Usually the capacitors used in high frequency and ultra-high frequency circuits should be selected mica capacitors, glass glaze capacitors or high frequency (Class I) porcelain dielectric capacitors. And paper dielectric capacitors, metalized paper dielectric capacitors, organic film capacitors, low-frequency (Class II, Class III) porcelain dielectric capacitors, electrolytic capacitors, etc., are generally used in medium and low frequency circuits. In tuned circuits, solid dielectric sealed variable capacitors, air dielectric capacitors and fine-tuning capacitors are available.
The main parameters of the selected capacitors (including nominal capacity, allowable deviation, rated operating voltage, insulation resistance, etc.) and external dimensions should be consistent with the application of circuit requirements. Selection of electrolytic capacitors Electrolytic capacitors are mainly used for power supply | regulator circuit or medium and low frequency circuits for power supply filtering (100~1000μF), decoupling (47~220μF), low-frequency circuit interstage coupling (1~22μF), low-frequency bypass, time constant setting, isolation of DC and other capacitors used.
General power supply circuits and medium and low frequency circuits, you can choose aluminum electrolytic capacitors. Speakers with crossover capacitors, TV S correction capacitors and motor starting capacitors, etc., can choose non-polarized aluminum electrolytic capacitors. Communication equipment and a variety of high-precision electronic equipment in the circuit, can use non-solid tantalum electrolytic capacitors or niobium electrolytic capacitors.
The choice of electrolytic capacitors, should pay attention to its outer surface should be smooth, no depression or mutilation, the plastic cover should be intact, the sign should be clear, the pins can not be loose, the pins to the Department of electrolyte leakage can not be.
Solid organic dielectric capacitors in the selection of solid organic dielectric capacitors, the most used organic film dielectric capacitors, such as polyester capacitors (CL series), polystyrene capacitors (CB series) and polypropylene capacitors (CBB series).
Polyester capacitors can be used in recorders, televisions and other electronic equipment for the low frequency circuit as decoupling, bypass, isolation of DC capacitors.
Polystyrene capacitors can be used in audio circuits and high-voltage pulse circuits, and cannot be used in high-frequency circuits.
Polypropylene capacitors have better high-frequency characteristics than polyester capacitors and polystyrene capacitors, in addition to television, audio and other electronic equipment can be used for DC circuits, high-frequency pulse circuits, can be used as a.c. motor starting and running capacitors. Solid inorganic dielectric capacitors in the selection of solid inorganic quality capacitors, the most widely used is porcelain capacitors, especially porcelain capacitors, monolithic capacitors and leadless porcelain capacitors.
High-frequency circuits and ultra-high-frequency circuits should be selected I dielectric capacitors, medium and low-frequency circuits can be selected II dielectric capacitors. III dielectric capacitors can only be used for low-frequency circuits, but can not be used for medium and high-frequency circuits.
High-frequency circuits in the coupling capacitor, bypass capacitors and tuning circuits in the fixed capacitors, can choose glass glaze capacitors or mica capacitors. Selection of variable capacitors Variable capacitors are mainly used in tuned circuits. Air dielectric variable capacitors were used in the early days of tube radios and communication equipment, etc., and are now rarely used in electronic equipment; while solid dielectric variable capacitors are still widely used.
Amplitude modulation radio can generally choose sealed double-concatenated variable capacitors, AM / FM radios and recorders can choose sealed four-concatenated variable capacitors (sealed four-concatenated variable capacitor shell is usually also with a film-type semi-variable capacitors).