EMI & placement of driver, interrupter and MIDI controller

[DaiKen]

Hi all,

Given the EMI from a Tesla coil I suppose it is sensible to create a distance between the primary and secondary circuits on the one side and sensitive electronics as found in the driver, interrupter and MIDI controller on the other.
If I'm right, what is an advisable distance and/or should I put the EMI sensitive boards in a (small) faraday cage?

Hope to get some advice on this!

Kind regards,

Ton

[Mads Barnkob]

Since the first SSTCs I built, where noise was an issue with drivers and interrupters being exposed to the electromagnetic field of the Tesla coil, I started to shield the controlling parts with a grounded metal sheet between primary coil and controllers. Later on I moved on to build it all into a full metal enclosure, that is grounded to RF earth.

You are correct to ask about distance to primary coil to avoid induction heating of the metal parts/enclosures. The most well known issue is properly Dr.Spark that burned off his wire mesh in a high power DRSSTC, at a distance of maybe 10-15 cm from the primary coil, so that is a obvious minimum distance for HIGH power, when we talk about 1-2 kW, you can maybe do with some 5 centimeters.

drspark.org is unfortunately closed, so I can not find the pictures from Christopher Hooper of the burned wire mesh.

[DaiKen]

Dear Mads,

Thanks again for your reply.

Please correct me if I'm wrong, I have the following set-up in mind:
Two square wooden boards, both coils on the top one, the power supply/full bridge/mmc on the bottom one.
Than the driver and interrupter with MIDI interface:
The driver on the lower board, and  interrupter/MIDI connected from a distance with an ST to ST fiber cable to it (I ordered a 20 meter cable, which should propably enough for a coil like your DRSSTC 1).
I gather I'll have to put full metal enclosure (with RF earth) on all electronics on the lower board,all with enough distance to avoid induction heating, right?

Next, how to protect the cables (mains, fiber) from unwanted strikes?

And last but not least: how do you 'earth' the lot when inside a building and only mains 'ground' is available?

Looking forward to your answer!

Kind regards,

Ton

[profdc9]

The only way I know of for sure to prevent strikes on the cables is to surround the cables with a ground sheath.  However, you may have just have to ensure that a ground strike is more much likely that a cable strike by placing the cables out of the way with a few cm of wood separating the coil and the electronics underneath it.

I found I had to place the controller into a metal box, twist and wrap the power supply wires around ferrite toroids (wrap both wires on the same toroid, not each wire on separate toroids!).   I also had to do the same for the current sense transformers.  The EMI produced by the strike can disrupt the controller board even if it does not directly hit it, and ferrite common-mode chokes on the wires going into/out of the controller can largely mitigate this.

When earth ground is not available to put a ground spike into, you can use a counterpoise.  This is a large flat piece of metal placed under the coil that is generally 10 X the capacitance of the topload or more, so probably 3 to 4 square meters at least.  You can use cheap aluminum foil for this, steel screen, chicken wire, or my preferred material, aluminum roof flashing, more durable that aluminum foil and available in large rolls.  You can use aluminum foil insulation tape to join separate pieces together as well which also connects the pieces electrically, or if its just wire just twist the wires together well.

Some surround the bottom table of the Tesla coil with a grounded metal screen to ensure strikes to the bottom part are grounded.  This extra area can add to the counterpoise capacitance as well.

Dan

Dear Mads,

Thanks again for your reply.

Please correct me if I'm wrong, I have the following set-up in mind:
Two square wooden boards, both coils on the top one, the power supply/full bridge/mmc on the bottom one.
Than the driver and interrupter with MIDI interface:
The driver on the lower board, and  interrupter/MIDI connected from a distance with an ST to ST fiber cable to it (I ordered a 20 meter cable, which should propably enough for a coil like your DRSSTC 1).
I gather I'll have to put full metal enclosure (with RF earth) on all electronics on the lower board,all with enough distance to avoid induction heating, right?

Next, how to protect the cables (mains, fiber) from unwanted strikes?

And last but not least: how do you 'earth' the lot when inside a building and only mains 'ground' is available?

Looking forward to your answer!

Kind regards,

Ton

[Hydron]

I would suggest using some sort of industrial control cable for the mains connection. You can get shielded stuff easily and cheaply from both the standard RS/Farnell/Mouser/Digikey/TME type suppliers, or off ebay (probably cheaper). Some is copper shield (type CY I think) and other stuff is galv wire shield - both should work but I'd go for copper if possible.

I had a strike hit a switchmode PSU (running my controller), causing a breakdown between two mains traces on the PCB. This lead to a big mains arc and breaker trip, with a lot of evaporated copper inside the PSU. Everything other than the PSU was fine, but I learnt my lesson - don't allow strikes to any unshielded mains or control circuitry. Even if the strike doesn't cause direct damage, providing an ionised path through air/cable insulation for mains to follow can make for a big bang. I don't see fibre needing any protection.