X2 Line capacitors

X2 capacitors per se are of no utility.

X2 caps are such that are wired between two live poles in the input of the power supply, and Y are such that are wired between each pole and earth. Both are designed to aid in the line filtering both to difficult the input of interference signals from and into the equipment under question. But between them usually is placed a common mode inductor that consist of two winding in a ferrite core in the same direction, and as hardly coupled as consistent with isolation permits. Then, from line to rig, first it is placed the X2 cap (usually about 1µF 400V for 220V AC line), then the common mode choke (about 20mHy per side) and then the Y caps (.01µF 1KV). Then, X caps complete the cancelling in the line, and the Y caps derives interference to earth. The Y caps are a compromise, because a high value will give better filtering, but may trip the earth leakage device at home (Here known as "disyuntor"), and a too low value give no filtering. Sometimes, in SMPS's where also there is a PFC unit, two or three of them are cascaded to improve filtering noises.
 
Can I expect any immediate improvements using X2 capacitors across my AC line? It was reccommend to me. I thought it would be best to have this confirmed by members of this forum.
Thank you

Main filters are usually used to prevent noise escaping OUT of electronic equipment rather than noise getting IN as most electronic kit is relatively immune to this.
Frank
 
Can I expect any immediate improvements using X2 capacitors across my AC line?

Often it will give an improvement, modest most of the times.

An X2 cap across the mains transformer primary can suppress arcing at the switch so lengthen its life and reduce switch-off noises.

To achieve maximum results shouldn't the cap be as near as possible to switch pins?
 
But between them usually is placed a common mode inductor that consist of two winding in a ferrite core in the same direction, and as hardly coupled as consistent with isolation permits.

X2 caps make very little or no difference by themselves, however when combined with a CM choke - especially one of segmented design for improved HF rejection - that's a very likely positive move in SQ.
 
ClaveFremen said:
To achieve maximum results shouldn't the cap be as near as possible to switch pins?
No, this is not necessary. With the capacitor in place across the transformer primary the frequency components in the switch-off transient are much lower than they would be, so distance of connection does not matter.

samoloko said:
I see different applications with a wide range from 0.0022 to 1 uF
That is because the exact value does not matter very much. Personally I would use somewhere between 5nF and 100nF. If you really want to you can calculate the value after measuring the transformer inductance and resistance, but few people bother.
 

mrVetz

Member
2010-03-04 6:42 pm
Putting a X2 type cap across the transformer primary or across the switch does help with the noise from turning the switch on & off, especialy if the switch is low quality (this works only if grounding is done properly). Adding a full mains filter with X2 and Y2 capacitors, a resistor and a choke will help if you have noisy equipment around the house. I can't say that I heard a sonic improvement, but the stray noises from the mains line seem to be less audible.

I use the volume control on my CD player (marantz cd 17) so I can hookup my lm3875 power amp directly to the CD player. This gives me the best overall sound but makes noise an issue. The filter comes in handy to minimize the mains noise.
 

FoMoCo

Member
2012-12-04 10:04 pm
... With the capacitor in place across the transformer primary the frequency components in the switch-off transient are much lower than they would be, so distance of connection does not matter.
This also makes a nice resonant tank. Unless you get really unlucky with component selection, I'd expect it to be of no real consequence.

Putting the cap across the switch means the device is never completely switched off, unless the cap is very small.
True. But, the capacitor value should be extremely small. Realistically, this isn't a concern. Just don't go throw a huge cap across there. 0.1uF or less is a good starting point.

If you are worried about this then it would be better placed across the switch contacts in series with 100R to form a snubber.
+1 100R and 0.01uF, if you're concerned about the switch contacts arcing. Otherwise, there's no purpose.
 
The transformer will probably have sufficient resistance to damp the oscillation on switch-off. Those seeking perfection may, of course, calculate the right values of snubber resistance and capacitance to ensure critical damping. The rest of us will just put 10-47nF across the transformer primary and be happy.

There is another way of looking at it. X2 capacitors can gradually lose their value, due to internal spark erosion. If the equipment spends more time off than on, then put the cap across the transformer. If more on than off, put the cap across the switch. This will maximise the life of the cap.
 
The primary purpose of an X2 capacitor across the Live and Neutral of the Mains supply is to attenuate the interference that comes in with the required 50/60Hz.

This attenuation is achieved by "filtering" the high frequencies that compose the interference.
The cables feeding the property have inductance. Adding the capacitor creates the LC filter with a 2pole roll off characteristic. This will be fairly effective from the 10s of kHz to the low MHz.

John Curl suggested Live to Neutral capacitor value of up to 10uF.