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BEAM From the Ground Up is a BEAM Reference Library site.

Basics of passive components -- capacitors
Storing your charge for you

A capacitor is a device that stores charge (namely, electrons). A capacitor functions much like a battery, but charges and discharges much more efficiently (batteries, though, can store much more charge).

A basic capacitor is made up of two conductors separated by an insulator, or dielectric. The dielectric can be made of paper, plastic, mica, ceramic, glass, a vacuum or nearly any other nonconductive material. Capacitor electron storing ability (it's capacitance) is measured in Farads. One Farad is actually a huge amount of charge (6,280,000,000,000,000,000 electrons to be exact), so we usually rate capacitors in microfarads (uF = 0.000,001F) and picofarads (pF = 0.000,000,000,001F ). Capacitors are also graded by their breakdown (i.e., smoke) voltage. Capacitors rated for lower voltages are generally smaller; you don't want to use too low a voltage rating, though, unless you enjoy repeatedly replacing capacitors in your creation.


Non-polarized fixed capacitor TBRTBR

A non-polarized ("non polar") capacitor is a type of capacitor that has no implicit polarity -- it can be connected either way in a circuit. Ceramic, mica and some electrolytic capacitors are non-polarized. You'll also sometimes hear people call them "bipolar" capacitors.


Polarized fixed capacitor TBR

A polarized ("polar") capacitor is a type of capacitor that have implicit polarity -- it can only be connected one way in a circuit. The positive lead is shown on the schematic (and often on the capacitor) with a little "+" 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 generally electrolytics, meaning that the dielectric is made up of a thin layer of oxide formed on the aluminum or tantalum foil conductor.

Note that you really need to pay attention to correctly hooking a polarized capacitor up (both with respect to polarity, as well as not pushing a capacitor past its rated voltage). If you "push" a polarized capacitor hard enough, it is possible to begin 'electrolyzing' the moist electrolyte. Modern electrolytic capacitors usually have a pressure relief vent to prevent popping the aluminum can (but don't bet your eyesight on this).


Variable capacitor

A variable capacitor is a capacitor whose capacitance (surprise!) can be varied. They are usually made up of one or more moving plates and one or more non moving plates separated by the dielectric. By moving the plates, we can vary the capacitance. These types of capacitor are usually used in radio tuners, oscillators, etc. (we won't be messing with them in BEAMbots ).


Capacitors are marked with their capacitance values in some more-or-less cryptic fashion. Most capacitors are marked with one code; surface mount capacitors are marked differently.

BEAM Usage
I have a writeup on capacitor usage in BEAMbots here. This also includes details on capacitor markings and more details on the various types of caps.

For more information...

"SatCure" has a tutorial page on capacitors here.

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