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Stan Zurek, FRM21/4/12,, {accessed 2020-10-01}
reviewed by Jeanete Leicht, 2014-01-21


FRM21/4/12 (also FRM21-4-12 or FRM 21 x 4 x 12) - a type of frame core made of soft ferrite, supplied by Ferroxcube.1)

The numbers refer to the dimensions of the frame in millimetres, so 21 = 21 mm, etc.

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The core comprises a cuboid bar, which slides into a bobbin. The outer frame of the core is assembled on top of the bobbin.

The bar has a symbol BAR22/2/6 (or BAR22-2-6). The bar is marked with “22” because it is slightly longer (21.8 +/- 0.3 mm).

The relative winding width is longer than in most of other ferrite cores, which allows for the bobbin to have multiple sections. The manufacturer offers the multiple-chamber bobbin, but there is no indication of how all the parts should be held together (e.g. a clip).

As of 2013 the core was offered in two materials: 3C90 and 3C91.


Frame core (FRM20 shown for illustration) with the bar inserted into a bobbin

by S. Zurek, Encyclopedia Magnetica, CC-BY-3.0

Disassembled frame core (FRM20), bar and bobbin

by S. Zurek, Encyclopedia Magnetica, CC-BY-3.0

Core parameters

The table below lists the core set parameters as given by Ferroxcube.2)

Symbol Parameter Value
Σ(l/A) core factor 5.06 mm-1
Ve effective volume 312 mm
le effective length 40 mm
Ae effective area 7.9 mm2
Amin minimum area 5.7 mm2
m mass of frame 1.5 g
m mass of bar 1.0 g

The values were measured for frame with a bar core, without an air gap.3)

Material grade AL value effective permeability μe
3C90 400 nH +/- 20% 1610
3C91 470 nH +/- 20% 1890

Data sheets

Ferrite core FRM21/4/12 Ferroxcube

Copyrights © Ferroxcube

See also


1), 2), 3) Ferroxcube, Soft Ferrites and Accessories, Data Handbook, 2013
frm21-4-12.txt · Last modified: 2020/07/19 00:55 by stan_zurek

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