At around 50-60 Hz copper and steel have about the same shielding abilities for magnetic fields. Below that frequency the conductivity of copper comes into play and above that frequency the permeability of steel comes into play. (significantly more so)
Using two layers of #26Ga of mild steel (at 0.75" separation) will get you a little over 10dB of magnetic field shielding at 60Hz.
Using two layers of 0.125" (aprox 3mm) of mild steel (at 0.5" separation...~12mm) will get you a little over 30dB of magnetic field shielding at 60Hz.
This was the construction details for a "Class 10" and "Class 30" shielded enclosures ( rooms )
An AC magnetic filed induced into your shielding will produce fields "inside" of your shield at any gaps in the magnetic path. (leaks)
If you can "shunt" the magnetic field around your susceptible device enough then you can get some shielding that way...despite gaps. This is where high Mu materials come into play. You may have seen some small electric motors in tape drives wrapped in a magnetic material.
Electric field shielding at audio frequencies is very much simpler and easier.
Using two layers of #26Ga of mild steel (at 0.75" separation) will get you a little over 10dB of magnetic field shielding at 60Hz.
Using two layers of 0.125" (aprox 3mm) of mild steel (at 0.5" separation...~12mm) will get you a little over 30dB of magnetic field shielding at 60Hz.
This was the construction details for a "Class 10" and "Class 30" shielded enclosures ( rooms )
An AC magnetic filed induced into your shielding will produce fields "inside" of your shield at any gaps in the magnetic path. (leaks)
If you can "shunt" the magnetic field around your susceptible device enough then you can get some shielding that way...despite gaps. This is where high Mu materials come into play. You may have seen some small electric motors in tape drives wrapped in a magnetic material.
Electric field shielding at audio frequencies is very much simpler and easier.
Your magnet experiment says otherwise.obiwankenobi said:
RFI, perhaps, but you need to avoid any slots or gaps that the RF can sneak through.
50Hz no, unless the foil is many cm in thickness.
Two different problems, so need two different solutions.
RF can be stopped using almost any thin conductor.
50Hz magnetic field is difficult to stop, so usually people use different methods such as minimising circuit loops and putting some distance between the 50Hz source and the sensitive circuit.
If this is a commercial project then maybe you need to pay for a competent designer?
Thank you DUG, excellent info, I am learning and taking notes !
My circuit is working very well but I would like to prevent 50/60 Hz hum as much as I can. I don't have grounding problems or noise resulting from loops, etc. The box can't be metallic in my case. The DC power supply goes outside, far from the box, and has an extra conductor that optionally connects circuit ground to mains earth. I think that some foil wrapping around the PCB will slightly attenuate these fields. This box could be near other equipment with transformers. Not simple issue.
WntrMute2:
DF96:
You didn't understand or you have missed some of my writing. I know the theory but I have a LOT to learn, and I am trying to learn here. Before this project becomes commercial, then I'll try to get the necessary information to make it work correctly. I am a hobbyist in the audio field, but not in the digital hardware and software design and computer science field. This is a DIY forum, so we usually don't hire "competent" designers for a hobby. So I will take the part of your reply that is really good and meaningful.
In some scenarios we are pupils and in some other we can be teachers.
My circuit is working very well but I would like to prevent 50/60 Hz hum as much as I can. I don't have grounding problems or noise resulting from loops, etc. The box can't be metallic in my case. The DC power supply goes outside, far from the box, and has an extra conductor that optionally connects circuit ground to mains earth. I think that some foil wrapping around the PCB will slightly attenuate these fields. This box could be near other equipment with transformers. Not simple issue.
WntrMute2:
DIY can be fun and also can result in excellent implementations, sometimes much better than professional. I have seen this many times.
DF96:
You didn't understand or you have missed some of my writing. I know the theory but I have a LOT to learn, and I am trying to learn here. Before this project becomes commercial, then I'll try to get the necessary information to make it work correctly. I am a hobbyist in the audio field, but not in the digital hardware and software design and computer science field. This is a DIY forum, so we usually don't hire "competent" designers for a hobby. So I will take the part of your reply that is really good and meaningful.
In some scenarios we are pupils and in some other we can be teachers. OK, you firmly believe that you know the theory.
I am still unclear whether this project is for fun or for profit.
I am still unclear whether this project is for fun or for profit.
I try to help hobbyists and students (provided they make it clear that they are students and not trying to cheat on their assignments). I am less willing to provide free consultancy to commercial enterprises who are attempting something beyond their ability; this would be unfair on all the consultants and contract designers trying to make a living from electronics.WntrMute2 said:
According to our supplier, the cheapest EMI shielding for ABS plastic enclosoures is a EMI paint based on Cu or Ag that is sprayed on the inside of the enclosoure, or an additive that can be mixed to the ABS itself to make it slightly conductive. The paint method is widely adopted. The additive is more expensive and cannot be used in some instances. I've rarely seen the copper foil method lately. This is true for radio frequency shielding, the major concern for today microprocessor-based circuits or circuits that can be near to them, such as low-level audo boards that are sitting next to CD/DVD players or Wi-Fi devices. 50/60Hz shielding is a completely different thing.
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