Capacitors_demystified_ee133

Capacitors_demystified_ee133 - EE133 Winter 2001 Capacitors...

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EE133 – Winter 2001 Capacitors Demystified 1 Capacitors Demystified: Practical information on capacitor usage in EE133 Why do I need to read this? Despite all we know about capacitors from previous exposure (including everyone’s favorite capacitor fact – ‘Well, I know the voltage across a capacitor cannot change instantaneously!’), there is a quite a bit about the (good) properties and (bad) non- idealities of these devices that effects the way they are actually used in RF circuits. So, with the practical predisposition of EE133, let’s take another look at our old friend… The Starting Line-up Like fine cheeses or candy bars, capacitors come in wide variety of sizes, shapes, and prices- and they can be made from a host of different materials (except for nougat). However, all of these characteristics are interrelated because, in classic Murphy fashion, a capacitor that ranks well in one or two of these properties is usually terrible in the other categories. The physical size and shape are only of marginal importance to us since the former is pretty much irrelevant at the size of our solder board and the latter is only for aesthetics (which is a foreign term to EE’s anyway). But, the composite material is an important factor in determining the behavior of the capacitor, so for the purpose of this class we will identify four major types (Refer to page 21 of “The Art of Electronics” by Horowitz & Hill for a more complete listing of capacitor families): Material/Comments Picture Ceramics (10 pF – 22μF): Inexpensive, but not good at high frequencies Tantalum (0.1μF – 500μF): Polarized, low inductance Electrolytic (0.1μF – 75μF): Polarized, not good at high frequencies (NOTE: longer lead is usually the positive terminal) Silver Mica (1 pF – 4.7 nF): Expensive with only small capacitive values, but great for RF
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EE133 – Winter 2001 Capacitors Demystified 2 Some Commonly Asked Questions about Capacitors Resistors are color-coded. How are capacitors labeled? I have attached a sheet that describes the various notations for labeling capacitors with their value. But, the system is not nearly as well standardized as the codes for resistors, so often it is a good idea to just measure them. Especially since students often return capacitors to the wrong bins. What was that loud popping sound and why is there goo on my circuit? Congratulations, you have just blown up your first capacitor! ‘Popping a cap’ as it is called by EE’s (though it has certain other connotations in the rap community) is almost always the result of ignoring its polarization. Polarized capacitors are designed to only function if one lead (generally labeled by a plus ’+’ sign) is always at a higher DC potential than the other lead (generally labeled by a negative ‘-‘ sign, or an arrow with a negative sign pointing to it). Only use polarized capacitors if you can actually assure, based on the operation of the circuit, that the two nodes between which the capacitor is
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Capacitors_demystified_ee133 - EE133 Winter 2001 Capacitors...

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