Question about polarized capacitors

Bararontok

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

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

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

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

Why does the dielectric film breakdown when the current travels in the wrong direction?

Dadface

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

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

Dadface

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

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

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

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

Dadface

That's right.Take care when you use them.

Bararontok

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

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

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

Dadface

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

tiny-tim

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.