Making an ultra high frequency Tesla coil...
Inducktion, Fri Jan 15 2016, 07:50AMOk. So.
I know this is going to be riddled with issues just because of the frequency.
However.
I have wound a very, very small tesla coil secondary. It has a resonant frequency of 20 Mhz.
Yes, 20 mhz.
Now, the problem I'm facing; how to drive this....
I have vacuum tubes, and I can buy MOSFET's.
I know vacuum tubes would probably be a better solution in some ways, but they are inherently inefficient (dat filament) and are not small.
I have found several mosfets offered by Cree... They're SiC. 900 volt capability, and I'm unsure of which two models to purchase for this coil.
Model 1 has a gate capacitance of 150 pF, can handle 11.5 amps and has an on state resistance of 0.28 ohms. It costs $3.46 per. linky
Model 2 has a gate capacitance of 350 pF, can handle 23 amps, and has an on state resistance of 0.12 ohms. It costs $6.42 per. linky
Model one also has a slightly slower turn off delay compared to model 2 but it's small so it shouldn't matter too much.
Now my question is, should I even bother with trying to drive this with FET's, and, if so, which should I get that would work most efficiently? If not, how should I go about it with a vacuum tube?
As for the driver circuit I'm going with a PLL. I found a model of 4046 that can do 30 mhz. Specifically this one
Thank you!
Re: Making an ultra high frequency Tesla coil...
DerAlbi, Fri Jan 15 2016, 08:08AM
Hmmh. I would first tackle the problem at a higher level like a good topology choise.
If you go for something like a Class-H Edit: i mean Class-E drive the gate-drive requirements go down a little since its implementing ZVS and therefore the miller plateau is minimized. One could also think about a resonant gate drive in that respect.
What i also see as a problem is skin effect. I currently do not see how you get a coil with sufficient Q-factor done. The question is if you go for a LC-Tank (with the typical top load) or if you just use a Lambda/4 or whatever length optimized wire for the coil. My idea there is to pump up a standing wave inside the wire itself. I am not sure how this relates to the coils SFR.
I think you need extensive simulation on this.
DerAlbi, Fri Jan 15 2016, 08:08AM
Hmmh. I would first tackle the problem at a higher level like a good topology choise.
If you go for something like a Class-H Edit: i mean Class-E drive the gate-drive requirements go down a little since its implementing ZVS and therefore the miller plateau is minimized. One could also think about a resonant gate drive in that respect.
What i also see as a problem is skin effect. I currently do not see how you get a coil with sufficient Q-factor done. The question is if you go for a LC-Tank (with the typical top load) or if you just use a Lambda/4 or whatever length optimized wire for the coil. My idea there is to pump up a standing wave inside the wire itself. I am not sure how this relates to the coils SFR.
I think you need extensive simulation on this.
Re: Making an ultra high frequency Tesla coil...
Sulaiman, Fri Jan 15 2016, 12:28PM
You could start with something like this
and a sig-gen like this
Sulaiman, Fri Jan 15 2016, 12:28PM
You could start with something like this
and a sig-gen like this
Re: Making an ultra high frequency Tesla coil...
WaveRider, Fri Jan 15 2016, 01:35PM
Keep in mind that the arc discharge will have a capacitance as well as a resistance. This capacitance can detune the coil significantly, so your PLL tracking system must have a tuning range that can accommodate the frequency shift range that takes place before and after arc ignition. (This depends on how big the arc actually is, as well.) Getting "perfect" ZVS is not always so easy either, because the changing arc size also changes the resistive loading as well. (No arc, small resistive/capacitive loading. Big arc, big resistive/capacitive loading.)
Have fun!
WaveRider, Fri Jan 15 2016, 01:35PM
Keep in mind that the arc discharge will have a capacitance as well as a resistance. This capacitance can detune the coil significantly, so your PLL tracking system must have a tuning range that can accommodate the frequency shift range that takes place before and after arc ignition. (This depends on how big the arc actually is, as well.) Getting "perfect" ZVS is not always so easy either, because the changing arc size also changes the resistive loading as well. (No arc, small resistive/capacitive loading. Big arc, big resistive/capacitive loading.)
