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Old 24th Jun 2012, 8:50 am   #4
Radio Wrangler
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Location: Fife, Scotland, UK.
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Default Re: Does reforming capacitors really work?

Electrolytic capacitors reform unless damaged. It's how they're made.

Aluminium foils along with permeable separators are impregnated with conductive gunge and wound together. This isn't a finished capacitor because both sides of both foils have aluminium oxide layers. A controlled voltage is applied to get current flowing. The oxide on one foil breaks down, leaving naked aluminium in contact with the electrolyte, while the oxide thickness on the other foil is built-up.

A surprising number of people think an electrolytic capacitor works electro-chemically like a battery. It's the aluminium oxide layer which is the dielectric. The foils get matted to increase their surface area and therefore capacitance, and the conductive gunge forms the other electrode, one that is able to flow to intimately follow the surface of the oxide.

The capacitor is formed electro-chemically, but it works normally with an insulating dielectric.

Left alone, the oxide slowly returns to its normal condition and the capacitor is depolarised. Reforming simply redoes the manufacturing process. Doing it properly is rather time consuming. I'd certainly use a reformer rather than running a whole radio on a variac, and plan on taking a few days over the job.

Interestingly, the capacitor is symmetrical as wound. It's the forming part of the manufacturing process which programs which terminal is the positive one.

The use of conductive liquid adds some series resistance to the capacitor, and it's this which ESR meters measure, giving an indication of old capacitors drying out through loss of water vapour. Don't knock ESR. it's quite useful in damping unwanted resonances in filter circuits. I've seen cases where high Q film and ceramic parts have been used in place of electrolytics and all sorts of troublesome resonances have arisen. With the modern trend of using high capacitance ceramics, we have to add resistors to damp resonances. In ye olde days, the electrolytic capacitor on the power rail did this job for us.

As reforming progressively changes the oxide thickness, you get a trade-off happening between voltage capability and capacitance. Aim to get the capacitance close to the number printed on the can and the voltage rating should be somewhere near where it ought to be.

David
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