It's easy enough to find the number of turns required for a particular
inductance on a known toroid core, but how do you know what size wire
to use so that the turns will all fit on a single layer? That's this
morning's geometry problem. I suppose you can find formulas, but a
quick Google search didn't turn up anything useful for me this
morning. What I worked out, that may be of some use to others, is:

Given D = inside diameter of the toroid core, and d = wire diameter,
same units, and N = number of turns:

N = integer( pi/arcsin(d/(D-d)) (arcsine in radians...)

d(max) = D*sin(pi/N)/(1+sin(pi/N))

If you want to calculate in degrees, replace pi by 180. To allow for
the inevitable little gaps and the wire not hugging the core ID
closely, pick a wire with a diameter at least 10% less than d(max).

Also, if you use a smaller wire so the turns can be spread out or
bunched together, you'll find that you can significantly adjust the
coil's inductance that way, especially with low-mu toroids such as -2
and -6 powdered iron.

Cheers,
Tom

"K7ITM" wrote in message
...
It's easy enough to find the number of turns required for a particular
inductance on a known toroid core, but how do you know what size wire
to use so that the turns will all fit on a single layer? That's this
morning's geometry problem. I suppose you can find formulas, but a
quick Google search didn't turn up anything useful for me this
morning. What I worked out, that may be of some use to others, is:

Nice, but the software "Mini ring-core calculator" (ver 1.2 free) will do
this for you, as will the Amidon manuals.

W4ZCB

On Apr 29, 11:41 am, "Harold E. Johnson" wrote:
"K7ITM" wrote in message

...

It's easy enough to find the number of turns required for a particular
inductance on a known toroid core, but how do you know what size wire
to use so that the turns will all fit on a single layer? That's this
morning's geometry problem. I suppose you can find formulas, but a
quick Google search didn't turn up anything useful for me this
morning. What I worked out, that may be of some use to others, is:

Nice, but the software "Mini ring-core calculator" (ver 1.2 free) will do
this for you, as will the Amidon manuals.

W4ZCB

:-) I knew if I posted that, someone would tell me about a nice
little calculator. For others who may have trouble finding the mini
ring-core calculator, try: http://www.dl5swb.de/html/mini_ring_core_calculator.htm

I did find another somewhat similar calculator as a Java applet, but
it seems to get the wire size wrong. The one Harold mentions gets the
wire size right, and does a whole lot more. Great to have around if
you're winding toroids, and it also does simple solenoid coils and
inductance of straight wire. You can even select US units (AWG;
inches) or rest of the world units (metric). Nice that it also tells
you the length of wire you'll need to wind the coil. Includes a
resonance calc and more.

Thanks, Harold!

Cheers,
Tom