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To: [email protected]
Subject: LF: Re: CORES
From: "Niels Rudberg J?rgensen" <[email protected]>
Date: Wed, 18 May 2005 12:23:05 +0200
Delivery-date: Wed, 18 May 2005 11:24:19 +0100
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----- Original Message -----
Sent: Tuesday, May 17, 2005 1:24 AM
Subject: LF: CORES

I have been winding some transformers recently and have been getting some odd results.
All cores have a prim of 9 turns and 27 turns sec.
1. 3C85 58 mm diam Prim inductance 167uh Sec 744 uh
2. Same material abt same size but E core Prim 292 uh Sec 2810 uh no gap.
3. Material unknown out of a smps similar size Prim 21 uh Sec 178 uh no gap
4. TV lopt core again abt same size Prim 79 uh Sec 865 uh no gap
The Prim wire is litz 2mm and sec 1mm copper enam wire in all cases.
1 and 2 seem to have a big difference considering the same material, 3 must have a low u.
Has anyone else tried comparisons of cores.
If cores are selected at random and not checked there would be a big variation in results even using the same turns ratio.
The above are intended as output matching transformers in the FET amps like the YXM and MRF class D/E design.
I normally use 1 in my amps but have tried 3 and in spite of the big difference in values it does seem to work and I have not noticed any difference in RF output, which seems strange.
73 de Mal/G3KEV
Hi Mal:
Your core no. 1 would be my choice. The best core configuration will always be the
toroid as it has the lowest leakage (translate that into: giving a transformer with a
minimum of spikes).Your secondary inductance figure is wrong, probably an error
of measurement, but never mind that.
Your core no. 2 , I guess it is a 56/24/19 E-core, is OK --. I assume you have an
inadverted air gap of probably about a tenth of a milimeter in that core.
Both 1 and 2 are OK with regard to ferrite magnetization up to at least 400 Watt.
Your core no. 3 is probably a low permeability (7-800 mu) one. I would not like
its low inductance (I always when designing conservatively start out with a
transformer impedance ten times the circuit impedance). 
No comments on your no. 4 transformer.
A  general comment: If posible choose a core with a short magnetic path (and
no air gap (however provided you do not have a DC-problem!)). Spread the windings
over  the core circumference. Keep the inductance as high as possible in
order to keep the magnetizing current (and core stored energy) as low as posible.
These precautions will help you to keep spikes (the transistor killers) at
a minimum.
Good luck with your further experiments!
73 =
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