Small-ish 3D printed SGTC via cheap ZVS flyback build, humbly asking a couple ?s

[Michelle_]

Good afternoon,

I've always wanted to build a tesla coil and after having built a tiny slayer exciter (I'll post it later) I'm moving up the food chain and wish to build a simple SGTC.

I bought a cheap ZVS/flyback to get off the ground with:

https://www.amazon.com/Driver-Module-Tesla-Ignition-Generator/dp/B0CG9VX1LK

For now I think I want to run it at 12V and 100W to keep the power levels sane, and run it with a static spark gap, I can bump the voltage up to 24V later if I want to but I might move on to a SSTC instead and give this one away - another reason I don't want it to be too powerful.

1.) Question: Are there any pitfalls to this idea?
2.) Question: I know I need to shield the spark gap from view, would a faraday cage over the electronics do any good to reduce EMI?

Really, I'd be quite happy with performance similar to this one:

https://highvoltageforum.net/index.php?topic=2442.0

My strategy will be to optimize the electrical parameters of the circuit by accurately calculating the resonant frequency and building an appropriately tuned resonant capacitor.

3.) Question: Can I estimate the output voltage and current of the flyback accurately enough to do so, or should I try to measure it?
4.) Question: Is it possible to measure the resonant frequency of the coil and top load assembly using an oscilloscope? Do I need a signal generator for that? I know I can estimate it in javaTC, is that accurate enough?

Attached are some pictures of what I have so far. This is only the 2nd coil I've ever wound and although there are no overlaps, it's not 100% perfect (there's some unevenness) because of how I constructed the secondary form. I coated it in clear epoxy and am also not totally happy with how it turned out, but again I don't think it will cause any issues performance wise. That said I'm going to redo it and make a perfect version. The second coil form I printed is thinner wall (1mm versus the original 1.5mm) and is one single piece of PETG versus the last one that is two pieces bonded. I wound the first secondary by hand but the hopefully perfect version will be power wound using my winding jig I built from junk and old prototype parts. I'm also going to investigate means to improve the coating.

The secondary itself is 1000 turns of 32awg wire, 10" tall and tapered from 3" to 2". The top load will be covered with aluminum tape. I'm going to 3D print a coil form for a pancake primary once I have a specification to work from.

[alan sailer]

Question 2)

A faraday cage will eliminate radiated EMI but cannot eliminate conducted EMI for the tautological reason that
conducted EMI  goes down a cable that goes outside the cage... in other words, the cables leading up to your
spark gap will also eventually radiate EMI.

Personally I'd not worry about it unless you plan to operate the coil frequently and for long periods of time or
live next to a rabid HAM radio operator :-)

It's an interesting question to me how much grief the tesla community creates with our uncontrolled emissions...

Question 4)

http://www.hvtesla.com/tuning.html

I have used these technique a few dozen times. The only thing to be careful is that the resonance can be
so narrow that you can miss it. Having the JAVATC estimate can help you "narrow" this down.

Cheers.

[davekni]

1.) Question: Are there any pitfalls to this idea?

Sudden discharge of flyback transformer secondary due to spark gap can destroy transformer (or internal diodes).  Depends on internal construction.  Either add a Terry filter or have a few spare flyback transformers for experimenting.  Search for Terry filter and/or study this thread:
    https://highvoltageforum.net/index.php?topic=822.0
Above thread includes other cautions about unused flyback transformer pins, which might be relevant to your design.

3.) Question: Can I estimate the output voltage and current of the flyback accurately enough to do so, or should I try to measure it?

Can be estimated from simulation, but only if you know transformer characteristics (inductances, coupling factor, and saturation current).  May be easier/faster to measure.  At least you are starting with a ZVS/transformer pair that presumably is known to work together.  It is easy to end up with little power due to ZVS frequency too high or low for transformer (or other issues).

Hope your project is successful and a good learning experience.

[Michelle_]

Question 2)

A faraday cage will eliminate radiated EMI but cannot eliminate conducted EMI for the tautological reason that
conducted EMI  goes down a cable that goes outside the cage... in other words, the cables leading up to your
spark gap will also eventually radiate EMI.

Personally I'd not worry about it unless you plan to operate the coil frequently and for long periods of time or
live next to a rabid HAM radio operator :-)

It's an interesting question to me how much grief the tesla community creates with our uncontrolled emissions...

