All About Circuits Forum electrolytic capacitor and polarity of AC
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#1
07-31-2011, 08:58 PM
 PG1995 Senior Member Join Date: Apr 2011 Posts: 753
electrolytic capacitor and polarity of AC

Hi

1: Please have a look here; you would find my question there. Thank you.

2: Can we use electrolytic capacitor with DC?

Regards
PG
#2
08-01-2011, 05:54 AM
 Kermit2 Senior Member Join Date: Feb 2010 Posts: 2,405

electrolytic capacitors are very similar to batteries as concerns how they react to the polarity of voltages. They should NOT be used with AC voltages.

Google can provide you many examples of the construction and chemistry of these capacitors. You simply give some key words and then look through the hundreds of thousands of results; click on one to read what it has to say.

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#3
08-01-2011, 06:14 AM
 Kermit2 Senior Member Join Date: Feb 2010 Posts: 2,405

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#4
08-01-2011, 08:54 AM
 PeeSeeBee Junior Member Join Date: Jun 2011 Location: London. UK Posts: 43

You can use electrolytics in ac circuits (audio amplifiers are full of them), but not in the way you have shown in that diagram. Don't build that circuit. A capacitor appears as a low resistance to ac (depending on value & frequency). The capacitor will get hot very quickly & may explode.

For electrolytics to work properly they have to have a voltage across the terminals for the chemical reaction to take place. This voltage must be correctly polarised (i.e. the +ve terminal must always be more +ve than the -ve terminal). So as a coupling capacitor in an amplifier, for example, you can use an electrolytic to couple the ac (audio) to the next stage, as long as the circuit is arranged such that a dc voltage is across the capacitors terminals.

As for using them with dc circuits. Look at most circuit diagrams & you will see them across the dc power supply connections.
#5
08-02-2011, 07:09 PM
 PG1995 Senior Member Join Date: Apr 2011 Posts: 753

Thank you, everyone.

Actually I was wrong in a way. Because electrolytic capacitor is in fact used with DC. For AC, there are special kind of electrolytic capacitors.

I have read that 'regular' capacitor can store very little amount of charge in comparison to a electrolytic capacitor and that's the reason they are preferred over 'regular' or 'normal' capacitors. Is this really so? Please let me know. Thank you.
#6
08-02-2011, 08:16 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,030

Just to make sure you're understanding it; there are "polarized electrolytic capacitors" and "non-polarized electrolytic capacitors".

The polarized type are by far the most common. When we mention electrolytic caps on here, we're talking about the polarized type, unless we specifically state non-polarized or "NP".
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#7
08-02-2011, 08:34 PM
 PG1995 Senior Member Join Date: Apr 2011 Posts: 753

Quote:
 Originally Posted by SgtWookie Just to make sure you're understanding it; there are "polarized electrolytic capacitors" and "non-polarized electrolytic capacitors". The polarized type are by far the most common. When we mention electrolytic caps on here, we're talking about the polarized type, unless we specifically state non-polarized or "NP".
Thank you for the clarification, Sgt.

Regards
PG
#8
08-02-2011, 10:01 PM
 Adjuster Senior Member Join Date: Dec 2010 Location: London UK (GMT) Posts: 2,147

Quote:
 Originally Posted by PG1995 Thank you, everyone. Actually I was wrong in a way. Because electrolytic capacitor is in fact used with DC. For AC, there are special kind of electrolytic capacitors. I have read that 'regular' capacitor can store very little amount of charge in comparison to a electrolytic capacitor and that's the reason they are preferred over 'regular' or 'normal' capacitors. Is this really so? Please let me know. Thank you.
In typical DC applications at moderate voltages, electrolytic capacitors generally can store more energy than other types. They are very popular for many uses, and may be the only practical choice in certain applications, such as some power supplies and compact photographic flash equipment.

On the other hand, they have a number of limitations which can make alternative types more appropriate. Here is my list - I expect I will have missed some things out.
1. The commonest types of electrolytics are polarised, as already mentioned.
2. They have relatively wide value tolerances, perhaps tens of percent.
3. They have larger leakage currents than almost any other capacitor type.
4. They have relatively low Q-values (aka large loss angles, or high esr). The above limitations restrict their use in filtering applications, apart from smoothing filters in power supplies. You would not expect to tune a radio with an electrolytic.
5. They can be more prone to generating distortion products than some other types. There is a degree of controversy here, but some designers of high-fidelity audio systems do not recommend their use in sound equipment, at least within the signal path.
6. They can be made for use up to several hundred volts, but are unavailable above about 1kV (unsure of the exact limit).
7. NP or AC rated electrolytics often cannot be used continuously at their maximum ratings, and may be more suitable for short-term work such as motor starting.
8. Their life expectancy and reliability is not as good as some other types. This has been improving over the years, but recently some very poor devices have come on to the market.
9. If left un-powered for long periods of time, some electrolytics may fail if powered up again abruptly without a re-forming process - but this applies mainly to old equipment.
10. Their operating temperature range is limited, often with a steep trade-off of lifespan versus temperature.
11. They are sealed devices containing fluid, often not truly hermetic. This can lead to pressure limitations, and can pose a contamination hazard.
Electric capacitors may therefore not be permitted under the design rules for certain very high reliability products.

http://electrochem.cwru.edu/encycl/a...electr-cap.htm

http://en.wikipedia.org/wiki/Capacitor_plague
#9
08-02-2011, 10:58 PM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,022 Blog Entries: 5

There is an exception to these rules, a way to use two polarized caps with AC. It works, though I can't really explain the mechanism. I think it is how they make non polarized electrolytics.

One of the explanations I've heard (and I don't claim it is true) is the correctly polarized cap takes up all the strain, while the other basically conducts slightly.
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General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
#10
08-03-2011, 08:43 PM
 PG1995 Senior Member Join Date: Apr 2011 Posts: 753

I genuinely appreciate your help, Adjuster, Bill. I understand it takes much times and effort to help others. Thanks a lot.

1: I think in the linked image, the caps on the left side of the red 'line' are electrolytics, and on the right regular/normal ones, please correct me if I'm wrong:
http://img641.imageshack.us/img641/7758/capshn.jpg

2: In the linked image on the left side of ICs you have transistors:
http://img818.imageshack.us/img818/3...conductors.jpg

Are ICs (integrated circuits) pre-fabricated circuits which work as a unit in a circuit in which they are used?

Thank you for your time and help.

Regards
PG

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