Why can some X2 caps not be used "in series with mains"? - Page 1
Forum section: electronics / beginners

author: elcaron
author_rank: Contributor
timestamp: February 17, 2019, 08:19:06 am
content: I am currently looking for an X2 150nF cap to use for current limiting when sensing mains (230V) with an appropriate optocoupler. (I know this will introduce a phase shift, I do not need zero-cross detection, and I have also added resistors that limit the surge current - schematic attached).I findthis cap, but the datasheet states 'Not for use in "series with mains" type applications.'. It is rated for across-the-line EMI supression, though. In my case, I do not really see a physical difference, only that the cap sees less voltage. Is this for cases where inductive or capacitive loads are involved?Does this apply to most caps of this type, even if they do not state it, e.g.this one?This oneexplicitly lists in-series use in the datasheet.

author: Gyro
author_rank: Super Contributor
timestamp: February 17, 2019, 10:12:10 am
content: Two possible reasons that come to mind:- Class-X capacitors achieve their safety by self-healing, ie. if a spot in the dielectric film breaks down, the sputtered metalization on each side is thin enough to burn back from that spot to clear the short. To do this, there needs to be sufficient current available to achieve the burning. In across-the-mains applications, this current is obviously available but not in high impedance series applications, so the short could pass a lot of current into your opto.- Class-X capacitors are deliberately made with thin metalization so that they will self-heal easily. This limits their series current capability and causes them to loose capacitance over time. This is a common failure mode in capacitor dropper PSU applications.I'm not sure which it is for this particular manufacturer, but I think it is one of the above.

author: Shock
author_rank: Super Contributor
timestamp: February 17, 2019, 11:10:06 am
content: I think this application note describes what you are asking.https://www.vishay.com/docs/28153/anaccaps.pdfEven though thedatasheet you providedon these caps has a section on series and parallel usage they are talking about other series, it states at the top "across the line", you have to be careful with that. But if we have alook at this serieswhich was mentioned in that datasheet, it states "for connection in series".I'm not recommending this specific capacitor, just pointing out what to look for.

author: elcaron
author_rank: Contributor
timestamp: February 17, 2019, 01:11:29 pm
content: Thanks, I indeed overlooked that. Damn. And of course that cap isn't available at my supplier of choice. Guess I have keep looking ...

author: rhb
author_rank: Super Contributor
timestamp: February 18, 2019, 12:25:04 am
content: Quote from: Shock on February 17, 2019, 11:10:06 amReading that application note I wouldn't trust anything I *thought* it said. The grammar is so bad as to be completely ambiguous. Gad! I would have thought a company as prominent as Vishay would hire competent tech writers, but clearly that's not the case.I'm planning on making a CISPR16 LISN without the low pass input choke so I'm looking for suitable capacitors. A capacitor short would fry my SA, so I plan to use two caps in series.