Author Topic: New to tesla coils, some questions before I start building a SSTC  (Read 1027 times)

Offline Blin

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Hello all!

Newbie in tesla coils (and in electronics) here, I want to build a SSTC (not a DRSSTC - I want to keep it as simple as possible for now). I think I understand most of the circuits around here, but I have some questions before I start building.

I plan to start with a single MOSFET SSTC to keep things simple (later I'll build a half bridge, then full bridge).

I've seen this design from E-Schmok and I want to build something similar, but I want to be sure I understand the purpose of some components in this circuit.



I got the interrupter part figured out already, my questions are about the power part:
* Can the power supply which powers the primary and MOSFET be a regulated switch mode power supply?
* What's the purpose of R13 and C23? Looks like a RC snubber?
* What's the purpose of D5 and D6? I guess it's to clamp the voltage to the MOSFET gate but 15v should be harmless for an IRFP540, so I'm wondering.
* Any reason to have 2 push-pull stages (Q5-Q6 and Q7-Q8)? Would a single one be enough?
* What's the purpose of C19 and C20?

I'm also a bit lost about the secondary design. I've seen different advices, sometimes contradictory. I've put some values in JavaTC and got a diameter, height and number of turns, but I don't want to make a secondary that won't produce sparks with my single transistor design. Is there a rule of thumb for secondary designs? How much does it depend on the power circuit?

The transistor I plan to use is a fcp20n60 (600V, 60A pulsed, RDSon=0.15) so not the best one out there, but I've got quite a few and I don't care if I blow them up.

Thanks for your insight, I still have a lot to learn...

Offline FilipŠebík

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #1 on: March 10, 2019, 08:21:47 PM »
Hello, to answer the questions first:
1. Yes, they can be driven/regulated using SMPS
2. Actually it is, so you are right
3. The zener diodes are there for gate protection and they should be used
4. Yeah it should be but the gain of just the BD139 and BD140 may not be high enough and can reduce the gate drive, which may not make the mosfet open fully but partialy and thus it will work in its linear region, in which losses are big, so you should use the first stage too
6. They are just used as a filters, to filter the voltage rail

The secondary could be any coil with 1000+ turns and it should be 1:5 width:Height for small coils under 30cm, 1:4 for bigger coils Under 1m and 1:3 for big coils 1m<
The fcp20n60 is ok to use here.

Offline Mads Barnkob

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #2 on: March 11, 2019, 09:46:17 AM »
Hi Blin and welcome to HVF

Filip answered all your questions, so I just give a word on the secondary design.

For a DRSSTC it is my opponion, showed with calculations that the smaller coils, the smaller W:H ratios is possible. I like 1:4 to 1:5 for DRSSTCs, depending on the power level, simply because the most power the better it is to have some distance to earth so all strikes are not just to ground. See table 3 and 4 here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/secondary-coil/

For a SSTC the W:H ratios should be smaller because a SSTC needs a much higher coupling to produce a fair amount of sparks. 1:2 to 1:3 for a SSTC is the optimal in my experience.
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Offline Blin

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #3 on: April 11, 2019, 10:18:07 PM »
Thanks so much for your answers.

I've been experimenting with the circuit and working on the PCB (which I'm all new to, so it's taken quite a bit of time). Now I need a good secondary...

Mads, I've read your guide, and decided to go for a 30 cm height, 8 cm diameter secondary and 800 to 1200 turns. Started looking for wire and then found a pre-wound secondary on ebay: https://www.ebay.com/itm/Teslaspule-32x7-5-cm-1000-Wdg-0-2mm-Tesla-Coil-Spule-TC-001-HVSHP/273279706078

Seems to be perfect for my usage, and to be honest I'd rather buy it pre-wound than do it myself. So I have a few more questions now :)
  • Is the secondary in the link above able to produce good sparks in my use case?
  • For the primary, I think I'll just use some insulated copper wire from an extension (rated 30 amps or so). Ok as well?
  • The grounding: I'd like to just use the earth connection from an extension. I suppose it's ok as well?

