How To Easily identify Ferrite Cores
Identifying the sort of material utilized in a ferrite core for use at radio frequencies is a common question. This article collects advice that the author has given in internet forums for over a decade, however, it appears to be unusual knowledge.
The most common method involves taking some measurements to estimate the initial permeability I usually at audio frequencies and comparing it to a table of I for common core materials. This method may well reveal a number of mixes with comparable I but each one may be quite different at higher frequencies.
The suitability for use at radio frequencies is usually determined by complex permeability rather than I at 10kHz.
The complex permeability characteristic of #43 ferrite material is depicted in the above plot from the Cosmoferrites’ data book.
In actuality, the data reported was measured on a toroidal core with a short winding; the datasheet claims that it was [m]easured on a 17/10/6mm toroid using the HP 4284A and the HP 4291A, and most likely with only one turn.
Because of this property, a tiny inductor wound on a core of #43 material with very minimal flux leakage will have a complex impedance with X/R=’/”. In the picture above, the frequency at which R=X is a good ‘signature’ for the material type, #43 crosses across at around 14MHz.
This measurement must be completed with as few revolutions as possible in order to get a reliable and accurate readout on the measuring device. Because of the capacitance, more turns equals more stray capacitance, and the impedance will no longer obey the basic model X/R=’/”. On the other hand, using the common MFJ-259B to measure an impedance on the order of ohms does not produce accurate findings.
If the Ferrite core inductor has significant flux leakage (for example, a rod), the flux leakage causes a divergence from X/R=’/”.
(The method is not relevant to powdered iron products because they often have lesser loss than ferrites, and grading by I is still the best option.)
Because ferrite materials are subject to production tolerances and temperature variations, don’t expect to apply datasheets to real cores with a 1% accuracy.
Table 1: Selected Cosmoferrites’ data
For various typical Cosmoferrites’ materials, Table 1 displays the cross-over frequency and i. Although #33 and #43 have similar I their RF performance differs significantly, owing in part to the fact that #33 is an MnZn ferrite and #43 is a NiZn ferrite. It would be quite easy to identify an unknown core of type #43 or #33 incorrectly based on me alone, even adjusting for manufacturing tolerances, temperature, and measurement error.
Similarly, different manufacturers’ cores may be made of comparable materials, thus measuring I alone do not provide a reliable indication of RF performance or a true comparison with a known core.
