Capacitors

Capacitors are passive electronic components that store energy in the form of an electrostatic field. In their simplest form, capacitors consist of two conducting plates separated by an insulating material called the dielectric. The capacitance is directly proportional to the surface areas of the plates, and is inversely proportional to the separation between the plates. Capacitance also depends on the dielectric constant of the substance separating the plates.  The standard unit of capacitance is the farad, abbreviated F. This is a large unit; more common units are the microfarad, abbreviated µF (1 µF = 10-6 F) and the Pico farad, abbreviated pF (1 pF = 10-12 F).

Capacitors can be fixed or variable.  Fixed capacitors have a fixed capacitance value, nonadjustable.  Variable capacitors are adjustable (typically via a potentiometer), specific capacitance values can be set via adjustment mechanism.  Capacitors that may be safely operated with only one DC polarity are called "polar" or polarized. Capacitors, which can be operated without regard to polarity, are "nonpolar." Sometimes "polar" caps are combined in special ways to allow a limited ability to cope with reversing polarity; these are called "bipolar."

Dielectric materials that are available for capacitors include air, aluminum electrolytic, ceramic, glass, mica, multilayer, oil, paraffin paper, polycarbonate, polyester, polypropylene, polystyrene, tantalum, and teflon.  The capacitance range is an important specification that dictates the range of the capacitance of the capacitor.  Fixed capacitors have only one value.  Variable capacitors should have a range of capacitance values.  The capacitance tolerance is the precision of the capacitance produced by the capacitor measured as a percent of the capacitance.

Important packaging specifications to consider for capacitors include lead types, mounting options and operating temperature.  Lead types include: axial leads, radial lead, flying leads, no leads (SMT), tab leads, screw leads, gull wing leads, and j-leads.  Through-hole capacitors connect to a printed circuit board by inserting a terminal or lead through a hole in the board and soldering it to the opposite side.  Surface mount components are a direct response to cost reduction efforts that center around improved circuit board production. Automatic or robotic pick and place equipment can pick up and place surface mount style components on a printed circuit board faster and more accurately than previous technology would allow. Instead of a pin or terminal passing through a printed circuit board and being soldered on the opposite side, surface mount components utilize a flat solderable surface that is soldered to a flat solderable pad on the face of the printed circuit board. The pad on the circuit board is usually coated with a paste like formulation of solder and flux. With careful placement, surface mount style components on solder paste will stay in position until elevated temperatures, usually from an infrared oven, melt the solder paste and solder the mount's flat terminals to the circuit board's pad.  Bracket mount capacitors come with a bracket for mounting.

• Capacitor Engineering Bulletin: Polarization and Leakage Current Characterizations of High Voltage, Film Capacitors
  It is an unfortunate fact that "real" capacitors differ from ideal capacitors in many respects. Real capacitors include parasitic inductance and resistance, and vary in capacitance with temperature,...
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• Performance and Safety of Large Energy Discharge Capacitors
  Large capacitors used in the pulsed power industry offer a unique challenge from both a design and application standpoint. The capacitors are getting larger, storing more energy, and delivering that...
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• Pulse Power Capacitors - US Army Research Program Has Yielded Millisecond Discharge High-Voltage Film Capacitors with Sub-Microsecond Discharge on the Horizon
  The US Army Research Laboratory has sponsored a capacitor development program for film dielectric capacitors. The program has evaluated dielectric materials for high energy density capacitors from...
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Part Numbers for Capacitors