I am using an ET54 core with a 0.5 mm gap.

Does anyone know the reluctance of this gap ?

 

I could not find any reference material specific to that core size that did not also mention core material and winding method with freq of interest.

I did find a good paper with many formula and design suggestions. Perhaps it could clear up some questions you may have or otherwise be helpful.

http://www.ieee.li/pdf/viewgraphs/fundamentals_of_magnetics_design.pdf

 

I could not find any reference material specific to that core size that did not also mention core material and winding method with freq of interest.

I did find a good paper with many formula and design suggestions. Perhaps it could clear up some questions you may have or otherwise be helpful.

http://www.ieee.li/pdf/viewgraphs/fundamentals_of_magnetics_design.pdf

I eventually found this but my maths is wrong somewhere.


l/ uo * ur * A

l=0.0005 metres (0.5mm gap)
uo = 4 pi 10power-7
ur = 1610 for 3c90 core
A = 0.00028metres squared

This gives 882.6 which is nonsense.

I did a search of the internet and someone was getting 1 ohm for a 1.5mm gap.

 

Reluctance is the relationship between magnetic flux and mmf. Nothing to do with ohms. Although some people find the electrical analog helpful.

I suggest you review your understanding of magnetic circuits.

Perhaps this might help

http://www.allaboutcircuits.com/worksheets/magnet2.html

Looking at the equation should show you that gap length isn't the sole determinant of reluctance.

An air gap has a relative permeability of 1 - not 1610. So it's even "worse" than you envisaged.

 

l/ uo* ur * A

That's the formula for the reluctance of the core.

The reluctance of the gap is lg/(μ0Ae) and the total reluctance of the core and gap is (le/μr+lg)/(μ0Ae) where le is the effective area of the core.

source: http://info.ee.surrey.ac.uk/Workshop/advice/coils/gap/ check it out it's very informative.
http://info.ee.surrey.ac.uk/Workshop/advice/coils/gap/