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I read in a textbook about electrolytic caps the following:

The foils are separated by paper saturated with an electrolyte for current conduction — thus the name electrolytic capacitors

I'm confused. I thought there's no current conduction inside a cap but only electric field between the foils which are layered by aluminum-oxide. I thought electrolyte soaked paper is used for to heal the foils.

Is this argument from the text correct?

Edit: If cap dielectric should be an insulator why it is conductive Im completely confused.

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  • \$\begingroup\$ Solid dielectric in electrolytic caps is incredibly thin. The electrolyte acts as one of the capacitor plates. \$\endgroup\$
    – wbeaty
    Commented Jun 7, 2017 at 0:55
  • \$\begingroup\$ So the positive anode foil's inner surface makes the thin dielectric as aluminium-oxide layer? The negative side foil and electrolyte soaked paper altogether forms the cathode? Is that correct? But what is the benefit of adding electrolyte to the cathode? \$\endgroup\$
    – user1245
    Commented Jun 7, 2017 at 1:02
  • \$\begingroup\$ The electrolyte itself is the cathode plate; the negative plate of the capacitor. The "cathode foil" isn't a capacitor plate, and does not touch the dielectric. Instead it provides a metal terminal that connects to the liquid. (Heh, if common electric circuitry was made of hoses full of salt-water, then no cathode foil would be needed.) \$\endgroup\$
    – wbeaty
    Commented Jun 7, 2017 at 1:09
  • \$\begingroup\$ is that true the electrolyte also heals the dielectric layer of the anode for longer life? \$\endgroup\$
    – user1245
    Commented Jun 7, 2017 at 1:17

2 Answers 2

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The 'electrolytic capacitor' is not called that because it contains an electrolyte. It's called that because a key step in its manufacture is the formulation of thin layer of dielectric oxide on the positive aluminium plate by electrolysis.

The electrolytic capacitor gets its large capacitance from the very thin dielectric layer and the large area of the roughened plate. Without a liquid, the air or vacuum b

The electrolytic capacitor gets its large capacitance from the very thin dielectric layer and the large area of the roughened plate. Without a liquid, the air or vacuum between the plates would cause a huge loss in capacitance. The electrolyte (that is, conductive liquid) fills the interstices of the anode, and is effectively the cathode to the thick oxide layer on the anode.

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  • \$\begingroup\$ Thanks but "Without a liquid, the air or vacuum between the plates would cause a huge loss in capacitance." C is proportional to Area and (1/Distance). What is the benefit of making paper layer thick here? I mean if the electrolyte acts as one of the capacitor plates(cathode) why is so thick(all that paper and gaps liquid ect)? What does it benefit comparing to aluminium foil cathode(imagine we remove paper and the the anode foil with dielectric layer and the cathode remains and they are stick each-other distance of the foils will be small so the capacitance)? Do you get where Im confused? \$\endgroup\$
    – user1245
    Commented Jun 7, 2017 at 8:50
  • \$\begingroup\$ With paper same distance. Why making cathode as a whole thick with the paper and liquid? \$\endgroup\$
    – user1245
    Commented Jun 7, 2017 at 8:58
  • \$\begingroup\$ the paper is there to separate the plates and to soak up the electrolyte, make it easier to handle. Without the paper, making the capacitor could damage the oxide coating on the plates, and it would need to be handled as containing a liquid during manufacture, rather than damp paper. There's no electrical reason for the paper, so it's as thin as possible. \$\endgroup\$
    – Neil_UK
    Commented Jun 7, 2017 at 9:39
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The charges must be brought to opposite sides of the aluminum oxide dielectric without intervening air also acting as a dielectric. Thus the empty space in an aluminum electrolytic capacitor must be filled with a conductive liquid or polymer formulation, the electrolyte.

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  • \$\begingroup\$ I dont understand. Do you mean the charges actually travel from one foil to another through that conductive electrolyte soaked paper? Thats opposite to my understanding of a capacitor. \$\endgroup\$
    – user1245
    Commented Jun 7, 2017 at 0:28
  • \$\begingroup\$ No, the charges gather around the dielectric formed on one of the foils. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jun 7, 2017 at 0:34
  • \$\begingroup\$ are those charges trapped in the soaked paper? same amount charges always? \$\endgroup\$
    – user1245
    Commented Jun 7, 2017 at 0:35
  • \$\begingroup\$ At least some are in the electrolyte. Spillover will be contained in the foil without the dielectric. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jun 7, 2017 at 0:39
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    \$\begingroup\$ Electrolytic capacitors have one plate made of aluminum, the other plate made of salt water (borate salt.) Between these two plates is the insulating dielectric film. That's the simplified/ideal version, ignoring the second (cathode) plate and its much thinner oxide film. \$\endgroup\$
    – wbeaty
    Commented Jun 7, 2017 at 1:02

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