My first QCW

[davekni]

The toroid heated up so much that it collapsed by a few millimeters (toroid is 3d printed).

Is there space between top of secondary winding and toroid?  If little or no space, induction heating of toroid is likely cause of heating (and of wasted power).  Magnetic field lines need a path to return to bottom.  (I've built a couple coils with ferrite plates between secondary top and toroid to minimize gap needed.  Much easier and lighter to leave a gap.  I'd recommend a gap of roughly secondary radius, half radius at very minimum.

Other less-likely possibility is poor electrical connection between foil tape and secondary top.  Most foil tape has non-conductive (or minimally conductive) adhesive.  Usually there are enough random points of contact at edges.  Sometimes requires folding under a narrow hem on tape edges to contact adjacent foil.

[Mathieu thm]

Is there space between top of secondary winding and toroid?  If little or no space, induction heating of toroid is likely cause of heating (and of wasted power).  Magnetic field lines need a path to return to bottom.  (I've built a couple coils with ferrite plates between secondary top and toroid to minimize gap needed.  Much easier and lighter to leave a gap.  I'd recommend a gap of roughly secondary radius, half radius at very minimum.

Yes, there is a gap between the secondary and the topload, the size of the gap is about the size of the secondary radius.

Other less-likely possibility is poor electrical connection between foil tape and secondary top.  Most foil tape has non-conductive (or minimally conductive) adhesive.  Usually there are enough random points of contact at edges.  Sometimes requires folding under a narrow hem on tape edges to contact adjacent foil.

I'll look into it to improve the contact.
I've also thought of a hypothesis: I've noticed that the 2.5 mm² copper that serves as my breakout point has changed color because it's very hot, and on the video, you can see that after a burst, the tip is red. So maybe this heat is also dissipating into the topload, which is enough to make it heat up.
As can be seen on this screenshot :

[Benjamin Lockhart]

My guess is simply induction heating from the primary and secondary current. Like Dave said, I'd raise the toroid a little bit.
Another thing that might help would be cutting a thin strip of foil out of the toroid so it doesn't act as a shorted turn.

[Mathieu thm]

My guess is simply induction heating from the primary and secondary current. Like Dave said, I'd raise the toroid a little bit.
Another thing that might help would be cutting a thin strip of foil out of the toroid so it doesn't act as a shorted turn.

I'll try it next time

[davekni]

I've also thought of a hypothesis: I've noticed that the 2.5 mm² copper that serves as my breakout point has changed color because it's very hot, and on the video, you can see that after a burst, the tip is red. So maybe this heat is also dissipating into the topload, which is enough to make it heat up.

My QCW copper breakout point tip also melts a bit when initially sharp.  I doubt that heat conducts enough from copper breakout to topload.  It is only momentarily that hot.

[Anders Mikkelsen]

Excellent performance! Glad to see you worked out the issues in the end. Toroid heating is very likely from induction heating by the fields around the coils as mentioned. Steve Ward mentioned that he had to slot his toroid for this reason, to avoid the shorted turn effect. This would have the added effect of not blocking the primary field near the breakout point, which might affect the degree of corkscrewing in the arcs and possibly the spark growth. It's also possible to use thicker metal for the toroid, to give less heating for a given induced current, but the benefit here will be limited by the skin depth of conduction at the operating frequency.

[Mathieu thm]

Excellent performance!

Thanks !

My QCW copper breakout point tip also melts a bit when initially sharp.  I doubt that heat conducts enough from copper breakout to topload.  It is only momentarily that hot.

Toroid heating is very likely from induction heating by the fields around the coils as mentioned. Steve Ward mentioned that he had to slot his toroid for this reason, to avoid the shorted turn effect. This would have the added effect of not blocking the primary field near the breakout point, which might affect the degree of corkscrewing in the arcs and possibly the spark growth. It's also possible to use thicker metal for the toroid, to give less heating for a given induced current, but the benefit here will be limited by the skin depth of conduction at the operating frequency.

I will make the changes on the toroid, starting by increasing the gap between the secondary and the toroid. And if it's not going to change much, I'll be looking to slot the toroid.

[Mathieu thm]

I increased the height of the toroid, all went well, and a few tens of seconds later I couldn't pass more than about 80cm of arc without triggering the ocd. so I put the toroid back on as before but the same problem occurs.
I thought it might be the mmc that had lost some of its capacity as I've been abusing it, but it has exactly the same capacity as when I started (12nF).

is it possible that the mmc loses some of its capacity after a few seconds of operation and then returns to its normal capacity?
Any suggestions?

EDIT : I tested the primary with a dummy load, and everything worked normally, maybe a problem with the secondary , but I don't see where it could be.

[Mathieu thm]

Good news !
The qcw works as before, I was afraid there might be a problem with the electronics, but no. The problem was that part of the arc was inside the secondary, creating a small carbon track.



I replaced the wooden support and improved the insulation at the level of the threaded rod.
No more toroid heating problems with this gap between toroid and secondary :

[Benjamin Lockhart]

Looks great! I hope to build a QCW someday.