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Toroidal Transformer Screening

Started by Syd Rumpo July 17, 2015
Hi

Another magnetics question!

I have a SMPS which uses a toroidal ferrite transformer.  It switches at 
~100kHz, works very well, it's not my design and I can't change it.

However, it introduces noise at about 50kHz onto/into my electronics 
which causes a problem.  After much experimentation, I found that 
putting an earthed screen around the transformer kills the problem quite 
dramatically.  The screen must be earthed, and doesn't need to be magnetic.

This is something I can do.  I don't have much room but I guess that 
wrapping copper tape around its exposed enamelled wire will cause 
problems for Edward Current.  Is there a rule of thumb for what sort of 
gap to leave between the windings and a screen?  How quickly does the 
field drop off?

(And yes, I know not to make a shorted turn.)

Cheers
-- 
Syd
On 7/17/2015 6:30 AM, Syd Rumpo wrote:
> Hi > > Another magnetics question! > > I have a SMPS which uses a toroidal ferrite transformer. It switches at > ~100kHz, works very well, it's not my design and I can't change it. > > However, it introduces noise at about 50kHz onto/into my electronics > which causes a problem. After much experimentation, I found that > putting an earthed screen around the transformer kills the problem quite > dramatically. The screen must be earthed, and doesn't need to be magnetic. > > This is something I can do. I don't have much room but I guess that > wrapping copper tape around its exposed enamelled wire will cause > problems for Edward Current. Is there a rule of thumb for what sort of > gap to leave between the windings and a screen? How quickly does the > field drop off? > > (And yes, I know not to make a shorted turn.) > > Cheers
It seems that you have capacitive coupling. You can probably use aluminum foil, if that is convenient. Any metal should work. If you have control over the assembly's layout, it would be wise to arrange things to nip it at the source.
On 7/17/2015 6:30 AM, Syd Rumpo wrote:
> Hi > > Another magnetics question! > > I have a SMPS which uses a toroidal ferrite transformer. It switches at > ~100kHz, works very well, it's not my design and I can't change it. > > However, it introduces noise at about 50kHz onto/into my electronics > which causes a problem. After much experimentation, I found that > putting an earthed screen around the transformer kills the problem quite > dramatically. The screen must be earthed, and doesn't need to be magnetic. > > This is something I can do. I don't have much room but I guess that > wrapping copper tape around its exposed enamelled wire will cause > problems for Edward Current. Is there a rule of thumb for what sort of > gap to leave between the windings and a screen? How quickly does the > field drop off? > > (And yes, I know not to make a shorted turn.) > > Cheers
The toroid will see your shield only if the mu is quite low. I don't know how low it would need to be, but you seem to be doing some good experimenting, so watch your results as you apply the shield at various distances. Consider such things as copper-clad kapton tape, or, for test purposes, you can get metalized tapes from the hobby stores.
On Fri, 17 Jul 2015 09:59:24 -0500, John S <Sophi.2@invalid.org> Gave
us:

