Quote:
Originally Posted by julie_m
Voltage dropping capacitors do not need to be Class X rated, since there is a series resistance. In fact, the self-healing property required for Class X is undesirable in this application.
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The jury is still out on this one. It's quite complicated when you start peeling back the layers...
Yes, some types of X2 caps can slowly lose capacitance with time - that issues crops up on the forum from time to time. But not all do. Good quality plastic film types that are properly encapsulated have been 100% reliable in my experience. But of course, those aren't what are used at this end of the market.
So if you decide to not use a self-healing X2 type, you've got quite a tricky design problem on your hands. Surges are inevitable, and there are 2 places where these end up.
The series resistance (low 100s of ohms) is the obvious one. But if you use a film resistor here - like these cheap products always do - it's guaranteed to fail in normal use (after only a year or two IME). So you need to use a carbon-comp or wire-wound, and a fuse (which should be there anyway, but often isn't).
The other recipient of the surge current is the circuit itself. If it's a good quality type, the electrolytic smoothing capacitor should cope well enough (when new), but it's worth making the zener (or whatever shunt device you're using) a relatively chunky sort. Again, not commonly seen on this sort of cheap tat.
In short, cheap capacitive droppers are guaranteed to fail, and not necessarily safely - and the circuitry being powered is likely to become "collateral damage" when this happens. At the very least a fuse should be added so that you're not relying on a film resistor to interrupt what could be a serious amount of energy.
Personally, with these sorts of cheap and cheerful Chinese things I like to - at the very least - replace the cheap dropper cap with a decent X2 type and replace the series resistor with a carbon-comp type. Did I also mention adding a fuse
I've got projects here with capacitive droppers that have been working for 20+ years continuously - for those, I used BC (now Vishay) MKP338-4 types, and would happily carry on using up my stocks for my own use. Today, for a new design I'd use Vishay F1772 types - these are still X2 types, but are sold as "high stability" with series connections in mind - so they are, by design, immune to the falling-capacitance problem (although the MKP338-4 seems to be fine in my experience), and it makes the issue of dealing with surges much easier.