Question 4)

http://www.hvtesla.com/tuning.html

I have used these technique a few dozen times. The only thing to be careful is that the resonance can be
so narrow that you can miss it. Having the JAVATC estimate can help you "narrow" this down.

Cheers.

Thanks for the link. I think I'm going to just get a function generator, it will bother me if I don't know the resonant frequency with some certainty, I agree estimating with javaTC to get in range will be the way to go.

I'll not worry about EMI for the time being and focus on finding a way to cool and shield the spark gap.

[Michelle_]

1.) Question: Are there any pitfalls to this idea?

Sudden discharge of flyback transformer secondary due to spark gap can destroy transformer (or internal diodes).  Depends on internal construction.  Either add a Terry filter or have a few spare flyback transformers for experimenting.  Search for Terry filter and/or study this thread:
    https://highvoltageforum.net/index.php?topic=822.0
Above thread includes other cautions about unused flyback transformer pins, which might be relevant to your design.

3.) Question: Can I estimate the output voltage and current of the flyback accurately enough to do so, or should I try to measure it?

Can be estimated from simulation, but only if you know transformer characteristics (inductances, coupling factor, and saturation current).  May be easier/faster to measure.  At least you are starting with a ZVS/transformer pair that presumably is known to work together.  It is easy to end up with little power due to ZVS frequency too high or low for transformer (or other issues).

Hope your project is successful and a good learning experience.

Thanks for the link and the heads up. I will look into what a Terry filter is and read the thread. Fortunately these flybacks are cheap and readily available but nonetheless I want to do it right and have a reliable system.

I do have a fluke HV probe which goes to 40KV supposedly, so I can try that. It's unclear to me in javaTC what "Rated
mA" means - if that's input or output, and how to measure that if it's not pulling a load already. I'm going to dig into the calculations more.

[Michelle_]

WAIT A MINUTE I think I just realized something that was confusing me in javaTC?

So for feeding a tesla coil DC like I'm going to do, the capacitance value of the resonant capacitor is only related to its tuning with the primary coil (making the primary + resonant capacitor frequency match the secondary resonant frequency)? In other words if I were to change the input voltage over a range of say 12-24V, it would have no effect on the required capacitance value of the resonant capacitor, I would just have to make sure it can handle the peak voltage?

[MRMILSTAR]

WAIT A MINUTE I think I just realized something that was confusing me in javaTC?

So for feeding a tesla coil DC like I'm going to do, the capacitance value of the resonant capacitor is only related to its tuning with the primary coil (making the primary + resonant capacitor frequency match the secondary resonant frequency)? In other words if I were to change the input voltage over a range of say 12-24V, it would have no effect on the required capacitance value of the resonant capacitor, I would just have to make sure it can handle the peak voltage?

That is correct.

[Michelle_]

WAIT A MINUTE I think I just realized something that was confusing me in javaTC?

So for feeding a tesla coil DC like I'm going to do, the capacitance value of the resonant capacitor is only related to its tuning with the primary coil (making the primary + resonant capacitor frequency match the secondary resonant frequency)? In other words if I were to change the input voltage over a range of say 12-24V, it would have no effect on the required capacitance value of the resonant capacitor, I would just have to make sure it can handle the peak voltage?

That is correct.

Thanks for confirming. That explains a lot, looks like NSTs have AC output which explains some of the differences between those circuits.

[Michelle_]

RESULTS

This is interesting. I didn't actually count the number of windings but,

Depending on if my javaTC numbers are more nominal or based on what can be measured:

JavaTC estimated resonant frequency: 430-494KHZ (with top load)
JavaTC estimated resistance: 106-122ohm

Measured (with signal generator and oscilloscope) resonant frequency: 472.5KHZ (with top load)
Measured resistance: 107ohm

That seems pretty good right? I'm not sure how exact other people's results are but I feel better that things match fairly closely (it seems close to me?)

[MRMILSTAR]

That looks like within about 10% of prediction which is pretty good. Certainly close enough for initial tuning. Did you account for the tolerance of your capacitors?

[Michelle_]

That looks like within about 10% of prediction which is pretty good. Certainly close enough for initial tuning. Did you account for the tolerance of your capacitors?

