Small SSTCs, RFI range & safety considerations
flannelhead, Thu Jul 02 2015, 03:46PMHello,
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.
Thanks in advance for any input! :)
[1]
[2]
Re: Small SSTCs, RFI range & safety considerations
Mads Barnkob, Thu Jul 02 2015, 06:56PM
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:
Mads Barnkob, Thu Jul 02 2015, 06:56PM
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:
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Thu Jul 02 2015, 07:14PM
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.
flannelhead, Thu Jul 02 2015, 07:14PM
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! 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:
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.
Re: Small SSTCs, RFI range & safety considerations
Sigurthr, Fri Jul 03 2015, 03:31AM
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.
Sigurthr, Fri Jul 03 2015, 03:31AM
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:
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.
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Wed Jul 08 2015, 10:22AM
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.
flannelhead, Wed Jul 08 2015, 10:22AM
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.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.
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.
Re: Small SSTCs, RFI range & safety considerations
Sigurthr, Wed Jul 08 2015, 08:50PM
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.
Sigurthr, Wed Jul 08 2015, 08:50PM
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.
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Wed Jul 08 2015, 09:03PM
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.
flannelhead, Wed Jul 08 2015, 09:03PM
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! 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.
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.
Re: Small SSTCs, RFI range & safety considerations
Sigurthr, Wed Jul 08 2015, 10:58PM
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.
Sigurthr, Wed Jul 08 2015, 10:58PM
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.
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Thu Jul 09 2015, 08:49AM
flannelhead, Thu Jul 09 2015, 08:49AM
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.All I can say is invest in the class-Y caps, and enough of them to do every circuit of your apartment
Re: Small SSTCs, RFI range & safety considerations
Sigurthr, Fri Jul 10 2015, 05:35AM
You could always build a faraday cage and run the coil inside it. Size your mesh right and you'll have nothing to worry about.
Sigurthr, Fri Jul 10 2015, 05:35AM
You could always build a faraday cage and run the coil inside it. Size your mesh right and you'll have nothing to worry about.
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Sat Jul 11 2015, 02:54PM
Now, let's say this filtering happens to be enough so that I don't notice any issues with my appliances (remember, I'm not able to install class Y capacitors anywhere in the house wiring). As the RF current travels along the three conductors, could the possible high RF impedance and differences between the conductors cause some phase shifts / potential differences between the conductors so that somewhere else along the line (like another apartment) there could suddenly be much more interference due to the current than in my apartment? In other words, am I always the first to notice if there's an RFI problem due to the grounding or could it happen somewhere else (another apartment) without me noticing anything? My intuition says that at least the interference wouldn't get worse further away from the coil. I've yet to properly check that the mains ground actually connects to mother Earth, but I'm fairly confident it does. At the very least my DMM shows quite a small DC resistance (< 1 ohm) between PE and the cold water tubes. I'll do the check with a coathanger and a wire anyway! The distance to earth is quite long, though – I'm living on the 7th floor.
Sorry for all the (probably simple) questions – but I'm really trying to understand what I am about to do before actually doing anything (possibly stupid). While I'm quite familiar with general electronics and safety (enough to safely build and run something like the mini SSTC at relatively low powers), the RF and EMI stuff are something I've really not that much experience with. I'm studying physics at the university for the third year so I've got a grasp about electricity, but the radio frequencies seem more like a special niche so there hasn't been much of that on the courses.
flannelhead, Sat Jul 11 2015, 02:54PM
Sigurthr wrote ...
You could always build a faraday cage and run the coil inside it. Size your mesh right and you'll have nothing to worry about.
That's definitely worth considering, thanks for the reminder!You could always build a faraday cage and run the coil inside it. Size your mesh right and you'll have nothing to worry about.
Sigurthr wrote ...
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.
