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High voltage capacitors are passive electronic components that store charge and energy for use in high voltage applications. They consist of two conducting plates separated by an insulating material called the dielectric. Film capacitors are high voltage capacitors made out of plastic. There are two basic types. Film-foil capacitors include one or more layers of a plastic film dielectric wound alternately with metal foil electrodes. Metallized film capacitors consist of a film dielectric on which the metal electrode has been vapor-deposited. The layers are wound into a convoluted roll so that the electrodes extend beyond the dielectric films. Capacitance, a measure of energy storage ability, is typically expressed as C = K A/D, where A is the area of the electrodes, D is their separation, and K is a function of the dielectric between the electrodes.
Selecting High Voltage Capacitors
Selecting high voltage capacitors requires an analysis of dielectric materials.
- Aluminum electrolytic capacitors are polar devices that feature a high volumetric density but cannot withstand reverse voltages.
- Ceramic capacitors are made of resistive ceramic materials and provide bonded metal contacts. Examples include ceramic Z5U, a Class III ceramic dielectric, and ceramic X7R, a temperature-stable material that is suitable for bypassing and coupling applications.
- Glass, mica, oil, air, and paraffin paper are other commonly used dielectric materials.
- Polycarbonate, polypropylene, and polystyrene are suitable for applications such as fillers and timers.
- Tantalum, a metallic element, is used to make a variety of alloys.
- Niobium, a hard ceramic material characterized by high conductivity, has similar chemical properties to tantalum.
Performance specifications for high voltage capacitors include capacitance range and capacitance tolerance, a percentage of total capacitance. Working DC voltage, insulation resistance, dissipation factor, and temperature coefficient are additional considerations.
- Working DC voltage (WVDC) is the maximum voltage that can be applied continuously at any temperature between a lower category temperature and the rated temperature.
- Insulation resistance is the ratio between an applied DC voltage and the resulting leakage current.
- The dissipation factor (DF) is the ratio between the resistive and reactive parts of a capacitor’s impedance when a sinusoidal voltage at a specified frequency is applied.
- The temperature coefficient is the change in capacitance measured over a range of temperatures.
High voltage capacitors can use axial, radial, flying, tab, screw, gull wing, or J-leads. Some devices bolt into place while others require or include mounting brackets. Pole-mounted capacitors are also available.
- Surface mount technology (SMT) adds high voltage capacitors to a printed circuit board (PCB) by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board.
- Through hole technology (THT) mounts components on a PCB by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board.
High voltage capacitors are packaged in tape reels, trays or rails, shipping tubes or stick magazines, and in bulk packs. Tape reel assemblies include a carrier tape with embossed cavities for storing individual components. A cover tape seals the carrier tape in place and the composite tape is wound on a reel that can be loaded into industry-standard, pick-and-place board assembly equipment. High voltage capacitors with leads on four sides are often packed in trays or rails that are made of carbon-powder or fiber materials and molded into rectangular outlines that contain matrices of uniformly spaced pockets. Shipping tubes or stick magazines are containers made of rigid polyvinylchloride (PVC) and extruded in industry-standard sizes. Bulk packs are used to distribute components as individual parts.
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Aluminum Electrolytic Capacitors
Aluminum electrolytic capacitors use an electrolytic process to form the dielectric. Wet electrolytic capacitors have a moist electrolyte. Dry or solid electrolytic capacitors do not.
Capacitors are electronic components used for storing charge and energy. In their simplest form, capacitors consist of two conducting plates separated by an insulating material called the dielectric.
Ceramic capacitors have a dielectric made of ceramic materials.
Chip capacitors or surface mount capacitors do not have leads.
Film capacitors are insulated with polyester, polycarbonate, polypropylene, polystyrene, or other dielectric materials.
Tantalum capacitors are used in smaller electronic devices including portable telephones, pagers, personal computers, and automotive electronics.
Ultracapacitors store charges (energy) by physically separating positive and negative charges (unlike batteries which do so chemically). Very high power densities can be achieved by this method.