Capacitors are as simple as two plates of metal separated by a insulator called dielectric. When you make large values of capacitors, just two films or plates will not do, so long strips of metalized films used to get bigger values. Electrolytic capacitors use an electro chemical phenomena to form a very thin dielectric in microns, this enables large values in small size.
Capacitors ares like reservoirs of charge. In the Water Analogy they are like your Overhead Tank or Sump. It can be like a Lake or Pond, when water is the charge.
Ceramic capacitors are the most economic in the lower values like pf and nf, Electrolytic Capacitors which have polarities that should strictly be observed, are used above 1uf. Plastic caps have temperature and environment stability and are more in use in Instrumentation and Industrial Electronics in place of ceramic, mica or paper caps. Mica and Paper caps have become rare today.
Ultracaps and supercaps are the new innovation for the years 2000+. They have values in farads but quite small in size. Electrolytic caps become too big in size beyond 10,000 uF, hence supercaps have a future.Tantalum electrolytic caps are used in Instrumentation and computers, a bit expensive but much smaller.
Important Point to note before circuit design
- Voltage Rating is the most important to ensure long operational life and reliability, overrate 1.5 times. If max voltage capacitor will face is 15V then use 25V rating capacitor.
- SMPS-ESR means Equivalent series Resistance at high SMPS frequency, the capacitor adds a inductance by the way it is constructed. So use Caps in Parallel for Lower ESR.
- For Oscillators and A-D Converters choose Plastic Multilayer Caps as they leak less and have low temperature sensitivity.
- Ceramic and Elcap are for Filtering and decoupling and are low priced too.
- Electrolytic Caps back2back in series can make non-polar caps for Low current AC like AF, as used in a cross-over network of loud speaker of stereo system.
- NPO “negative-positive-zero” caps and most plastic caps can be used for Oscillators as the negative and positive temperature coefficients are very less.
- Most Plastic Caps are wound, so have inductance, which makes them unsuitable for filtering RF.
- X and Y caps are special EMI-RFI filter caps, suppress interference and are safe at high voltage.