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Tesla coil grounding

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DekuTree64

Sun Aug 02 2015, 12:00PM

Registered Member #54596 Joined: Fri Mar 06 2015, 11:31AM
Location:
Posts: 19

Two questions:
1. What exactly is the purpose of connecting the bottom end of the secondary to ground?
2. What would happen if you put another capacity sphere/toroid at the bottom instead?

I'm somewhat of an electrical noob, but just thinking it through logically, it seems like when you pulse the primary, and that induces a current in the secondary, the wave would go up the coil, reflect off the top capacity, go back down the coil, out into the Earth, never to be seen again. No energy accumulation in the coil. That is, unless it's the size of Tesla's Colorado Springs model and can send a powerful enough wave into the Earth that it reflects off the other side and comes back.

On the other hand, if your ground connection is weak, then the wave would partially reflect off the end of the ground wire and back into the coil, which would be a similar result to putting another capacity at the bottom end.

Am I thinking right, or is there more to it?

doctor electrons

Sun Aug 02 2015, 12:38PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

You missed one important item. The primary is pulsed in a timed sequence, when that wave is on its way back down
the coil the primary is pulsed again. Before the energy can get to ground. That is where you get the accumulation of
energy, hence the breakout on the toroid.

Ask yourself this, if the wave was pushed up the coil, why would it want to come back down? (to your question #1)

An Experiment, take a vacuum cleaner and stretch out the cord, then cut off the plug and wire on another vacuum. (#2)

Ground is important for numerous reasons. One is to complete a circuit, current flows in loops. If you open that loop no
current can flow. "Open circuit"
Think about a capacitor, it in itself is an open circuit. The only way to get any use out of its charge is to make it a part of
a closed circuit.
I know that i didn't directly answer your two questions but its more important that you have that "ah ha" moment by
figuring it out your way ;)

Dr. Slack

Sun Aug 02 2015, 12:56PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Not all your thinking is right, pulses, waves etc. Although the primary is pulsed, the resonant nature of both primary and secondary, and their loose coupling, means that the best model for understanding the excitation of the secondary, at least during the important 'ring-up' phase prior to breakout, is a gradual feed of energy from the primary.

The primary induces an oscillatory voltage in the secondary, at a frequency that is resonant in the secondary. There are various models for understanding this resonance, but the simplest is to consider a capacitive topload, connected by an inductive secondary to a zero-impedance ground point. This forms a quarter-wave resonator.

The equivalent half-wave resonator can be made by connecting two TCs together at the bottom of the secondary and driving them in anti-phase. Alternatively, use a floating secondary driven at the middle, with a top-load (as it were) on each end. In the first case, there is no voltage at the junction of the secondaries. No voltage === perfect effective ground. In the second case, the zero-volt point is at the middle of the secondary.

So it doesn't matter whether you use a quarter or half-wave setup, the quarter wave needs a true ground point, the half-wave has an effective ground point in the middle of the secondary.

Now, let's look at your standard quarter-wave TC. Is a voltage developed at the bottom point of the secondary? If yes, then it's not at ground. If the bottom point is capacitive, that is you've not connected it to any of the wiring, and maybe added a top-load type capacitor to it, then your zero-volt point will move up into the secondary somewhere. If the bottom point is inductive, so connected by a length of wire to grounded objects in the vicinity, like the ground, your garage metal roof, mains wiring in the walls, a mesh counterpoise underneath the coil etc, then the zero-volt point will move down, further down the wire, perhaps into the wiring. Typically when you employ an ad-hoc ground like this, it will be difficult to determine where the zero-volt point is, there may be several.

Does this matter for coil operation? Usually, not much. There are two situations where it matters. One, when you manage to get a high resistance between the bottom of the secondary and your zero-volt point, this will damage the resonant Q and reduce your coil output. This is quite difficult to do accidentally, the easiest ways I can think of would be to use a ground spike driven into dry ground, or have a bad connection in your ground wire. Two, where your adhoc ground allows current flows in conductors that couple energy to yours and your neighbours' sensitive electronics. That's why it's a good idea to deliberately bring mains wiring and their ground conductors into the TC 'ground', with Y caps at the sockets, and multiple connections back to your secondary bottom terminal.

The point is that there is always a zero-volt point. Always. Figure out where it is, and that will be a good start to understanding whether your grounding scheme is adequate.

DekuTree64

Sun Aug 02 2015, 03:08PM

Registered Member #54596 Joined: Fri Mar 06 2015, 11:31AM
Location:
Posts: 19

doctor electrons wrote ...
You missed one important item. The primary is pulsed in a timed sequence, when that wave is on its way back down the coil the primary is pulsed again. Before the energy can get to ground. That is where you get the accumulation of energy, hence the breakout on the toroid.

