Breakout rod material and size vs streamer resuls

[donnersm]

While test driving my DRSSTC I used a solid copper wire of 2,5mm2 with a lenght of approx 20cm...I had some nice streamers… Because the rod was all bended on not so nice to look at, i replaced it with a Steel wire of 0.3mm and 30cm in lenght… perfectly shaped ...so I expected beter streamers….Well guess what? I ended up having almost no streamers at all.

So my question, is about the breakout rod. What whould be the perfect length, rod diameter and material to use? Is there a guideline or formula, or is it all just trial and error?

[Phoenix]

The solid copper wire was better because it had about 1.8mm in diameter, which is more than your steel rod. A thinner wire leads to more corona losses because the curvature of the cross section is sharper. I am using a 4mm copper rod with a smooth surface on my drsstc. I also added two heat shrink tubes to decrease corona losses.

If you take a long time exposure with a camera of the steel rod, you will see corona glow all over it.

Greetings
Phoenix

[coilinator]

Hi,

I prever tungsten rod, such that is used in tig-welding. Until 4mm or so its quite effortable ! You need to grind it because its super hard but gives nice pointy tips and also has superior thermal properties !

About the size: I realized that a short b.o.p. seems to give longer streamer- mayby because of the fewer looses, Phoenix mentioned above but also the break out of the spark seems more heavier !

At some high f TC such as VTTC or SSTC I like to put the b.o.p. in the middle of the torus, and also took them not to long. Its also quite a difference, compared to putting it somewhere on the outside- don´t know maybe its the electrical field... ?

have a nice day.

Andi

[Mads Barnkob]

The larger the Tesla coil / topload is, the more you need a larger diameter break out rod to prevent corona losses.

I am planning to make some aluminium tubing at maybe 30-40 mm diameter, with a brass insert that can hold a tungsten rod for tig welding with a set screw, that should take care of the corona and wear on the break out point tip due to heat.

[Uspring]

The length of the breakout rod has a significant effect on the operation of a DRSSTC. Some DRSSTCs might not even break out at all, if they use e.g. a spun smooth toroid without any breakout rod. This is due to the high voltages required to break out from smooth surfaces. TCs usually have primary tanks tuned lower than the secondary. That implies, that, as long as the secondary resonance frequency is not lowered by arc capacitance, it will be somewhat out of tune. Primary current can then rise up to large values due to the missing secondary loading, causing the overcurrent detection to trip. This might happen before the arc breaks out.

The length of the breakout rod or more precisely, the distance that it extends over the rim of the toroid, determines the breakout voltage. The shorter the breakout rod, the higher the breakout voltage becomes and with it, the larger the primary current will need to be for breakout. I suspect, that for the shorter rod you use, the primary current will ramp up higher before the arc begins. A high primary current also implies, that there is much energy stored in the primary, which will quickly be dumped into the secondary, when the secondary gets into tune by arc loading. That might cause the bigger arcs you are seeing then.

If you want to use the longer rod, you probably can obtain a similar effect by tuning your primary a bit lower. That will defer breakout and thus also allow for higher primary currents at breakout time. If you tune too low, the OCD might trip and the arc might not break out at all.

All of the above assumes, that your interrupter won't terminate charging up the primary too early.

[davekni]

Anyone have experience comparing similar-geometry breakout points of different materials?  Due to its magnetic permeability, steel has an extremely shallow skin depth.  I haven't personally noticed any difference with steel.  Even the high resistance caused by the thin conduction skin may still be insignificant at high voltage and comparatively-low current.  I haven't done any direct comparisons with the same geometry, however.

[Mads Barnkob]

I think you are right about the business end for the Tesla coil, the high voltage and amount of charged up electrons is at a point of no return. Even a wet stick, plant, leaf, stick, steel or silver rod will have little difference if they had the same physical size. There is nowhere else to go, but the amount of corona and field shaping seems be affected the most by the diameter of the break out point.

I have used steel, brass, copper and tungsten. Bare copper wire is by far the worst corona wise, but that also have a much more bent surface than the stiff rods of steel, brass and tungsten.