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I'm trying to order components, so I can make a board following this circuit. My problem is, I cannot find any 0.1uf capacitors, that are rated at 275VAC. The highest voltage I can find, in stock, is only 50VAC, unless I want to buy a couple thousand, which I only want 2 to 3.

I can how ever find a film capacitor with the exact ratings I need, with the exception of it being bipolar.

Can I add a diode in series with C1, and C2, to achieve the results I want? And if that wont work, could someone please recommend another cap from digikey?

Much appreciated.

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LittleRain
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  • Did you even look? Polyprop and film are both readily available: https://www.digikey.com/products/en/capacitors/film-capacitors/62?k=capacitor+275VAC&k=&pkeyword=capacitor+275VAC&sv=0&pv1989=0&pv2049=u0.1%C2%B5F&sf=0&FV=ffe0003e&quantity=&ColumnSort=0&page=1&pageSize=25 – Jack Creasey Mar 08 '19 at 00:13
  • That's exactly what I was looking at, have been looking for about half an hour. But aren't those bipolar? – LittleRain Mar 08 '19 at 00:14
  • You can use bipolar caps in place of unipolar. Since a bipolar cap can tolerate either polarity being applied to it, it works just fine if you only ever apply one polarity to it. You can't do the reverse though for obvious reasons. – DKNguyen Mar 08 '19 at 00:14
  • Why are you looking for a polarized cap for an AC application?! – ThreePhaseEel Mar 08 '19 at 00:18
  • Will it work with this circuit though? Specifically C1, because its supposed to be charged with R1, and if its bipolar wouldn't it charge both ways? Sorry I've never worked with AC before. – LittleRain Mar 08 '19 at 00:19
  • ThreePhaseEel, I'm just following the schematic. Trying to make a dimmer circuit for lights/motors. – LittleRain Mar 08 '19 at 00:19
  • @LittleRain what on that schematic is telling you that that cap is polarized? – ThreePhaseEel Mar 08 '19 at 00:20
  • Doh. You are right, I was looking at the bent line. – LittleRain Mar 08 '19 at 00:21
  • Polar caps are "run it backwards and it'll die with a bang", not "this is desired behavior". You can pretty much always substitute a non-polar cap. – TimWescott Mar 08 '19 at 00:21
  • Ok thanks a lot guys. Right Tim, that's what I thought at first until I read this schematic, and I started to doubt myself. A quick google search also didn't help with that either, something along the lines of Can vs Should. – LittleRain Mar 08 '19 at 00:26

1 Answers1

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That's not a polarized cap you're seeing on the schematic

The universal sign of a polarized capacitor, regardless of whether it is using straight-plates-with-hatching, straight-plate/bent-plate, or plate-in-plate symbology, is the + sign that marks the actual polarity of the capacitor on the schematic. From the lack of that plus sign here and the fact this is an AC filter, we can infer the cap is a non-polarized type (such as a film capacitor).

But don't just put any old cap in there!

However, since this is an AC mains circuit, the capacitor you need to use here is what is called an X-rated capacitor. These are tested by UL and other agencies to withstand mains voltage surges without catching on fire, and carry an AC voltage rating (unlike most caps, which only have a DC voltage rating) as a result, in addition to being festooned with agency approval markings. Furthermore, the "275VAC" rating on an X capacitor is a maximum nominal voltage rating, not a maximum working voltage rating -- in practice, a 250VAC capacitor is fine for 240VAC applications, and either will work for 120VAC applications.

ThreePhaseEel
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  • 'festooned' is a good description of what the X and Y rated caps have on them. –  Mar 08 '19 at 04:14