So, as I understand the matter, the high impedance to RF will cause some potential differences in the mains ground conductor, right? If I add an EMI filter on the side of the coil like in this schematic (so there's a common mode choke in addition to the class Y/X capacitors which are from an ATX PSU) and connect the secondary base to the mains ground, the RF current from the base should get equalized on all the three conductors. As a consequence, nearby appliances don't "see" the current as all the three potentials are shifted equally by the RF current.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.
Now, let's say this filtering happens to be enough so that I don't notice any issues with my appliances (remember, I'm not able to install class Y capacitors anywhere in the house wiring). As the RF current travels along the three conductors, could the possible high RF impedance and differences between the conductors cause some phase shifts / potential differences between the conductors so that somewhere else along the line (like another apartment) there could suddenly be much more interference due to the current than in my apartment? In other words, am I always the first to notice if there's an RFI problem due to the grounding or could it happen somewhere else (another apartment) without me noticing anything? My intuition says that at least the interference wouldn't get worse further away from the coil. I've yet to properly check that the mains ground actually connects to mother Earth, but I'm fairly confident it does. At the very least my DMM shows quite a small DC resistance (< 1 ohm) between PE and the cold water tubes. I'll do the check with a coathanger and a wire anyway! The distance to earth is quite long, though – I'm living on the 7th floor.
Sorry for all the (probably simple) questions – but I'm really trying to understand what I am about to do before actually doing anything (possibly stupid). While I'm quite familiar with general electronics and safety (enough to safely build and run something like the mini SSTC at relatively low powers), the RF and EMI stuff are something I've really not that much experience with. I'm studying physics at the university for the third year so I've got a grasp about electricity, but the radio frequencies seem more like a special niche so there hasn't been much of that on the courses.
Re: Small SSTCs, RFI range & safety considerations
Sigurthr, Sun Jul 12 2015, 02:49AM
Just a note: RFI filters at the coil are useless; the capacitive coupling of the EM field to the environment causes orders of magnitude more EMI than what travels through the power cord to the wall outlet (unless you're operating the coil in the center of an airport hangar, lol!). That's why you have to make equalization nodes at the wall outlets. I just made little RF equalizer boxes that plug into the outlets and have the caps and wiring inside them, then placed them around the house.
I'm not sure if the impedance mismatches will cause maxima nodes at different points down the lines relative to each conductor, but it seems reasonable to assume so if the wiring's conductors are of uneven inductances with respect to each other. Maxima nodes will form on the mains wiring even with equalization throughout, but unless two points connect to a singular device, no device will see the potential difference.
Radiated EMI should be low, since most TC wavelengths are still much longer than wiring length. A HF TC could approach the 1/4 wave wire length in a distant circuit though, but it's unlikely.
Sigurthr, Sun Jul 12 2015, 02:49AM
Just a note: RFI filters at the coil are useless; the capacitive coupling of the EM field to the environment causes orders of magnitude more EMI than what travels through the power cord to the wall outlet (unless you're operating the coil in the center of an airport hangar, lol!). That's why you have to make equalization nodes at the wall outlets. I just made little RF equalizer boxes that plug into the outlets and have the caps and wiring inside them, then placed them around the house.
I'm not sure if the impedance mismatches will cause maxima nodes at different points down the lines relative to each conductor, but it seems reasonable to assume so if the wiring's conductors are of uneven inductances with respect to each other. Maxima nodes will form on the mains wiring even with equalization throughout, but unless two points connect to a singular device, no device will see the potential difference.
Radiated EMI should be low, since most TC wavelengths are still much longer than wiring length. A HF TC could approach the 1/4 wave wire length in a distant circuit though, but it's unlikely.
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Sun Jul 12 2015, 02:46PM
Thanks Sigurth. I really appreciate the time you've taken to answer my questions. Thanks to you, now I think I understand the EMI/RFI problem at least to some degree.
It seems my best bet really is to build a Faraday cage to reduce the interference, as I don't even accidentaly want to cause harm to my appliances, let alone my neighbours'. You said the mesh size should be chosen right. As I understand it, the diameter of the apertures should be smaller than the smallest wavelength I want to block (up to some harmonic, probably? Which one?). Are there any other things I'd have to take in account? Also, the cage and the secondary should be both connected together AND to the mains ground to prevent them from climbing up to some high potential, right?
