sci.physics.electromag

On 19 mar, 20:31, Theo Markettos
wrote:
> Wimpie wrote:
>
> [big snip]
>
> > Hope this helps a bit.
>
> Thanks, it does a lot.
>
> A bit of explanation about the other half of my question. I'm thinking of a
> fluxgate-type device here, where it's possible to measure the background
> field by seeing how much flux you have to apply before the core goes into
> saturation. They're pulsed at a fairly low frequency (say 10KHz) and I was
> wondering what the upper limit was set by - is that because of
> inductance/capacitance of the device (let's assume linearity isn't
> important)? Ignoring that (assume the core is in some strong
> preexisting AC field), what controls how quickly a core can go in and out of
> saturation? Or is it not possible to get to this situation and other EM
> effects take over?
>
> Theo

Hello Theo,

When talking about low frequency varying current and fields,
Saturation depends on many factors. For closed magnetic circuits
without air gap (for example a ferrite or wound ring core), the
magnetic path length, Bsat and permeability determines the amount of
ampere*turns to get saturation. So for closed circuits, magnetic
properties of the material and path length dominate.

When an air gap is present, or a completely open circuit (like a
ferrite bar as used in AM/LW radio receivers), the larger the length/D
ratio, the less Ampere turns are required. Search for effective
permeability of ferrite bars (check Ferroxcube, Epcos, Amidon, etc).
For open circuits, field path through air can play a dominant role and
will always lead to higher Ampere*turns to get core saturation.

The AC response depends on the cross section of the magnetic material
and the material itself. Real metals, except the very thin ribbons,
have poor frequency performance. This is mostly because of eddy
current phenomena. The thinner the ribbon (in case of wound cores or
half cores), the better the high frequency performance (up to above
100 kHz). Thin (amorphous) metal strips are used as tags in Electronic
Article Surveillance.

Ferrite comes in a many varieties. The choice in ferrites with steep
rectangular BH curve, is limited. The low permeability materials have
good frequency response (above MHz frequencies), but BH curve isn't
square shaped.

In generally, you can calculate the flux inside a core from the
inductance, magnetic cross section and number of turns.
B = I*inductance/(turns*crosssection).

For your application, I do not expect capacitance to be a problem
(unless you are going to used many turns in combination with very low
drive current).

Best regards,

Wim
PA3DJS
www.tetech.nl
remove abc from the mail address.