Hello,
In the attached AC filter we can see that L1 is a common mode choke.
We are trying to reduce the component count of this filter. Do you agree that we can get away with omitting the Cy1 capacitors , but that we must keep the Cy2 capacitors?
In fact, why would the Cy1 capacitors be needed at all?.....i mean, any noise that has escaped out out to earth, would always want to take the least resistance path, and that would definitely be through the Cy2 capacitors….because then the noise would be going back through the common mode choke and thus would not see the high common mode impedance of the common mode choke.
So why are the Cy1 capacitors included? If anything the Cy1 capacitors should just be used as extra capacitors at the same position as the Cy2 capacitors. Do you agree?
In the attached AC filter we can see that L1 is a common mode choke.
We are trying to reduce the component count of this filter. Do you agree that we can get away with omitting the Cy1 capacitors , but that we must keep the Cy2 capacitors?
In fact, why would the Cy1 capacitors be needed at all?.....i mean, any noise that has escaped out out to earth, would always want to take the least resistance path, and that would definitely be through the Cy2 capacitors….because then the noise would be going back through the common mode choke and thus would not see the high common mode impedance of the common mode choke.
So why are the Cy1 capacitors included? If anything the Cy1 capacitors should just be used as extra capacitors at the same position as the Cy2 capacitors. Do you agree?
Attachments
Thanks, I question why Cy1 is needed. Any noise which has coupled out of the product that will return back into live or neutral would choose the Cy2 route…because that takes it back through the other half of the common mode choke…where it sees the inductance of the common mode choke cancelled out…..if the noise were to return via Cy1, then it would miss out the other half of the common mode choke, and it would see a very high inductance, so therefore it would certainly not take that route…electricity generally takes the lowest impedance route, which in this case means returning back to mains via Cy2 and the other half of the common mode choke.
So why is Cy1 needed?
As you know, Common mode noise is reduced by having a common mode choke…..the common mode choke “forces” noise which has coupled out of the product to come back into the live or neutral wires, via the common mode chokes other half……so it makes the noise into differential noise, which gets acted on by the differential filter.
So, Y capacitors do not filter common mode noise……Y capacitors actually transport noise back into the mains live or neutral from earth…..making it “differential mode noise”....and allowing it to be acted on by the differential mode filter
So why is Cy1 needed?
As you know, Common mode noise is reduced by having a common mode choke…..the common mode choke “forces” noise which has coupled out of the product to come back into the live or neutral wires, via the common mode chokes other half……so it makes the noise into differential noise, which gets acted on by the differential filter.
So, Y capacitors do not filter common mode noise……Y capacitors actually transport noise back into the mains live or neutral from earth…..making it “differential mode noise”....and allowing it to be acted on by the differential mode filter
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A common mode choke acts as a choke for common mode HF. It does not convert common-mode to differential mode.
The X caps do not affect common-mode stuff. Only the Y caps do that.
If you don't need what the filter does, why not redesign it to do what you need?
The X caps do not affect common-mode stuff. Only the Y caps do that.
If you don't need what the filter does, why not redesign it to do what you need?
Thanks but a common mode choke diverts would-be common mode emissions back into the neutral wire from ground....once its back into the neutral return, it is no longer common mode noise...because its returning via the neutral........so its differential mode noise now.
There is no LC filtering of common mode noise.
Y caps do not filter common mode noise....Y caps act as a "bridge" over which would-be common mode emissions come back into the mains neutral.....ie make the noise become differential mode again.....where diff mode filters can act on it.
Of course a small bit does escape as common mode noise...and that is the bit that does not go through the second half of the common mode choke.......but thats only a small bit...and it certainly doesnt get filtered by a Y cap circuit...because if it went near a y cap then it would use it as a "bridge" to get back into the neutral return, where it would go through the second half of the common mode choke....because the common mode choke presents no inductance to a noise that goes through it in live and back through it in neutral.
No.kipper1 said:
Wrong.
No.
As you clearly have your own ideas about how filters work, why ask people who follow the traditional circuit model for advice?
There are no websites with anything on this. It is industrial secrey, surely you agree?
We agree that common mode chokes have a high inductance for common mode emissions?,.... and so reduce common mode emissions by providing a lower impedance path for noise when it goes through the second half of the EMI filter. (becuase of field cancellation within the common mode choke).
If noise goes through both "halves" of the common mode choke, then it is no longer common mode noise, surely we agree?
...why would significant quantities of noise NOT go through both halves of the common mode choke?......because surely that is the lowest impedance path.
I certainly cant find any text that says the opposite of what i am saying. And i cant find any document about LC filtering of common mode noise.
