Tech Forum

Capacitors...there are MANY types, physical size/confirguration, chemistry, range and ratings. Choosing capacitor types depends on many variables. Size constraints, environment (i.e. voltage, temperature and frequency).  Most capacitor applications are either in AC circuits or used as timing circuits and storage. Application of the types depends a lot on cost also. Tantalums are more effective in some cicuits due to size and tight tolerance, but are also much more expensive over electrolytics. In RF circuits, typical types are mica, ceramic, glass, vacuum, air and even oil dialectrics. (there are 100s of them) Each application again depends on cost considerations, size and power levels.

I suggest if the reader wants more data on capacitors to consult one of many texts covering the theory and application of capacitors. A good start would be -Electronic Communications- by Robert Shrader (McGraw-Hill)

I 'think' Nuts & Volts did a series on capacitors many months ago that was good reference material.

There are many types of capacitors in common uses today, with many different properties. The key difference is the dielectric, or insulating material, between two metal plates. Here are some common types. All values shown are just rough approximations. Maximum capacity is expressed as nF (nanofarad), (µF) microfarad and F (Farad), maximum voltage as V (volt) or kV (kilovolt), and maximum frequency as Hz, (Hertz), kHz (kiklohertz) or MHz (megahertz).

   Air variable capacitor: used in radio receivers and low-power transmitters. Value may be changed, but bulky for capacity. Max. ~1 nF, ~1,000 V, ~100 MHz
   Vacuum variable capacitor: used in high-power transmitters. Value may be changed, but bulky for capacity. Max. ~1 nF, ~10,000 V, ~1,000 MHz
   Mica capacitor: used in receivers and transmitters. Value is stable. Max. ~1 nF, ~1,000 V, ~1,000 MHz
   Ceramic capacitor: used in digital electronics, receivers and transmitters. Value is less stable than mica, but more compact. Max. ~1 µF, ~1,000 V, ~1,000 MHz
   Al or Ta electrolytic capacitor: used in audio-frequency and power supplies. Al is cheap, Ta more efficient. All require a DC offset and are destroyed by true AC or reverse voltage. Max. ~100,000 µF, ~300 V, ~30,000 Hz
   Ultracapacitor: double-layer dielectric capacitors for power supplies and power backup. It offers very high capacity, and might eventually approach that of electrochemical cells, but has low voltage per capacitor, so are usually placed in series. Max. ~100 F, ~1 V. ~100 Hz

Other dielectrics are used, such as paper, plastics, glass and water (yes, pure water has a high dielectric constant, low conductivity and is environmentally friendly):
[url=http://www.rle.mit.edu/cehv/documents/34-Phy.Tech..pdf]http://www.rle.mit.edu/cehv/documents/34-Phy.Tech..pdf[/url]

Ceramic caps are small, low loss, low leakage and inexpensive but the temperature coefficient varies from COG (very stable, 2% is available) to X7R (moderately stable, 10% is available) to Z5U (typically +20%, -80% over temperature). There is another type of ceramic called porcelain that is used in microwave applications; ordinary ceramics are too lossy at those frequencies.

Aluminum Electrolytics are widely used because they are low cost and smaller than a film capacitor. Electrolytic caps are leaky (measured in microamps), have internal resistance that may be significant in some applications, and vary with temperature (+- 20% typically) and frequency (not useful at high frequency). After 6 or more months on the shelf, aluminum electrolytic caps should be re-formed to regain their voltage rating.

Tantalum caps are also an electrolytic but smaller than aluminum, lower leakage, better temperature coefficient and operate at higher temperature. Cost is more than aluminum but does not de-form (loose its voltage rating) with non use.

There are other capacitor types to consider: mica, polypropylene, metalized polyester,  polystyrene, and paper. Each has pluses and minuses to consider.