DRSSTC Questions

[flyingperson23]

It likely has less to do with coupling and more with physical distance. Your secondary is pretty short and your ground strike ring is pretty high compared to other coils.

[davekni]

Other than raising the top-load I'm not sure what to do about this.

Looks like raising top-load a bit might be good for another reason, to avoid shorting upper turns of secondary.  A bit of gap between top of secondary and top load allows secondary magnetic field to exit top and return to bottom.  Will reduce secondary frequency a bit, so may need retuning.

[alan sailer]

Thanks for the ideas. I will raise the top load a bit, make a longer breakout and reduce the coupling by raising the secondary.
These all should move the top load away from the primary/strike ring.

I have done no real tuning as of yet. I have to finish getting the coil into it's final mechanically stable condition.

I am curious if anyone has an idea how effective strike rings and that de-coupling cap on the bridge are?
Do they prevent bridge death or just make it less likely?

Once again, the help is very useful.

Cheers.

[flyingperson23]

Ground strike rings are pretty good at keeping the arc from hitting the primary if properly placed. If it's too close, the arc to the ground ring can ionize the air in between the ground ring and primary and cause an arc there. I've heard of decoupling caps being useful for smaller coils, and also them doing nothing for large coils, so I'm not sure for that.

[alan sailer]

Ionizing the air creating a path to the primary. Interesting idea.

All of the strike rings I have seen are smooth rings of metal. Another protective device is a lightening rod, which
uses a sharp point to create a potential concentration which directs the lightening return strike to a ground rod.

The only reason I can think of that this effect is not used in tesla coils is that it would encourage secondary to
ground arcs.

Cheers.

[Benjamin Lockhart]

You want to design the primary coil and strike ring geometry to discourage any ionization in that area, it can cause primary to secondary flashovers and excessive strikes to the ring like you're seeing.

I would add a much larger toroid sitting on top of your existing one if tuning allows for it. This will probably help the most, aside from winding a taller secondary coil.

Larger diameter tubing for the strike rail might help but not much.

I try to avoid coil designs where the toroid is smaller than the strike rail diameter, especially when they are super close together like this.

The ramped nature of the QCW coils with similar dimensions as yours helps a lot with controlling the flashover, as well as those coils have a pretty low topload voltage.

[alan sailer]

Benjamin,

Don't know if I have enough primary inductance to have a larger toroid. I will do some JavaTC.

I have two QCW and as you say the toroids are much larger.

The picture I took may not show it but my toroid and the strike rail are darn close to the same diameter.

I di however, raise the torrid up about 3/4 of an inch. I also made the breakout about an inch longer.

I spent a little while coming up with a simple but effective way to measure arc length to do tuning. It is
set-up right now and I was able to make my first arc measurements. The results surprised me. The
primary is set at some guess for decent initial tune. For a 130usec interrupter pulse and 150Hz I am getting
solid 22 inch/56 cm arcs at 150VAC input.

I'm pretty amazed. The Vce waveforms are so clean that they look like a textbook.





A slow motion video of the coil hitting 22 inches.

https://www.youtube.com/shorts/E11fc_pDfyw

Cheers.

[davekni]

The only reason I can think of that this effect is not used in tesla coils is that it would encourage secondary to
ground arcs.

Yes.  Before finding this forum, my initial DRSSTC construction had a strike ring with small points.  Exactly that problem, strikes from part way up secondary to strike ring.

[alan sailer]

The coil is pretty close to final and still working fairly well. The one worry is the secondary arc over at low pule rates.
I am just going to avoid low pulse rates.

I am super pleased with the arcs. 24 inches is more than I expected by about 50%.

I am thinking about adding a soft start circuit to the coil so that I don't need a variac to run the thing. I did a quick
(15 minute) search for some circuit ideas and didn't run across anything likely. Does anyone have a circuit that they like?
If not, I have a few time delay relays that I'll look into...

Cheers.

[Benjamin Lockhart]

For the soft start circuit I've used a 50A 24V coil relay that bypasses a 12 ohm resistor in line with the mains supply. The relay coil is powered directly from the 24V smps for the driver.

The 24V power supply has about a half second turn on delay and the 12 ohm resistor limits inrush to 20A which is enough to mostly charge the bus before the power supply turns on.

It's only 2 extra parts and has worked flawlessly for me.

[alan sailer]

Interesting idea. I'll try and measure my power supply delay.

Cheers.

[Benjamin Lockhart]

Even a quarter of a second might be enough to get the inrush current down to acceptable levels.