On 17.09.15 09:39, Gene Heskett wrote: > On Thursday 17 September 2015 05:09:07 Erik Christiansen wrote: > The fact that you shorted it while charged with only the meters volt or > so charge, and "fixed" it, which punches thru the foil oxides and > restores a good but temporary connection, making that cap look good > again, also says it will be bad again quick enough to eat your warranty > budget.
It is reforming the dielectric which was performed. It deteriorates during prolonged disuse, and can be regenerated by applying a low voltage at a modest current. Discharging the capacitor was merely to avoid risk to the tester electronics afterwards, nothing else. OK, a weak dielectric manifests primarily as increased leakage (shunt resistance) and susceptibility to failure at less than full voltage, suddenly applied. The effect on series resistance is harder to qualify. > Making a good, very low resistance connection between the external > terminal metal, normally made out of something solderable like copper, > and the alu foils that make up the capacitors "plates" is a high art > form because the alu is so reactive with the oxygen in the air we > breath. Throw in that it is etched chemically to increase its surface > area 10 to 50 fold over plain alu, which increases its capacitance by > the same factor, and you can begin to imagine the difficulty in actually > getting a "gas tight" joint between the terminal and the alu foils. Not a lot of imagination needed. The big aluminium electrolytics I've examined over the years had plain aluminium strip which looked to be welded to the etched plates (prior to etching, I expect.) and also to the terminals. In this case, the terminals are aluminium posts, internally threaded. The internal terminal connection, presumably also welded, isn't going to experience oxidation, I suggest. In any event, the resistance of an unetched oxide layer on aluminium ought not be overestimated. Whether the mechanism is electron tunnelling or not, the massive current which flowed through two series oxide layers on the outside terminals, with only a volt or two, confirms at high current what an ohmmeter shows on placing two probes on an aluminium surface - at least an order of magnitude less than my 0.02 ohm resistor. Most of the ESR is in the electrolyte, AIUI. Erik ------------------------------------------------------------------------------ Monitor Your Dynamic Infrastructure at Any Scale With Datadog! Get real-time metrics from all of your servers, apps and tools in one place. SourceForge users - Click here to start your Free Trial of Datadog now! http://pubads.g.doubleclick.net/gampad/clk?id=241902991&iu=/4140 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