I don't have any capacitors yet, I wanted to measure the output voltage of my power supply at 24v first (tomorrow) and then I think I will have enough information to specify them, as far as I understand it what I was going to do was:

1.) measure the actual secondary frequency
2.) Measure the actual output voltage
3.) Model a primary coil in javaTC and get a suggested capacitor
4.) Find a real one that's close to or slightly higher than the suggested value
5.) Re calculate the primary and tapping point, check coupling coefficient
6.) Buy capacitors and make primary capacitor with 10Mohm resistors to drain
7.) 3D print form for building pancake primary and wind primary


EDIT:

Fly back voltage = 10,000 at 10v in
24,000 at 24v in

Unloaded of course.

[Michelle_]

So, the two big resistors near the three output terminals start creeping over 60 degrees if I’m putting in more than 16v, (no fan yet obviously) does that seem wrong to anyone? The rest of the setup looks much cooler from my thermal camera. At 24v in the resistors kept heating up into the high 60s before I turned power off.

[NyaaX_X]

Hello, Here is my thinking.

First, At the common ZVS oscillator circuit, there're usually two resistors with bigger package that have around hunderds of resistance.
They, and the two of fast diodes and the resonant tank are the MOSFETs gate driving circuit. On the other hand, the two smaller ~10k resistors as the pull-down resistors, parallel to the zenor diodes (usually 12V).

So, if you increase the input voltage. This two driver resistors can pass more current for some of the more current pass through Zenor (guess 1)
And the MOSFETs gate signal is like 0V/12V (by zenor) ~square wave so the more voltage can constantly shift up Vin-12V the voltage across this two driver resistor (guess 2)
Then, I don't know for this kind of resonant circuit, if the Miller effect still plays a big part when the input power been increase (guess 3)

For your sample post SGTC. I was Lucky~ I only use a simple LCR meter to measure the L and C of primary coil, secondary coil, and primary resonant capacitors to approach the resonant frequency.

I think it is a more detailed sample way~

(1) Think of the shape of your secondary coil (it make you have to consider about the coupling), for a SGTC I think you need a longer secondary compares to DRSSTC for avoid the arc flash from the high voltage in the primary to secondary (SGTC tens of kV compares to DRSSTC hundreds V to kv).

(2) Assume the coupling factor like typical 0.1~0.3 and simulte them in SPICE to check how much inductance the primary need (The leakage inductance and capacitance make the resonant freq.)

Then make your circuit and tune them. For the resonant capacitor. I use MS15000V film-capacitor from EACO. I think CDE or WIMA or KEMET or other manufacturer are common on the market.

The current or the voltage as the leader in the primary resonant tank (Same resonant frequency, big L small C or big C small L). I don't know the actual different of their behavior to a SGTC.

By the way, https://www.youtube.com/watch?v=__IYgKmSxGM this was my goal.

[Michelle_]

Hello, Here is my thinking.

First, At the common ZVS oscillator circuit, there're usually two resistors with bigger package that have around hunderds of resistance.
They, and the two of fast diodes and the resonant tank are the MOSFETs gate driving circuit. On the other hand, the two smaller ~10k resistors as the pull-down resistors, parallel to the zenor diodes (usually 12V).

So, if you increase the input voltage. This two driver resistors can pass more current for some of the more current pass through Zenor (guess 1)
And the MOSFETs gate signal is like 0V/12V (by zenor) ~square wave so the more voltage can constantly shift up Vin-12V the voltage across this two driver resistor (guess 2)
Then, I don't know for this kind of resonant circuit, if the Miller effect still plays a big part when the input power been increase (guess 3)

For your sample post SGTC. I was Lucky~ I only use a simple LCR meter to measure the L and C of primary coil, secondary coil, and primary resonant capacitors to approach the resonant frequency.

I think it is a more detailed sample way~

(1) Think of the shape of your secondary coil (it make you have to consider about the coupling), for a SGTC I think you need a longer secondary compares to DRSSTC for avoid the arc flash from the high voltage in the primary to secondary (SGTC tens of kV compares to DRSSTC hundreds V to kv).

(2) Assume the coupling factor like typical 0.1~0.3 and simulte them in SPICE to check how much inductance the primary need (The leakage inductance and capacitance make the resonant freq.)

