Re: gdt

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

On 14 Jul 2004, at 11:37, Tesla list wrote:

 > Original poster: herwig.roscher-at-gmx.de
 >
 >
 > David Speck <dave-at-davidspeckmd-dot-org> wrote:
 >  > Any simple way to distinguish a powdered iron from a ferrite toroid
 >  > core?
 >
 > Dave,
 >
 > One way to distinguish powdered iron from ferrite is the Curie-
 > temperature. This is the temperature, the permeability drops to 1. For
 > RF-ferrite this temperature usually is in the range from 160 deg C/320
 > deg F to 200 deg C/392 deg F. Powdered iron changes its permeability
 > at much higher (e.g. 750 deg C/1382 deg F) temperatures.
 >
 > So you could wind several turns of teflon (PTFE) insulated wire onto
 > the core and measure the inductance. Then place it in an oven and heat
 > it up as high as possible. Remove the core and measure the inductance
 > again. If it is in the same order, you probably have a powdered iron
 > core.
 >
 > Cheers,
 >
 > Herwig

Another way is to simply wind a few turns of wire, measure the
inductance and calculate the induction factor (Al) by:

Al = L/N^2

You will typically get >1000 nH/turn^2 for ferrites and just a few
10's for iron powder. The permeability of iron powder toroids is
inherently low - they have a distributed airgap and are specifically
designed to be used for inductors which carry high levels of DC
without saturating.

Malcolm