[SOLVED] Could use some help understanding ferrite core spec, and suitible core measurments.

[David_]

Hello.

I have a wide asortment of different unknown toroidal cores and in an attempt to find suitible methods to characterise these i've really lost my way.

I've read about the subject a lot from all sources i could find around the internet but have now ended up feeling less knowledgeable than when i started.

My intension was to create a work list that i could follow to get the most accurate and useful parameters of a core down on paper and when free time came around go through all my cores and document said results in a kind of index.

Avalible equipment:
2 channel 60MHz DSO
2 channel 20MHz, 100MSa/s arbitrary waveform generator
DMM
Solder iron, component asortment, yada yada yada.

So far i've measured(with caliper) hight, inner diameter, outer diameter and from there calculated magnetic path -area and length.
But now i am drawing a blank.
Where should i go from here and which properties can i realistically measure?
Ex: initial- relative- absolute permeabilty, inductance factor, saturation limit...
I could really use some guidence.

Mvh David_

 

[David_]

A better question whould be:
In trying to measure core material properties of a singel core, what characteristics are the most useful to know in order to be able to properly reuse said core?

Regards

 

[WimRFP]

Hello, A nice measurement is the low frequency BH curve. The basic setup is relatively simple, but you need some math to convert the measured properties back towards magnetic field properties.

You can find an example measurement setup at: http://info.ee.surrey.ac.uk/Workshop/advice/coils/BHCkt/index.html

If your Digital Oscilloscope has integration together with XY mode, you can skip the integrator. You need the integrator to "undo" the d.../dt from the induction formula

Vind = n*d(flux)/dt, Vind = induced voltage [V] n = number of turns , flux = core flux [Wb, Vs]

With the same setup you can measure initial permeability, maximum permeability and saturation flux density.

If you want to use your ferrite cores in an EMC/filter application, you need to measure the small signal permeabillity (real part and imaginary part) versus frequency also. That can be done via measurement of the complex impedance versus frequency at small signal level. From the impedance you can find the inductance (also real inductance and loss inductance based on number of turns and core geometry). From there you can derive u' (the real "inductive" part of rel.permeability) and u" (the imaginary "resistive/loss" part of rel. permeability).

When you are familiar with AC network calculations (complex or real), you can determine impedance with your setup by measuring amplitudes and phase shift. Depending on your setup and frequency, you may need to correct your measurements for the input capacitance of your probes.

 

[David_]

Thank you!
This really helpt me a lot.
I'v intended to reply sooner but life got in the way.

In the coming days i will take pictures and post results, theres a couple things i dont get but i think they can partly be explained by that my scope isn't really up for the task.

Theres few things as pleasing as when after many atempts a experiment finaly succsedes:-D

Cheers!

 

[David_]

I have taken some pictures of the measurment, however the are very large and to big to post my self but they are available in a public album on my account.
There's not really much info to gain nor much to be said but i said i would do it, can't remember enough to describe them with any resulting numbers but they depict the first part of the measurment from: http://info.ee.surrey.ac.uk/Workshop/advice/coils/BHCkt/index.html

 

[David_]

I remembered that I started to wright a C++ program to make measuring core parameters easier, it is not done but it does work but I don't know how I would make it available for download but when I do know how I will post a link here. the program(windows .exe) asks for you to type in different parameters one by one, first
h =core hight(mm)
D =outer diameter(mm)
d =inner diameter(mm)
C1 =integrator capacitor value(uF)
R6 =integrator resistor value(ohm)
Np =primary turns(dec)
Ns =secondary turns(dec)
Ip =primary winding current(mA)
Vo =integrator output voltage(mV)

Then it takes those numbers and in a few steps calculate and then show the results, the part below is the explanation for another column that show these parameters but with the result given from the measured values. If one want to measure core parameters, and on a few cores then this sure make the hole procedure both faster and easier. I will post the program here as soon as i have a viable way of doing it, I am not sure that I have got everything right and it would be nice to know if the calculations might be correct.
"Core Hight: h = mm
"Core Outer Diameter: D = mm
"Core Inner Diameter: d = mm
"Outer Radian: Or = D/2 mm
Inner Radian: Ir = d/2 mm
"Effective Path Length: le = 2pi*ln(O/I)/(1/I-1/O) mm
"Effective Path Area: Ae = h*ln^2(Or/ir)/(1/Ir-1/Or) mm^2
"Core Constant: CC = le/Ae m^-1
"Magnetic Field Strength: H = Np*Ip/le amperes per metre
"Magnetic Flux: flux = -Vo(C1*R6)/Ns webers
"Magnetic Flux Density: B = flux/Ae tesla
"Absolute Permeability: u = u0*ur=B/H henrys per metre
"Relative Permeability: ur = u/u0
"Reluctance: Rm = Fm/flux per henry
"Magneto-motive force: Fm = H*le Amperes
"Inductance: L = u*Ae*N^2/le Henrys
"Inductance Factor: AL = 10^9/Rm Henrys/turns^2