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Messages - Blin

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1
Things have improved quite a bit since my last post (partially thanks to a proper scope). Hopefully my experience will be useful for other people.

The RC snubber is very important. With a good scope, I used this spreadsheet to calculate the required values for R and C (instead of using random small values). After adding the calculated values to the circuit (for a damping factor of 1), the ringing stopped completely, and the current draw doubled, together with the spark length!

However I still couldn't get my interrupter to work. Or rather, it worked as long as the coil was off. When the coil was on, I just got a 0v output (due to interferences, I guess). I tried everything I could with no success, and eventually decided to go for a separate supply for the control circuit, hoping to solve the issue but also to later on use a higher voltage for the primary coil circuit.

The separate supply didn't change anything on the interrupter part (still a flat 0V signal), until I increased the voltage from 15V to 32V on the primary coil circuit. Somehow this made the interrupter work, I'm still unsure why. The control circuit runs at 12V.

So I now have quite reasonable sparks (I would say 4 to 5 cm), and I can play tones, which is nice. I got also a huge improvement in noise/interferences by grounding the circuit (using mains grounds): the sound is much clearer (and the sparks much sharper).

Next, I would like to try to change the coupling to get more current draw (and longer sparks of course), i.e. 1 turn only at the primary instead of 2, and try to place it at different heights. I'd like to also play with the PWM duty cycle to see if it changes anything (I'm using 50% duty cycle at the moment).

Thanks again for your great advices.

2
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.

3
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.

4
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.

5
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).

6
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.

7
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...).

8
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...

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