I couldn't find specific guidelines for designing a Faraday cage for a TC (despite googling), but I'm thinking of something like shown in this thread. I assume leaving the top open for convenience doesn't pose too much of a problem as it seemed to work very well for WaveRider's coil.
flannelhead, Sun Jul 12 2015, 02:46PM
Thanks Sigurth. I really appreciate the time you've taken to answer my questions. Thanks to you, now I think I understand the EMI/RFI problem at least to some degree.
It seems my best bet really is to build a Faraday cage to reduce the interference, as I don't even accidentaly want to cause harm to my appliances, let alone my neighbours'. You said the mesh size should be chosen right. As I understand it, the diameter of the apertures should be smaller than the smallest wavelength I want to block (up to some harmonic, probably? Which one?). Are there any other things I'd have to take in account? Also, the cage and the secondary should be both connected together AND to the mains ground to prevent them from climbing up to some high potential, right?
I couldn't find specific guidelines for designing a Faraday cage for a TC (despite googling), but I'm thinking of something like shown in this thread. I assume leaving the top open for convenience doesn't pose too much of a problem as it seemed to work very well for WaveRider's coil.
Re: Small SSTCs, RFI range & safety considerations
Dr. Slack, Sun Jul 12 2015, 04:55PM
Building a full Farady cage is rather a big undertaking. You may not need to. If you do build one, and get other things wrong, you can still have a problem.
Step by step ...
You must connect all the mains grounds together, and to the secondary ground. It doesn't matter whether this is connected to soil at ground level, this is irrelveant, even though it will be, it matters for lightning and for system-wide fault currents, it doesn't matter for TCs. The point is that all metal buried in the walls, floors, ceilings should be connected to coil ground. This reduces EMI carried on one bit of grounded cable with reference to another bit.
You must have a RFI filter at your coil supply. It is a very good idea to 'class Y' all of your mains sockets. As a tenant you can't do this from behind the sockets, but you can put caps in plug tops, and plug them in. This reduces EMI carried between the wires and ground.
It's an improvement to have an extended ground plane under the coil (counterpoise), which must be connected to the secondary ground. This intercepts many of the electric field lines coming from the coil, reducing its far field radiation efficiency (ability to upset the neighbours).
At any point, if this reduces emissions to a satisfactory level, you can stop there. A TC is a very inefficient radiator of RF to start with, if you wanted to design an antenna for these frequencies, it would not be the size and shape of a TC. There's no point in over-engineering a solution, when it's good enough, stop. Are you able to ask your neighbours 'does it interfere with anything *now*?'
If it still does, then you could extend the sides of the counterpoise up the walls with chicken-wire, and continue closing a Faraday cage. This is where the difference between a counterpoise, and a partial Faraday screen begins to become one of degree. As you go up the walls, you intercept more and more of the field, until with a fully closed screen, it is all stopped.
As a rule of thumb, as long as the dimensions are << wavelength/10 (which they are for room dimensions and TC frequencies), a circular hole of diameter d in the screen that's more than 3d from the radiator is going to leak damn-all. That said, the same goes for a slot of length s. So if you have a door of height 2m, width 800mm, and you don't run conductors across to maintain continuity, the critical dimension is the 2m, not the 800mm. There is little point putting mesh on your door, even if it is all connected at the hinge side, unless you can make multiple connections across the opening side when you operate the coil, croc clips would be fine.
Dr. Slack, Sun Jul 12 2015, 04:55PM
Building a full Farady cage is rather a big undertaking. You may not need to. If you do build one, and get other things wrong, you can still have a problem.
Step by step ...
You must connect all the mains grounds together, and to the secondary ground. It doesn't matter whether this is connected to soil at ground level, this is irrelveant, even though it will be, it matters for lightning and for system-wide fault currents, it doesn't matter for TCs. The point is that all metal buried in the walls, floors, ceilings should be connected to coil ground. This reduces EMI carried on one bit of grounded cable with reference to another bit.
