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Graham Armitage

Tue Apr 01 2014, 01:58AM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Based on all I have read on this forum, it is suggested that the strike objects be grounded directly to the base of the secondary (also to ground wire in the 120v source) so as eliminate extraneous ground loops running through the house wiring. Makes sense in theory, but when I do that, a strike to the grounded object causes fluorescent lights to flicker, tv's goes haywire etc. When I let the arc strike to an object just sitting on the concrete floor it does not interfere with any of the house wiring.

Any explanation for why this seems to be opposite from what I read? I have the strike ring around the primary connected to the base of the secondary and ground, but I can see I would want that.

Steve Conner

Tue Apr 01 2014, 07:50AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The capacitance that stores the Tesla coil's energy is between the topload and the surroundings (walls of the room etc) Any sudden discharge of this capacitance will make a high transient current flow in the surroundings. It will invariably get into the house wiring as this is the most conductive part of the surroundings.

If the arc strikes an ungrounded object, there is no low-impedance return path to the surroundings, so the topload doesn't discharge suddenly.

You might think that grounding the strike object to the secondary base "completes the circuit" and all the discharge current circulates in that loop without getting into the wiring. This is only true for current at the coil's resonant frequency. The spike of current from a ground strike contains lots of high frequencies which can't pass through the secondary coil because of its high inductance. So they flow from the strike target straight out of the secondary base ground wire, and return to the topload via the surroundings.

Graham Armitage

Tue Apr 01 2014, 10:00AM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

So if a strike to an ungrounded object does not fully discharge the top-load that would suggest a lower current draw and as you say, less high frequency current returning via the surroundings. Two good things I imagine, so why is it always suggested that we ground the strike object? What is the benefit?

Steve Conner

Tue Apr 01 2014, 10:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I have no idea. When doing public demos I try to use a full Faraday cage, and if that's not possible I try to avoid arcing to ground altogether. Blazing away at a grounded target is a sure-fire way to generate lots of nasty EMI.

Graham Armitage

Tue Apr 01 2014, 11:02AM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Thanks Steve, that's good to hear, and to confirm what I was seeing and doing. How about the base of the secondary and strike ring - do you connect those together and ground them to the mains supply or do you use a separate RF ground?

Steve Conner

Tue Apr 01 2014, 11:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Secondary base and strike ring grounding is a tricky issue. For protection of DRSSTC electronics, you want all of the grounds tied together, but for EMI you want to separate the RF grounds from the incoming power supply.

Nowadays I err on the side of protection, since as I explained above, separating the grounds doesn't really help EMI as most of the really bad stuff gets capacitively coupled straight into the surroundings from the topload anyway.

So, I tie all of the grounds together at the base of the coil. Secondary base ground, strike rails, IGBT heatsink, EMI line filter, and the AC line green wire. I also bypass the DC bus directly to the IGBT heatsink using a couple of 0.1uF capacitors, to give a direct return path for any primary strikes. Otherwise they can punch through your gate drive transformers or the IGBTs' isolation barriers and the resulting follow-through current from the DC bus will blow your setup to smithereens.

When running in a Faraday cage (more often than not nowadays) I also bond all of the above grounds to the cage.

Graham Armitage

Tue Apr 01 2014, 11:19AM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Wow - that's good info. I like the idea of the protecting the circuits with the 0.1uF caps - that's a bonus. Other than that I do have a similar grounding config. Would love to see your faraday cage setup sometime.
Thanks

Kizmo

Tue Apr 01 2014, 03:12PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I can confirm that the grounding method Steve suggested works very well with big DRSSTC too. And by big i mean my BiggerDR (

)

Graham Armitage

Tue Apr 01 2014, 05:10PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

That's an impressive coil. Looking at the video it seems you have a cable from the ladder going back toward the coil. If that is not a grounding wire, as Steve advises against, what is it and where is it going?

Steve Conner

Tue Apr 01 2014, 05:15PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think it is a grounding wire and Kizmo just doesn't care about EMI as he is in the middle of a forest in Finland.

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