The bottom of a Tesla coil's secondary has to connect to something, and the wall socket is the wrong thing. Outside the coil, ground is one side of the resonant circuit. Inside the driver there are three separate grounds that must not be joined, and getting those wrong kills more QCW drivers than anything else.
Why the secondary needs a ground at all
Every streamer that leaves the topload is current, and that current has to come back. It travels through the air into whatever it strikes, or into the surroundings by displacement, and then back to the bottom of the winding. In the usual arrangement, with one end of the winding at high voltage and the other at the base, the resonator is not a closed loop of wire. The loop goes out through space and comes back through the ground connection.
The ground is part of the circuit, not a safety afterthought.
More precisely, the ground is one plate of a capacitor. The toroid's capacitance is capacitance to ground: 10.88 pF on one coil, which is about 34 kΩ of reactance at 426 kHz [derived, X = 1/(2πfC)]. The resonance runs on all of Ces, that 10.88 pF plus the winding's own 3.15, so 14.03 pF and 26.6 kΩ. That is the resonator's own impedance, the number that, with the winding's inductance, fixes the frequency. The secondary article reaches the same 26.6 kΩ by a different route, √(L/C) rather than 1/(2πfC), and the two agree because at resonance ωL and 1/ωC are one quantity [derived]. The figure is worked out there and only quoted here.
The clearest evidence is in the calculator everyone already uses. Before JavaTC returns a number it asks for ground radius, wall radius and ceiling height. It is asking about the room, because without a counterpoise the room is the other plate: the floor, the walls, the bench, you. The coil still resonates, but the capacitance is not one you can repeat, and it moves when you walk towards the coil.
Not mains earth
A counterpoise instead
A large sheet of metal under the coil, with the secondary's base connected to it. Roofing aluminium beats foil or mesh, because it does not crumple and does not have a thousand joints. Round the corners, or they will corona.
Keep the lead short, but not because of the impedance it adds. Ten metres of wire is about 27 Ω at these frequencies [derived, ten metres at the usual rule of thumb of a microhenry per metre is 10 µH, and 2πfL at 426 kHz is 26.8 Ω], against a toroid reactance of 34 kΩ. That is eight parts in ten thousand, and it changes nothing about the resonance. What matters is what the lead radiates. At a hundred kilovolts on the topload, that wire is carrying about three amps at 426 kHz [derived, I = 2πfCV on 10.88 pF]. Run it beside a signal cable and the signal cable carries it too. Keep it short, and run it on its own.
What the lead is made of is less certain. There is no published gauge for it. At 426 kHz the current runs on the surface, so a strap or a length of braid carries it better than a round wire of the same cross-section. Size it for surface, not for area.
The base of the secondary connects to both the counterpoise and the strike ring.
The strike ring
A grounded ring around the coil, there to take the arc hits that would otherwise reach the ceiling, the operator, or the electronics. Run it to the RF ground on its own wire, not the counterpoise's and not through the frame. The two carry different currents and should not share a lead.
Give it a generous tube diameter, keep it clear of the winding, and leave it no sharp ends: cap them with balls or bend them back. A point on the ring coronas, and then the ring is a source instead of a sink.
How big it has to be
Half the topload's field leaves downward. A sheet of radius R sitting h below it intercepts ½(1 - h/√(h²+R²)) of the whole sphere. Of the half that goes its way, it catches:
R / h | of the downward field |
|---|---|
| 1× | 29 % |
| 2× | 55 % |
| 3× | 68 % |
| 10× | 90 % |
A radius of twice the height is where it stops being cheap, and three times is where going bigger stops mattering. Going from three to ten gains twenty two points for eleven times the aluminium.
The word counterpoise comes from radio, where it means something a coil builder cannot build. There it names a ground system sized against the wavelength, and at 426 kHz a wavelength is about 700 metres [derived, c/f], so nothing a coil builder puts on a floor is a counterpoise in the original sense. The sizing above exists because the radio rule is out of reach.
There is also a rule that a sheet about four times the toroid's diameter lands in the same place. It has no source, and it does not land in the same place. With the topload a metre up, R = 2h is a sheet four metres across, and four times a toroid's diameter only reaches that if the toroid is a metre wide, which no coil described here is. The two rules are a factor of three or more apart on any coil in this department, so the four-diameter rule is not used.
So size the sheet for the geometry.
Joining strips is fine, and it is not a close call. A 5 cm overlap along a metre of seam is 0.05 square metres of plate against plate. Hold the two a whole millimetre apart with nothing but air between them, which is far worse than any real lap joint, and it is still 0.44 nF and under an ohm at coil frequencies [derived, C = ε₀A/d then X = 1/(2πfC) at 426 kHz, giving 0.84 Ω]. Against 34 kΩ of toroid reactance, at three amps, that is a couple of volts under a hundred kilovolts. A figure of "ten to fifteen ohms" for the same seam with nothing under it is out of reach of any plausible gap. Compare a seam against the toroid's reactance, not against zero. The same check says taped toroid seams are acceptable.
Whether a ground can be too good is an open question. The worry is the whiplash: a ground strike empties the topload faster than charge can move in the winding, and the front that runs down the coil might punch through the turn-to-turn varnish near the base. Nothing on file puts a number on it, and the one published argument about grounding and that front, below, points the other way.
