Re: Topband: measuring transformer core inductance

[Nick Hall-Patch <nhp@ieee.org>]

At 05:36 PM 11/13/03, Steve KK7UV wrote:
> I picked up an LCR meter for the main purpose of winding transformers and 
> being able to nail down the actual inductance and transformation 
> ratios.  The meter was advertised as "Frequency to 15MHz" but as it turns 
> out it meant that it measures frequency rather than generates 
> frequency.  It actually generates a 1 kHz signal for measuring inductance 
> which probably makes it worthless for measuring inductance at 1.8 MHz.  Is 
> this the case?  I read where core permeabilities change with 
> freuquency.  My question to the group is what do you use to measure the 
> inductance of the windings so you know when you reach the desired 
> ratios?  I've had miserable luck doing it by formula and thought the meter 
> would cinch it for me.

It's a big problem measuring true inductance at a given frequency unless 
you can guarantee that the core permeability is exactly the same at the 
desired frequency as it is at the frequency that your measurement equipment 
uses.    1 kHz is usually too low for inductors that will be used at 
1.8MHz, at least for any of the ferrites that produce reasonable inductance 
with a small number of turns (I use a 100kHz LCR bridge at work which gets 
me a little closer to the ball park)


> I cannot seem to locate website charts that show inductance/frequency 
> curves for the cores on Amidon or Fair-Rite websites.  This would help to 
> estimate where one needs to go when they wind up a core.

The first few pages of the Fair-Rite catalog (available from their website) 
gives curves of complex permeability vs. frequency for various core 
materials.  AL is  dependent on permeability, among other things, and one 
can generate AL from these curves and from core dimensions (and inductance 
is dependant on AL).  But, generally, you want to wind your inductors on a 
core where the u' curve is still flat at the frequency of interest, and the 
u" value is still insignificant.

If nobody comes up with a better answer,  perhaps we could consult Lowfer 
Bill Bowers, my guru on such matters.

best wishes,

Nick
VE7DXR

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