Tesla Coil Build Thread

[Sigurthr]

Secondary and topload size correlate directly to operational frequency, which has an effect on the transistors you use to drive the coil. Lower frequency = less stress on the transistors. IGBTs are no good over about 90kHz (which is very low and requires a very large topload and secondary). A six inch tall coil will probably run around 900kHz+ depending on topload considerations. At that frequency you have to use MOSFETs and would probably have to stick to under 400W of power.

The simplest drivers are based on 74HC14 (a basic Schmitt trigger) feeding a pair of gate drive chips like the ubiquitous UCC37321 and UCC37322. The output from the gate drive chips drives a GDT (gate drive transformer) which then couples the output to the half/full bridge. Steve Ward's SSTC-5 ("mini SSTC, not "micro"!) is the best place to get an overall idea, just google it. There are lots of substitutions and improvements to be made on Steve's schematic, but they all involve harder to find and more expensive parts, even if the over all complexity and parts count is reduced (like in my version).

Here's my schematic; I've tested it up to 2MHz with no reliability issues, and it will handle 3kW with enough cooling.
http://img818.imageshack.us/img818/4581/tqdn.png

 

[benmwv]

Awesome! Thanks. This is exactly what I was looking for, a schematic recommended by someone here I can trust There are so many online I didn't know what to use.

So do I have to match capacitors or primary to the coil/topload or does the feedback antenna take care of all the resonation stuff?

With my first one I'm not looking to make anything really powerful, just something small with a little breakout, able to light up flouros, draw some short arcs. I dont think I will have a problem with staying under 400W. I dont know how much that will do as far as tesla coils go, but it seems like quite a bit of power. I'm thinking like 100W max for a little desktop coil? Or do you think it will take more power than that?

 

[DashApple]

No capacitors need to be matched to the secondary / topload in a SSTC , the antennae is used to pick up the secondary EMF feild signal and drive the primary at the secondary resonant frequency .

Primary turns in a SSTC only really determines the current draw from the mains ,

The Caps on the half bridge are used to make the primary not show as a short ( DC blocking Caps )

 

[Sigurthr]

What Twirly (Ion) said ^.

You can light up fluoros with as little as about 100mW (google the "slayer exciter" - note; design does not scale up). 100W @ 120V mains input is a bit less than 1A of current draw. Magnetic field strength (amps x turns) is important part of tesla coil design, and while you can make a coil of any power level, only 1A is not a lot of current (field strength when applied to reasonable amount of turns in primary). So there tends to be a sweet spot of around 3A and up of primary current for mains fed 120V coils. 3A x 120V = 360W, so I'd aim for around there. Typically this means using 8, 10, or 12ga wire rated for at least 80degC @ >300V wrapped around the base of your secondary coil or around a pvc pipe slightly larger than your secondary. Play around with turns until you get the right amount of current draw. If you use 10ga on a 4" pvc pipe I would guess that 15-20 turns would be about right.

How closely the primary and secondary are coupled to eachother also determines the current draw, so raising the primary to be centered more increases the current. It also does bad things once the primary's top turn reaches past about 3/8 the height of the secondary so generally this method can only be used to increase current up to a certain point before secondary stresses get too high and you risk flames. It is a goo way to lower the current though; lower the primary in relation to the secondary (or raise the secondary) until the current draw you want is reached.

Btw, while my schematic is pretty damn solid as far as drivers go, it can die an early death if the antenna is too long or too close to the secondary's topload. The input protection diodes can only handle about 75V, so a good test to determine how close the antenna can be at maximum is to set the antenna as far away as you can where the coil still runs and then use an ac multimeter as a voltage probe. Ground one probe and wave the other around as a probe. Digital meters don't like TCs so grab a cheap hardware store analog one. The directions I put in the schematic for the antenna are generally accepted as foolproof for antenna placement though.

 

[benmwv]

Awesome! I was hoping it would work this way.

I have noticed most sstc seem to use a cylindrical primary, instead of the flat/cone shape on a standard TC. Any particular reason for this? I think its something to do with coupling but I really am not sure.

@sig

Hmm ill have to build that slayer exciter. It will be the precursor to my sstc I suppose. Thanks

I do have an oscilloscope, will that help with some tuning or antenna placement or anything?

I think I will probably be keeping the primary turns high then. Could I use a half wave rectifier pulling twice the current to make it perform better at a lower power level? Instead of high voltage low current.