There is one number for the event itself. Steve Ward, on the Tesla Coil Mailing List in 2012, asked whether ground strikes could damage the bottom of a winding: "During a ground discharge, the voltage at the topload has been measured to collapse within 100nS (worst case) and 250nS (more typical)", and during that event "a wave front propagates down from the top of the coil". That makes the front real and fast, a quarter of a microsecond on a machine whose bang lasts hundreds.
The suspect in that thread is a weak ground, not a strong one. Bert Hickman's answer was ground bounce. If the path from the base and the strike rail to local ground has real inductance, the base spikes as the topload's charge dumps through it, and his cure is a grounded counterpoise tied to the strike rail so that the discharge goes straight back to its own plate. Ward agreed, said it was already done, and gave his own rough figure for a coil with no local ground capacitance: up to 50 kV on the base for 100 to 200 ns (TCML, 22 September 2012).
What the front does when it arrives is not established. Ward's own guess in the same exchange was that end-to-end capacitance, topload to winding included, would disperse it and take the steepness out. Paul Nicholson's answer concedes the dispersion and not the conclusion: "Yes dispersion spreads the down-going transient although it could still build up quite a high volts/turn near the base. This has been discussed before as one possible cause of racing arcs" (TCML, 23 September 2012). In 2012 it was still one possible cause, discussed before and open.
The failure that started that thread turned out to have a duller explanation. The coil had been rained on through a leaky roof and then baked with about 660 W to drive the water out. When the failed winding was cut open, the wire had separated from the PVC form. Nicholson's verdict on the whiplash story for that coil: "I guess this is a more likely explanation for the fault." A dramatic mechanism was proposed, a mundane one explained the actual damage, and the dramatic one went back to being a hypothesis. If you suspect whiplash for a dead bottom turn, check the form first.
The frame under the primary is a shorted turn
The cure is to break the loop: nylon bushes, washers, spacers and Kapton, so that each upper rail is grounded at one end only. The break has to stand whatever the primary induces around the frame at kiloamps, not the few volts a bolted joint suggests, and there is no measured figure for that. The 150 V per side that used to be quoted for it had no source anywhere. It would not have supported the rule attached to it either, that two millimetres of gap is not enough, because two millimetres of air stands off about 6 kV [derived, 3 kV per millimetre] and 150 V is forty times inside that. Break the loop and ground each rail once. Sizing a gap is the wrong question.
The frame belongs to both halves of the subject. It is outside the electronics and carries the coil's own current, and it is also the part most likely to put that current somewhere the driver can feel it.
Three grounds inside the driver
Apart from the coil's ground, the driver has three of its own, and they are not interchangeable.
- GND2 is the high side device's emitter. It floats with the device. Do not join it to PGND.
- PGND is the low side's emitter, which is fixed.
- GND1 is logic ground. Join it to PGND at one star point, Kelvin.
The star is there so that power currents have no physical path through the logic ground.
Return a clamp or TVS to the Kelvin emitter, not to the noisy power node, and put the clamp right at the gate pin, bypassing Rg.
One builder's answer to the same boundary, drawn as a block diagram:
The gate supply crosses the same boundary
And when the arc hits ground, a QCW does the opposite
On an ordinary DRSSTC a strike to ground is a low resistance short across the secondary, and the primary current slams into its limit.
It is often said that on a QCW the current falls on a ground strike, the reverse of a DRSSTC. That is half true, and it is the wrong half to remember. Loading a coil harder does at first pull the primary current down, but that fall is used up before the strike arrives. The turn sits near 41 kΩ of load and an ordinary 1.5 m arc already presents 50, so a real coil starts beside the minimum and goes up from there. What a ground strike does works the whole curve.
The 50 is from Anders Mikkelsen's parallel arc model, 8 pF per meter in parallel with 75 kohm*meter, fitted to his own measured phase angles (HVF 3140) and divided by a metre and a half. It is also the harshest arc on file. The five other published figures convert to 77 to 258 kΩ at the same length, and on any of those the coil starts well up the falling side and crosses the minimum on the way instead of starting beside it. What holds for all six is that the minimum sits at 41 kΩ and the one strike channel anybody has published is 25. By the time a real strike has established itself, the current is on the rising side of the curve, whatever it did on the way there.
Either way the topload voltage collapses and the arc takes the load off the secondary. How fast is not pinned down. Ward's 100 to 250 nanoseconds comes without the measurement behind it, and it is an order of magnitude shorter than the one thread with a strike actually measured on it. In HVF 117 Uspring reads the duration as "around 1-2 us", and Hydron's capture of the same event is where the 25 kΩ comes from. Use the microseconds, since they are the measured ones. What the overcurrent detector does with that is in the same ground strike article. In short, whether it fires depends on the threshold rather than the strike. Half again over the normal ramp needs a channel of 22.1 kΩ and a fifth over needs 37.3, while the one strike anybody has published bottoms out at 25, between the two.
One thing that will surprise you
Metal closer than half the secondary's diameter kills the Q. A chassis, a case, a rack, a radiator: they load the resonator and steal energy into induction heating, and the coil simply performs worse for no reason you can see on a scope.
The earth symbol under the secondary on every department diagram means this, and what it is not connected to matters more.