primary spacing

[thedoc298]

Getting ready to make my primary for my 25 centimeter drsstc and have a question about the spacing between the windings. Its just a flat pancake using 10mm tubing, I was planning on using 10mm-12.7 mm spacing between the turns, but would like some opinion's, good, bad or what do you think.

[davekni]

Getting ready to make my primary for my 25 centimeter drsstc and have a question about the spacing between the windings. Its just a flat pancake using 10mm tubing, I was planning on using 10mm-12.7 mm spacing between the turns, but would like some opinion's, good, bad or what do you think.

My first DRSSTC was designed by guess and seeing a few examples I'd found on youtube.  Hadn't found this forum yet.  For my new QCW coil, I ran many simulations using the free FEMM magnetics simulation program before constructing primary and secondary.  I've found FEMM very useful for answering many questions covering ferrite transformers and induction heating and even levitating aluminum in an induction coil (besides TC design).  It was quite worth time to learn for me.  So, I'd suggest using FEMM to see what primary geometry maximizes coupling factor without raising height of primary coil.

Of course, the easier option is to look at many examples available here and pick something that appears common among coils what perform well.  I suspect the exact spacing doesn't make a huge difference.  Just need room for enough turns to meet your inductance requirement.

Hopefully you will get other opinions from other's experience.

[MRMILSTAR]

You can use JAVATC to test many parameter combinations. It was made for spark gap coils but it will also work for any disruptive-type coil such as a DRSSTC. Just ignore the spark gap inputs and outputs.

[davekni]

You can use JAVATC to test many parameter combinations.

Yes, of course, much easier than learning FEMM, and plenty accurate enough.  I must have had a brain lapse.
Look for high coupling while keeping primary at or close to height of bottom of secondary and inside primary turn diameter significantly larger than secondary diameter.  How much larger diameter (and how far above secondary bottom) are answers you'll need to get from other examples or other forum members answering your question.  Small primary-to-secondary gap and primary above secondary bottom both increase coupling, but also increase likelihood of racing sparks along secondary and/or arcs from secondary to primary.

[Mads Barnkob]

If you wanted and the electrical properties suited you, you could properly pack it tight with insulating layer between turns. Coupling issues, like racing sparks, would properly be harder to fix. So it's all a trade-off between physical space it takes up   

So primary design is in my opinion based on the following:
- Enough room for a primary tap, not shorting onto the adjacent turns
- Placement of primary turn that connects to resonant capacitor, is the high voltage point, take care there, needs more spacing and dont place current transformer there.
- Enough room for secondary earth terminal, not to spark to primary coil. Place the terminal at 0.9 turn and you have the most room.
- Enough room for your fingers, that is a varying factor from human to human, it sucks to make something that is not serviceable.
- Avoid excessive heating, by enabling good room for heat dissipation
- Wider flat primary coil is easier to adjust coupling with, changes in it's height to secondary coil, are less radical coupling changes than with f.ex. a helical/cylindrical primary coil.

[thedoc298]

I tried to delete post after posting because of the wealth of info and examples, I have already built 2 srsg and two spark gaps and gave not much thought to spacing and all worked very well. My larger drsstc has been waiting for the copper and have never did any research of spacing till the copper came. I ended up using 10mm spacing. Thanks for all the feedback and I will check out
FEMM. < is a cool place I had never been to.

Will be posting the final build and testing.