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.
Thanks
Thanks
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.
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.
Just for interest,
X and Y type caps uses..
Link:
http://www.epcos.com/web/generator/...erty=Data__en.pdf;/PDF_EMIGeneralStandard.pdf
Regards
M. Gregg
X and Y type caps uses..
Link:
http://www.epcos.com/web/generator/...erty=Data__en.pdf;/PDF_EMIGeneralStandard.pdf
Regards
M. Gregg
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%C...4%EE%F0%EC%E0%F6%E8%FF/corona%20phenomena.pdf for more information about AC corona.
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/supplier/library/pdf/MurataAppSpecific.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_capacitor_information.pdf
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/supplier/library/pdf/MurataAppSpecific.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_capacitor_information.pdf
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!Bill_P said:
Is that roughly it?
X Caps and Y Caps are designed to be self protecting.
I cant recall what makes each one specific for its task but they are designed to fail SAFE.
Only X Caps and Y Caps should be used across the mains (in their specific roles). Using a non X/Y type could result in excessive heat if a standard cap is used in their place.
The same is not true the other way round. An X/Y type can be used in place of a standard cap, even if its dynamics may not be best suited for that application.
I cant recall what makes each one specific for its task but they are designed to fail SAFE.
Only X Caps and Y Caps should be used across the mains (in their specific roles). Using a non X/Y type could result in excessive heat if a standard cap is used in their place.
The same is not true the other way round. An X/Y type can be used in place of a standard cap, even if its dynamics may not be best suited for that application.
Caps
Y2 is safest , it is two caps in series and has very strict standards . We use NEMCO standard and them as a testing house .
You can find the DC voltages sometimes if the certification is given in detail . I assume it to be about 50 % above AC peak voltage . However I can not guarantee that .
I have always suspected they would sound good and never got around to trying some . 4n7 and 10 nF in Y2 could be useful ..
The rule to remember is only Y2 to ground in mains voltage .
Never fit VDR's to earth / ground . Someone is bound to think it a good idea .
Y2 is safest , it is two caps in series and has very strict standards . We use NEMCO standard and them as a testing house .
You can find the DC voltages sometimes if the certification is given in detail . I assume it to be about 50 % above AC peak voltage . However I can not guarantee that .
I have always suspected they would sound good and never got around to trying some . 4n7 and 10 nF in Y2 could be useful ..
The rule to remember is only Y2 to ground in mains voltage .
Never fit VDR's to earth / ground . Someone is bound to think it a good idea .
This has been posted before but it worth re-listing for newbies.
http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/KEMET%20Kollege%20Presentations/$file/EvoxRifaRFIandSMD.pdf
http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/KEMET%20Kollege%20Presentations/$file/EvoxRifaRFIandSMD.pdf
To DF96: Yes, roughly. The metallization in the cap vaporizes at each arcing. The capacitance falls with each such event until it approaches zero.
Paper Rifa class X2
We were told Rifa Evox paper has a good reputation on punch through and self healing . To quote you can drill through them and they still work . I have found them to fail at solder joint . Also they react to mains noise with nasty buzzes .
We were told Rifa Evox paper has a good reputation on punch through and self healing . To quote you can drill through them and they still work . I have found them to fail at solder joint . Also they react to mains noise with nasty buzzes .
Even though in the OP's terms we are talking general purpose film caps NOT X2 caps.. I thought the usual rule of thumb was that to deal reliably with AC, the DC volt rating needs to be ~2.82 x nom voltage applied - simply because the AC peak (= 1.41x Vdc) is then reversed at each cycle.
Yes, VAC pk only SQRT(2) times the relevant Vdc, reversing - but the implicit 50% de-rating then deals with the potential stored-charge/corona issue raised above.
So a DC film cap used on AC needs , say >600Vdc rating to cope with 230VAC generally. Not that I'd like to try it idly, I'd spring for X2 because they are cheap ... and guaranteed to fail gracefully in such use.
Yes, VAC pk only SQRT(2) times the relevant Vdc, reversing - but the implicit 50% de-rating then deals with the potential stored-charge/corona issue raised above.
So a DC film cap used on AC needs , say >600Vdc rating to cope with 230VAC generally. Not that I'd like to try it idly, I'd spring for X2 because they are cheap ... and guaranteed to fail gracefully in such use.
Rifa
http://www.farnell.com/datasheets/300514.pdf
This is a snubber of 0.1 uF and 100 R used for switches . It is to the general Rifa formula of design . They are good enough to give a voltage of 630 V DC in the PDF ( hope I remembered correctly ) . A good component to know .
I have seen higher figures sometimes . BC ( Philips ) I think do 300 VAC as standard . There are 440 VAC also ( old UK 3 phase ) .
2 x root 2 is the formula . We take 230 V to be 300 V where I work .
That's almost 850 V . We test many things at 1500 V .
I have used these in tube amps with good results .
http://uk.farnell.com/ducati/4-16-10-13-64/capacitor-8uf-450vac/dp/1219055
http://www.farnell.com/datasheets/300514.pdf
This is a snubber of 0.1 uF and 100 R used for switches . It is to the general Rifa formula of design . They are good enough to give a voltage of 630 V DC in the PDF ( hope I remembered correctly ) . A good component to know .
I have seen higher figures sometimes . BC ( Philips ) I think do 300 VAC as standard . There are 440 VAC also ( old UK 3 phase ) .
2 x root 2 is the formula . We take 230 V to be 300 V where I work .
That's almost 850 V . We test many things at 1500 V .
I have used these in tube amps with good results .
http://uk.farnell.com/ducati/4-16-10-13-64/capacitor-8uf-450vac/dp/1219055
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No, the improvement in corona resistance is not much greater for a 600VDC cap than for a 400VDC cap. I would not use a DC only rated cap on the AC mains under any circumstances.
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