>Consider such things as copper-clad kapton tape, or, for test purposes, >you can get metalized tapes from the hobby stores.
Home depot Copper roof flashing by the roll. http://www.homedepot.com/p/Sandell-Flash-Guard-8-in-x-20-ft-Copper-Step-Flashing-FG030820/202090786 or amazon makes these, which are quite a bit cheaper than a 20' roll. www.amazon.com/Sheet-Metal-Caps-Copper-Flashing/dp/B00D0U4GH0 Or better still a variety pack from ebay: http://www.ebay.com/itm/Copper-Strip-50mmx0-6mm-PER-METRE-C101-Roofing-Flashing-Moss-Valley-/141631726911?pt=LH_DefaultDomain_0&hash=item20f9e8a53f
Syd Rumpo wrote:
> Hi > > Another magnetics question! > > I have a SMPS which uses a toroidal ferrite transformer. It switches at > ~100kHz, works very well, it's not my design and I can't change it. > > However, it introduces noise at about 50kHz onto/into my electronics > which causes a problem. After much experimentation, I found that putting > an earthed screen around the transformer kills the problem quite > dramatically. The screen must be earthed, and doesn't need to be magnetic. > > This is something I can do. I don't have much room but I guess that > wrapping copper tape around its exposed enamelled wire will cause > problems for Edward Current. Is there a rule of thumb for what sort of > gap to leave between the windings and a screen? How quickly does the > field drop off? > > (And yes, I know not to make a shorted turn.) > > Cheers
OK, i will bite: what is Edward Current? and where do i get a description? The quacker DuckDuckGo fails.
On 7/19/2015 8:13 PM, Robert Baer wrote:
> Syd Rumpo wrote: >> Hi >> >> Another magnetics question! >> >> I have a SMPS which uses a toroidal ferrite transformer. It switches at >> ~100kHz, works very well, it's not my design and I can't change it. >> >> However, it introduces noise at about 50kHz onto/into my electronics >> which causes a problem. After much experimentation, I found that putting >> an earthed screen around the transformer kills the problem quite >> dramatically. The screen must be earthed, and doesn't need to be >> magnetic. >> >> This is something I can do. I don't have much room but I guess that >> wrapping copper tape around its exposed enamelled wire will cause >> problems for Edward Current. Is there a rule of thumb for what sort of >> gap to leave between the windings and a screen? How quickly does the >> field drop off? >> >> (And yes, I know not to make a shorted turn.) >> >> Cheers > OK, i will bite: what is Edward Current? and where do i get a > description? > The quacker DuckDuckGo fails. >
He is using the formal name rather than the nickname (Eddie).
On 7/19/2015 9:14 PM, John S wrote:
> On 7/19/2015 8:13 PM, Robert Baer wrote: >> Syd Rumpo wrote: >>> Hi >>> >>> Another magnetics question! >>> >>> I have a SMPS which uses a toroidal ferrite transformer. It switches at >>> ~100kHz, works very well, it's not my design and I can't change it. >>> >>> However, it introduces noise at about 50kHz onto/into my electronics >>> which causes a problem. After much experimentation, I found that putting >>> an earthed screen around the transformer kills the problem quite >>> dramatically. The screen must be earthed, and doesn't need to be >>> magnetic. >>> >>> This is something I can do. I don't have much room but I guess that >>> wrapping copper tape around its exposed enamelled wire will cause >>> problems for Edward Current. Is there a rule of thumb for what sort of >>> gap to leave between the windings and a screen? How quickly does the >>> field drop off? >>> >>> (And yes, I know not to make a shorted turn.) >>> >>> Cheers >> OK, i will bite: what is Edward Current? and where do i get a >> description? >> The quacker DuckDuckGo fails. >> > > He is using the formal name rather than the nickname (Eddie).
Oops! Should be Eddy.
On 20/07/15 03:15, John S wrote:
> On 7/19/2015 9:14 PM, John S wrote: >> On 7/19/2015 8:13 PM, Robert Baer wrote: >>> Syd Rumpo wrote:
>>> OK, i will bite: what is Edward Current? and where do i get a >>> description? >>> The quacker DuckDuckGo fails. >>> >> >> He is using the formal name rather than the nickname (Eddie). > > Oops! Should be Eddy. >
I thought it was rather clever!
On 20/07/2015 03:14, John S wrote:
> On 7/19/2015 8:13 PM, Robert Baer wrote:
<snip>
>> OK, i will bite: what is Edward Current? and where do i get a >> description? >> The quacker DuckDuckGo fails. >> > > He is using the formal name rather than the nickname (Eddie).
Well, I haven't met the guy, so it seems a bit forward to use his diminutive. Cheers -- Syd
On 17/07/2015 15:59, John S wrote:
> On 7/17/2015 6:30 AM, Syd Rumpo wrote: >> Hi >> >> Another magnetics question! >> >> I have a SMPS which uses a toroidal ferrite transformer. It switches at >> ~100kHz, works very well, it's not my design and I can't change it. >> >> However, it introduces noise at about 50kHz onto/into my electronics >> which causes a problem. After much experimentation, I found that >> putting an earthed screen around the transformer kills the problem quite >> dramatically. The screen must be earthed, and doesn't need to be >> magnetic. >> >> This is something I can do. I don't have much room but I guess that >> wrapping copper tape around its exposed enamelled wire will cause >> problems for Edward Current. Is there a rule of thumb for what sort of >> gap to leave between the windings and a screen? How quickly does the >> field drop off? >> >> (And yes, I know not to make a shorted turn.) >> >> Cheers > > The toroid will see your shield only if the mu is quite low. I don't > know how low it would need to be, but you seem to be doing some good > experimenting, so watch your results as you apply the shield at various > distances. > > Consider such things as copper-clad kapton tape, or, for test purposes, > you can get metalized tapes from the hobby stores.
Thanks for your input, I'll try the tape. Cheers -- Syd