I was using a Ferroxcube calculator to work out some choke windings. These have to be multiple RM12, about 5mH and 2.5A DC, so I was looking at the maximum inductance before saturation for different cores.
A 400nH core means 4 series cores each with 57 turns giving 1.3uH each. or 3 x 250nH cores with 89 turns, 2000uH each or 10 x 1000nH cores with 22 turns, 500uH each.
The 400nH cores are probably best, 89 turns won't fit with reasonably thick wire and 10 cores is painful.
However - it seems that the /total/ number of turns is about constant. Is that a rule of thumb, just normal or just lucky?
Cheers