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Many applications that need a low voltage supply that is not isolated from the mains use a capacitive dropper circuit to get it -- an example is shown in the schematic below.

schematic

simulate this circuit – Schematic created using CircuitLab

In this type of circuit, Clim is usually a metallized-film cap (in the 100nF-10uF range depending on current draw) with a 400 or 630WVDC rating. Can an equal value X-class safety capacitor with a 250 or 275WVAC rating be substituted instead, or would the continuous current flow pose a problem for the X-class capacitor/a hazard to the outside that is greater than the current capacitors used? Would using an X-class capacitor for Clim provide any benefits over the current non-safety-rated capacitors used in that position, even?

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  • \$\begingroup\$ Current flows continuously in such a capacitor regardless of whether it is in series with a load. In fact, the load reduces the current slightly. \$\endgroup\$ – Dave Tweed Aug 25 '16 at 11:09
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Can an equal value X-class safety capacitor with a 250 or 275WVAC rating be substituted instead

An X class safety capacitor rated for your incoming AC supply (not stated in the question), can be placed across live and neutral without fear of it shorting the AC out (if it fails) and thus causing a fire: -

enter image description here

Putting it in series with a bridge rectifier and load can only reduce the voltage the X capacitor sees so clearly it is under less stress.

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  • \$\begingroup\$ Clim should be an X2 for reliability. This question has come up before, this year in fact, so do a search. \$\endgroup\$ – Robert Endl Aug 25 '16 at 7:40
  • \$\begingroup\$ @RobertEndl I think you mean X1 dude (or even Y). \$\endgroup\$ – Andy aka Aug 25 '16 at 9:46
  • \$\begingroup\$ Andy...Uh, no, X2 should do I think. Why X1? \$\endgroup\$ – Robert Endl Aug 25 '16 at 12:59
  • \$\begingroup\$ X1 has higher voltage withstand and hence higher "reliability". Y is "safer" which could mean "reliability". \$\endgroup\$ – Andy aka Aug 25 '16 at 13:03
  • \$\begingroup\$ An X2 should be near bullet-proof in this application. When's the last time you saw an X2 fail, except maybe on a line with exceptional line spikes. \$\endgroup\$ – Robert Endl Aug 25 '16 at 14:10
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Class X doesn't make that circuit any less dangerous. You have to assume that it will fail to a short circuit. When it does there's a path from the neutral through the rectifier to the load. This will gleefully kill you.

You MUST assume that the neutral will be energized. Maybe the plug can be reversed (Europe). Maybe there's a failing neutral pin (RV hooked up to a beat-up campsite receptacle). Maybe someone messed up the wiring. Always plan for neutral-ground voltage.

If the load is properly fused and insulated, you don't need a rated capacitor. That's the Right Way to use a capacitive dropper. The better tutorials will make it clear that you are NOT building touch-safe equipment.

If touch-safety is your goal, the safety-police answer is to buy a listed SELV power supply. Or just use batteries. Either are commodity parts used in all kinds of electronic gadgets so there's not a great reason to roll your own supply, but if you insist:

The bare minimum would be to draw a low-voltage boundary through the limiting resistor, dropping capacitor, and bleed resistor. Those parts need to fail open - fusible resistors, class Y cap - and there needs to be sufficient clearance between any hot and low-volt conductor. This is an unusual application for a class-Y cap, they come in tiny values, so it's only practical if you're drawing a similarly tiny amount of current.

If you buy cheap no-marking or possibly counterfeit power supplies, it's good to know what to look for: clear delineation around the hot block only bridged by safety components. They might even be potted, covered, or taped. "Looking Mains Voltage in the Eye" in Hackaday is good reading.

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  • \$\begingroup\$ "Class X doesn't make that circuit any less dangerous. You have to assume that it will fail to a short circuit." - Are you sure? As far as I know, X-Class caps are intended for applications where the cap failing short would present a fire risk, so these caps are designed to fail open or not fail at all. "If the load is properly fused and insulated, you don't need a rated capacitor." - I think you do, because of the fire risk. "you are NOT building touch-safe equipment." - I'm under the impression that the OP is aware of this (but I could be wrong). \$\endgroup\$ – marcelm Jan 4 at 22:18

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