Thanks, and I'll post photos when I have something working (which might take quite some more time...).

Offline Blin

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #4 on: November 02, 2019, 10:57:50 PM »
Hi again,
So I've finally had time to put it all together, with a few modifications with respect to the original schema. It works but the performance is very underwhelming (I can only get very small sparks, less than 1 mm, when I approach a grounded metallic object).

I could use some help with troubleshooting. I've already checked and fixed some obvious defects, and here's where I'm at.

First, some differences with the original schema:
  • The primary voltage is 15 volts (my smoothing capacitors have a limit of 16v). It's powered by a lab power supply.
  • The control circuit voltage is 5 volts. I'm using a CD74ACT14 hex schmitt trigger instead of a CD40106.
  • I have only one push-pull stage, with a STA353 and a STC352 transistor pair.
  • There is no interrupter.
  • Secondary: height: 32 cm, diameter: 7.5 cm, ~1000 turns, 0.2mm wire, no topload.
  • Primary: 5 turns, diameter: 9.5 cm, placed around 1 cm from the base of the secondary.
  • For the snubber network I use a 100 ohm resistor and a 47 nF ceramic capacitor.
I've checked everything I could and all looks ok, so I can't understand why I don't get sparks. My power supply indicates a current of 100 to 150 mA (at 15V) which seems quite low.

The resonant frequency of the seconday is around 500 KHz. I've tried driving the mosfet with a PWM output from an Arduino. The best results I got was with 10% duty cycle at 60 KHz (2 to 3 mm sparks with a grounded metallic object). The power drawn was around 2A at 15V.

Here are some oscilloscope traces (sorry for the poor quality, I only have a small handheld oscilloscope). The time base is 1us, and the 0v line is the blue triangle on the right of the screen.

Probe at around 5 cm of the top of the secondary


MOSFET gate voltage


Secondary feedback



Output of the CD74ACT14



MOSFET drain (this one looks weird, but I'm not sure what a good signal looks like)


Any help is appreciated.

Offline Weston

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #5 on: November 03, 2019, 03:14:47 AM »
If I am reading the oscilloscope waveform correctly, the mosfet gate waveform scale is 1V/div? It seems that you do not have enough voltage on the MOSFET gate. That waveform is only ~5V peak and you want a higher voltage to turn it on, something like 12V. You can also see that the MOSFET is not turning on fully by the fact that the drain voltage on the mosfet never gets close to zero. If the mosfet is fully on it should be a low resistance and there should be relatively low voltage across it.

You said the control circuit voltage is 5V, does that include the push pull stage driving the gate? The push pull stage needs a supply voltage equal to what you want for your gate voltage (~12 - 15V). Additionally, the push pull stage does not provide any voltage gain, only current gain, so the input signal needs to be 12V and right now you are using 5V logic. If you want to keep using 5V logic you could add a common emitter amplifier (which would invert the signal but also provide gain).

Alternatively you could use a gate drive IC and do away with the totem pole. Something like this could be directly driven by the logic signal and drive the MOSFET gate faster than the push pull stage: https://www.digikey.com/product-detail/en/ixys-integrated-circuits-division/IXDN630CI/CLA374-ND/2623260

Offline Blin

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #6 on: November 03, 2019, 05:26:59 PM »
If I am reading the oscilloscope waveform correctly, the mosfet gate waveform scale is 1V/div? It seems that you do not have enough voltage on the MOSFET gate. That waveform is only ~5V peak and you want a higher voltage to turn it on, something like 12V. You can also see that the MOSFET is not turning on fully by the fact that the drain voltage on the mosfet never gets close to zero. If the mosfet is fully on it should be a low resistance and there should be relatively low voltage across it.
You're reading it right, indeed the MOSFET gate is turned on by the 5V circuitry. I didn't think this would be a problem since the datasheet of the FQPF9N50C that I'm currently using states that the gate threshold voltage is 4V maximum. But I neglected to look at the graph of drain current at a Vgs of 5V, and indeed, it's around 1A. That must be the issue, thanks a lot!