...................................................................
Also, this is realted to the above post, so please may i post it here?...
Page 17 (RHS of page) of the following document…
https://ac-dc.power.com/system/files_force/product-docs/an15.pdf
….states the following about EMI filter design for offline power supplies….
We agree that common mode chokes have a high inductance for common mode emissions?,.... and so reduce common mode emissions by providing a lower impedance path for noise when it goes through the second half of the EMI filter. (becuase of field cancellation within the common mode choke).
If noise goes through both "halves" of the common mode choke, then it is no longer common mode noise, surely we agree?
...why would significant quantities of noise NOT go through both halves of the common mode choke?......because surely that is the lowest impedance path.
I certainly cant find any text that says the opposite of what i am saying. And i cant find any document about LC filtering of common mode noise.
...................................................................
Also, this is realted to the above post, so please may i post it here?...
Page 17 (RHS of page) of the following document…
https://ac-dc.power.com/system/files_force/product-docs/an15.pdf
….states the following about EMI filter design for offline power supplies….
….The wavelength at 1MHz is 300 metres, is it really the case that noise at around 1MHz could “couple around the EMI filter” and get straight back into the mains?
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There are lots of websites teaching basic electronics. A number of good textbooks. Nothing secret.kipper1 said:
Yes.
No.
No. If it goes through the choke then it never was common-mode noise.
Textbooks do not specifically seek to dispute all possible false statements about how circuits work. Instead, they teach true statements about how circuits work. My experience is that people demanding disproof of their statements would, in most cases, not understand such disproof because if they could they would not need to ask for it.
Stray capacitance, or inductive coupling between circuit loops.
It is clear that you do not understand mains filters. As this is a safety-critical item you really should leave it to people who know what they are doing.
My boss cant find anybody. Are you in UK? We can pay to a degree. How much woudl you charge? BTW , the schurter filter 5500.2052 passes it , but we dont want the expense and dont like the connections...so we can actually copy that...anyway, until we get separate diff mode and common mode plots, we cant start calculating it, i think we agree on that.
Thanks DF96, i have read your CV, you have a PhD in something very complicated, have published books in RF Engineering, and i am not sure we could afford you even if you would care to entertain the idea.
I think you get the picture why my company is trying this themselves. There are very few engineers left in the UK like yourself.
Should we all give up and rely on the Far East to do it all for us?
I think you get the picture why my company is trying this themselves. There are very few engineers left in the UK like yourself.
Should we all give up and rely on the Far East to do it all for us?
I can only assume he is not willing to pay enough. There are lots of good engineers (and some bad ones) in every country. You can buy an expert, or buy a filter. The latter will be cheaper. You can always pay to have an existing filter modified to meet your precise needs.kipper1 said:
I am retired, not looking for work, and do not consider myself sufficiently expert on mains filters to be able to charge money with integrity. This is DIYaudio.
Thanks, i agree with this, i will have to avoid twisting things in order to be overly brief and avoid poetic license in technical matters.
Today, the number of UK citizens opting for electronics degrees is around a hundred or so per year...paltry...the office of national stats wont even give you the number because its so low they are ashamed...the UK is finished in engineering unless they bring back those tech type schools that you went to...plus start operating UK engineering in the same way the Germans do their engineering co's
Here is a good website on the matter..ie, why nobody in UK can find an engineer any more.....
https://massey276.wixsite.com/ukelectronics
https://massey276.wixsite.com/ukelectronics
There is certainly a problem in UK education, especially technical education. This is because grade inflation has been enthusiastically pursued for at least 50 years by the educational establishment and politicians of all parties. In the past it was expected that around 25% of the 16 years age cohort would achieve a good set of O-levels (school leaving certificate); now we expect approaching 50% of the 21 years age cohort to achieve an honours degree. The outcome is predictable: a typical modern UK graduate is less educated than a school leaver 50 years ago. A modern Masters degree is at a lower level than a Bachelors degree used to be.
However, there are still bright people and, as always, they will educate themselves.
However, there are still bright people and, as always, they will educate themselves.
It appears to me that not only happens there. Here, in Argentina, things are going wrong from about 70 years to the date. Each year is worse than the previous.
When I started university in 1991, a companion of me, didn't know how to add fractions providing he comes from a renowned private college from Lanus City, while me graduated in a more humble school (ENET N°1 Banfield), in a still more humble building, knew limits, derivatives and integrals.
When I started university in 1991, a companion of me, didn't know how to add fractions providing he comes from a renowned private college from Lanus City, while me graduated in a more humble school (ENET N°1 Banfield), in a still more humble building, knew limits, derivatives and integrals.
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