I'm still not seeing how the energy remains confined to the coil. The Earth is conductive, right? So electrically, wouldn't it behave more like a bottomless pit than, well, "solid ground"? Each wave from the primary would follow the same path, up the coil, reflect off the top, down the coil, out to sea. You can keep feeding it, but until the whole sea is moving, the coil's not going to vibrate any harder.

Or is it that the energy goes back into the primary by induction, and reflects off its capacitor? And whatever energy didn't transfer to the primary does go "out to sea", and that's a good thing so it doesn't interfere with the resonating system?

wrote ...
Ask yourself this, if the wave was pushed up the coil, why would it want to come back down? (to your question #1)

Well it's not going to just sit there at the top. Whatever energy doesn't escape into the air has to go somewhere.

wrote ...
An Experiment, take a vacuum cleaner and stretch out the cord, then cut off the plug and wire on another vacuum. (#2)

If I wind the joined cords into a coil and induce a current in it, they should work just fine, but will be in opposite phase to eachother... right?

wrote ...
Ground is important for numerous reasons. One is to complete a circuit, current flows in loops. If you open that loop no current can flow. "Open circuit"

How does that close the circuit? It's still a straight line, Earth -> coil -> top capacity. Unless the top capacity produces a spark long enough to reach the ground.

wrote ...
I know that i didn't directly answer your two questions but its more important that you have that "ah ha" moment by figuring it out your way ;)

That's what I'm hoping for. Thanks for trying :)

Dr. Slack wrote ...
The primary induces an oscillatory voltage in the secondary, at a frequency that is resonant in the secondary. There are various models for understanding this resonance, but the simplest is to consider a capacitive topload, connected by an inductive secondary to a zero-impedance ground point. This forms a quarter-wave resonator.

The zero impedance ground point is what's getting me. Doesn't that mean the energy in the secondary can flow right out unimpeded?

Also, I can intuitively understand half-wave resonators, but I may be missing some obvious aspect of quarter wave. I'm currently visualizing it that the top capacity alternates between high positive voltage and high negative voltage, while the bottom/ground point alternates between high positive current and high negative current. Is that right?

Dr. Slack

Sun Aug 02 2015, 03:58PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

DekuTree64 wrote ...

Also, I can intuitively understand half-wave resonators, but I may be missing some obvious aspect of quarter wave. I'm currently visualizing it that the top capacity alternates between high positive voltage and high negative voltage, while the bottom/ground point alternates between high positive current and high negative current. Is that right?

Absolutely, spot-on.

Now take your half-wave resonator, with its zero voltage high current node in the middle, and see it's just two quarter-wave resonators, back to back, operating in anti-phase. It doesn't matter what feeds the current into the bottom terminal of a quarter-wave, whether it's the current coming out of another one in anti-phase, or the arbitrary current that a ground point can supply to hold its voltage at zero.

I wouldn't worry too much about vacuum cleaners for the moment.

I think I can guess where you're coming from. I don't think the wave/reflection aka transmission line model is the best one to use for TCs. However, it does give a very intuitive understanding of a half-wave dipole resonator. Of course a TC secondary+topload is neither a lumped L+C (it's longer) nor is it a quarter-wave dipole (it's shorter). It can be modelled as either (for fundamental operation, neither is a good model for the harmonics), as long as you use effective values and all the rules for your particular model consistently. Now in the wave/reflection model, a wave is a voltage wave AND and current wave travelling together. It is reflected in voltage phase, current anti-phase at an open circuit, so the open end of a dipole or the TC secondary, or both ends of a half-wave dipole. At a short-circuit, that is a low impedance point like the connection to ground, and the wave is reflected in voltage anti-phase, but in-phase in current, which is why there's zero volts and high current there.

So having figured that out, I can now explicitly answer your question #1. What's the purpose of the ground connection? In the transmission-line model of the TC secondary, to create a low impedance point to reflect the downwards travelling wave back up again, but with reversed voltage.

DekuTree64

Sun Aug 02 2015, 05:13PM

Registered Member #54596 Joined: Fri Mar 06 2015, 11:31AM
Location:
Posts: 19

Ok, I think I finally got it. And I think you're right, wave reflection is not the best way to look at it. It's more like a fluid problem. High pressure in the topload does bounce back downward, and becomes high current rushing into the ground. But the part I wasn't accounting for is that the current will continue rushing down until a vacuum (opposite voltage) is created in the topload, which is how the energy remains in the system. That opposite voltage then sucks current back up through the ground connection, and the cycle is complete.

Thanks for the help!

Chris Maness

Mon Aug 03 2015, 10:46PM

Registered Member #55389 Joined: Mon Jul 06 2015, 12:38AM
Location: Grand Terrace, CA
Posts: 29

This thread helped me trouble shoot why my TC was not working at a camp demonstration. The soil was probably too dry. I was just using a tent stake for a ground.

Chris

Chris

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