You must have a RFI filter at your coil supply. It is a very good idea to 'class Y' all of your mains sockets. As a tenant you can't do this from behind the sockets, but you can put caps in plug tops, and plug them in. This reduces EMI carried between the wires and ground.
It's an improvement to have an extended ground plane under the coil (counterpoise), which must be connected to the secondary ground. This intercepts many of the electric field lines coming from the coil, reducing its far field radiation efficiency (ability to upset the neighbours).
At any point, if this reduces emissions to a satisfactory level, you can stop there. A TC is a very inefficient radiator of RF to start with, if you wanted to design an antenna for these frequencies, it would not be the size and shape of a TC. There's no point in over-engineering a solution, when it's good enough, stop. Are you able to ask your neighbours 'does it interfere with anything *now*?'
If it still does, then you could extend the sides of the counterpoise up the walls with chicken-wire, and continue closing a Faraday cage. This is where the difference between a counterpoise, and a partial Faraday screen begins to become one of degree. As you go up the walls, you intercept more and more of the field, until with a fully closed screen, it is all stopped.
As a rule of thumb, as long as the dimensions are << wavelength/10 (which they are for room dimensions and TC frequencies), a circular hole of diameter d in the screen that's more than 3d from the radiator is going to leak damn-all. That said, the same goes for a slot of length s. So if you have a door of height 2m, width 800mm, and you don't run conductors across to maintain continuity, the critical dimension is the 2m, not the 800mm. There is little point putting mesh on your door, even if it is all connected at the hinge side, unless you can make multiple connections across the opening side when you operate the coil, croc clips would be fine.
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Sun Jul 12 2015, 08:28PM
Instead of turning a whole room into a Faraday cage, I was thinking of building a smaller, partial cylindrical Faraday screen of diameter ~40cm with walls and bottom made of mesh. I'd make it just large enough to accomodate the coil (see the link in my previous post). The walls of the cage would extend some tens of centimeters above the topload, with the top left open. This seems less of an effort than making a large enough local ground plane, and would intercept a good amount of the electric field lines as well (to my understanding).
flannelhead, Sun Jul 12 2015, 08:28PM
Instead of turning a whole room into a Faraday cage, I was thinking of building a smaller, partial cylindrical Faraday screen of diameter ~40cm with walls and bottom made of mesh. I'd make it just large enough to accomodate the coil (see the link in my previous post). The walls of the cage would extend some tens of centimeters above the topload, with the top left open. This seems less of an effort than making a large enough local ground plane, and would intercept a good amount of the electric field lines as well (to my understanding).
Re: Small SSTCs, RFI range & safety considerations
Ash Small, Sun Jul 12 2015, 08:44PM
As a last resort you 'could' hire a small generator and take it somewhere else to run it, if you can't take it somewhere with mains power?
Ash Small, Sun Jul 12 2015, 08:44PM
As a last resort you 'could' hire a small generator and take it somewhere else to run it, if you can't take it somewhere with mains power?
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Mon Jul 13 2015, 12:49PM
A generator would be a good last resort, as you said, but I've got no means of transporting it as my only vehicle is a bicycle. And of course I'd prefer a more permanent solution.
I created a schematic illustration of what I'm thinking of doing based on this discussion. Does this make sense in terms of reducing the coupling of RFI via the EM field to the mains wiring? A small partial Faraday cage with the top open would be quite easy to construct and probably would also "complete" much of the secondary LC circuit for RF and thus reduce emission by a good amount. There would still be some interference coupling to the power cord, but that would be filtered by class Y capacitors outside.
I don't see this as over engineering but rather just playing safe (is there a difference, though?) At the same time this would be a nice experiment.
flannelhead, Mon Jul 13 2015, 12:49PM
A generator would be a good last resort, as you said, but I've got no means of transporting it as my only vehicle is a bicycle. And of course I'd prefer a more permanent solution.