Have fun!
Re: Making an ultra high frequency Tesla coil...
woodchuck, Fri Jan 15 2016, 04:33PM
I know of an Aussie amateur radio operator who made a 6M (52MHz) power amplifier by paralleling a bunch of IRF540's.
Craziest dang thing I ever saw.
Your choice of silicon (or silicon carbide) may not be the biggest obstacle you face.
woodchuck, Fri Jan 15 2016, 04:33PM
I know of an Aussie amateur radio operator who made a 6M (52MHz) power amplifier by paralleling a bunch of IRF540's.
Craziest dang thing I ever saw. Your choice of silicon (or silicon carbide) may not be the biggest obstacle you face.
Re: Making an ultra high frequency Tesla coil...
Hazmatt_(The Underdog), Fri Jan 15 2016, 06:43PM
I was characterizing my coils for a while with my GR 1233 power amplifier, and all I was getting out of the coil was a short flame arc, not much unlike that of the plasma speakers.
So your arc is not going to be all that fantastic, it will be a dancing flame that propagates vertically, and length will depend on input power.
Getting down to the real nitty-gritty however, you most likely do not want to go with semiconductors
Why is that?
Simply put, the VSWR of the coil changes dramatically at the resonance point, this is because of the phase change right at resonance. Semiconductors have a difficult time with the large mismatches presented to them by the coil as the frequency changes slightly. I would expect VSWR changes over 10:1, and semiconductors typically have issues with 2:1, the reflected power causes dramatic heating and device failure.
Now you can better "match" your coil with a push-pull driver into a transformer, this has been done many times, and there is a "Popular Electronics" article which does this, but the output is a foot or less.
If I were you, I would change my design criteria to 13.56 MHz rather then 20 MHz. I have a 13.56 MHz amplifier in the garage right now that can deliver 2KW into a load, and these are fairly common for semiconductor processing, and the designs are out there, it's more realistic.
However, the mismatch will be difficult to manage, and you will have to be on top of your game to not blow-up the final stage if the coil has problems.
I would anticipate using a heavier gauge wire (doesn't matter anyway, the coil is "short") like 26 AWG, and a tungsten output terminal. The output terminal will be important so that the "electrical length" of the coil does not change much as it operates (maintain constant VSWR) and the heavier gauge wire will give more surface area for the heavier RF current you will be pushing.
You will also want to be able to control the input power to the final stage so that you don't just blow it up on turn-on, this can be done with a volume control (attenuator)
And you will also want a decent directional coupler that can handle the input power and will give you visual feedback to your degree of mismatch (VSWR). My PA has the coupler on-board, so look for a PA with the coupler built in, then all you have to do is attach a couple of meters to the output terminals (the meters will need a log scale as it is power, it's really voltage in terms of loaded into 50 ohms, but we calibrate the voltage scale to output power.)
There's more to it of course, but I think that's the major point.
I would probably go vacuum tube, they tend to be more forgiving, but for push pull output you will need a custom ferrite OPT.
Hazmatt_(The Underdog), Fri Jan 15 2016, 06:43PM
I was characterizing my coils for a while with my GR 1233 power amplifier, and all I was getting out of the coil was a short flame arc, not much unlike that of the plasma speakers.
So your arc is not going to be all that fantastic, it will be a dancing flame that propagates vertically, and length will depend on input power.
Getting down to the real nitty-gritty however, you most likely do not want to go with semiconductors
Why is that?
Simply put, the VSWR of the coil changes dramatically at the resonance point, this is because of the phase change right at resonance. Semiconductors have a difficult time with the large mismatches presented to them by the coil as the frequency changes slightly. I would expect VSWR changes over 10:1, and semiconductors typically have issues with 2:1, the reflected power causes dramatic heating and device failure.
Now you can better "match" your coil with a push-pull driver into a transformer, this has been done many times, and there is a "Popular Electronics" article which does this, but the output is a foot or less.
If I were you, I would change my design criteria to 13.56 MHz rather then 20 MHz. I have a 13.56 MHz amplifier in the garage right now that can deliver 2KW into a load, and these are fairly common for semiconductor processing, and the designs are out there, it's more realistic.
