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 Post subject: 250 VAC Caps
PostPosted: Feb Wed 11, 2004 8:34 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 707
Location: Merrick,NY,USA
Hi Guys,<P>I just came across some 250 Volt AC rated polyester caps. The caps do not list a DC rating. Is there a way to tell? Is the DC rating a Multiple of the AC rating? <P>Mike<P>------------------<BR>


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 Post subject: 250 VAC Caps
PostPosted: Feb Wed 11, 2004 9:35 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warwick,RI
tubesrgood wrote:
<font>quote:</font><HR>Originally posted by Michael Simos:<BR><B>Hi Guys,<P>I just came across some 250 Volt AC rated polyester caps. The caps do not list a DC rating. Is there a way to tell? Is the DC rating a Multiple of the AC rating? <P>Mike<P>Somebody correct me,if I am wrong,but I believe a cap rated for a particular "AC working voltage" will work well(in theory anyway) with DC voltage 1.4 times higher in voltage than the AC rating shown on the cap.<BR>But,I have in my cap stock many AC caps which I bought from surplus catalogs that advertised them as having actual DC capabilties considerably higher than 1.4 times the stated AC rating,so I guess it depends on the individual cap design,as well.<BR>AC caps are capable of withstanding stresses not normally found in straight DC circuits,I do believe,such as greater heating effects due to the nature of alternating current.<BR>Malcolm<P><BR></B><HR>
<P><P>------------------<BR>


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 Post subject: 250 VAC Caps
PostPosted: Feb Wed 11, 2004 9:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Vieques, PR, USA
I agree with Malcolm. 1.4 would be a safe bet. I see ones labelled both ways and sometimes the DC rating is up as much as 2.5 times the AC but it does vary, apparently with the construction of the cap.<P>Just curious, what kind of values did you find?<P>-Bill M<P>------------------<BR>


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 Post subject: 250 VAC Caps
PostPosted: Feb Wed 11, 2004 7:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 30698
Location: Pocasset, Cape Cod, MA
I think 2.5 times is safe, provided the peak voltage never goes above about 900V (corona point for foil electrodes). But I see a lot of variations, and it probably also depends on the expected lifetime.<P>------------------<BR>


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 Post subject: 250 VAC Caps
PostPosted: Feb Thu 12, 2004 9:02 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 707
Location: Merrick,NY,USA
Hi Bill,<P>Found them in the MCM catalog and ordered a couple of different values. They had a broad range of values for sale.<P>Mike<P>------------------<BR>


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 Post subject: 250 VAC Caps
PostPosted: Feb Sat 14, 2004 7:08 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 30698
Location: Pocasset, Cape Cod, MA
??? K is 10%, presumably these are 0.12µF; I doubt they're made for line bypass unless they're marked X2 (across the line) or Y2 (line to neutral). AC-rated caps are normally pretty good. There's no logical reason why they should have affected low-frequency response in an amp. Do they measure 0.12?<P>------------------<BR>


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 Post subject: 250 VAC Caps
PostPosted: Feb Tue 17, 2004 6:28 pm 
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Posts: 3162
Location: Cockatoo, Victoria, Australia
When capacitors are used across AC lines they may be subjected to voltage spikes much higher than the normal AC voltage (inductive switching spikes, etc.) and are made for much higher working voltage than their AC rating would indicate. Hence the 270 volt AC / 1000 volt DC rating which sounds about right.<BR>Don Black.<BR>PS if you try using a capacitor DC rated at 1.4 times the AC voltage supply it will almost certainly break down.<P>------------------<BR>


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 Post subject: 250 VAC Caps
PostPosted: Feb Tue 17, 2004 9:33 pm 
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Posts: 34255
Location: Maryland 20709, USA
Leigh wrote:
<font>quote:</font><HR>Originally posted by Don Black:<BR><B>When capacitors are used across AC lines they may be subjected to voltage spikes much higher than the normal AC voltage (inductive switching spikes, etc.) and are made for much higher working voltage than their AC rating would indicate.</B><HR>
<P>Hi Don,<P>I was asked to troubleshoot a computer problem some years ago. It turned out they were getting 700-volt spikes on ground. It was inductively coupled from some large equipment in the basement below.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


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