Quote
You said the control circuit voltage is 5V, does that include the push pull stage driving the gate? The push pull stage needs a supply voltage equal to what you want for your gate voltage (~12 - 15V). Additionally, the push pull stage does not provide any voltage gain, only current gain, so the input signal needs to be 12V and right now you are using 5V logic. If you want to keep using 5V logic you could add a common emitter amplifier (which would invert the signal but also provide gain).

Alternatively you could use a gate drive IC and do away with the totem pole. Something like this could be directly driven by the logic signal and drive the MOSFET gate faster than the push pull stage: https://www.digikey.com/product-detail/en/ixys-integrated-circuits-division/IXDN630CI/CLA374-ND/2623260

Thanks, I'm gonna look at different options; I think I'll get a gate driver IC, but I'll try to see if I have a schmitt trigger that supports 15V first. I'll update when it works (or doesn't).

Offline Mads Barnkob

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #7 on: November 05, 2019, 08:38:15 PM »
I am sorry that I missed your follow-up questions about the secondary coil, that was is good buy at a decent price.

What is the bandwidth of your oscilloscope? It looks like those Chinese DSxxx scopes that promises much more than they can uphold: http://www.e-design.com.cn/?product-247.html All your waveforms are either triangle or going into sinusoidal and it is simply because those scopes can only show sinus waveforms up to their "specifications". For the 200 kHz versions, I have seen it as bad as square waves starts to round at 20 kHz and are totally unusable at 50 kHz.

You would be better off with a old 20MHz analog oscilloscope from a HAM market at 5$, my best guess is that your 1 MHz DSO is not trustable above 50-100kHz when it comes to square waves or fast rise times.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline Blin

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #8 on: November 07, 2019, 10:28:38 PM »
Hi,

So I finally got some success, and indeed using a 15V driving voltage did the trick (thanks Weston!). I went the easy way and just replaced my 5V schmitt trigger with a one that accepts up to 18V at Vcc. I now have 2-3 cm arcs at 15V, the heating on the MOSFET is acceptable.

The only remaining problem is that the arcs sometimes just stop, but I'm pretty sure it's because of noise on the schmitt trigger. Grounding the unused inputs and outputs made things much better, and I'll put a decoupling capacitor which should further improve things.

As for my scope, I agree with Mads, it's not very trustable, but I've still got acceptable results which helped me debugging. I can borrow a 200 MHz scope from a friend should I really need it.

My next steps are:
* Make a topload. I'll try a styrofoam toroid with aluminum tape, or get some aluminum savarin molds. Not sure what is best - any advice is appreciated.
* The interrupter part.

Thanks again for the help.

Offline Weston

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #9 on: November 08, 2019, 05:23:48 AM »
Congrats on getting it working! You should post pictures of the coil.

This design is a free running oscillator. If something causes it to lose the signal it will stop oscillating and not produce any sparks. Does it stop oscillating when you are trying to draw arcs or is it completely random? Are you using an antenna or a current transformer on the secondary ground for feedback? It could also be be interference from the tesla coil causing issues with the power supply and making it cut out.

Some additional filtering / shielding may help it be more stable. If it's antenna feedback changing the antenna location may help. Some designs also very weakly couple in some sort of oscillator onto the feedback pin so the oscillations can start up reliably, an interrupter would also have the same effect. 

Toploads are not that important for small tesla coils and can make achieving breakout at low powers more difficult. But if you want to add one, both the options you listed are good. For non DRSSTC coils you generally need a breakout point so the smoothness of the topload is not that important. In the past I have taped together metal bowls or used drip pans from electric stoves. Nice spun metal toploads are also pretty cheap on ebay nowdays https://www.ebay.com/itm/Alloy-Top-Load-Tesla-Coil-DIY-SSTC-DRSSTC-parts/183667718619

Offline Blin

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #10 on: November 08, 2019, 10:38:04 AM »
It only stops oscillating when I try to draw arcs. The feedback is direct feedback from the secondary base to the schmitt trigger input (with diodes to clamp it), as in the circuit schema in my first post. There are a lot of interferences in the air near the circuit; I plan to shield it in a grounded ATX power supply case when I'm done. And indeed, an interrupter will fix this issue, so I'm not too worried about it.

