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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Good points, Sulaiman.
Even commercial foil capacitors have ESR, though.
One point that is troubling me a bit is skin effect, ie if the current flows in as shown in the above diagrams, and the aluminium foil is rolled up, the current will only flow across the surface of the top layer(s), and at the points where the subjacent layers are exposed to air, rather than where they are in contact with superjacent/subjacent layers of foil.
I appreciate that this will only be a problem at high frequencies, but it could add significant resistance, at least in theory.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
since you are connecting to the ends of the roll, along the entire edge of the foil(s) esr isn't an issue.
The length of the capacitor means that the shortest possible current loop will be at least 2x 525 mm (capacitor length + external wiring) I guess at least 1uH, (2.pi.F.L will be greater than esr) so e.s.l. will be more of a problem at higher frequencies and of course dielectric loss.
I'd expect dielectric loss and esl to be more significant than esr for a rolled capacitor if the entire edge of the foils are connected to.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Thanks for the reply, Sulaiman.
As I mentioned above, my main concern now is with skin effect in the 'scrunched up ends', but I think I now have a solution. If I add an extra layer of (thinner) polythene, which covers the foil completely, this will insulate the layers of foil at the 'terminals'. This should allow for wires to be inserted, in contact with each layer, thus eliminating the skin effect issues. These wires will be incorporated at the rolling stage, and will be rolled with the capacator. This will also eliminate skin effect from the wires leading to the capacitor, as there will be lots of small wires carrying the current.
I'll post some photo's of the rolling operation, which I hope to get done over the next few days.
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HV capacitor question (Rolling my own)
