Andy G4JNT wrote:

Use the formula    Vrms = 4.44.F.N.A.B   to calculate the Minimum turns
needed on any given core, using a B of around 0.1 Tesla for a good safety
margin.   All non-gapped ferrites designed for SMPSU transformers will have
more than enough inductance - up in the tens to hundreds of milli-henries -
when wound with enough turns to satisfy this equation.

Thanks Andy. I suspect I am not the only one on this reflector who
regards the design of cored inductors as something of a black art!

73, Tom G3OLB