This circuit from the SMPS circuit challenge of a year ago(!) is the "LED
blinker/dead battery milker/3909 replacement" from Russell McMahon:

http://www.piclist.com/postbot.asp?id=piclist\2001\10\10\084451a
http://www.piclist.com/postbot.asp?id=piclist\2001\09\27\023449a

It uses an RF choke of 300 uH.  I've spent hours trying to figure out how to
wind this inductor myself.  Alot of info is available
(http://www.mag-inc.com/library.asp#top, http://www.ferroxcube.com/ and
others) but it's hard for me to tell which is relevant or not.

It appears an aircoil would require 300+ turns which my guess is not
practical (and will other properties of the coil be correct?).  [ L = (A^2.
x N^2) / (9A + 10B) ]

Should a core be used?  What type?  Could some type of toroid be used (kids
might like winding it??).  What formulas can I use for this application?
Are any of the design guides applicable?  (There seem to be alot of info for
designing SMPSs but this application seems to be on the fringe.)

A similar application (http://www.emanator.demon.co.uk/bigclive/joule.htm)
uses a ferrite bead.  Can that be used?

Thanks.
---
David  Scott
http://www.piclist.com/member/DS-sss-C40
PIC/PICList FAQ: http://www.piclist.com

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