However, the mismatch will be difficult to manage, and you will have to be on top of your game to not blow-up the final stage if the coil has problems.
I would anticipate using a heavier gauge wire (doesn't matter anyway, the coil is "short") like 26 AWG, and a tungsten output terminal. The output terminal will be important so that the "electrical length" of the coil does not change much as it operates (maintain constant VSWR) and the heavier gauge wire will give more surface area for the heavier RF current you will be pushing.
You will also want to be able to control the input power to the final stage so that you don't just blow it up on turn-on, this can be done with a volume control (attenuator)
And you will also want a decent directional coupler that can handle the input power and will give you visual feedback to your degree of mismatch (VSWR). My PA has the coupler on-board, so look for a PA with the coupler built in, then all you have to do is attach a couple of meters to the output terminals (the meters will need a log scale as it is power, it's really voltage in terms of loaded into 50 ohms, but we calibrate the voltage scale to output power.)
There's more to it of course, but I think that's the major point.
I would probably go vacuum tube, they tend to be more forgiving, but for push pull output you will need a custom ferrite OPT.
Re: Making an ultra high frequency Tesla coil...
Inducktion, Fri Jan 15 2016, 08:16PM
Hm.
I found videos of Zilipoper with his 17 mhz vacuum tube tesla coil, and I have to say his arcs are ridiculous.
However that being said I don't have even remotely the same power tubes he had. He was using some big radio frequency mommas. The biggest vacuum tube I have is a 6L6GC.
Would BJT's function any better than MOSFET's, or should I just go with vacuum tubes?
Inducktion, Fri Jan 15 2016, 08:16PM
Hm.
I found videos of Zilipoper with his 17 mhz vacuum tube tesla coil, and I have to say his arcs are ridiculous.
However that being said I don't have even remotely the same power tubes he had. He was using some big radio frequency mommas. The biggest vacuum tube I have is a 6L6GC.
Would BJT's function any better than MOSFET's, or should I just go with vacuum tubes?
Re: Making an ultra high frequency Tesla coil...
DerAlbi, Fri Jan 15 2016, 08:34PM
BJT are great linear devices when it comes to low distortion power amplification. Thats what you need for good signal quality for High datarate RF but they allways yield extremely bad efficiency. There is actually a lot of development for mobilephone base stations going on because the power consumption is the highest cost for the communication companies.
I dont think you need linear behavior.. you need just fast switching. The benefit of the BJT is therefore worth nothing to you imho. For switching you might even run into saturation problems which cripples every BJT.
I think SiC Mosfets are the way to go. (In combination with an optimized topology) However i now understand as your frequency will change dramatically designs like Class-E will be complicated because they rely on a more or less constant frequency.
Please ignore "class-H" above. that was a brainfart, sry
I meant Class-E. Class E is easy. but it puts some stress on the switch. (it quadruples the needed voltage capability of the mosfet :-/ )
DerAlbi, Fri Jan 15 2016, 08:34PM
BJT are great linear devices when it comes to low distortion power amplification. Thats what you need for good signal quality for High datarate RF but they allways yield extremely bad efficiency. There is actually a lot of development for mobilephone base stations going on because the power consumption is the highest cost for the communication companies.
I dont think you need linear behavior.. you need just fast switching. The benefit of the BJT is therefore worth nothing to you imho. For switching you might even run into saturation problems which cripples every BJT.
I think SiC Mosfets are the way to go. (In combination with an optimized topology) However i now understand as your frequency will change dramatically designs like Class-E will be complicated because they rely on a more or less constant frequency.
Please ignore "class-H" above. that was a brainfart, sry
I meant Class-E. Class E is easy. but it puts some stress on the switch. (it quadruples the needed voltage capability of the mosfet :-/ )Re: Making an ultra high frequency Tesla coil...
Hazmatt_(The Underdog), Sat Jan 16 2016, 12:59AM
If you are going to use semiconductors, I would recommend MOS, not BJT.
I was working on this problem with a class AB biased BJT power amp. It was push pull combined for roughly 150W output power into 50 ohms.
