CORE FOR GDT AND CT

[prabhatkumar]

Hey there everyone !! I had successfully made my first sstc and I am very pleased with it's performance . I know I am not making big arcs but then this was only a learning experience and I had it. But during the lockdown I was thinking to study more on drsstc and then also choose a core for the CT, GDT and capacitors for the mmc. I don't want to make like deadly arcs which are 4 feet long . My secondary is only 10 inches tall and has a diameter of around 4.2 inches. It has got 1400 turns. In real life ,I get a resonating frequency of 238.5 kHz. I will run the coil without the toroid.
So now for the main question , I was reading the forums for the core for GDT and CT , and found that mads replied that he uses this :
https://www.mouser.in/ProductDetail/EPCOS-TDK/B64290L674X830/?qs=sGAEpiMZZMt1hubY80%2Fs8DEvGx1xc7EcUsoMlWTaffs=
I also found that many people use this core:
https://in.element14.com/ferroxcube/tn32-19-13-3e25/ferrite-core-toroid-3e25/dp/3057008
I also found another toroid from ferroxcube which has a different coating ( epoxy ), compared to the above cores . These two cores have the same material 3e-25 . But strangely though the coated one has a bigger diameter and a higher Al value still being a little cheaper from the other . I don't know a lot about the cores so please enlighten me on this . I would prefer to order the cheaper one with epoxy coating as I could save like 4 usd on 20 pieces . Also will the high Al of 7.3 uH of this core be suitable for the gdt. Link for the 2nd ferroxcube core
https://in.element14.com/ferroxcube/tx36-23-15-3e25/ferrite-core-toroid-3e25/dp/3057021
Also sorry for the inconvenience as the prices are mentioned in INR and not in USD

As a standard rule of thumb ( as far as I know ) , the Al value should be above 5000 as mads has said in his drsstc design guide.
Now I could also source any of the above cores , the epcos being a little bit more expensive because it has got a slightly larger diameter, I am thinking on the ferroxocube one.
But while browsing I found a page on digikey where there is a large number of cores given with materials like N30, T-35, T-65 etc
Now I would go for the core diameter and the Al value mentioned aside each core , it is right or also do we need to look for the other properties of the core . If yes then please can someone share a link to a page or briefly explain the main difference between the types like T-35, T-38 etc . I will attach the link to that page which has the different cores given.
https://www.digikey.com/catalog/en/partgroup/toroid-core-type/23563
THANKS !!

[ritaismyconscience]

The second core is thicker (has more cross sectional area) so its inductance should be higher. Both of them should work though.

[prabhatkumar]

Second one as in which core please mention the name ??

[klugesmith]

3E25 is one of many ferrite formula standards.  Others have different permeability and saturation flux density.  Permeability and high frequency loss are temperature-dependent functions of Bmax.

You can get the single-turn inductance directly from core dimensions and permeability. 
Those determine its magnetic reluctance (ampere turns per weber), whose inverse is the single turn inductance.  Might help to get an understanding how the core variables affect you.

For example, Let's consider that TN32-19-13-3E25. 

Put 1 turn of wire through it, carrying 1 ampere of current.
The toroid's effective length (of magnetic flux path) is 76 mm.
With 1 ampere of MMF spread uniformly along that path, the H value is I/l = 1.0/0.076 = 0.076 amperes/meter.  YOu can convert that to oersteds if you want.

The relative permability of 3E25 is given as 5500.  I'm no expert, but that seem like a nice high value for a ferrite.   Flux density in the core is B = H * u0 & u(relative), where u0 is always 4e-7*pi.  So B = 0.0909 teslas (909 gausses), well below saturation.

The toroid's effective area (of magnetic flux path) is given as 76.5 mm^2.
Total flux is the product of B and flux path area. 
In SI units, 0.091 * 7.65e-5 = 6.96e-6 webers = 6.96 uWb.

If the current were changing at 1 ampere per second, the flux would change at 6.96 uWb per second, automatically inducing 6.96 uV per turn.  Hence the inductance is 6.96 uH.
Verstehen Sie?

[davekni]

"Second one as in which core please mention the name ??"  I think that refers to the TX36/23/15-3E25 core you listed as being a bit cheaper and having an epoxy coating.  It looks good to me.  The link you reference lists inductance as 7.39uH / turn^2, which is above Mads' recommended minimum of 5000nH (5uH).  For your 238kHz frequency, even a 2-turn GDT would avoid saturation.  (I'd recommend a few more turns to reduce magnetization current.)

[prabhatkumar]

Thanks a lot klugesmith and davekni for your kind replies. Well I really appreciate how klugesmith taught how to calculate the inductance per turn just by formulas. And it's a shame for me as I still don't understand that level of details of electronics. And davekni also thanks for your advice on the core.
And something more bad also that I can't source anything from element 14 since they require a GST no so individuals and hobbyists like me can't buy straight from them . I will be updating this thread when I buy the cores from some other seller locally and trying it out ( the local vendors don't have the ferroxcube cores , they have only epcos cores like N-30 and T-38  and maybe few others ). Thanks everyone for now !

[Didik]

"Second one as in which core please mention the name ??"  I think that refers to the TX36/23/15-3E25 core you listed as being a bit cheaper and having an epoxy coating.  It looks good to me.  The link you reference lists inductance as 7.39uH / turn^2, which is above Mads' recommended minimum of 5000nH (5uH).  For your 238kHz frequency, even a 2-turn GDT would avoid saturation.  (I'd recommend a few more turns to reduce magnetization current.)

hi guys.. i'm also just learning a small drsstc. which one do you think is more suitable for ferrite? number 1. outer diameter 40mm thick 15mm.and one winding of wire produces 5.2uH no.2.outer diameter 20mm thick 8mm and one winding wire produces 20uH.

[davekni]

40mm core would be better in general.  Smaller 20mm one would be fine for a high frequency coil, say >200kHz.  Could help if making a small coil where build size matters.  Detailed calculation requires knowing core saturation flux density and core area, number of turns that fit, etc.

[Rafft]

David

time to introduce the generic 'vga' gdt  with its standardized dimensions, im pretty sure the specs wont have large differences from other manufacturers.

im sure the op could find a few

[Didik]

.thanks , i bought it right away and started building it , your guides are all very valuable to me , i plan to build sstc using seccataco interrupter , and maybe my secondary will run at 450khz without top load . as reference of ferrite core i mentioned above , i have to how many turns to turn for a full bridge gate drive (irf 460)