacitor, including paper capacitors, mica capacitors, air capacitors, tantalum capacitors, thin film capacitors (polyethylene film, polystyrene film, polypropylene film, etc.), aluminum capacitors and ceramic capacitors.
Electrolytic capacitors are usually used when very large capacitance values are required. Instead of using a very thin metal film layer as one of the electrodes, the electrolytic capacitor uses a semi-liquid electrolyte solution in the form of gel or paste as the second electrode.
Then, what parameters should be paid attention to in capacitor selection?
1. Nominal capacitance parameters.
The nominal value of the capacitance, C of the capacitance, is the most important of all capacitance characteristics. Usually picofarads (PF), nanofarads (NF) or microfarads（ μ F) Unit, and mark on the capacitor body in the form of numbers, letters or color strips. The selection of the capacitance is determined by the circuit configuration, but the value read on the capacitance side may not necessarily be its actual value.
2. Operating voltage parameters.
The operating voltage is another important capacitance characteristic, which is the maximum DC or AC voltage that can be applied to the capacitor without failure during its operating life. Generally, the working voltage printed on the side of the capacitor body refers to its DC working voltage (WVdc).
The common working DC voltages are 10V, 16V, 25V, 35V, 50V, 63v, 100V, 160V, 250V, 400V and 1000V, and are printed on the main body of the capacitor.
3. Precision tolerance parameter
Like resistance, capacitance has a positive and negative value, that is, accuracy tolerance. For low value capacitance generally less than 100 PF, it is expressed in picofara (± PF), and for higher value capacitance generally higher than 100 PF, it is expressed in percentage (±%).
The rated values of the capacitors are determined according to their proximity to the actual value compared with the rated nominal capacitance, and are provided with color strips or letters to indicate their actual tolerances. The most common tolerance variation of capacitance is 5% or 10%, but some plastic capacitors are rated as low as ± 1%.
4. Leakage current
The dielectric used to separate the conductive plates inside the capacitor is not a perfect insulator. When applied to the capacitor, a very small current will flow or leak through the dielectric due to the influence of the strong electric field established by the charge on the electrode plate.
This small DC current flowing in the range of nanoamperes (NA) is called capacitor leakage current. Leakage current is the result of electrons physically passing through the dielectric, around its edges, or through its leads. If the power supply voltage is removed, this will completely discharge the capacitor over time.