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Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Hello,
I'm on the verge of doing my first proper SSTC build. That means I'm probably going to stick to a power of < 150 W and keep the coil quite small (height < 6"). The resonant frequency would be around 500 – 1000 kHz.
What I'm a bit concerned about is not so much EMI or RFI causing damage to nearby electronic equipment but the RFI blocking some important signals. I searched around a bit on the topic and learned the following [1]:
SSTCs have quite a narrow RF bandwidth compared to SGTCs of equal power
The RF field is also somewhat stronger
Now I know that on certain bands there are some important radio signals like emergency communication in my country. I also know that if not the base frequency, the harmonics could well hit some of those bands. The last thing I'd want to do, even accidentally, is to interfere with such signals, so I'm asking what the general consensus about the range of RF interference caused by SSTCs is (also considering the quite low power I'm intending to use). I found this post by Richie [2] which is quite relevant. He says the interference is very localized.
At my parents' I ran a very small coil I built a couple of years ago, which is using a crystal oscillator at 6 MHz and thus has a very well defined frequency. My dad tuned his amateur radio receiver to that frequency and got very strong signal from the coil even on the third harmonic. The antenna was about 20 metres away from the coil.
Of course people have been building and running SSTCs for quite many years, and probably if there were significant interference problems they'd have already come up many times. I'd also like to know what to do about the coil design to mitigate outside RFI as much as possible (aside from building a Faraday's cage, which I'll probably do anyway).
A bit of a side question, what kind of designs do people nowadays recommend for the power range I mentioned? I'm a bit inclined to stay at quite low (<50 V) primary voltages, but perhaps a half bridge design run off mains would also do if the duty cycle is kept low enough? Also I'm a bit worried about grounding the secondary to the mains ground as I'm living in an apartment and would rather not to fry my neighbours' electronic devices... But that topic has been already discussed many times here and elsewhere so I won't delve into that here.
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Mads Barnkob wrote ...
I have only experienced knocking my ADSL internet router out, some times it would need a complete reset and run setup again.
This is no wonder for a SSTC working at 300kHz if you look at the frequency spectrum used by ADSL lines:
Thank you for the reply! Just to clarify, though, do you mean the router device inside your house, not something else further along the line?
My concern is more about the long range radio frequency impact of a small SSTC, as in possible interference on bands where important (eg. emergency) communication takes place. I'd like to know if there is actual information about the strength of that impact and/or any ways of mitigating it.
I have only experienced knocking my ADSL internet router out, some times it would need a complete reset and run setup again.
This is no wonder for a SSTC working at 300kHz if you look at the frequency spectrum used by ADSL lines:
I had a similar problem with my router, but that was before I redid the ground system in my electronics lab. I used class-Y and class-X to add RF bypassing/equalization to all three conductors of the mains wiring at several nodes on each circuit present near the coil. I also ran a dedicated low impedance at RF ground cable to an amateur radio style ground rod array, and tied the mains earth line to it in addition to running the new ground directly to the tesla coils. I can now run my big 3kW CW SSTC at full power without disrupting DSL or even WiFi service to my house, and without disrupting my expensive PC a mere 2.5 meters away from the coil.
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Sigurthr wrote ...
I used class-Y and class-X to add RF bypassing/equalization to all three conductors of the mains wiring at several nodes on each circuit present near the coil. I also ran a dedicated low impedance at RF ground cable to an amateur radio style ground rod array, and tied the mains earth line to it in addition to running the new ground directly to the tesla coils. I can now run my big 3kW CW SSTC at full power without disrupting DSL or even WiFi service to my house, and without disrupting my expensive PC a mere 2.5 meters away from the coil.
That's good to hear! In some ATX power supplies there seems to be an EMI filter circuit similar to what you described + some inductors and a NTC thermistor as an inrush current limiter. That circuit seems pretty useful, I could reuse the parts in a SSTC (I have a spare ATX supply just meant for utilising the parts, chassis, etc.) Thanks for the advice.
Now I'm still facing the problem of finding a good RF ground. On the 7th floor of a block of flats I don't seem to have many other choices than the mains ground wire or a counterpoise made of tin foil. Given the low average power (< 150 W) I'm intending to run at, do you think using the mains ground would be much of a risk (insulations in the house wiring not tolerating RF currents etc.), given proper EMI filtering / equalization? Of course the peak power would be higher if using an interrupter. Design-wise I'm thinking of something similar to Steve Ward's Mini SSTC.
1) current won't be shared equally through the conductors unless tied together with RF-bypass caps as I described. Current will get to the H and N lines from capacitive coupling between the conductors too, so this is -always- an issue.
2) mains ground may be high impedance to RF, and thus poorly conduct to earth. Or it could be high impedance to even DC, and be a really poor earth anyway.
when either of these two conditions occur you get currents going where you don't want them; your appliances. If you're willing to disconnect anything you don't want to possibly have to immediately replace, then I'd say you can live with using mains ground, but you may get performance issues on the TC if it doesn't impedance match well.
I've had no luck with counterpoises, but others have. If you do use a counterpoise you need to fully isolate from mains power too, so make sure you run from battery or have a good isolation transformer.
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Sigurthr wrote ...
The problem with mains ground is twofold:
1) current won't be shared equally through the conductors unless tied together with RF-bypass caps as I described. Current will get to the H and N lines from capacitive coupling between the conductors too, so this is -always- an issue.
2) mains ground may be high impedance to RF, and thus poorly conduct to earth. Or it could be high impedance to even DC, and be a really poor earth anyway.
when either of these two conditions occur you get currents going where you don't want them; your appliances. If you're willing to disconnect anything you don't want to possibly have to immediately replace, then I'd say you can live with using mains ground, but you may get performance issues on the TC if it doesn't impedance match well.
I've had no luck with counterpoises, but others have. If you do use a counterpoise you need to fully isolate from mains power too, so make sure you run from battery or have a good isolation transformer.
Thank you for the comprehensive answer!
It seems, now, that running an SSTC in my apartment is quite a tough problem to solve. I'd have no problem with unplugging sensitive devices while running the coil, but obviously I can't have my neighbours do the same every time I want to run it. I'll have to think about the matter for a while. If anyone has experiences (positive / negative) or tips to share about SSTCs (grounding them) in apartments, I'd appreciate them.
All I can say is invest in the class-Y caps, and enough of them to do every circuit of your apartment, and make sure your mains ground actually does connect to earth (even if you just check DC resistance with a DMM by using a coat hanger as a ground rod and a line of wire tossed out the window). If you place enough of the balancing caps nodes around it should be ok.
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Sigurthr wrote ...
All I can say is invest in the class-Y caps, and enough of them to do every circuit of your apartment
That's where I have a limitation – I'm only a tenant in this apartment and most certainly not allowed to modify the existing wirings in any way. So basically I'm limited to whatever I can do locally in the TC circuit itself. Too bad, this seems like a difficult setting... It's starting to seem I'd better put the TC stuff aside until I have a house of my own (or, at the very least, a good connection to an Earth ground!). Yet the idea is still bugging me that many people seem to be running small coils grounded to the mains ground with no problems, even without EMI filtering. It seems to be a matter of how much risk of interference one wants to take.
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Small SSTCs, RFI range & safety considerations
Just to clarify, though, do you mean the router device inside your house, not something else further along the line?