[Amps] (Perhaps) A plate choke too far ...

Radio WC6W wc6w_amps at yahoo.com
Thu Jan 14 17:33:53 PST 2010


  No one appears to have addressed my original query re: servo cancelling the DC in a toroid core.  Rather wandering back to the general plate choke topic...  So, I'll wander a bit too!

  While experimenting with all those chokes, one of the curious things I noticed was that if I took a large ferrite washer (4.4 cm dia. x .4 cm thick of unknown material) and placed the flat side adjacent to a 70uH solenoid wound coil (3" of B&W 3016), it almost completely dampened the series resonances (first one circa 25 MHz)!!  

  I went looking around the 'net for an explanation and found US patent #3,835,370 that states that this effect is well known! 

  The magic washer exhibits rather high resistivity (circa 10^7 ohms per cube) and is only weakly attracted by a magnet.  Otherwise, it looks like plain ol' ferrite though, it is unlike any other core material in my inventory.  It is also very low mu -- 32 turns of wire produced only 3uH!  Anyone know what material this might be??

  I'd wager that this scheme could be usefully employed with almost any stock solenoid plate choke to eliminate those pesky resonances.

  Further searching found #3,982,814 & #2,692,372 both of which use a secondary Nichrome winding to similar effect!

  A final point of interest: #3,980,975 employing concentric coils.  I tried this with a pair of B&W coils and it kinda of worked.  A bit of judicious turns counting and positioning may be in order.

  Note: Patents may be easily downloaded via:  free.patentfetcher.com  

73 & Good evening,
   Marv WC6W

http://wc6w.50webs.com/







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