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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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Old 10th May 2021, 1:31 am   #1
klugardo
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Default Capacitors across the mains supply?

How can I tell if the capacitor I have is across the mains or not?

What does this mean?
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Old 10th May 2021, 5:19 am   #2
joebog1
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Default 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
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Old 10th May 2021, 6:26 am   #3
Cruisin Marine
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Default 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.
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Old 15th May 2021, 9:17 pm   #4
Oldcodger
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Default 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.
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Old 15th May 2021, 9:31 pm   #5
Hartley118
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Default 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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Old 15th May 2021, 9:50 pm   #6
Cruisin Marine
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Default 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.
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Old 15th May 2021, 9:53 pm   #7
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Default Re: Capacitors across the mains supply?

Quote:
a powerful low impedance source
Which increases very quickly with frequency, a a few kHz it is quite high, more than enough for a small 10's of nF capacitor to reduce it significantly.
 
Old 15th May 2021, 10:22 pm   #8
Hartley118
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Default Re: Capacitors across the mains supply?

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
a powerful low impedance source
Which increases very quickly with frequency, a a few kHz it is quite high, more than enough for a small 10's of nF capacitor to reduce it significantly.
Isn’t there plenty of distributed parallel capacitance anyway in the mains wiring? I just question the practical benefit of those simplistically positioned troublesome WIMA capacitors. Has anyone noticed any practical difference after one has explosively expired - as they often do?

Martin
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Old 15th May 2021, 11:37 pm   #9
Cruisin Marine
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Default 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.
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Old 16th May 2021, 12:13 am   #10
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Default Re: Capacitors across the mains supply?

Quote:
Originally Posted by Hartley118 View Post
So how much RFI reduction does a simple capacitor directly across the mains actually provide in practice?
As others have said, the mains isn't low Z at all frequencies. I've done a fair bit of EMC testing over the years. An X capacitor across the mains has been seen to make no effect on some conducted spurs, a few dB on many and 20+ dB on some.

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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