Question about polarized capacitors

  1. 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?
     
  2. jcsd
  3. vk6kro

    vk6kro 4,059
    Science Advisor

    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.
     
  4. 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?
     
  5. Dadface

    Dadface 2,084
    Gold Member

    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.
     
  6. Why does the dielectric film breakdown when the current travels in the wrong direction?
     
  7. Dadface

    Dadface 2,084
    Gold Member

    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.
     
  8. So, does the supply current used to build up charge go into the metal plate or into the electrolyte plate?
     
  9. Dadface

    Dadface 2,084
    Gold Member

    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
     
  10. 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.
     
  11. Dadface

    Dadface 2,084
    Gold Member

    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.
     
  12. Exactly, the current must flow from the anode to the cathode and not in the reverse direction. Otherwise the film will break down.
     
  13. Dadface

    Dadface 2,084
    Gold Member

    That's right.Take care when you use them.
     
  14. 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.
     
  15. Dadface

    Dadface 2,084
    Gold Member

    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.
     
  16. 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=/wikipe...glish.png/120px-Elko-Wickelaufbau-english.png

    [​IMG]
     
    Last edited: Feb 21, 2012
  17. Dadface

    Dadface 2,084
    Gold Member

    There are two layers but only one of these,the aluminium oxide,is labelled as the dielectric.
     
  18. tiny-tim

    tiny-tim 26,043
    Science Advisor
    Homework Helper

    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.​
     
  19. Dadface

    Dadface 2,084
    Gold Member

    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.
     
    Last edited: Feb 22, 2012
  20. tiny-tim

    tiny-tim 26,043
    Science Advisor
    Homework Helper

    Hi Dadface! :smile:
    I got that from what may be a "bible" for capacitors, http://www.faradnet.com (it's in the "See Also" links at the top of the Library page).

    At http://www.faradnet.com/deeley/chapt_02.htm#theory
    The electrolytic capacitor can only be used with a flow of current in one direction. The aluminum electrode must therefore always be connected to the positive side of the applied voltage, and the electrolyte must always be negative. With the current flowing through the capacitor in this direction, the current intensity is small. If the direction of current flow is reversed, a large current will flow through the capacitor and the capacitor as such becomes useless.

    From this, it can be readily seen that the system exhibits the characteristics of a rectifier, and an electrolytic capacitor does not then differ in any way from the well known electrolytic rectifier.

    (I know nothing about electrolytic rectifiers :redface:)
     
  21. Dadface

    Dadface 2,084
    Gold Member

    Thanks tiny-tim,
    the faradnet article is really interesting.I have yet to check out the other links.I am not sure how the rectifying action works but I have a few initial ideas.
    Firstly the oxide film may be semiconducting and the interface between anode and oxide film may be acting as a rectifier.Please note that there is no interface between oxide film and electrolyte for the non submerged part of the anode.I don't know if this is significant.
    Secondly,the electrolytic cell shown in the article seems to be of sturdy(and open) construction (unlike the flimsy electrolytics using aluminium foil) and can probably withstand the high currents resulting from any film breakdown.If so the capacitor action slightly resembles rectifier action.In forward bias the current stops when the capacitor is charged and in reverse bias a "high" current can flow.There are time limitations,for example a reverse current can start to build up an oxide film on the opposite electrode.
    The more I look at this the more I realise how little I understand about electrolysis and such things.:confused:
     
    Last edited: Feb 23, 2012
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