Aluminum electrolytic capacitor ESR measurement, excessive reverse voltage? - Page 1
Forum section: electronics / projects

author: matthuszagh
author_rank: Frequent Contributor
timestamp: February 04, 2023, 01:32:54 am
content: I'm reading an application note about aluminum electrolytics from Cornell-Dubilier (attached) where they state the following about performing an ESR measurement:QuoteThat would put the negative peak voltage at -1.4 V. Is that not excessive? If not, why not and what would be excessive? Does the signal frequency play into this.They mention the same 1Vrms signal for the impedance measurement across frequency. They don't explicitly mention bias voltage here, but I would expect it's also 0 bias.

author: moffy
author_rank: Super Contributor
timestamp: February 04, 2023, 01:52:18 am
content: Page 180 of the document you reference states: "By comparison, aluminum electrolytic capacitors are polarized and cannot withstand voltage reversal in excess of 1.5 V."So it appears the 1V rms just fits within this limit.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: February 04, 2023, 02:03:03 pm
content: Thanks for pointing that out. It seems like there are a lot of different opinions on acceptable reverse voltage. I've seen anywhere from -0.5 to -1.5.

author: moffy
author_rank: Super Contributor
timestamp: February 04, 2023, 10:43:31 pm
content: I was surprised at how large it was, I've always imagined about -0.6V but with no real data.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2023, 11:01:00 pm
content: I've seen figures from that, to 5 or 10% of rating, to 5V or so...There's not an electrode potential associated with it; or, if there is it's probably related to the electrolyte, maybe as an ionic bilayer or something, which will be good for a volt or two depending on electrolyte. So, it's not like it's exactly the 1.229V of water. (I don't actually know how much if any water they use in electrolytics, anyway? Mmh, probably mostly, still; just with enough stuff dissolved in it to raise the boiling point, at least for applicable (high temp) types.)Also, forming is a process that still works on finished capacitors; you can reverse one, and as long as it's done slowly enough (to avoid overheating, and allow gas to diffuse out), it will survive. If maybe not still within ratings (the cathode isn't textured so won't have as much capacitance as the anode does; the material is thinned in the process, perhaps increasing ESR, and certainly decreasing life; etc.).Which is basically how bipolar electrolytics are made, as far as I know -- symmetrical (textured and formed) foils.Electrolytics are certainly less sensitive than say tantalums, which can tolerate only a small reversal and don't form (at least not in the same way) (so, are more likely to fail shorted [or worse]).Tim

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 05, 2023, 12:40:21 am
content: The actual voltage across the cap is likely quite small- few bridges could supply the current to generate much AC voltage across a large cap.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: February 06, 2023, 12:53:51 am
content: Quote from: Conrad Hoffman on February 05, 2023, 12:40:21 amOh good point. I checked the specifications for the HP 4192A, which I would imagine is a typical instrument for this sort of thing, and it gives an output impedance of 50ohms.