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RM Series Ferrite Cores
For winding one-off inductors to suit electronic projects

The RM series of cores with their associated bobbins and clips form an ideal starting point for the design and construction of small inductors and transformers.

They are manufactured by many companies worldwide and it is not sensible to try and list all the different versions here... A search for 'RM series cores' will reveal a manufacturer that is local to you, and give the specific data for the cores that they manufacture.

I personally have used this style of core, in hundreds of applications. I originally believed the first ones were made of 'dust iron' (powdered iron in USA) before it was possible to sinter ferrite material. But I am told that Ferrite technology is 75 years old and iron dust, or powder, technology is even older. My first encounter with annular coils surrounded by a compressed core of two halves was with 'pot cores', made out of 'dust iron', this was in 1950 or so in audio filter components in some surplus military equipment that was, at the time, considered 'miniaturised'.

The illustration at right shows a pair of simple cores. There is another version that has a hole axially through the centre limb with either a brass thread or a threaded nylon tube so that an adjusting slug can be screwed in and out.

Generally the mating surfaces are ground flat for minimum reluctance, but some types have deliberately shorter central limbs in order to provide a central air gap... This increases reluctance markedly, but narrows the tolerance on the reluctance considerably.
typical RM ferrite cores

typical RM winding bobbin The moulded bobbin shown here fits the annular space inside a pair of cores. The pins are arranged as "dual in line" or sometimes in groups as shown.

Whatever the arrangement of pins it is common for the inter pin distances to be arranged on a 2.54 mm (0.1") grid.
The completed assembly is held together by spring steel clips of the shape indicated at right. The clips have extensions that can be soldered into a circuit board at the same time as the bobbin pins lending mechanical strength and stability to the inductor or transformer concerned. typical RM ferrite core assemby clips
typical RM ferrite core, with adjuster... assembled A completed inductor is shown in this drawing. It is of a type that has a central, threaded, adjuster slug.

The drawing is a little stylised, but it is typical and was drawn from an actual RM10 example.

Throughout the rest of this page the examples quoted and types and dimensions are those of the particular versions offered by RS Components Simply because they are a supplier that is local to where I live and has been plundered for some of the information given here. If you use components from a different supplier then you must use the design parameters published by that particular supplier rather than any figures quoted here.

Size DesignationPara
Inductance factor (nH/turns2) (gapped cores)AL160250250250400
Centre limb gapmm0.
Core Material P11P11P11P11P11
Permeability (effective)e109171146100159
Range of adjustment+%2014151720
Frequency range (recommended)kHz5.5-8003.5-7003-6502-6501.2-500
Path length (effective)mm26.926.929.641.741.7
Path area (effective)mm231.331.340.383.283.2
Winding area (single section bobbin)mm215152141.541.5

Un-gapped information
MaterialALSizeeTolerance (%)
P112000RM61385-20 +30
F94625RM63200-20 +30
F106200RM64295-20 +30
F398600RM65955-30 +40
P112800RM71560-20 +30
F94690RM72610-20 +30
P113960RM101575-20 +30
F97600RM103025-20 +30
F9C8060RM103205-20 +30

RM6 Bobbin dimensions
RM 6 bobbin
  Reduce the winding area by 1 mm2 for a two section bobbin.

There is a 4 pin version that has pins in the 2, 3, 5, and 6 positions.

The plastic bobbin is sometimes a dark grey colour rather than the brick red shown in the drawings.

RM7 Bobbin dimensions
RM 7 bobbin
  Pin layouts for 8 & 5 pins are shown.

The prongs on the clip that form the earth stakes are nominally 0.6 mm wide, but I have seen examples that were 0.95 mm and some others that had tapered spikes.

RM10 Bobbin dimensions
RM 10 bobbin
  Reduce the winding area by 2.5 mm2 for a two section bobbin.

The black dots on the grid represent 8 pin layout and the pink ones are added for 12 pin version.

How to design a simple inductor...
This is a simplified description suitable for audio frequencies and ignores losses (they should be so small that any effect they have will be well within adjuster range).

To illustrate the process I will describe making a replacement L3 inductor for our Apidictor... This is a 7 Henry choke and is centre tapped.

As we have an adjuster that only adds to the inductance we can subtract half of the adjuster range from the total inductance required. The adjuster ranges vary with frame size, but for our initial calculations we can assume that we are going to produce a 6.5 H choke.
The turns required can be calculated by using the formula below.

Total turns =  Where L is the inductance required expressed in nH (10-9H) and AL is the Inductance factor from the table further up this page.

As 250 occurs as a value for AL in all of the 3 frame sizes it is a convenient number to use. Dividing 6,500,000,000 by 250 gives 26,000,000, Taking the square root of that gives us a figure of 5,099 for the turns required.

This is unacceptable on three counts, first the number of turns would be tedious to wind and count by hand, secondly the wire would need to be very thin to accommodate the number of turns in the window space of even the large RM10 core. The third reason is that the long length of very thin wire would have a significant resistance and this would introduce losses that were too large to ignore.
To overcome these problems we will need a core without an air gap in the centre limb so that we can have a higher figure for AL (but a wider tolerance on final inductance). If we use an RM10 (adjustable) in an F9 type ferrite we can have 7,600 for our value of AL Which modifies the figure to 925 turns (we will actually wind 463 turns, make a tapping and then wind a second lot of 463 turns to get our centre tap). This is still a high number of turns, but is more achievable. Our tolerance on final inductance is plus 30% or minus 20% and we will have to make a second attempt if our finished choke does not test out correctly.

We still have to find a wire gauge that we can wind our required turns in the winding area of the bobbin (41.5 mm2 for RM10). 41.5 divided by 926 gives a figure of 0.045 mm2 to accommodate each conductor. The wires are round and layers of turns will nest to a certain degree, also any irregularity in winding will work against us, so for our initial calculations we will assume that this area is circular, but that the actual copper area is only 50% of the available window this gives the cross sectional area of our wire as 0.0225 mm 2 = 0.16 mm diameter.

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Written... 13 December 2001, Revised... 18 December 2001, Revised... 20 February 2002, Amended... 16 April 2002, New Domain 12 April 2004,

Electronics Favicon Coding Standard 2003 Issue 5