Capacitor

Polarized Capacitor

A Polarised Capacitor’s plates are polarity sensitive and are normally electrolytic. The positive lead is shown on the schematic with a "+" symbol. The negative lead is generally not shown on the schematic, but may be marked on the capacitor with a bar or "-" symbol. Polarized capacitors are the electrolytic type or tandalium type. Polarized capacitors have large leakage current if the voltage is inverted.

The reason an electrolytic capacitor is polarized is due to the fact that the aluminum oxide layer is held in place by the electric field, and when reverse-biased, it dissolves into the electrolyte. Connecting an electrolytic capacitor backwards

The farad

The farad (symbol: F) is the SI derived unit of electrical capacitance. It is named after the English physicist Michael Faraday. The Farad is the capacitance which stores a one-coulomb charge across a potential difference of one volt.

There are various increments of the unit of a farad.

• 1 mF (millifarad, one thousandth (10−3) of a farad) = 1000 μF = 1000000 nF
• 1 μF (microfarad, one millionth (10−6) of a farad) = 0.000 001 F = 1000 nF = 1000000 pF
• 1 nF (nanofarad, one billionth (10−9) of a farad) = 0.001 μF = 1000 pF
• 1 pF (picofarad, one trillionth (10−12) of a farad)

There is some confusion in various means of representation of these units. For example the mFd is the same as uF, which is also the same as the symbol "µ" as seen in "µF". Although the "mfd" represents "milliFarad" while "uF" represents the smaller "microFarad," some older capacitors show "mF" where it should be "µF".

Sometimes you see the millifarad marked as mF and sometimes mFd. The proper values are usually specified in farads (F), microfarads (μF), nanofarads (nF) and picofarads (pF). A value of 0.1 pF is about the smallest available in capacitors for general use in electronic design, since smaller ones would be dominated by the parasitic capacitances of other components, wiring or printed circuit boards. Capacitance values of 1 pF or lower can be achieved by twisting two short lengths of insulated wire together.