 

[Sigurthr]

You can use half wave rectification instead of full wave but generally it is not advised for feedback based coils (anything other than a precision oscillator based driver). The problem is that under about 10V on the DC Bus, which happens near every zero crossing of the mains line, the feedback signal to the antenna or current transformer is lost. During this time the gate drive circuitry defaults to parasitic oscillations in the multi-MHz range. This causes massive heating in the gate drive and in the bridge transistors and is a common mode of failure for first time coilers. The solution is to add enough bus capacitance to prevent the bus voltage from dropping below 10V at all times. Using half wave rectification doubles the capacitance needed. In the end you almost always do better with full wave rectification and minimal capacitance, in terms of reducing average power.

Scope will help immensely with cleaning up the gate signal and phasing the GDT properly, that's about it.

Re: vertical instead of horizontal primary coils: the reason for this is coupling factor. Flat spiral and conical primaries have a lower coupling factor due to less of the magnetic field intersecting the secondary. These are used on SGTCs and DRSSTCs as there is a time component of many tens of cycles where the primary resonant circuit has to ring up and down a higher voltage than the bus provides. If you increase the coupling above about 0.18K you get racing sparks as energy is pumped into the secondary faster than it can receive it. In a SSTC there is no ring up or ring down at all so you benefit from pumping as much energy as you can as fast as possible into the secondary. I've done some experiments with super high coupling (0.6K) and found it to be a massive performance increase, but generally above 0.35K there is so much eddy current in the secondary that it will self destruct. That and you wind up with major dielectric breakdown issues between the turns and the two coils.

FWIW; I never could get the damn slayer exciter to work. Bipolar technology does not like me for some reason.

I'd recommend these as starting points:
-secondary: 3" PVC coliform 7.5" long with 5.5" of 30ga winding (1.5" of bare form on bottom, 1/2" at top.
-primary: 4" PVC coliform with 18turns of 10ga finely stranded copper wire rated for >300V. Start the winding approximately 1" below the level where secondary winding starts.

Keep primary leads from bridge to the primary coil as short as possible, and do not twist them together. Make all connections on the bridge as short as possible with as thick of wire as possible, 10ga or thicker. Twist the leads going to and from the GDT into pairs corresponding to the individual windings and keep them as short as possible. Keep the bus lines between the rectifier and the bridge as short as possible and place the bus cap(s) as close to the transistors as possible. Leave at least 1cm of space between all high side connections (gate resistor and heatsinks, etc) to prevent arcing. Use at least 14ga wire for the Secondary ground connection, and don't connect it to the negative bus or to the negative/ground of the driver. (I did the last part once and discovered that I could NOT turn off the coil by unplugging it! It was running off of leakage/induction/antennae currents between the ground line and the unisolated mains Neutral line. It was hilarious but scary. I switched it off and it stayed on so I yanked the plug and it still stayed on!)

 

[MrMcChicken]

Looks really great! Could you draw up a quick schematic of your gate side subsection? I've never heard of zeners being used to clean up the gate signal. Most zener diodes are considered too slow for common TC frequencies. Judging by your secondary size you've got to be around the 400kHz+ neighborhood.

Really makes me want to finish my mini halfbridge SSTC. I've got the secondary, toroid, GDT, and a decent driver (but I really -should- make a better one) all done for nearly a year now. I just need to get around to making a new primary and bridge.

Hi, sorry for the late response.
Here's the shematic i use:

I have changed the MOSFETs. Instead of IRF 740 I am using IRFP 460. I added some zener diodes too, to keep them a bit colder.

Maybe I increase the capacitors a bit to get more power because the halfbridge stays too cold :eg:

 

[Sigurthr]

Thanks! Very interesting! So you've got active clamping every cycle. Just about every design out there uses zeners with a higher Zener knee voltage than you ever expect to see across the gates, this makes them work like TVS diodes. Essentially they're invisible unless something goes wrong. But yours are actively waveshaping, bravo!

 

[MrMcChicken]

Glad you like it .
I've read that zener diodes improve the signal quality a lot but they will get pretty hot too.
Actually ther isn't such a high power so I think they will survive pretty well .
Pleas notice, that this isn't my shematic.
I got if from here: SSTC Solid State Tesla Coil.

I've also measured the frequency of the coil.
Here's a pic (1V/div - 0.5µs/div) :

The frequency isn't stable, it drifts when ther are different things laying arround or if it's warmer or colder around the coil.
My scope stood about 1,5m away.