The bus voltage was a little over 100VDC. The amp was fairly stable into a fixed load, but as soon as the load changed dramatically one of the BJT's would explode, which took out the other pair because it was imbalanced. So a small imbalance in a high voltage amp causes severe imbalances.
FET on the other hand is pumping huge currents, like 30A from one device into a step-up balun. You have two of those so you're effectively pushing 60A at 28V or so into a step-up. What this gains you is if the fets see a change in say 10% of their operating current they can tolerate it without a meltdown. And if you combine two amps, it is more load tolerant to mismatches.
The consequence of this is that semis for RF are fairly pricey, and you would want a compensated design, which is why I'm recommending you just buy a pre-built brick amp.
Hazmatt_(The Underdog), Sat Jan 16 2016, 12:59AM
If you are going to use semiconductors, I would recommend MOS, not BJT.
I was working on this problem with a class AB biased BJT power amp. It was push pull combined for roughly 150W output power into 50 ohms.
The bus voltage was a little over 100VDC. The amp was fairly stable into a fixed load, but as soon as the load changed dramatically one of the BJT's would explode, which took out the other pair because it was imbalanced. So a small imbalance in a high voltage amp causes severe imbalances.
FET on the other hand is pumping huge currents, like 30A from one device into a step-up balun. You have two of those so you're effectively pushing 60A at 28V or so into a step-up. What this gains you is if the fets see a change in say 10% of their operating current they can tolerate it without a meltdown. And if you combine two amps, it is more load tolerant to mismatches.
The consequence of this is that semis for RF are fairly pricey, and you would want a compensated design, which is why I'm recommending you just buy a pre-built brick amp.
Re: Making an ultra high frequency Tesla coil...
Sigurthr, Sat Jan 16 2016, 02:01AM
If you're going to go semiconductor stick to Class-E or other "dual resonant" topologies. You can't hard switch 20MHz at any appreciable power level. Unmodulated, highly filtered DC fed Class-E can perform quite well even at 20MHz, it just becomes critical that you choose the right switch. If you try modulating it you're going to have reflected power issues.
Personally I'd go with a VTTC. You face literally no additional design problems for 20MHz than you do for 200KHz other than having to use ribbon instead of wire. I know 811A tubes can handle quite well and they're not really that pricy on eBay (~$30 ea). That's what I'd go with.
Sigurthr, Sat Jan 16 2016, 02:01AM
If you're going to go semiconductor stick to Class-E or other "dual resonant" topologies. You can't hard switch 20MHz at any appreciable power level. Unmodulated, highly filtered DC fed Class-E can perform quite well even at 20MHz, it just becomes critical that you choose the right switch. If you try modulating it you're going to have reflected power issues.
Personally I'd go with a VTTC. You face literally no additional design problems for 20MHz than you do for 200KHz other than having to use ribbon instead of wire. I know 811A tubes can handle quite well and they're not really that pricy on eBay (~$30 ea). That's what I'd go with.
Re: Making an ultra high frequency Tesla coil...
Inducktion, Sat Jan 16 2016, 03:50AM
I ordered some parts from Mouser for this tesla coil; If the semiconductor route doesn't work properly I'll go with vacuum tubes instead.
I already have a couple of different ideas to try out, so we'll see how it goes...
edit:
Well, since I want to be prepared in case semiconductors don't work properly I've been looking at vacuum tubes.
I found a listing for 10 2C39A vacuum tubes for $27 shipped. They're air cooled transmitter tubes, and rated at 100 watts each with only 1 amp for the filament at 6.3 volts.
They're also pretty small, too. Sound like I should jump on them?
Inducktion, Sat Jan 16 2016, 03:50AM
I ordered some parts from Mouser for this tesla coil; If the semiconductor route doesn't work properly I'll go with vacuum tubes instead.
I already have a couple of different ideas to try out, so we'll see how it goes...
edit:
Well, since I want to be prepared in case semiconductors don't work properly I've been looking at vacuum tubes.
I found a listing for 10 2C39A vacuum tubes for $27 shipped. They're air cooled transmitter tubes, and rated at 100 watts each with only 1 amp for the filament at 6.3 volts.
