From: "Ivan Makarov" on Thurs, Dec 1 2005 11:47 pm

Hi,
does anybody know if coating on Amidon toroidal cores has any dielectric
properties, such as a particular breakdown voltage rating, or it is not
meant for electrical insulation.

Since Bill Amidon sold his business to another company, the "new"
Amidon company has been reselling another company's toroidal core
forms.

The original Amidon company resold Micrometals toroidal forms.*
Micrometals still very much in business in Anaheim, CA; website:

http://www.micrometals.com

Micrometals also makes power-application toroid forms for 60 Hz
and those are described as coated with Parylene C with a given
insulation rating of 500 V RMS at 60 Hz. By casual observation of
some Micrometals powdered iron toroid forms I have on-hand, the
coating appears to be the same. Parylene is a relatively inert
polymer which is unaffected by typical hydrocarbon solvents found
in varnishes and lacquers. I use McCloskey "Gym Seal" varnish
(oil based) as an over-coating of finished toroid windings with
great success in withstanding humidity and general aging. That
is available at most do-it-yourself hardware chain stores. The
same could be applied to the "standard" toroid forms if more
insulation of the core is desired.

The coatings on most toroidal cores is applied mainly to protect
the core material from oxidation. One can get uncoated cores
on special order but those unused cores should be sealed away
from air movement to avoid oxidation. The second reason for a
coating is to make winding easier and reduce sharp edges that
could stress the wires (true even with special toroid winding
machines)...especially if winding by hand.

There's not much need to worry about core insulation with
powdered iron materials. The powder itself is in a binder
which adds some isolation from breakdown in addition to holding
it all together. On actual application, check out one of the
early Amidon kits available in electronics stores, a 1 KW
Balun kit with huge core (to handle the RF power) and rather
ordinary coated solid wire. The RMS voltage at 1 KW in a
600 Ohm open-wire feedline application is 775 Volts. Given that
real-world antennas can get mismatched, high VSWR situations
could have double that voltage fairly easily. I've never
heard of any such Amidon baluns that flashed-over in normal
use (disregarding lightning strikes).



* Micrometals has a minimum quantity requirement on orders, used
to be minimum of 100 of any type core...that might have changed.