Then make your circuit and tune them. For the resonant capacitor. I use MS15000V film-capacitor from EACO. I think CDE or WIMA or KEMET or other manufacturer are common on the market.

The current or the voltage as the leader in the primary resonant tank (Same resonant frequency, big L small C or big C small L). I don't know the actual different of their behavior to a SGTC.

By the way, https://www.youtube.com/watch?v=__IYgKmSxGM this was my goal.

Thanks for sharing your thoughts.

Right now for my primary I have a pancake designed in javaTC.

The diameter of the bottom of my secondary is 3" and the ID of the primary is 5". I will probably make a short wall in between them for good measure. The primary OD is set to 7-8" depending on if I have 1/8" tubing or 1/4" tubing selected (I bought both). It ends up being around 7-8 feet of wire but I have more than that and will tap in. It's saying something like .8-1" between coils on the primary but I will also 3d print recesses/walls to help isolate the windings from one another but leave enough room for the tap.

My coupling coefficient is .15, I figured I can raise the secondary relative to the primary to lower it if needed?

For 5 turns (Seems like either size tubing) I am getting a .02uf primary capacitor, and for 4 turns a .03 uf primary capacitor. I figured I will go shopping and look for something in that range and adjust the turns of the primary to match what I can get.

Based on what I can see if I have capacitors in that range I can tap the primary and get directly in the range of the secondary's resonant frequency and have quite a bit of room for adjustment either way.

ALSO: I am reading that you're not supposed to run the ZVS/flyback with no load which could be why it was overheating? That would be good news because it would mean it is otherwise working correctly and the output voltage is high.

[Michelle_]

https://www.digikey.com/en/products/detail/kemet/C4CAYUD3220AA3J/6556360

EDIT: ordered some of these to try as something to start with.

[davekni]

So, the two big resistors near the three output terminals start creeping over 60 degrees if I’m putting in more than 16v, (no fan yet obviously) does that seem wrong to anyone? The rest of the setup looks much cooler from my thermal camera. At 24v in the resistors kept heating up into the high 60s before I turned power off.

Power resistors like these are designed to run hot.  200C surface temperature is common.  Only real concern is their heat warming adjacent capacitor.  A fan will help with that.

[Michelle_]

So, the two big resistors near the three output terminals start creeping over 60 degrees if I’m putting in more than 16v, (no fan yet obviously) does that seem wrong to anyone? The rest of the setup looks much cooler from my thermal camera. At 24v in the resistors kept heating up into the high 60s before I turned power off.

Power resistors like these are designed to run hot.  200C surface temperature is common.  Only real concern is their heat warming adjacent capacitor.  A fan will help with that.

200! wow, ok good to know. I have begun a habit of looking at everything with the thermal camera so I can catch problems before they start a fire lol.

[davekni]

I have begun a habit of looking at everything with the thermal camera so I can catch problems before they start a fire lol.

Yes, me too.  A very useful tool.  Even more useful with a closeup lens.  ZnSe lenses are available at reasonable cost (~$20) intended for CO2 laser output focusing.  I find 100mm focal length most useful.
After a while you will get used to ignoring resistor hot spots.

[Michelle_]

I'm not sure exactly how I feel about this, the proportions look a bit strange but the numbers all look good and I'm fairly sure there won't be any arcing between the primary coils or the primary and the secondary base junction lol. I'm going to have a lower deck beneath the primary holding deck that is on standoffs, and since it's so big (14" OD) I'm pretty sure I can fit all of the electronics in there including a PSU and a spark gap with a fan, etc... There's room to expand and add a bigger secondary for sure. Unfortunately I will need to split the primary holder into multiple pieces to print it but whatever. I'll probably add a vertical breakout point to the top of the toroid as well as a strike ring that will mount onto where the 5 standoff holes are, and ground it through one of the standoffs. I don't know if the primary needs to be as wide as it is or have extra turns (this has 7 full turns for potential tuning) but I'm afraid of the 24KV input voltage arcing across it so I have 1/2" spacing between primary windings with the depth of the winding hole also 1/2".

[alan sailer]

Looks great.

I'd wonder about making contact to the primary ring for tuning. From the top you have to get
down into that narrow canyon. From the bottom cutting holes...

Not a criticism just curious...

Cheers.