They're also pretty small, too. Sound like I should jump on them?
Re: Making an ultra high frequency Tesla coil...
Hazmatt_(The Underdog), Sun Jan 17 2016, 05:42AM
You're going to need sockets and the chimneys for a forced air cooled tube. You will probably need to do a fairly involved chassis too because the fan will have to pressurize the base of the tube with the cooling air, which will be forced up through the chimney and through the cooling fins of the anode.
Me personally I would go for something a little less complicated like the 811A, a bunch of 807's (I like this one), 572B's etc. (graphite anode beastie)
Hazmatt_(The Underdog), Sun Jan 17 2016, 05:42AM
You're going to need sockets and the chimneys for a forced air cooled tube. You will probably need to do a fairly involved chassis too because the fan will have to pressurize the base of the tube with the cooling air, which will be forced up through the chimney and through the cooling fins of the anode.
Me personally I would go for something a little less complicated like the 811A, a bunch of 807's (I like this one), 572B's etc. (graphite anode beastie)
Re: Making an ultra high frequency Tesla coil...
Inducktion, Sun Jan 17 2016, 05:47AM
my worry with going with larger regular tubes is supplying the filament. I don't have a PSU lying around that can do the power needed to give the filament life.
Inducktion, Sun Jan 17 2016, 05:47AM
my worry with going with larger regular tubes is supplying the filament. I don't have a PSU lying around that can do the power needed to give the filament life.
Re: Making an ultra high frequency Tesla coil...
Hazmatt_(The Underdog), Sun Jan 17 2016, 05:52AM
You can't find or wind a filament transformer?
Maybe even a beat-up ATX power supply that you can hack for 6.3V?
The filament supply is probably the easiest part of lighting the tube.
A MOT pretty much has it covered for a 572B.
Hazmatt_(The Underdog), Sun Jan 17 2016, 05:52AM
You can't find or wind a filament transformer?
Maybe even a beat-up ATX power supply that you can hack for 6.3V?
The filament supply is probably the easiest part of lighting the tube.
A MOT pretty much has it covered for a 572B.
Re: Making an ultra high frequency Tesla coil...
Sigurthr, Sun Jan 17 2016, 08:38PM
For the 811A you can use these:
Or you can do what I do for filament supplies. Grab a 10A buck smps off of ebay and run it off a cheap chinese ac-dc 12v or 24v converter.
Sigurthr, Sun Jan 17 2016, 08:38PM
For the 811A you can use these:
Or you can do what I do for filament supplies. Grab a 10A buck smps off of ebay and run it off a cheap chinese ac-dc 12v or 24v converter.
Re: Making an ultra high frequency Tesla coil...
Inducktion, Sun Jan 17 2016, 09:37PM
Alright;
I bought a Type 803 vacuum tube for $13. Seemed like a much better deal than trying to get a 811A, plus it has higher plate dissipation.
I'll look around for a filament supply.
Inducktion, Sun Jan 17 2016, 09:37PM
Alright;
I bought a Type 803 vacuum tube for $13. Seemed like a much better deal than trying to get a 811A, plus it has higher plate dissipation.
I'll look around for a filament supply.
Re: Making an ultra high frequency Tesla coil...
Hazmatt_(The Underdog), Mon Jan 18 2016, 04:34AM
Sounds like a good choice. You can run it hot without complicated ventilation, it has plenty of headroom on the B+ side, and lots of power handling.
Hazmatt_(The Underdog), Mon Jan 18 2016, 04:34AM
Sounds like a good choice. You can run it hot without complicated ventilation, it has plenty of headroom on the B+ side, and lots of power handling.
Re: Making an ultra high frequency Tesla coil...
Inducktion, Tue Jan 19 2016, 12:55AM
So I modified a SMPS I had lying around. It was originally rated for 5 volts at 8 amps.
I changed some resistors (and disconnected the OVP) and got an adjustable voltage from 7 to about 10.50 volts. Perfect for the 803.
Inducktion, Tue Jan 19 2016, 12:55AM
So I modified a SMPS I had lying around. It was originally rated for 5 volts at 8 amps.
