Wound torroids not useable as scope lead common mode chokes

• Wound torroids not useable as scope lead common mode chokes
  Posted by Faringdon on 31 Oct, 2023 22:26
• Hi,
  We often wrap our scope leads  round and through a torroid to reduce common mode noise.
  However, we only now have torroids which are covered in copper windings. It takes ages to get all the windings off, but would you agree, they will be very less useful as common mode chokes if we leave the windings on them?...due to shielding. (the windings are "Open" circuit, with free ends)
• #1 Reply
  Posted by tggzzz on 31 Oct, 2023 22:44
• Quote from: Faringdon on 31 Oct, 2023 22:26

  Hi,
  We often wrap our scope leads  round and through a torroid to reduce common mode noise.
  However, we only now have torroids which are covered in copper windings. It takes ages to get all the windings off, but would you agree, they will be very less useful as common mode chokes if we leave the windings on them?...due to shielding. (the windings are "Open" circuit, with free ends)
  

  
  
  Classic faringdon/treez question
  
  I expect most "people would agree" that the best answer would you be to test that for yourself. You are a professional SMPS engineer, aren't you?
  
  If you can't be bothered to do that, why should anybody here waste their time responding to your under-specified question - e.g. Frequency, voltage, current, distance, orientation of the source, and part number of your toroid.
• #2 Reply
  Posted by Konkedout on 01 Nov, 2023 03:49
• I think that the toroids with unused windings might work well enough but will not be quite as good as bare ones.  The reason is that the unused windings contribute winding capacitance which will degrade the high frequency rejection provided by the toroid.  Even though those windings are not connected to anything, they will act like capacitance from input-output of the scope probe lead winding.
  
  Of course the added practical issue is that the unused wire will fatten the core so that you cannot apply as many turns of the winding that you do want.
  
  I use a big common mode toroid in the output of my cheap bench power supply.  The common mode toroid really knocks down the common mode noise produced by the power supply.
  
  But for such an instrumentation purpose, I think that the cost of a few pieces should not be a big deal.  Why not buy a few large high perm amorphous cores?  I tested some amorphous cores a few years ago, and they seemed to offer wider bandwidth than high perm ferrite.
• #3 Reply
  Posted by tggzzz on 01 Nov, 2023 09:08
• This question is double posted at https://www.eevblog.com/forum/projects/probing-noisy-signals-to-the-scope/msg5145303/#msg5145303
  
  For background, see https://www.eevblog.com/forum/renewable-energy/mains-zero-cross-detector-pulse-issue/msg5127057/#msg5127057
  
  Think about the better place to reply. Or not reply.
• #4 Reply
  Posted by jonpaul on 01 Nov, 2023 13:00
• TROLL WARNING see other posts/replies
  
  j
• #5 Reply
  Posted by jonpaul on 01 Nov, 2023 13:02
• FTTS
  
  GO WAWAY JUST BUY TOTOIDS   FERRITE CORES  $0.50
  
  in 55 yrs I NEVER NEEDED TO DO THIS
  
   
  j