Physics Forums

Physics Forums (http://www.physicsforums.com/index.php)
-   Electrical Engineering (http://www.physicsforums.com/forumdisplay.php?f=102)
-   -   Question about polarized capacitors (http://www.physicsforums.com/showthread.php?t=579727)

Bararontok Feb21-12 03:20 AM

Question about polarized capacitors
 
There are two types of capacitor, non-polarized and polarized. The non-polarized capacitor holds equal amounts of charge in each of its two plates so that a difference in charge is only achieved when electrical current from a power supply transfers electrons from one plate to another to cause a potential difference, storing electrical energy in the process. When the power supply is cut off and a load is connected to the non-polarized capacitor, the electrons flow from the charged plate, to the load, and back to the other plate until both plates hold an equal amount of charge and the capacitor once again becomes neutral. But in the case of polarized capacitors where one plate has excess amounts of charge compared to the other plate, will the plate with excess amounts of charge not spontaneously transfer electrons to the other plate if connected to a load or shorted, even without the aid of a power supply because there is already an imbalance of electric charge? And will this transfer of electrons not balance out the charges on the two plates and cause the polarized capacitor to become non-polarized?

vk6kro Feb21-12 03:24 AM

Re: Question about polarized capacitors
 
Polarized just means the capacitor has to be connected a certain way.

It does not already carry a charge when you buy it from the store.

If you charge it, it has a charge which you may then discharge if you wish.

Bararontok Feb21-12 03:30 AM

Re: Question about polarized capacitors
 
So why does the polarized capacitor have to have a positive and negative terminal designation if the capacitor uses two plates that both have the same capacity to store electric charge? Is there something in the structure of this type of capacitor that will destroy it if the polarity of the supply current is reversed?

Dadface Feb21-12 03:47 AM

Re: Question about polarized capacitors
 
Quote:

Quote by Bararontok (Post 3775203)
So why does the polarized capacitor have to have a positive and negative terminal designation if the capacitor uses two plates that both have the same capacity to store electric charge? Is there something in the structure of this type of capacitor that will destroy it if the polarity of the supply current is reversed?

I think your "polarised capacitors" are what I know as "electrolytic capacitors".These capacitors have a very thin dielectric film which is maintained by connecting the power supply the right way round.If the power supply is connected the wrong way round the the dielectric breaks down and the capacitor blows.

Bararontok Feb21-12 03:54 AM

Re: Question about polarized capacitors
 
Why does the dielectric film breakdown when the current travels in the wrong direction?

Dadface Feb21-12 04:17 AM

Re: Question about polarized capacitors
 
The film is built up by electrolysis and by use of a suitable electrolyte and a d.c. current.If the power supply is connected the wrong way round the electro/chemical reaction reverses and the film breaks down.

Bararontok Feb21-12 05:01 AM

Re: Question about polarized capacitors
 
So, does the supply current used to build up charge go into the metal plate or into the electrolyte plate?

Dadface Feb21-12 07:27 AM

Re: Question about polarized capacitors
 
There is an electrolyte between metal plates and the thin film makes the dielectric.The plate with the film deposited on it can be considered as the anode and the plate plus the unreacted electrolyte can be considered as the cathode.The charge is numerically equal on both electrodes one side being positive and the other side negative.Try googling for constructional and other details

Bararontok Feb21-12 08:33 AM

Re: Question about polarized capacitors
 
So that means that sending the supply current into the plate with the film will reverse the electrochemical reaction, causing the film insulation to breakdown and short the capacitor.

Dadface Feb21-12 08:51 AM

Re: Question about polarized capacitors
 
Quote:

Quote by Bararontok (Post 3775507)
So that means that sending the supply current into the plate with the film will reverse the electrochemical reaction, causing the film insulation to breakdown and short the capacitor.

The current flows through both plates and it is a current which is responsible for building the film in the first place.Any supply current flowing in the right direction helps to maintain the film but a current flowing in the opposite direction will cause the film to break down.

