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Advice on designing your own mains filter

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Dan K

pfm Member
Chaps, I've come to the conclusion that my iMac is very noisey and I'd like to do something about it.

A few questions..

In the past I have used a mains conditioner that had 19uF of blue Epcos Polyprop caps between Line and Earth and again between Neutral / Earth.

Is there a limit to the capacitance amount?

I understand that any cap must be 'X2' rated, ie guaranteed to break 'open' at mains voltage. The caps used must also have absolutely minimal leakage.

Is the X2 rating equivalent to the VDE rating? Are they both safe for UK mains use? -VDE regs appear to be very lengthy.

Assuming that I use a properly rated cap, then is there a different limit if it's across the Live/ Neutral, rather than as above?

Can I just use a washing machine starter cap in this application? Ie 50uF or so of very low leakage polyprop?

For a RC filter I should combine this shunt cap with a series resistor, 25W, 0R3 WW or similar and use a suitably large bleed resistor to keep it safe after shut down.

Many thanks for your help
 
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Dan K said:
and again between Neutral / Earth.
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NO!
Capacitance Neutral to Earth is very limited or your RCCB will trip and really dangerous if Live and Neutral get swapped. This position has to be Class Y and limited to 4.7nF and usually less to keep the leakage current down.

Mains filters have to be designed by people who know the regulations well
 
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davidsrsb said:
NO!
Capacitance Neutral to Earth is very limited or your RCCB will trip and really dangerous if Live and Neutral get swapped. This position has to be Class Y and limited to 4.7nF and usually less to keep the leakage current down.

Mains filters have to be designed by people who know the regulations well
Click to expand...
Thanks David, that was a commercial product! I didn't think much of it TBH
 
So, The filter calc is exactly the same as any other..

If I use a 3amp breaker, a 0R6 resistor and 50uF I'll have a low pass filter with a cutoff freq of 4.8kH. A voltage drop of 1.8V and 5 Watts to dissipate.

I could treble the (washing machine) caps and get 1.8kH.
 
NO Dan

Motor run caps are NOT suitable - they are not continuously rated, let alone X2; and you'll have about 10A of circulating reactive current. And a very ineffective filter, btw.

TBH I think - given the iMac will be using an SMPS - I'd start with a medical series commercial filter - Schaffner or similar - these do not have an filtration to Mains E, to avoid leakage current to mains Earth, but still provide a lot of common-mode suppression via the internal choke. Case it up properly.

Or for plug-in, no work required, a reputable conventional filter device like this: https://uk.rs-online.com/web/p/iec-filters/1229448
 
Paul R said:
What do you mean by your 'iMac is very noisy'?
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Its SM PSU chucks a ton of electrical noise back into my system. It's a rather old 11,3 Mac and (rather annoyingly) responds very obviously to different mains filters / leads. I run Audirvana Plus on it and use the optical out to a DAC, I've tried many options over the years and this is my preference. The USB outputs are cr@p in comparison.
 
I use a dedicated mains conditioner for my setup and have done so for many years (20) and it does make a difference.

I bought a James Audio conditioner back in the day https://www.tnt-audio.com/accessories/james_conditioner_e.html and glad I did, sadly the company's no longer. I did speak to the designer - builder (Alan Clarke) after about 5 years with regards to service requirements. He assured me their were no serviceable parts in the unit and it would last a lifetime without issue.

As previously noted buy one...
 
laverda said:
I use a dedicated mains conditioner for my setup and have done so for many years (20) and it does make a difference.

I bought a James Audio conditioner back in the day https://www.tnt-audio.com/accessories/james_conditioner_e.html and glad I did, sadly the company's no longer. I did speak to the designer - builder (Alan Clarke) after about 5 years with regards to service requirements. He assured me their were no serviceable parts in the unit and it would last a lifetime without issue.

As previously noted buy one...
Click to expand...
Do you still use it? I have one that I've loaned to a friend. I may have to get it back.

Are all the outputs rated the same?
Are there any pics of the inside available?

Thanks
 
I still can't see where the circulating reactive current is coming from. DC resistance of a large polyprop is 30M!..bleed resistor maybe 10K
 
Calculate the reactance of the cap Xc = [ 1/ (2*pi*f*c) ]. Divide AC rms voltage by that number to get AC rms current.
  • At 240Vac at 50Hz a 1uF cap is passing about 75mA of reactive AC current.
  • 150uF means c.10A of AC current is flowing in the Mains wiring, via the cap.
  • It's reactive current i.e. out of phase by (very nearly) 90degrees from applied ac voltage so you don't get billed for it, but it will be heating your house and equipment lead mains wiring none the less; about 60W in that little resistor (which you will pay for). And given IEC leads are only 6A rated*, really bad idea...
  • It won't be a good filter because the total mains wiring will approach the same resistance, give or take, as the series R value you proposed. So maybe 6-9dB of attenuation max for a lot of waste. One Heck of a spark when you plug it in, too.
  • And without serious thought given to how you discharge the caps on power-down, great potential for a fatal shock off the mains plug end: 150uf at up to a peak 340vdc is a lot of joules.
= Seriously bad idea.


( * under 2m they can be 10A rated, but that's just a 'bye' in the regs - its the same 1.5sq.mm wiring)

ETA an hour later for afterthought, belonging in same post:

I missed Dan's post above re: 10K bleed resistor.
Well with the largest posited cap value, 150uF into 10K will take about 2*pi*10K*0.00015) = 9.5 secs to bleed down to 63% of final voltage on un-plug. 5RC = 99% decay: so from the worst case 340V, a whole minute to get down to >10v. A very long way from safe - even at 50uF, 20seconds+

(@Dan K - this edit is absolutely not a dig at you, for the question you asked in good faith: but hopefully a useful illustration for anyone who searches sim in future.
The discharge risk is just as true for things like, say, Valve Amp psu supply design! ATB)
 
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As usual Martin C has the answers :)

48uF (measured) gives 3.6A at 240V/50Hz, and that is why the 3A Klixon circuit breaker goes pop after 2.5 minutes
 
Dan K said:
Its SM PSU chucks a ton of electrical noise back into my system. It's a rather old 11,3 Mac and (rather annoyingly) responds very obviously to different mains filters / leads. I run Audirvana Plus on it and use the optical out to a DAC, I've tried many options over the years and this is my preference. The USB outputs are cr@p in comparison.
Click to expand...
Does this manifest audibly?

Is it worth looking inside the SMPS for degrading components?
 
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