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Indoor DRSSTC grounding

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Avi

Mon Sept 16 2013, 01:27AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

I will start by saying there is no access to outside, and no room for a large metal plate under it.
according to the last line of

states "through an EMI filter to the power line's ground" a quick look at the schematic seems to show it routes the RF current into both active and neutral? is that wise?
How exactly should it be connected up? (TC is powered from a transformer) should this also go though the same EMI filter or should that just go to its own mains plug and the active and neutral output of the EMI filter be left unconnected?

Dr. Dark Current

Mon Sept 16 2013, 10:47AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Just get relatively large caps - depends on if you have a GFCI - if yes, try around 100-200 nF, if not, you can go into the uF range - and connect them from the live wire to PE and from neutral to PE. This will force the RF current to flow through all the power conductors, decreasing the RF impedance and protecting nearby appliances from common mode noise. The line filters contain such caps, but they are very small (few nF) and will do nothing with the large Tesla coil base currents.
Not a perfect solution, but probably the best you can do.

P.S. What about central heating or water pipes?

Avi

Thu Sept 19 2013, 01:33AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

GFCI where? at the fusebox? or directly at where i will be using it?

Dr. Dark Current

Thu Sept 19 2013, 11:29AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well, usually here it is in the box where the main breaker and power meter sits. I think nobody would purposely use an extra GFCI in front of a Tesla coil

Avi

Wed Sept 25 2013, 08:28AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

like this?

Steve Conner

Wed Sept 25 2013, 09:06AM

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

No, that's more complicated than it needs to be (and wrong anyway)

The whole point is that you have decided to send your RF current from the secondary base down the mains wiring because you don't have anywhere else to send it. So you need to make sure it goes down all three conductors equally. Then the three conductors will have the same RF voltage, so the other appliances won't see anything wrong.

You do this by linking the live and neutral wires to the earth wire with capacitors. Despite what Dr. Dark Current says, I've had good results with EMI filters, though I've used big hefty ones intended for use with inverter drives.

Your schematic is wrong because the common mode choke in the EMI filter seems to be in the way of the secondary base current getting into the mains. Normally we would be trying to keep RF out of the mains, but in this case there is nowhere else for it to go.

Dr. Dark Current

Wed Sept 25 2013, 08:59PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The use of a common mode choke a Tesla coil could lead to a nasty surprise... A relatively large portion of the RF secondary current flows back into the primary coil. This is seen as a common mode current on the bridge input, and because the chokes are designed to have the highest inductance possible, high voltage might develop on it. In better case, it just sparks over, but there are more things which can spark over - for example your GDT or the internal insulation in IGBT bricks.

Usually the number of turns on the common mode choke will probably not be enough to cause such problems, but it doesn't change anything on the fact that use of such filters in a TC is not the best thing to do.

Steve Conner

Wed Sept 25 2013, 10:26PM

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

Yes, and if a spark strikes the primary things go from bad to worse. To avoid this kind of catastrophe I decouple the DC bus to the IGBT heatsink using two capacitors with short leads, and ground the driver board to the heatsink too.

This doesn't really have anything to do with using a coil in an apartment though.

Avi

Thu Sept 26 2013, 02:50AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

so like this?

Steve Conner

Thu Sept 26 2013, 08:21AM

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

No. Why is the ground "NC"? It should be connected to the midpoint of the two 200nF capacitors.

Also according to Dr. Dark Current you should have some way for RF to get across the isolation transformer.

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