Bararontok Feb21-12 08:55 AM

Re: Question about polarized capacitors
 
Exactly, the current must flow from the anode to the cathode and not in the reverse direction. Otherwise the film will break down.

Dadface Feb21-12 09:02 AM

Re: Question about polarized capacitors
 
That's right.Take care when you use them.

Bararontok Feb21-12 09:46 AM

Re: Question about polarized capacitors
 
So when the electrolytic capacitor is manufactured, there is a designated side with the cathode plate and beside this plate is the microscopic metal oxide layer, followed by the absorbent spacer material soaked in electrolytes, then the dielectric insulator and lastly the anode plate.

Dadface Feb21-12 02:04 PM

Re: Question about polarized capacitors
 
The dielectric is the oxide layer.I'm assuming that the easiest manufacturing process would be to form the oxide layer by means of a current after everything else has been constructed.The direction of the current chosen determines what side would be the anode.

Bararontok Feb21-12 10:34 PM

Re: Question about polarized capacitors
 
There is a diagram specifically stating that there are two dielectric layers on each side of the electrolyte, one is an aluminum oxide compound placed beside the anode and the other is an air oxide compound placed beside the cathode. At the very center of these layers of material is the absorbent material soaked in electrolyte. The diagram is shown below, courtesy of

http://rpmedia.ask.com/ts?u=/wikiped...au-english.png

http://img19.imageshack.us/img19/784...ccapacitor.png

Dadface Feb22-12 03:26 AM

Re: Question about polarized capacitors
 
There are two layers but only one of these,the aluminium oxide,is labelled as the dielectric.

tiny-tim Feb22-12 05:52 AM

Quote:

Quote by Dadface (Post 3775219)
If the power supply is connected the wrong way round the the dielectric breaks down and the capacitor blows.

Does that mean that a polarised capacitor can't take an AC current?

Is this, from the PF Library, wrong
Polarised capacitor (or polar capacitor):

This has one metal and one electrolyte plate (instead of two metal plates), and its dielectric is the oxide of the metal.

It has nothing to do with polarisation current.

It simply means that it behaves as a capacitor in one direction only (with the metal plate at the positive potential), and as a conductor in the other direction.

So it only works one way round in a DC circuit, and behaves as a rectifier in an AC circuit.

Dadface Feb22-12 10:49 AM

Re: Question about polarized capacitors
 
Quote:

Quote by tiny-tim (Post 3777517)
Does that mean that a polarised capacitor can't take an AC current?

If too large a reverse current flows for too long a time the chemical reaction that built the oxide dielectric reverse and the film breaks down.This results in a short circuit,overheating and the problems associated with that.I'm guessing,therefore, that the answer depends on the voltage and the frequency of the supply and the characteristics of the capacitor such as the maximum reverse voltage it can withstand and the time limitations of this.I think that higher voltage AC supplies can be used if the circuitry is such that there is a d.c. component that maintains the integrity of the film.

Is this, from the PF Library, wrong …
Polarised capacitor (or polar capacitor):

This has one metal and one electrolyte plate (instead of two metal plates), and its dielectric is the oxide of the metal.

I would add that the second plate consists of the electrolyte and the metal plate that makes contact with it

It has nothing to do with polarisation current.

It simply means that it behaves as a capacitor in one direction only (with the metal plate at the positive potential), and as a conductor in the other direction.

So it only works one way round in a DC circuit, and behaves as a rectifier in an AC circuit.

I agree it works one way only in a dc circuit but I am not so sure about any rectifying action.

I should add that my comments in this thread have been with reference to electrolytic capacitors and before reading the opening posts I did not know that these could also be referred to as polar capacitors.After doing some searching I found that there are also non polar electrolytics which can stand the rated current in both directions.

Sorry tiny-tim.I've added some of my comments in your quotes.I dont know how to fix it and have to go out now.


All times are GMT -5. The time now is 06:28 AM.

Powered by vBulletin Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
© 2014 Physics Forums