Electronics notes/Capacitors

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See also Category:Electronics.

The symbols used in circuits


Capacitors, regularly abbreviated as caps, take an electrostatic charge, which will remain until discharged.

They are used in signal conditioning, noise reduction, filtering, and some short-term energy storage (e.g. dealing with transient loads, which for fast-switching ICs also amounts to noise reduction), and more.


Some basics and theory

Charging

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


See also:




Capacitor marking

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Rating considerations

Voltage

You'll often know the maximum voltage a capacitor will be exposed to, though you may wish to consider your circuit's ripple current, peak current, fault current, possible voltage reversal, and more.

The voltage spec seems a nominal one, in that it can probably handle more, but you may shorten its lifespan, and with significantly more you will make it fail.


Temperature (spec / behaviour)

Dielectrics / types / designs

Electrolytic capacitors (type)

Some electrolytic capacitors - various radial lead (both leads on one end) and some axial lead (both ends), and SMD variants ( top left)


  • polarized - reverse biasing these will cause an electrochemical reaction in the electrolyte, causing it to heat, expand, vent, and likely short in the process. The common cylinder shape has indentations on the top that will bulge and fold open (a safety feature, and one that makes failed electrolytic caps easy enough to identify)
  • higher inductance than many other types, so not so useful for high-frequency applications
  • cheap, so common wherever the frequency or stability limitations are not a problem
  • often can-shaped, and regularly black, grey, blue, and with a stripe indicating its negative lead
  • Capacitors with more than ~1µF are regularly electrolytic capacitors because of cost. Other types are also used, but often when electrolytic's polarization or lower stability is a problem.



Ceramic capacitors (type)

Various types of ceramic capacitors


http://en.wikipedia.org/wiki/Ceramic_capacitor


Tantalum capacitors (type)




Silver mica capacitors (type)

http://en.wikipedia.org/wiki/Silver_mica_capacitor



Plastic film

See also


Variable Capacitor (design)

Adjustable capacitors, often looking similar to potmeters.


De-coupling capacitor, a.k.a. bypass capacitor (application)

The problems we're trying to solve are a few properties/implications of transient loads, where transient variably means 'occasional', 'potentially extremely short', 'perhaps high-frequency', 'perhaps quite a bit of current'.


The main reason behind this is that when any part of a circuit starts or stops drawing power, or changes the amount of draw, this has a small but real effect on the (global) voltage, often a bit more on specific PCB traces, because they have low but still non-zero resistance, but more importantly, some inductance.


This effect can be noticeable as two different issues: dropping voltage, and being noise.


Voltage drop and current buffers

Voltage noise

See also:


Coupling capacitor (application)

Charging and discharging

See also