Al vs Cu in capacitor plates? - Page 1
Forum section: electronics / beginners

author: nsled
author_rank: Contributor
timestamp: April 11, 2023, 01:59:52 pm
content: Can someone tell me the expected change from going from Aluminium to Copper in a rolled foil capacitor?What is the difference in discharge between a rolled foil capacitor and a flat sheet capacitor?If I had to guess, the Cu would discharge quicker due to less resistance, and the flat would discharge quicker due to less inductance, but what is the truth?

author: wasedadoc
author_rank: Super Contributor
timestamp: April 11, 2023, 02:41:02 pm
content: I would say in answer to both your questions, if the difference matters then the circuit is abusing the capacitor.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2023, 02:59:52 pm
content: Rolling up the capacitor has not much effect. The current in both foils comensates and there is thus no net magnetic field to add up.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 11, 2023, 03:41:02 pm
content: The inductance is almost entirely a function of the geometry so that will be the same. The aluminum will have a higher resistance.In practice the electrical properties of the metal itself are usually not what guides the design. Aluminum forms a thin, tough, self reforming insulating oxide layer that passivates the surface and has a very high breakdown strength. This allows the construction of electrolytic capacitors. Aluminum can easily be deposited in thin films on plastic substrates for film capacitors and again the oxide layer protects the film from further chemical degradation.Copper and silver form thicker conductive oxides and sulfides that don't protect the surface as well. They are also more expensive and heavier than aluminum. Silver still used in for example silver mica capacitors for example but is basically a niche use case only.

author: johansen
author_rank: Super Contributor
timestamp: April 12, 2023, 03:17:20 am
content: Silver is used in induction heater water cooled film capacitors.But copper and silver would never work in an electrolytic capacitor.

author: TimFox
author_rank: Super Contributor
timestamp: April 12, 2023, 03:34:40 am
content: With plastic film capacitors, especially polypropylene film, there are (physically larger) capacitors with layers of metal foil (aluminum) between film layers, and others (smaller and more popular) with thin aluminum deposited directly on the plastic.Generally, the film-foil units have higher current ratings and somewhat lower loss (ESR) than the metallized-film units.

author: barshatriplee
author_rank: Regular Contributor
timestamp: April 13, 2023, 04:54:43 am
content: This may interest you too:https://www.academia.edu/21456626/ESD_Testing_of_Aluminum_and_Copper_Vertical_Parallel_plate_VPP_Capacitor_Structures

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2023, 11:16:50 am
content: Rolled, with single contact, or wide (schoopage)?As said, geometry dominates.Single-contact type (usually with a ribbon welded or crimped to the foil in the roll) are suitable for mains frequency, and that's about it. The roll acts like a broad transmission line, so has various resonant frequencies (at low impedances where you might not mind it), and its overall capacitance resonates with the connecting leads which being relatively long and thin, dominate the ESL.Excellent capacitors are possible with mere aluminized film...Quote from: johansen on April 12, 2023, 03:17:20 am...even for induction heating. I don't know that I've seen any caps for that purpose using foil, actually. Perhaps the larger boxed types, or higher frequencies, use different construction, or materials.The smaller conduction-cooled types are basically machined copper end plates, with cavities for "cells" to fit into: usually the floating-electrode type (three metal layers, one in the middle, unconnected), polypropylene, schoopage contacts, bonded to the plates with low-melting solder for obvious reason. The surrounding space is filled with potting. The cells are oil impregnated, and usually "sweat" a little on first use, which they note is normal behavior.There is a wide span between EMI caps where the aluminizing is so thin the ESR is several ohms in 0.1uF sizes, and oxidizes away in about a decade (moisture slowly but surely diffuses through the potting and polypropylene, "corroding" the metal; the metal layer is so thin, it corrodes through, in time), and high power or pulse caps (with thick aluminizing, or foil).There's also the matter of RMS current handling vs. frequency, which depends on cooling and construction. A larger roll or stack is worse than multiple smaller rolls/stacks in parallel, because skin effect participates. High-frequency currents need to penetrate into the roll, but are shielded by it, due to the schoopage contact and other losses.Stacked films are available, most often as SMD chips, and ceramic chips are always stacked construction AFAIK. There's no particular advantage to stacked vs. rolled.Tim