About toploads, I hope it will improve performance because the arcs I get are still very short compared to the ones in these videos, where the same design is used:
and
/>
I can think of a few reasons why I get relatively small arcs with respect to these videos:
* No topload
* Running in continuous mode (without an interrupter)
* Running at 15V (the coils in the videos probably run at at least 30V, maybe more).
* Number of turns of the primary (I have 5 turns vs 1 turn in the second video)

I plan to work on all of these issues, but I was assuming (maybe incorrectly) that the lack of a topload was the main culprit. I'll also post pictures.

Offline Mads Barnkob

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #11 on: November 11, 2019, 03:52:25 PM »
Please do post some pictures / video of your setup, it will make it much easier to help you. It is not a beauty contest, so do not worry too much about how it looks, we all made coils that was more or less appealing to the eye :)

When you pull an arc, you are grounding or short circuiting the secondary coil, which is a very hard load on it. This pulls the frequency of the secondary LC circuit down, perhaps your circuit looses track and can not generate a new pulse and it stops. 

Pulsed mode will allow you to run with higher voltages than CW, so that is often a reason for getting longer sparks, we pulse the coils in order for the electronics to cool down between pulses of overload!

5 vs 1 turn is a huge difference! Calculate your primary reactance and you will see why you get smaller sparks. I actually put together a small SSTC design guide: http://kaizerpowerelectronics.dk/tesla-coils/sstc-design-guide/
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Offline Blin

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #12 on: November 11, 2019, 10:52:29 PM »
So here are some pictures (finally!).

First a general view of the setup (note that I went from 5 turns to 1 turn for the primary - it did improve things a bit but not by much; drawing now 1.7 amps at 15V instead of 1.3 amps previously):


The "shields" are from ATX power supplies. They are grounded to the mains ground.

Close-up of the primary and feedback:


The arcs (not that I had to put a weight on the wire, otherwise it starts moving back and forth, and I couldn't get an acceptable picture):



And finally the annotated circuit. Note that it's mainly "mid-air" soldering for now. It looks bad but all the connections are tested:


I'll gladly provide more pictures of specific parts if requested. The tank capacitors are 2200uF total.

I wasn't successful with the interrupter yet (hence why it's not in the picture; it's on a breadboard for now). I am using a 5V PWM signal, I use a comparator to "step it up" to 16V, which works perfectly fine when the coil is off. But with the coil on it produces rubbish. It's probably because of interferences on the +15V line, but decoupling caps don't seem to be able to get rid of them.

Offline Weston

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #13 on: November 13, 2019, 07:06:38 AM »
I don't think I have pictures of my first tesla coil, but I think everyone's first tesla coil looks roughly the same. Congrats on getting it working!

As a simplified explanation, the inductance of the primary is determined by the area inside the loop of wire while the energy coupled to the secondary is determined by how much of that area is occupied by the secondary. The long wires that are far apart from each other as they go from the primary to your circuit are part of that. Those wires add inductance but do not contribute to energy being coupled to the secondary, reducing performance. Try twisting / taping the two primary wires together as they go between the primary turn(s)/coilform and your circuit, this should decrease inductance and increase power. You might want to increase the turns to more than one initially to test this to not blow anything up as it will increase the current draw.

Those videos with longer sparks used a higher supply voltage. Power will roughly go as the square of the voltage. They also seem to have a primary with closer coupling.   

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Re: New to tesla coils, some questions before I start building a SSTC
« Reply #13 on: November 13, 2019, 07:06:38 AM »

 


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