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type of material RFC coil - ferrite versus iron powder

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LAN_403

tritium

Tue Dec 14 2010, 05:37PM

Registered Member #2591 Joined: Fri Jan 01 2010, 11:58AM
Location: netherlands
Posts: 76

what would be the best material fore class-e rfc coil@4mhz-500watt, for tesla coiling.
I have some big ferrite cores o.d. = 80mm (3.1 inch), ore do it has to be iron powder.

I read a lot of people use n2 material, but I can't find anything about that at the farnell/newark (my electronics source)

GeordieBoy

Tue Dec 14 2010, 06:13PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Carbonyl Iron powder is the best material to use for high-power RF chokes at HF frequencies.

I use Number-2 material for my RFC's... (And before anybody makes any wisecracks, it's not that sort of "Number Two".)

Coloured red-clear for identifications when bought from Micrometals:

-Richie,

Sulaiman

Tue Dec 14 2010, 07:28PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If the required inductance allows, I'd go for a single-layer solenoid coil,
a little large but cheap
AND you don't need to calculate core-loss etc.

As above, Micrometals -2 material is good at 4 MHz

You can also use NiZn ferrites (-61 or 4C65 etc.) but the choice of shape and size is limited, and surprisingly expensive,
e.g. today I received this

from RS for my Ham experiments.

tritium

Thu Dec 16 2010, 04:18PM

Registered Member #2591 Joined: Fri Jan 01 2010, 11:58AM
Location: netherlands
Posts: 76

I have these nice core's, they are blue

o.d. 58mm
i.d. 40mm
thickness 18mm
weight 110 grams
meassured with my lcr meter@68 turns it gives me an inductance of 26mH
so the Al = 5622 nH

is it possible to determine what type of material this core is made? and if I can use them...

update: i think it is this core http://www.ferroxcube.com/prod/assets/tx584118.pdf]
material 3c11 is 5400nH core, only the color is not white

Sulaiman

Thu Dec 16 2010, 10:36PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you go to the Ferroxcube website

and look up 3C11 datasheet,
(well done identifying the core, even if the core is of a different formula from a different manufacturer, the performance will be almost identical.
The difference between manufacturers is quite subtle,
In my opinion ferrite of a particular relative permeability behaves pretty much the same irrespective of manufacturer),

Look at Fig.1 u' and u" vs. Frequency.
u' is the part that we want, the effect on inductance.
u" is the part that we only want for rfi filtering etc. - effective loss.
I interpret it like this, the "Q" this material/core is u'/u" e.g.
@100 kHz Q=4500/300=15 which is usually adequate for power transformars etc.
around 400 kHz the inductance starts to fall
around 450 kHz the effective series resistance will be the same as the inductive reactance .... Q=1
@ 4 MHz an inductor using this core would 'look' like pure resistance, negligible inductance.
In itself this is not really a problem as it would 'look' like a VERY high impedance so it could work ..... at the flux levels used for the graph ... typically 0.5 or 1.0 mT = quite a few turns/volt.
Core losse increase something close to the cube of flux density !

So, if you need inductance at 4 MHz as above, a lower permeability core with less loss will be required, metal powder, low-permeability ferrites, mpp etc. OR large cross-section ferrite with an airgap OR air.

e.g. copper tubing or wire wound into 6 turns 4" diameter 2" long would be about 3.8 uH.
About 15 turns on a T400-2 would give the same inductance.
BUT
you don't have to worry about core saturation or losses.
You do need more space as the air-core inductor must not have magnetic materials nearby....usually not a problem for hobby stuff.

tritium

Fri Dec 17 2010, 04:28PM

Registered Member #2591 Joined: Fri Jan 01 2010, 11:58AM
Location: netherlands
Posts: 76

Sulaiman wrote ...

You do need more space as the air-core inductor must not have magnetic materials nearby....usually not a problem for hobby stuff.

ok, I wan't to use the air core, plenty of room for that, but...
there is a big aluminium watercooled heatsink beneath the rfc coil, is this a problem?

and it is nearby my mosfet gatedriver (deic421), maybe i'm getting more problems with an aircore than with a core-coil
or is it possible to shield the core?

coil specs are:
50mm diameter
58mm length
39 turns

that gives me an inductance of 47uH

Sulaiman

Fri Dec 17 2010, 06:59PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

39 turns x pi x 59mm diameter = 7.2 meter, less than 1/10 wavelength at 4 MHz so that's ok.
I didn't realise you'd need 47 uH at 4 MHz
(j 1181 Ohms ... are you sure you need 47 uH?)
That's a lot of Amp.Turns - very strong radiated field if air-cored
Out of my comfort-zone - I'll keep quiet.

tritium

Fri Dec 17 2010, 07:21PM

Registered Member #2591 Joined: Fri Jan 01 2010, 11:58AM
Location: netherlands
Posts: 76

it is simmular to this:

here is the circuit, it is a copy of richieburnett, it uses a 47uH RFC

GeordieBoy

Sat Dec 18 2010, 02:39PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Just use an iron-powder toroid. High permeability ferrite will saturate due to the DC supply current when used as an RFC unless it has a big air gap in the magnetic path.

-Richie,

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