More information about this kind of capacitor construction? - Page 1
Forum section: electronics / general

author: seqvenzer
author_rank: Newbie
timestamp: October 11, 2024, 10:21:55 pm
content: Attached an image.I took apart an old film capacitor (0.0325 uF, rated for 900 VAC). Seems like it's just simple potting on top and bottom, a transparent dielectric and aluminum foil. Very wide terminals for, I assume, high current situations.There are two rolls side by side, each roll is one terminal of the capacitor (each roll has a winding of dielectric and foil). I haven't come across this configuration before, does anyone have more information about it? Most film capacitors out there that I see sandwich the two terminals and roll them together. The two halves here are stuck together by an adhesive (easy to remove). I wonder what are the advantages of this approach.Thank you!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2024, 12:44:48 am
content: Hmm...Well, the radial leads (and voltage rating) may be a bit of a giveaway.Multiple cells (usually in parallel) might be used to improve eddy current or heat dissipation properties; it's a common method for high-current induction heating capacitors.Multiple dielectrics (in series) might also be used to handle more voltage, improve losses, etc. (e.g. double dielectric + floating electrode), though this still ends up as a single cell (winding).Aspect ratio, in general, is just kind of whatever. I'm not aware of any particular reasons why manufacturers choose one form or another -- other than that there's a market for it (a recent very compact PSU design of mine, uses two different shape 1uF X2 capacitors to optimize layout). You'd have to ask the manufacturer -- but chances are, the FAE you'd reach, doesn't know, or can't tell anyway.These are probably the flattest caps I've ever purchased:https://www.seventransistorlabs.com/Images/DCDC_800V.jpgThey're out there, they exist, but they're not particularly common; that may suggest lower density or higher cost to produce (e.g. lower yield?).But that's all a bit aside from the particular example shown, with two cells in series. Probably, the folded arrangement was of the most interest, but it's impossible to know for sure.Tim

author: seqvenzer
author_rank: Newbie
timestamp: October 12, 2024, 03:52:57 am
content: I'm not sure if we are talking about the same thing, not sure if your flat capacitor is configured in this way.A normal configuration is that the rolls of both terminals are spun together, separated by a dielectric, with a slight offset so they aren't making contact at the terminals. This capacitor has two independent rolls that sit side by side. It's as if they were rolled independently, and then put together. I was able to pull one off away from the other (so i separated one terminal from another). The two are held together by a bit of double-sided tape between them and whatever epoxy/potting managed to sneak in there, very little. I am not sure how the electric field nor displacement current is supposed to work in this configuration. This means that there is a "flat" portion where the two rolls are closest together, but then the rest of the material gets further and further out as the move away from each other. Hence my confusion for how is this supposed to work.Attached a photo of the full capacitor, next to the "half" capacitor (one roll - the yellow bit in the middle is an adhesive that peels off). Note that the terminal is attached to the roll. And the terminal width spans the diameter of the roll.Is your capacitor made this way? :|

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2024, 06:45:52 am
content: Like I said, all a bit aside from your example.The folded geometry reduces stray inductance, as the path is wide, and closely spaced. That, and the wide leads, suggests high current rating.Those flat caps, might have similar internal construction, but just two thin leads, not the wide flat things. Don't recall if they use schoopage or are just bonded to foil. (Would imagine yours do, use schoopage.)Tim

author: Slh
author_rank: Regular Contributor
timestamp: October 12, 2024, 09:24:48 am
content: I'm probably misunderstanding it but if the potting at the top is conductive then it could be series connected to make a safety capacitor. If one fails, you still have a capacitor rather than a short circuit.What's it from? Some sort of motor start capacitor?