Author Topic: Aluminum Electrolytic Capacitor - Dielectric Reforming  (Read 714 times)

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Offline PaulJFTopic starter

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Aluminum Electrolytic Capacitor - Dielectric Reforming
« on: October 30, 2019, 03:27:31 pm »
Hi

I've been doing some research on the mechanisms of liquid-electrolyte based aluminium electrolytic capacitors. I was hoping for a sounding board to confirm whether or not the things I've learned make sense or not. I also had some questions about reverse-bias conditions.

So, I believe an aluminum electrolyte capacitor with liquid-based electrolyte is constructed via:

  • An anodic aluminum foil with etched surface
  • A cathodic aluminum foil
  • A spacer, often paper, soaked in proprietary electrolyte mixture
  • A thin, naturally occurring layer of aluminum oxide on both aluminum foils due to galvanic reaction between the foils and the working electrolyte
  • A "manufactured" dielectric, commonly Aluminum Oxide (but can be different?), induced via applying positive voltage to anode foil and negative voltage to cathode foil causing oxidization between the anode foil and electrolyte, and is attached to the etched surface on the anode foil

I also understand that the working electrolyte, and the electrolyte used to manufacture the core dielectric on the anode foil are different.

The way I understand these processes work, is that the dielectric can be reformed indefinitely so long as it's done carefully. I.E., Apply low current with slowly increasing voltage levels. Is this true?

The slower, methodical process of reforming dielectrics are for capacitors which have almost essentially depleted their oxide layers. I understand that if voltage is applied periodically, then a methodical process of dielectric reforming isn't needed as the periodically applied voltages would be enough for the working electrolyte + aluminum foil to "heal" the few impurities in the dielectric. To me, the applied voltage required to induce the self-healing process must be rated voltage, otherwise the dielectric would not be restored to factory conditions. Is this true, what are your thoughts?

Why, in reverse-bias, does a significant aluminum oxide layer not form on the "cathode" foil? Is it because there is no etching on the foil to collect enough aluminum oxide and create a strong enough dielectric? I feel as though every one of these polarized capacitors could have been made bipolar if both of the foils were etched "correctly".

Thanks in advance for your thoughts!
I am excited to get a better understanding of what's going on.
« Last Edit: October 30, 2019, 05:50:33 pm by PaulJF »
 

Offline mcovington

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Re: Aluminum Electrolytic Capacitor - Dielectric Reforming
« Reply #1 on: October 31, 2019, 05:05:25 am »
My understanding is the following.  I am not an expert.

(1) The dielectric can be re-formed forever IF the capacitor never loses any of its moisture.  That's why we have working 50-year-old capacitors, and also why so many old capacitors fail!

(2) Reverse bias *does* form an insulating layer on the wrong electrode, reducing the performance of the capacitor.
 


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