Metal shields for capacitors. Why are they needed? - Page 1
Forum section: electronics / beginners

author: Meshka7
author_rank: Contributor
timestamp: January 19, 2015, 05:40:39 pm
content: On some boards I see capacitors in metal enclosures. Why and when are they needed? I've seen 47uF 16V electrolytic caps that don't have that gray metallic dome on top of them. Any idea? One guess is that they are sorta like RF shields? Would that be needed in a digital circuit? Maybe for EMI suppression?I know it's a very silly basic question. Thanks.

author: tautech
author_rank: Super Contributor
timestamp: January 19, 2015, 05:52:00 pm
content: Quote from: Meshka7 on January 19, 2015, 05:40:39 pmIt is an Aluminium can used to contain the liquid dielectric.It sometimes is also one of the plates of a capacitor.

author: mariush
author_rank: Super Contributor
timestamp: January 19, 2015, 06:05:21 pm
content: Most electrolytic capacitors are in metal containers, usually aluminum.Larger capacitors usually have plain metal top with a pattern "engraved" which acts as a stress relief, if there's gas forming inside the capacitor then the top will rupture where those grooves are and gas will come out in a safe way instead of just making the capacitor explode.Small capacitors like those 47uF 16v ones can't form enough gas to make a capacitor explode, so they don't have grooves or patterns on top.Note that some quite large capacitors used with mains voltage , like let's say 220-560uF 450v used in computer power supplies ) still have a pattern on top to behave like gas relief, but manufacturers put some round plastic or paper material (or tape) on top to protect people from touching the metal (which can be live, danger of death etc) ... that cover is still loose enough to allow gas to vent in case it's needed.There's also polymer capacitors which don't have those grooves on top because there's no liquid electrolytic inside capable of forming that gas which can make capacitors blow up.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2015, 06:12:18 pm
content: In the old days, tubular (foil and film or waxed paper, usually) capacitors were made with a band denoting the end connected to the outer foil. This end can be used as a shield, when the capacitor is used as a filter or bypass in sensitive circuitry.All modern examples of exposed metal are simply because they're easier or better to manufacture that way, usually because of electrochemical reasons as mentioned.Tim

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 19, 2015, 06:17:06 pm
content: Quote from: T3sl4co1l on January 19, 2015, 06:12:18 pmAnd even in modern times, some capacitors are still marked that way...

author: Meshka7
author_rank: Contributor
timestamp: January 19, 2015, 10:14:30 pm
content: Thanks for all the info. So is the only difference between this:https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcQS2ziaTcX_5SADx92z-2F99HYhmLKP-xkEIgv1TU-YtYam_2Ojand this:https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSIh7KLY9k-g_7Tic0bgel3IgzddUtIg6JXWdjrFi09nxxCSb8Lis the surface mounts vs through hole thing?PS: how do I embed a picture in my msg? Do I have to use "img src" html code?

author: c4757p
author_rank: Super Contributor
timestamp: January 19, 2015, 10:23:31 pm
content: Those two capacitors have the same construction - the through-hole one even looks the same, but then it has a plastic label wrapped around the metal can. Cut the label and it's metal too.

author: dannyf
author_rank: Super Contributor
timestamp: January 19, 2015, 10:27:13 pm
content: QuoteAgainst the possibility that those capacitors may explode?