I created a schematic illustration of what I'm thinking of doing based on this discussion. Does this make sense in terms of reducing the coupling of RFI via the EM field to the mains wiring? A small partial Faraday cage with the top open would be quite easy to construct and probably would also "complete" much of the secondary LC circuit for RF and thus reduce emission by a good amount. There would still be some interference coupling to the power cord, but that would be filtered by class Y capacitors outside.
I don't see this as over engineering but rather just playing safe (is there a difference, though?) At the same time this would be a nice experiment.
Re: Small SSTCs, RFI range & safety considerations
Sigurthr, Mon Jul 13 2015, 10:54PM
Looks fine to me, but have a removable top mesh you can easily place and remove. No reason not to include that, really. Given that the cage is so close to the secondary it will significantly load the coil and reduce visible output, bear that in mind.
Sigurthr, Mon Jul 13 2015, 10:54PM
Looks fine to me, but have a removable top mesh you can easily place and remove. No reason not to include that, really. Given that the cage is so close to the secondary it will significantly load the coil and reduce visible output, bear that in mind.
Re: Small SSTCs, RFI range & safety considerations
flannelhead, Tue Aug 11 2015, 12:06PM
The SSTC is now working Thanks for helping me understand the RFI issues.
I decided to test the coil without the Faraday cage at first. I have 100nF class X/Y2 capacitors coupling the three mains wires together on the extension cord to which the coil is also connected. The coil works very well and doesn't seem to cause any interference to my electrical appliances. The only sign of interference was when I had an AVR in-circuit programmer connected to my laptop and to the ATtiny based interrupter. When the coil was on, the ATtiny would repeatedly reset itself. I suppose the programmer cable was picking up interference. That was about half a metre from the coil and the interrupter is unshielded.
So radiated interference doesn't seem to be much of a problem. I'm still a bit confused about interference further down the line caused by possible high impedance of the mains wiring. As I happen to live in a 7th floor apartment, the RF current has a long way to go to the ground, and I'm puzzled if it could cause some havoc in the lower floor apartments.
I measured the DC resistance between the mains ground and some different pieces of water pipe going through my apartment. The resistance was near zero (shown by a multimeter). Shouldn't that be a good sign of the mains ground having a good connection to earth?
flannelhead, Tue Aug 11 2015, 12:06PM
The SSTC is now working
Thanks for helping me understand the RFI issues.I decided to test the coil without the Faraday cage at first. I have 100nF class X/Y2 capacitors coupling the three mains wires together on the extension cord to which the coil is also connected. The coil works very well and doesn't seem to cause any interference to my electrical appliances. The only sign of interference was when I had an AVR in-circuit programmer connected to my laptop and to the ATtiny based interrupter. When the coil was on, the ATtiny would repeatedly reset itself. I suppose the programmer cable was picking up interference. That was about half a metre from the coil and the interrupter is unshielded.
So radiated interference doesn't seem to be much of a problem. I'm still a bit confused about interference further down the line caused by possible high impedance of the mains wiring. As I happen to live in a 7th floor apartment, the RF current has a long way to go to the ground, and I'm puzzled if it could cause some havoc in the lower floor apartments.
I measured the DC resistance between the mains ground and some different pieces of water pipe going through my apartment. The resistance was near zero (shown by a multimeter). Shouldn't that be a good sign of the mains ground having a good connection to earth?
Re: Small SSTCs, RFI range & safety considerations
Sigurthr, Mon Aug 17 2015, 12:26AM
That means there's a good DC connection from mains earth to ground, but not any indication of a low impedance RF connection. Sounds like what you have going should be ok though. If you get any complaints, then you know you've got to change something.
Sigurthr, Mon Aug 17 2015, 12:26AM
That means there's a good DC connection from mains earth to ground, but not any indication of a low impedance RF connection. Sounds like what you have going should be ok though. If you get any complaints, then you know you've got to change something.
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