Capacitors

Discussion in 'General Electronics Discussion' started by [email protected], Dec 19, 2012.

1. [email protected]

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Mar 26, 2012
what is the main difference in Polar and Non Polar capacitors and where they should b used
Also give a brief introduction of Farad

Thanks for it

2. duke37

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Jan 9, 2011
The main difference is size (and cost). Electrolytic capacitors can be used when the voltage does not reverse and frequencies are low to medium.

A Farad is a measure of capacity and 1F will change voltage at the rate of 1V/second when fed with 1A.

3. Yoa01

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Jun 18, 2012
I'm going to add to the original question, if I may.

Is there a way to make a non-polarised cap polarised (maybe using a diode)? Also, why is it that polarised caps can work properly independant of direction?

4. BobK

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Jan 5, 2010
You can make a non-polaraized capacitor from polarized capacitors by putting 2 of them in series back-to-back. The resultant capacitor is 1/2 the value of each capacitor. I have tried several times to understand why this works, but it escapes me. If you search the net, you will find multiple opinions about whether or why this works. The thing that convinced me that it really does work was a link to a datasheet from a reputable manafacturer that said you could do that.

As I understand it, electroytic capacitors will work for a while with a low, reversed voltage, but they will deteriorate. With a reverse voltage close to their limit they will fail more quickly, but how quickly, I don't know.

Bob

5. Yoa01

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Jun 18, 2012
Huh. That's strange. How about making a non-polarised cap into a polarised one?

That is just strange. Kinda makes you wonder how these things actually work.

6. duke37

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Jan 9, 2011
A capacitor has two 'plates' with a dielectric in between. If the dielectric is air, the capacity will be low. The capacitor can be connected either way round, it works just the same.

If the air is replaced by a substance with a dielectric constant of greater than one, then the capacity will be increased. Small plastic film capacitors are in this category and are very common. The closer the plates, the higher the capacitance but the lower the permitted voltage.

An electrolytic capacitor has the two plates made of foil and some electrolyte is placed between. A voltage is applied in one direction only and an oxide film is built up on one plate (anodising), this film is very thin so the capacitance is high. There is always some current leakage and if the voltage is reversed, the film is destroyed so the capacitor is ruined.

Why should you want to make a polarised capacitor out of a non polarised one?
Are you going to smash the reverse gear of your car so it only goes forwards?

7. Yoa01

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Jun 18, 2012
Well I guess the good news is I knew most of that, but that 3rd paragraph was highly interesting. Thanks!

Well, what if I need a polarised cap and don't have one, but I have a cap of the same specs that's non-polarised? I don't really know how polarisation with caps works, to be honest.

How does a capacitor relate to shifting?

8. duke37

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Jan 9, 2011
Why ever should you need a polarised capacitor when you have a non polarised one?
Why would you chose a car which can only go forwards? This is the same logic.

Polarised capacitors (electrolytics) are much smaller than non polarised ones so they tend to be used for different puposes. A 1uF non polarized capacitor is quite bulky but a 10uF electrolytic can be quite diddy, it can however only be used with one polarity and cannot be used in reverse.

9. Yoa01

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Jun 18, 2012
Say, I don't know, you need to use a polarised cap for something. You're saying a normal cap would work just as well in its place?

And if it were used in reverse the previously stated thin film would be destroyed, right?

Thank you for going through this. Learn something new every time I come here.

10. duke37

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Jan 9, 2011
Read up the information on Wikipedia on electrolytic capacitors and anodising.
There are subtle differences other than capacity, polarity and voltage which may need to be taken into account for fussy circuits.

11. Yoa01

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Jun 18, 2012
...Probably should have done that to begin with. Thanks, and sorry