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tuurbo · Posted: Sat Nov 15, 2008 7:45 am Post subject: How does a capacitor work?

I'm wondering if anyone can tell me how one works? I'm trying to read up on them but I don't have any really good explanations. Does anyone know any good instructional links?
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Smoothie · Posted: Sat Nov 15, 2008 8:50 am Post subject:

Basically, they're very roughly similar to batteries. They store power, but for a shorter period of time, and release it much quicker. One example of a use is in electronic flashes or strobe lights. There, a capacitor is gradually charged from battery or another DC source, then the power is released in a fraction of a second to produce the quick bright burst of light.
Another use is to block DC current in a circuit where both AC and DC are present. They're also used for noise reduction.
Exactly "how they work" I believe depends on the application, but on a basic level, they're similar to batteries.
Try http://www.howstuffworks.com/search.php?terms=capacitor&x=21&y=36 for more.
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datatrain · Posted: Sat Nov 15, 2008 8:53 am Post subject: CApacitors

A capacitor consists of two or more conductive materials separated by an insulating medium. Typically this could be paper or mica or some other exotic material.

A capacitor has different properties for AC and DC currents.

A DC current will be blocked by a cap after it charges to the voltage being applied to it. It will retain this charge until it either leaks thru the dielectric (the insulator) material or is discharged in some way.

This is an effective way to block DC current or to store a charge to provide a current to perform a task. It will also act as a filter to smooth DC voltages.

With AC voltages applied, it works differently. The variations in the alternating current will pass through the CAP without passing the voltage itself.

Depending on the value of the cap in microfarads it will present different reactances to various frequencies so can pass or block AC signals typically at radio frequencies.

In a nut shell thats how they are made and what they do.
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macBdog · Posted: Sat Nov 15, 2008 12:34 pm Post subject:

If you are in the habit of thinking about electricity like water in pipes (which is a big help for some people) then caps are like water tanks.

The input side can have a varying flow rate but the output side will be comparably constant,
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RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

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

http://www.jaycar.com.au/images_uploaded/rescode.pdf

Just a general question or do you have something specific in mind?

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Dude, Wiki knows all.

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tuurbo · Posted: Sat Nov 15, 2008 5:01 pm Post subject:

I think it's a general question with some examples. I want to understand 'capacitance' systems. Thanks for the replies. I've never looked into this stuff before. Thanks for the links too. Lightening is very cool.

With electronics, how do they discharge so much so quickly as compared to a battery? Why do batteries take longer to discharge their 'load' (how crude)? Is there less resistance at the output end of things?

Example - When you use a capacitor in a car audio system, the cap allows more power or punch. So why doesn't it deplete it's resources very quickly? It gets replenished. And if it does deplete, how does it recharge quickly enough that you never 'miss a beat' when you're listening to music? If the battery is doing the work, wouldn't it be easy to just use another battery? If the drum solo is particularly long and loud, does the capacitor deplete faster?

Example - DRAM - signal capacitors hold a bit of data in each capacitor, and it lasts a brief time. Before it fades, it needs a refresh. It's volatile. When the power is removed it loses all data. Hence, it gets refreshed once in a while to buttress against data loss, leakage, or fading, whatever you want to call it.

How does reading from DRAM refresh it's charge? How does resistance fit in to the capacitor's discharge in the case of DRAM? A friend was telling me that capacitors in parallel behave like resistors in series (high capacitance), and capacitors in series behave like resistors in parallel. I'm still trying to figure this out.

I tried reading Wikipedia Rasta but my reading it it kind of raised questions. Here's what they said about DRAM:

datatrain · Posted: Sun Nov 16, 2008 7:47 am Post subject:

I'll answer the first couple of questions.
How do they discharge so quickly ? The discharge rate is totally dependant on the load. It's a matter of ohms law and the relationship between resistance, current and voltage.
Capacitors are not batteries. They do not generate an electrical current by their own resources. They are externally charged and that is their only source of power.

Batteries are chemical devices that produce electrical current by chemical reaction. A battery can be discharged very quickly but usually damage will result. A battery is designed to produce electrical current at a specific rate and voltage for a predetermined length of time. After that it's either dead or if rechargable can be recharged and used again.

Batteries and capacitors are completely different animals.
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tuurbo · Posted: Sun Nov 16, 2008 4:29 pm Post subject:

Thanks very much, that's helpful.

Here's a link to a simulation I'm finding helpful too...

http://www.google.ca/url?sa=t&source=web&ct=res&cd=1&url=http%3A%2F%2Fjlearn.mit.edu%2Fsimulations%2Fcapacitor.jnlp&ei=bb0fSfOnJIjKNODGkM8J&usg=AFQjCNHHp_SNzCIRYoSnsV6gp7b1ZIQXLw&sig2=ZZvJsxEih_NTz40-UHhctw

Nicely done.
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RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

tuurbo · Posted: Mon Nov 17, 2008 7:27 pm Post subject:

Hey RC - yeah it's a problem with a lot of teaching analogies - with this one because they don't store + manufacture, but it's helpful on the side of getting across the storage for supplying needed bursts when the main high capacity device is reloading.

I like the lightening one - the idea of oppositely charged particles sort of milling about on separate spaces, until they discharge. And the water tower one which I resort to when I get confused. Though I admit it might be slowing me down in getting the deeper stuff.

Is the charge, discharge pattern for a battery the same as for a capacitor?

Like, I get that it takes a while to charge a battery, longer than a capacitor, I would guess. But the discharge - the latter part of the curve, would you stretch that out from the bottom with a battery and keep the shape of the general curve, or would it be a relatively flat, linear long decline in output compared to a capacitor? I haven't ever compared them in the real world.
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Rich H · Posted: Mon Nov 17, 2008 7:41 pm Post subject:

Remember that in a battery you are changing the form of the electrical energy to chemical energy and this takes time and has losses, so you put in 10Ah you will only recover a portion of that.

A capacitor does not change the state so will charge/discharge very quickly indeed. However a capacitor will leak current quite quickly aswell so you cannot for example charge a capacitor and expect it to have any charge the following week. Batteries are better for storage, capacitors for bursts.
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RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

John Brown · Joined: 07 Nov 2002 Posts: 903 Location: Leesburg VA

Lot's of cooks in here already. But ... can't resist.

Suggest you forget the analogy between capacitors and batteries. It's limited at best. They are entirely different beasts and the analogy such as it is only applies to certain characteristics that ALL electrical devices have in common to greater or lessor extent. Then focus on just learning the basics about what each device does and how it works.

A battery is a chemical device that generates electricity. Period. Some (kinds of) batteries can be recharged by reversing the process. The common 'lead acid' car battery for example. Batteries are designed to a given voltage and current standard. For example car batteries are nominally 12 volts and each is rated to provide some current for some time at that voltage. There are several different ratings for car batteries you can google them if you like. The most common in use now is the CCA which is a defined short term rating for starting. Any resemblance to a capacitor is strictly incidental. All electrical devices have internal resistance that does get involved in just how fast they can provide or exchange energy with an external circuit. The internal resistance is why the battery gets hot starting the car.

A capacitor can store electrical energy. It can not generate electrical energy. Like the battery it will have some internal resistance and with the same effect but it's an order of magnitude lower than the battery and a capacitor is therefore more dependent of the external circuit to which it is attached to determine it's behavior. In general though a capacitor should exchange much less energy but much faster than a battery.

The capacitor in the silly big car stereo installations should properly be thought of as the final filter section of a power supply feeding the stereo/load. It's working pretty much as a giant capacitive output filter section.
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