I changed some resistors (and disconnected the OVP) and got an adjustable voltage from 7 to about 10.50 volts. Perfect for the 803.
Re: Making an ultra high frequency Tesla coil...
Perezx, Wed Jan 20 2016, 09:48AM
SSTC is almost incompatible with 20 MHz. You will just waste energy opening and closing your fet.
Try VTTC basefeed, I've described one here and it is staying close to 20 MHz.
And yes, have to mention: you wont get any streamer at this frequency. Just a bunch of flame.
Perezx, Wed Jan 20 2016, 09:48AM
SSTC is almost incompatible with 20 MHz. You will just waste energy opening and closing your fet.
Try VTTC basefeed, I've described one here and it is staying close to 20 MHz.
And yes, have to mention: you wont get any streamer at this frequency. Just a bunch of flame.
Re: Making an ultra high frequency Tesla coil...
Inducktion, Thu Jan 21 2016, 02:44AM
Yeah I'm realizing this. I tried making a starting oscillator at 20 mhz, and attempted to square it up with a high speed schmitt trigger.
Yeah no it wasn't happening. Still very much a sine wave and not pretty.
Guess vacuum tubes it is!
Inducktion, Thu Jan 21 2016, 02:44AM
Yeah I'm realizing this. I tried making a starting oscillator at 20 mhz, and attempted to square it up with a high speed schmitt trigger.
Yeah no it wasn't happening. Still very much a sine wave and not pretty.
Guess vacuum tubes it is!
Re: Making an ultra high frequency Tesla coil...
Perezx, Fri Jan 22 2016, 02:35AM
Found a diagram of HF SSTC. This was working at approx. 10 MHZ. However, it is pretty hard to tune. VTTCs are way better :)
Perezx, Fri Jan 22 2016, 02:35AM
Found a diagram of HF SSTC. This was working at approx. 10 MHZ. However, it is pretty hard to tune. VTTCs are way better :)
Re: Making an ultra high frequency Tesla coil...
hen918, Fri Jan 22 2016, 02:05PM
Any reason why the "MOSFET" is labelled VT1?
hen918, Fri Jan 22 2016, 02:05PM
Perezx wrote ...
Found a diagram of HF SSTC. This was working at approx. 10 MHZ. However, it is pretty hard to tune. VTTCs are way better :)
Found a diagram of HF SSTC. This was working at approx. 10 MHZ. However, it is pretty hard to tune. VTTCs are way better :)
Any reason why the "MOSFET" is labelled VT1?
Re: Making an ultra high frequency Tesla coil...
Inducktion, Sat Jan 23 2016, 09:06AM
i got the tube today and i'm seriously wondering how it can handle 125 watts of plate dissipation..... i'll still certainly try it out though.
Inducktion, Sat Jan 23 2016, 09:06AM
i got the tube today and i'm seriously wondering how it can handle 125 watts of plate dissipation..... i'll still certainly try it out though.
Re: Making an ultra high frequency Tesla coil...
Perezx, Mon Jan 25 2016, 04:00AM
Forgot to advise: secondary of such high frequency TCs should be made heatproof. Best way is to wind them on glass/ceramic
Perezx, Mon Jan 25 2016, 04:00AM
hen918 wrote ...
Any reason why the "MOSFET" is labelled VT1?
Any problems, officer? :)Any reason why the "MOSFET" is labelled VT1?
Forgot to advise: secondary of such high frequency TCs should be made heatproof. Best way is to wind them on glass/ceramic
Re: Making an ultra high frequency Tesla coil...
Inducktion, Mon Jan 25 2016, 05:30AM
Ok....so i did more research and there's no way in hell this tube is an 803. It's about the same size as an 811A, and looks to be an 807 instead.
Edit;
I got a refund but i still get to keep the tube, lol. Weird.
Inducktion, Mon Jan 25 2016, 05:30AM
Ok....so i did more research and there's no way in hell this tube is an 803. It's about the same size as an 811A, and looks to be an 807 instead.
Edit;
I got a refund but i still get to keep the tube, lol. Weird.
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