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This is probably the dumbest question ever, but I am an electronics nublet. I understand what capacitors do, and I've been reading beginner Electronics books and such, but I don't quite understand when to use them? Sometimes in these books they just seem kind of thrown in. I understand they are meant to smooth out current, but I still am not sure I understand when to use them.

Like I said, this is probably a nublet question to the max. But most information I find is more about what they are rather than when to use them.

Edit: For clarity, I mean in SMALL electronic applications. Think simple circuits and such.

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4 Answers 4

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When I first started out in electronics I struggled with the same question. The problem is that capacitors are used in a vast number of different ways.

However, as you're just starting out in electronics you probably only need to know about a few of these to start with. The most widely used and basic of these are:

Power Supply Smoothing

This is the easiest and very widely used application of a capacitor. If you stick a big beefy electrolytic capacitor (the bigger the better), it will fill in all the gaps created by rectifying an AC waveform, to create a relatively smooth DC. It works by repeatedly charging during the peaks, and discharging during the gaps. However, the more load you put on it, the quicker it will drain the capacitor and the more ripple you'll get.

Timing

If you supply power to a capacitor through a resistor, it will take time to charge. If you connect a resistive load to a capacitor, it will take time to discharge. The key thing to understand here about timing circuits is that capacitors appear as though they are short circuit while they are charging, but as soon as they are charged, they appear to be open circuit.

Filtering

If you pass DC through a capacitor, it will charge and then block any further current from flowing. However, if you pass AC through a capacitor, it will flow. How much current flows depends on the frequency of the AC, and the value of the capacitor.

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    \$\begingroup\$ This is super helpful (sorry for the late comment) \$\endgroup\$
    – user3073
    Apr 8, 2011 at 14:21
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    \$\begingroup\$ @Sauron: No problem. Glad to be of help. When I get some time I may edit my answer and add some more information. \$\endgroup\$
    – BG100
    Apr 8, 2011 at 15:15
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    \$\begingroup\$ Late to the party as well, but wanted to let you know that your answer is still helping people. Thank you for making this stackexchange awesome. \$\endgroup\$
    – kb.
    Sep 22, 2012 at 21:49
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    \$\begingroup\$ Even later to the party and agree with @kb ^^ \$\endgroup\$
    – Marko
    Jan 6, 2014 at 4:09
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Uses:

ac coupling –- blocking -- isolation

timing -- Time for a capacitor to charge or discharge is very roughly RC where R is the resistor in series with the capacitor.

Filter ( often power supply filter )

decoupling

tuned circuits

http://opencircuits.com/Capacitors

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How Stuff Works says

Sometimes, capacitors are used to store charge for high-speed use. That's what a flash does. Big lasers use this technique as well to get very bright, instantaneous flashes.

Capacitors can also eliminate ripples. If a line carrying DC voltage has ripples or spikes in it, a big capacitor can even out the voltage by absorbing the peaks and filling in the valleys.

A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges. However, any alternating current (AC) signal flows through a capacitor unimpeded. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing.

Wikipedia lists the following applications:

  • energy storage
  • pulsed power
  • power conditioning
  • power factor correction
  • signal coupling
  • decoupling
  • noise filters and snubbers
  • motor starters
  • signal processing
  • tuned circuits
  • sensing
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    \$\begingroup\$ However, any alternating current (AC) signal flows through a capacitor unimpeded. Any sources for that? From what I've heard, impedance of a capacitor is $R+ \frac {1}{j \omega C}$, where R is resistance of the leads and $\frac {1}{j \omega C}$ is reactance of the capacitor. \$\endgroup\$
    – AndrejaKo
    Apr 4, 2011 at 19:12
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    \$\begingroup\$ @andrejaKo The comment is a simplification assuming an ideal capacitor which has zero ESR, among other non-realistic attributes. Your equation is also an idealized simplification which doesn't factor is all real world properties of capacitors. For instance, you've ignored ESL which is a very important property in many applications. \$\endgroup\$
    – Mark
    Apr 4, 2011 at 19:45
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    \$\begingroup\$ Capacitors don't block direct currents. The voltage just climbs to infinity. :) \$\endgroup\$
    – endolith
    Apr 4, 2011 at 21:01
  • \$\begingroup\$ @Mark What's ESL? \$\endgroup\$
    – AndrejaKo
    Apr 4, 2011 at 22:21
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    \$\begingroup\$ @andrejaKo ESL = Equivalent Series Inductance, it represents the series inductance of the capacitor and is mostly a result of the package leads. ESL can result in resonances and is also critical in high frequency operation for applications like decoupling digital logic. ESR, Equivalent Series Resistance, the R in your equation above, is not only the result of lead resistance but also loses in the dielectric, additionally it is variable over frequency. There is also a parasitic capacitance to consider in a real world model when operating at high frequency. \$\endgroup\$
    – Mark
    Apr 4, 2011 at 22:45
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A few more applications:

  1. to create phase difference between voltages as in case of ceiling fans.Even though the main supply is AC to make the single phase motor work, you need to split phase.
  2. Capacitors can be used to store energy, super capacitors specifically designed for these application. These caps will have very small charging time compared to batteries.
  3. reactive power compensation: to improve the over all power factor of system so that kw/kva ratio is more.
  4. Filtering : you can always see an MLCC capacitor at the input of ICs, this is to limit dv/dt and protect IC.
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