Amidon (
http://www.amidoncorp.com) has been the standard place to buy this
stuff for amatuer radio operators, and they have a reasonable selection of
cores. They seem to list the fair-rite numbers in their ferrite cores (they
also have iron powder cores).
Most of the posters are on track as far as I can tell, here's my summary and
two bits:
- Design for the inductance you need _at_ your maximum current. The
effective inductance of your coil will go down as the core saturates, take
that into account.
- If you put a constant voltage on your core you can watch the current and
see if it's saturating: a constant inductance will be indicated by a
constant slope as the current increases. As the core starts to saturate the
current will curve upward slightly. If the current _really_ curves upward
before you've reached your design current you've got problems.
- If you're making a core for a switching power supply you probably don't
have to worry about gapping it -- gapping the core reduces the inductance
variation with variations in current by limiting the overall reluctance of
the core. If you need controlled inductance for some reason, run away.
- The winding will exhibit loss due to current x voltage drop. The core
itself will also show a loss due to hysteresis. The core loss is
proportional to the coil voltage x frequency (actually the core flux x
frequency). You need to size both winding and core, but if you use a
material that's recommended for your frequency and application you'll
probably get the right size core.
- One final note: if you do this again for much higher frequencies
(100-500kHz), make homeade litz wire to reduce skin effect: calculate the
wire cross section you need, and make it up with three to five strands of
smaller wire, loosely wound. This will bring the effective AC resistance of
your wire down closer to the DC resistance.