THE JELLY ROLL

Foil, film, film, foil

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The stack Foil A, two films, foil B, two films, then repeat as it winds

Hundreds of turns Metres of film rolled into one section

Both sides count Each foil works with the films above and below it, doubling the useful area

Not deposited Nothing is grown between the plates: the dielectric is separate sheets of plastic

A film capacitor is a sandwich of sheets, rolled up so square metres of plate fit in your hand.

TWO THIN BEATS ONE THICK

Why weak spots rarely line up

Every film has a few weak spots. With two or three thinner films in each gap, a weak spot in one film almost always sits against good film in the next.

Splitting the dielectric into layers turns rare defects into almost-never defects.

MARGINS

Keeping the edges safe

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The margin The film is wider than the foil, so bare film sticks out past the foil edge

Why Without it, high voltage would arc around the film edge from one foil to the other

Foil edges Sharp edges concentrate the field; some high-voltage designs fold or smooth them

Holding it If the layers wander sideways, the margin shrinks and the edge can flash over

The margin is a small strip of bare plastic doing a big job, and the winder must hold it every turn.

THE WINDER

A precise, fast machine

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Spools Several spools of foil and film feed one mandrel at once

Tension Each web runs at controlled tension so the roll is tight but not stretched

Alignment Edge guides keep foils and films where the design puts them

Stop point Winding stops at a set turn count or measured capacitance

Flattening Round rolls are often pressed oval so they stack into rectangular cans

Winding is where a capacitor's capacitance and much of its reliability are decided.

WHAT GOES WRONG

Winding defects

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Wrinkles Folds trap air and thin the film locally

Telescoping Layers slide along the roll's axis, shrinking a margin

Misalignment Foil too close to an edge: arcing or hipot failures later

Particles Dust or debris trapped between films becomes a weak spot

Tension swings Loose or uneven rolls shift capacitance and trap voids

Most winding defects don't show up at the winder; they show up as hipot or life-test failures days later.

UNIT 5 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen how the layers stack, why thin films are doubled up, what margins do, and what a winder must control.