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Old 2nd May 2012, 11:54 AM   #1
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Default X-capacitor

Just a small question, does anyone know if a 250V, 10nF Polyester capacitor would be suitable to use as an X-capacitor across +-50V (100V) of a transformer secondary? I've not had much luck finding any articles which recommend any types of capacitors to use.

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Old 2nd May 2012, 02:28 PM   #2
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Yes.
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Old 2nd May 2012, 04:23 PM   #3
DF96 is online now DF96  England
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If it is across a secondary then it doesn't have to be an X capacitor. Note that X and Y are capacitor types safe for appropriate mains use, not circuit positions.

As a general rule, capacitors which will have permanent AC stress need to be derated from their DC ratings although I am not sure why.
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Old 2nd May 2012, 05:22 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Just for interest,

X and Y type caps uses..

Link:

http://www.epcos.com/web/generator/W...alStandard.pdf

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Old 2nd May 2012, 05:44 PM   #5
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Usually X caps are used between line poles, and Y2 in each pole to earth.
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Old 4th May 2012, 12:16 AM   #6
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Thanks for your help
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Old 4th May 2012, 02:28 PM   #7
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Old 4th May 2012, 03:56 PM   #8
Bill_P is offline Bill_P  United States
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Quote:
Originally Posted by DF96 View Post
As a general rule, capacitors which will have permanent AC stress need to be derated from their DC ratings although I am not sure why.
AC corona effects can severely reduce the reliability of a capacitor connected across a 220 volt mains if the capacitor is not specially constructed for this connection. See ftp://ftp.vegalab.ru/%CA%CE%CC%CF%CE...0phenomena.pdf for more information about AC corona.
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Old 4th May 2012, 04:06 PM   #9
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Default Worth a try .

In my work I use X2 Y2 caps . Others have already covered the basics . Some Y2 ceramics used to state silver ceramic , Murata comes to mind . Sorry I can not say which and only saw it stated a few times when determining the certification . Perhaps some here as I used the words silver ceramic .

http://www.mouser.com/catalog/suppli...ppSpecific.pdf

As high voltages requires special and usually desirable dielectric they might be of Audio grade . They are cheap and should be better than low grade caps . I don't know because I never tried . As long as they don't exceed their ratings they should be fine . I see no reason why they wouldn't work at low voyages .

Never use 1000 V caps as a substitute in mains voltage work . Yes they will work . However they have no certificate for that use . The exception seems to be motors . I do not know if motor caps can be used on the mains direct .

In tube amps well worth a try .

If my hunch is right silver ceramic might be an alternative to silver mica .

Good luck with it .

The regulation for X2 across the mains is no more than 0.1 uF without a discharge resistor . After that discharge within 2 seconds and 5CR time constant is taken as fully discharged . Y2 is best avoided if to ground .

http://www.seered.co.uk/sunvic_capac...nformation.pdf
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Old 4th May 2012, 06:57 PM   #10
DF96 is online now DF96  England
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Quote:
Originally Posted by Bill_P
AC corona effects can . . .
Thanks for the link. The story, as I understand it, is that for a normal capacitor there is an AC voltage limit which is only loosely connected to the DC voltage limit, as it does not directly relate to dielectric thickness. Charging a capacitor to a highish voltage can involve a temporary corona discharge due to tiny air gaps between plate and dielectric, which does not do much harm unless you do it again 60 times a second!

Is that roughly it?
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