 

[Sigurthr]

That looks to be about 150KHz if my math is right. Lower (slightly) than my big coil, very nice! I wonder how your got such a low f0 because that secondary looks pretty small. What gauge wire is your secondary?

 

[MrMcChicken]

Are you sure?
F=1/T so this would be 1/0.0000035s = 285714.something Hz.
I use 0.15mm wire. I thing this should be almost like Gauge 35.

SORRY WRONG PICTURE. Please exuse.
The setting in this picture was 0.5us/div.
:yabbem::yabbem::yabbem:

 

[DashApple]

Nice gate signals , Cool idea using the Zeners , When I build mine I used 20V zeners and adjusted the resistor value to get the gate drive signal I wanted .

I wonder , with a 1.2Kv Rated IGBT in the tubes Cathode connection of my VTTC running a full wave smoothed supply ( 5Kv DC ) then PWM the IGBT to play music from the TC arc .

The tube should have most of the voltage across it so the 1.2Kv IGBT should survive

* I think *

Sig , Im goning to be brave and try a 80 - 90 KHz SSTC using a 200A/600V IGBT half bridge brick I have , But im stumped on how to attach the snubber , Ive seen people put one sunbber across each IGBT , but as this brick only has 3 connections would the sunbber go accros the + / - Rail ?

 

[Sigurthr]

Are you sure?
F=1/T so this would be 1/0.0000035s = 285714.something Hz.
I use 0.15mm wire. I thing this should be almost like Gauge 35.

SORRY WRONG PICTURE. Please exuse.
The setting in this picture was 0.5us/div.
:yabbem::yabbem::yabbem:

Yep that'll do it.

Nice gate signals , Cool idea using the Zeners , When I build mine I used 20V zeners and adjusted the resistor value to get the gate drive signal I wanted .

I wonder , with a 1.2Kv Rated IGBT in the tubes Cathode connection of my VTTC running a full wave smoothed supply ( 5Kv DC ) then PWM the IGBT to play music from the TC arc .

The tube should have most of the voltage across it so the 1.2Kv IGBT should survive

* I think *

Sig , Im goning to be brave and try a 80 - 90 KHz SSTC using a 200A/600V IGBT half bridge brick I have , But im stumped on how to attach the snubber , Ive seen people put one sunbber across each IGBT , but as this brick only has 3 connections would the sunbber go accros the + / - Rail ?

Yep across the DC Bus rails. You can put them across the Drain/Source / Collector/Emitter but you might get unwanted resonances.

 

[DashApple]

Thanks , Well for a secondary using 0.145mm Wire ( 35 WAG ) and with a topload of 10pF , to get 85 kHz resonance it needs to be 6 inches in diameter and 12 inches long .

So a fat short SSTC secondary , Now if I can find a ceramic tube with them dimensions

 

[Hydroas]

Nice Coil but this isn't 740kv. You can Calculate arround 1mm/1-3kv @ 50-60hz, but just up to 10kv.
By Teslacoils it is completely different. The Streamers have arround 12cm which is at 1Mhz may 50kv.
But I still like ist

 

[benmwv]

You guys know of any SSTC schematics using SCRs?

Reason I ask is that my dad does electronics work on locomotives and he got me these huge SCRs they use for I guess regulating the power production. I'm sure you guys know a locomotive produces TONS of power so these things can handle a lot.

They are rated for 1800v, and 1300A average with max of 8000A at a 50hz rep rate. I have been saving these for a BIG project since just one of these can switch a maximum average of 2,340,000 Watts (2.34 MEGA Watts) of power! And they come in pairs on some big ole bus bars.

Im thinking like a MOT with a very very large filter cap, then an inductor feeding between that big cap to a much smaller high amperage cap, using the SCR to repeatably dump the small cap into the primary. (I assume the primary has enough inductance that it would not exceed the 8000A rating? I will ahave to do some calulations and figure out how many turns I need to be sure.) The inductor between the caps should be sufficient to limit current flow from the big cap to the small one and the primary while the SCR is on so that only the small cap dumps through the primary. This big SCR needs a couple hundred ma running through it to stay on, so it should easily turn off while the small cap charges. Basically this would work like a SGTC, since SCRs have such a similar function to spark gaps, except they are triggered.

Does that sound plausible?

I just don't know how to make a circuit that will switch it at the right times for resonation. Could I use the same antannea feedback circuit as for mosfets?

This would be like stage 3 of my project. First I must tackle the slayer exciter and the desktop sstc.