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Old 14th March 2006, 01:00 AM   #1
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Default Chassis material Q

Hi,

I'm building a voltage converter/RFI filter. Basically, this is a 2kVA torroid isolation transformer for powering some equipment I purchased in Europe.

The output of the transformer then feeds some cascaded RFI filters, which are located on a panel directly above the power transformer. Components use point-to-point wiring for the most part.

All of this is mounted in a 12x12x9 Hammond enclosure.

My question is, should the panels for mounting the RFI chokes be made from aluminum or some permeable metal (e.g., steel)? And why?

Thanks and regards,

Rob
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Old 14th March 2006, 06:38 AM   #2
infinia is offline infinia  United States
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Hi
Yes you are correct.

I'm assuming the RFI choke you mention is actually a line filter module. They need to have a good solid electrical connection to shield chassis.
This is for suppression of common mode RFI and noise rejection.
Inside the filter there are two or more "Y capacitors" bypassing the hot and neutral to ground.
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Old 14th March 2006, 06:47 AM   #3
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Quote:
Originally posted by infinia
Hi
Yes you are correct.

I'm assuming the RFI choke you mention is actually a line filter module. They need to have a good solid electrical connection to shield chassis.
This is for suppression of common mode RFI and noise rejection.
Inside the filter there are two or more "Y capacitors" bypassing the hot and neutral to ground.
Thanks, but I didn't understand your response. Should the enclosure or chassis mounting plate be made out of aluminum or steel?

Some more details -- I didn't use a commercial line module. Instead, I saw a Monster power filter with a clear plastic cover and became inspired. This lead to some research, and I wound my own cores. Right now, there is one line filter and two common mode filters, all designed for minimal voltage drop at 60 Hz.

Thanks and regards,

Rob
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Old 14th March 2006, 01:43 PM   #4
infinia is offline infinia  United States
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Quote:
Originally posted by weinstro


Thanks, but I didn't understand your response. Should the enclosure or chassis mounting plate be made out of aluminum or steel?

Your choice, mostly steel for heavy XFMER?

Some more details -- I didn't use a commercial line module. Instead, I saw a Monster power filter with a clear plastic cover and became inspired. This lead to some research, and I wound my own cores. Right now, there is one line filter and two common mode filters, all designed for minimal voltage drop at 60 Hz.

Thanks and regards,

Rob
What about shields for RF performance on your designed filters?
Do you use x & Y capacitors? Your design is not going to be a see through chassis as the demo that inspired you on this endevour? Have you designed magnetics before. Have you designed power line filters before? A saturated core provides little filtering. Maybe there are many other factors you haven't condidered yet?
Maybe you should look for these as not DIY but get CSA UL approved parts. If you have answered no to any of my questions this could be a dangerous endevour. Check with your insurance before preceeding.

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Old 14th March 2006, 01:46 PM   #5
eLarson is offline eLarson  United States
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Just some general tips:

A magnetic material (e.g. steel, or one of the more exotic alloys like permalloy or "mu-metal") is useful for blocking magnetic fields.

Metals that conducting better (copper, aluminum, etc.) are better at blocking electrical fields, but not good at all at blocking magnetic fields.
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Old 14th March 2006, 04:54 PM   #6
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Quote:
What about shields for RF performance on your designed filters?
That's more or less my purpose of this thread. What about shields for the filters? And what about interaction between the isolation transformer and the RFI chokes? More on this below, though...

Quote:
Do you use x & Y capacitors?
Yes

Quote:
Your design is not going to be a see through chassis as the demo that inspired you on this endevour?
No, enclosed metal box. I can't see E-M fields, anyway.

Quote:
Have you designed magnetics before?
Yes. Mostly for switchmode power supplies and variable frequency motor drives.

Quote:
Have you designed power line filters before?
Nothing quite like this. But, I've followed the technical guides from Magnetics, Inc. I also was a practicing engineer in the power industry for a number of years.

Quote:
Maybe there are many other factors you haven't condidered yet?
Maybe. I believe I've done my homework here. But, there might be a million factors to consider. If I'm a genius, I've probably addressed half of them.

Quote:
Maybe you should look for these as not DIY but get CSA UL approved parts. If you have answered no to any of my questions this could be a dangerous endevour. Check with your insurance before preceeding.
I have, for the most part. The "box" uses listed/recognized cabling, switches, fusing, XY caps, and receptacles. Insurance company (health or fire) couldn't care less, unless I do something like bypass the breakers in the distribution panel.

Anyway, I appreciate you trying to be helpful, but I wasn't looking to do a deep dive on every design aspect. But, allow me to distill my question into a simple form.

Because of the sizing of the enclosure, and the isolation transformer (Plitron 2kVA), the xfmr mounts in the bottom of the enclosure. The enclosure is made by Hammond, and has a steel body with aluminum front/rear panels. I then have a sheetmetal "deck" or subpanel directly above the xfmr where the RFI cores are mounted. DPDT on/off switches and panel mounted fuseholders are on the face. AC receptacles are on the rear. Now that the mental scenario is established, the question is should I make the subpanel from aluminum or steel?

Both have advantages and disadvantages:

Aluminum - easy to work with, can serve as a low z ground plane, expensive, soft, hardened alloys not readily available to the hobbyist, can't shield against induced noise.

Steel - hard to work with, dulls my tools, not the best conductor for chassis grounding, relatively cheap, strong, material permeability will help on inductive shielding.
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Old 14th March 2006, 06:17 PM   #7
infinia is offline infinia  United States
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From an RFI filtering point of view in 2 parts

a) steel deck holding differential filters.

b) Last stage of common mode filter should be completely shielded and as close to exit point as possible. Use thick copper foil or aluminum depends on how much performance you need. Are you filtering for FCC class B or the tougher TUV standard? Your call. Medical is toughest of all, usually requires a farady shield between pri. and secondary. Its better to stop noise at the source. So biggest differential filter would located before Xfmr pri.
hope this helps you on your new project.
regards,
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Old 14th March 2006, 06:38 PM   #8
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Oop forgot,
Most importantly
Keep the input and output differential filters as far away from each as possible. This certainly is more critical than any material used to support them, I usually assume the toroid transformer provides little if any filtering. (interleaved windings).
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Old 15th March 2006, 12:43 AM   #9
wwood is offline wwood  United States
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A couple of general comments about shielding.

Electric fields are best shielded with materials that have as low a resistivity as practical.

Low resistivity materials are also good AC magnetic field attenuators via "Eddy currents" . Surprisingly, aluminum is quite good at attenuating magnetic fields above 1 KHz and even help at mains frequencies (the thicker the better).

Ferrous materials can be very good at attenuating magnetic fields, but their resistivity tends to be a higher than Al and thus not the best electric field or Eddy current shields.

Aluminum is a great compromise material for ac magnetic fields and for electric fields. Mains shielding will be so, so at the fundamental, but with each harmonic it gets better.

I am not an rfi expert, but my guess is that Al has the edge. The aluminum will be a good magnetic field shield at high frequencies and will afford some reduction at mains frequency, again, the thicker the better.
Also, my guess is that other factors such as how you do your grounding, orienting of components and component selection will have a bigger effect than the material differences.

My two cents.

Bill
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Old 15th March 2006, 01:51 AM   #10
infinia is offline infinia  United States
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Knew this was going turn into a psuedo-academic argument
Not worth my energy right now I'm sorry.
I have done a few these designs and have passed a few regulatory agencies after a few times to get the knack of the art. I've been there and done that. BFD right?
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