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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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10th May 2021, 1:31 am | #1 |
Triode
Join Date: Oct 2020
Location: Lakeland, Florida, USA.
Posts: 35
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Capacitors across the mains supply?
How can I tell if the capacitor I have is across the mains or not?
What does this mean? |
10th May 2021, 5:19 am | #2 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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Re: accross the mains capacitors
You got two "hot" wires coming in from your power socket!!. Black and White, last time I looked.
If the cap is ACROSS the mains, ( check with multimeter on OHMS scale X1 ) its across the mains. IF its American switched between Hot and Neutral, its called a DEATH cap. Fender amps are VERY famous for it!!. The only country that used that system, THAT I am aware of, is USA. Joe |
10th May 2021, 6:26 am | #3 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 994
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Re: accross the mains capacitors
They are usually filter capacitors, and certain types like RIFA are renowned for going very bad (do a search on those).
They can be wired across the two AC feeds (live and neutral in the UK) or also from either AC feed to the ground or earth connection. Here are some video's showing how nasty those RIFA Cap's can be https://duckduckgo.com/?q=RIFA+capac...deos&ia=videos This is interesting, it shows the ratings of the different X and Y rated type capacitors https://www.ehow.co.uk/info_8649387_...apacitors.html Last edited by Cruisin Marine; 10th May 2021 at 6:55 am. |
15th May 2021, 9:17 pm | #4 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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Re: Capacitors across the mains supply?
Could be RF filtering (usually sub nano farad value, as used on amps to stop RF breakthrough) or higher values on switch mode PSU (possibly to stop switching transients getting back to house wiring). I've been amazed ar how many so called big names PA amps are prone to local station pick up.
Son had this problem on a GAMES PSU marked 100 - 250v AC on a product made for USA use. He'd noted the supply range (but it was a present from his father-in-law on a LA holiday). First time he plugged it in, X/Y cap went bang. |
15th May 2021, 9:31 pm | #5 |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,198
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Re: Capacitors across the mains supply?
So how much RFI reduction does a simple capacitor directly across the mains actually provide in practice?
The mains is such a powerful low impedance source, isn’t it like trying to dam a river with a housebrick? Martin
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15th May 2021, 9:50 pm | #6 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 994
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Re: Capacitors across the mains supply?
So how much RFI reduction does a simple capacitor directly across the mains actually provide in practice?
The mains is such a powerful low impedance source, isn’t it like trying to dam a river with a housebrick? Martin The mains cannot be a low impedance to all frequencies, that is impossible- add to that the effective reactances added by the feed cables and you have unwanted radiation, both in and out of the unit being used. It is easier to "decouple" as many frequencies by brute force than to try and do it by taking all (many complex) factors in to account. |
15th May 2021, 9:53 pm | #7 | |
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Re: Capacitors across the mains supply?
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15th May 2021, 10:22 pm | #8 | ||
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,198
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Re: Capacitors across the mains supply?
Quote:
Martin
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15th May 2021, 11:37 pm | #9 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 994
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Re: Capacitors across the mains supply?
This all depends on what equipment you refer to. There is a massive difference between an old analogue Valve radio and a modern switch mode supply (even now in high level test gear) and radiation over an almost resonant length of one or two metres of mains cable can wipe our FM reception for example.
I must stop getting drawn in to these never ending discussions lol Last edited by Cruisin Marine; 15th May 2021 at 11:50 pm. |
16th May 2021, 12:13 am | #10 | |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
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Re: Capacitors across the mains supply?
Quote:
They're a good tool when you need to get something to pass a limit. Never look a gift dB in the mouth... Also, sticking a larger one in even when you already have a corcom/schaffner filtered mains receptacle can be helpful. David
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