A capacitor consists of two conductors separated by an insulator, or dielectric. Its ability to store charge is the capacitance C. Applying voltage to store charge is called charging the capacitor; short-circuiting the two leads or terminals of the capacitor to neutralize the charge is called discharging the capacitor. Schematic symbols for C are summarized in Fig. 16–30. <a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg::::/sites/dl/free/0072988215/363997/fig16_30.jpg','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (17.0K)</a>
The unit of capacitance is the farad. One farad of capacitance stores one coulomb of charge with one volt applied. Practical capacitors have much smaller capacitance values from 1 pF to 1000 F. A capacitance of 1 pF is 1 x 10-12 F; 1 µF = 1 x 10-6F; and 1 nF = 1 x 10-9F.
Q = CV, where Q is the charge in coulombs, C the capacitance in farads, and V is the potential difference across the capacitor in volts.
Capacitance increases with larger plate area and less distance between plates.
The ratio of charge stored in different insulators to the charge stored in air is the dielectric constant of the material. Air or vacuum has a dielectric constant Kε of 1.
The most common types of commercial capacitors are air, plastic film, paper, mica, ceramic, and electrolytic. Electrolytics are the only capacitors that require observing polarity when connecting to a circuit. The different types are compared in Table 16–2.
Capacitors are coded to indicate their capacitance in either microfarads (µF) or picofarads (pF).
For parallel capacitors, CT + C1 + C2 + C3 + ··· + etc.
For series capacitors, <a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg::::/sites/dl/free/0072988215/363997/16_09sum.jpg','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (2.0K)</a>
The electric field of a capacitance has stored energy ξ = ½ CV2, where V is volts, C is in farads, and electric energy is in joules.
When checked with an analog ohmmeter, a good capacitor shows charging current, and then the ohmmeter reading steadies at the leakage resistance. All types except electrolytics normally have very high leakage resistance such as 100,000 MΩ or more. Electrolytics have more leakage current; a typical leakage resistance is about 500 kΩ to 10 MΩ.
