Author Topic: A few (stupidly basic) questions  (Read 556 times)

Offline curtislemay

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A few (stupidly basic) questions
« on: March 29, 2020, 09:32:01 PM »
I've been scared of building a SGTC because I don't want to play with unregulated mains. Is this a reasonable fear? I have a high-quality spark gap that I found in the laboratory waste bin at my university, a microwave transformer, and the capacitor from the same microwave. Are SSTC's safer?

In an SSTC, how does the current end up so high, if we're sending mains into an inverter, wouldn't the maximum current be 15A? What am I missing here?

My EE friend is worried I'll be breaking the law with stray RF. Is this a concern, and should I build some sort of faraday cage around the circuit to minimize stray RF?

For an SSTC, can I use my DC supply (up to 60V/30A) instead of using AC mains and a bridge to feed the primary circuit?

I have two pre-wound coils that I also scavenged from laboratory ewaste bins, as well as a functioning variac, an oscilloscope, dozens of large capacitors, and some extremely nice heat sinks. I'd like to use as many of these as possible, but if I need to buy components (such as diodes and igbt's) I absolutely will.

Offline davekni

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Re: A few (stupidly basic) questions
« Reply #1 on: March 30, 2020, 02:23:23 AM »
Curtis,

Line voltage can be dangerous, but microwave oven transformers (MOTs) are MUCH more dangerous.  A man died here in Wilsonville a few years ago using an MOT for burning lichtenberg figures, accidentally touching the wires.  (I made an SRSGTC using two series-connected MOTs, 2.5kW average power.  That's my toy that scares me the most to run.)

Yes, SSTCs are generally safer.  And, yes, you can run one easily from 60V 30A - that's plenty of power.  Just requires higher current FETs or IGBTs, larger DC blocking capacitor, and fewer turns on the primary.

Can't comment much on RF regulations, which generally start at 30MHz for radiated, lower for conducted down line cord.  I suspect most Tesla Coils violate such regulations, at least when making strikes to a ground target.

If running with an interrupter, so pulsed operation, it's quite possible to have higher peak currents while keeping the average under 15A.

If you are using a bunch of scavenged parts (which is great), the first step is to measure them.
David Knierim

Offline MRMILSTAR

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Re: A few (stupidly basic) questions
« Reply #2 on: March 30, 2020, 05:31:27 AM »
I wouldn't worry too much about spurious emissions from Tesla coils. Yes its true that they all probably emit a lot of interference. However, you will not be operating a Tesla coil that much. It really won't be on long enough to annoy people. I've never heard of anyone being cited by the FCC for spurious emissions from a Tesla coil. Another thing to consider is that the devices that would annoy people the most if they exhibited interference are radios and TVs using antennas for reception. These aren't used much anymore, at least at home. Everything to your house is mostly over cable now.

I've been operating a 6.5 KVA pig-powered SGTC for several years and I have never received any complaints about interference. But then again, I only operate it several times per year.
« Last Edit: March 30, 2020, 09:25:56 PM by MRMILSTAR »
Steve White
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Offline Mads Barnkob

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Re: A few (stupidly basic) questions
« Reply #3 on: March 31, 2020, 08:54:35 AM »
Welcome to HVF :)

Safety
In the high voltage hobby, I find it much more life prolonging to view everything as live if it is not grounded. This way there is no questions about what is safe, what can I touch if the voltage is low enough, if its isolated, if its floating, one hand on the back etc.

Accept that everything is dangerous, even low currents at seemingly low voltages can stop your heart if it hits you at the right time in the cardiac cycle.

Adapt your work habits around this philosophy and you will never find yourself in a dangerous situation. Learn to make things safe by design. Now I do not want to sound as a saint, a quick glance on my website shows I have not always done this. But I have learned a lot and still have a lot to learn about safe by design.

EMI... Most of the energy should get dissipated in the spark itself, as it is your load. I have experienced that a 300 kHz SSTC can lay down ADSL modems, as the upload band of ADSL is just in that range. I also wrote some about all this in this article: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/grounding-circuit-protection-and-emi/ which should answer all your questions about EMI.

SSTC
A SSTC is not directly drawing the current from mains to the primary coil, you have a DC bus capacitance to deliver peak current in the middle. That being said, the primary circuit current is close to what you can pull fro your mains, you still have to charge those capacitors. With interrupters you are able to deliver higher currents, but maybe only 200 times a second, this is done to get around the mains power limitations and to stress the switches less to give them time to dissipate the switching losses.

I wrote a short SSTC design guide here: http://kaizerpowerelectronics.dk/tesla-coils/sstc-design-guide/

I have only built mains powered SSTCs as I find the low voltage versions to be harder to get to work and they also seem to have lower spark-length-efficiency, I am a sucker for long sparks :)

It is good that you have a variac, oscilloscope and some parts. I really recommend that you build something like the Steve Ward Mini SSTC, it can both be half- or full-bridge. It is simple, few parts and it will also explode once in a while due to slack antenna feedback control :) But once you get some high voltage experience with a SSTC, you can easily move into DRSSTC topologies and gain the benefits of stricter control.

A variac is crusial to slowly ramp up voltage, notice if there is sparks or flash-overs where there shouldn't be, then go to full voltage. Once the Tesla coils bridge explodes, repair it and turn the variac 20% down :) Always keep in mind that much of the Tesla coil design theory is with +/- 3dB due to many assumptions and experimental models.

Start out with some cheap IRFP460 MOSFETs (or a better more modern alternative with about the same ratings). Build a SSTC: http://kaizerpowerelectronics.dk/tesla-coils/kaizer-sstc-ii/

If you can afford it, get a differential probe, important tool to have when working with inverters. To avoid damaging your oscilloscope or short circuiting your inverter through your oscilloscopes ground path.   
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline davekni

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Re: A few (stupidly basic) questions
« Reply #4 on: April 01, 2020, 01:32:54 AM »
Mads,

I'm curious about your line: "I have only built mains powered SSTCs as I find the low voltage versions to be harder to get to work and they also seem to have lower spark-length-efficiency, I am a sucker for long sparks".  If the power levels are equivalent and the part impedances are all scaled appropriately (larger capacitors, lower inductances, etc.), what makes the difference?  The best guess I'd have is that wiring inductance through the primary circuit (H-Bridge to primary coil) isn't reduced enough.  Low impedance would require twisted pairs and tight layout.  Do you have other possible reasons?

Thank you.
David Knierim

Offline John123

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Re: A few (stupidly basic) questions
« Reply #5 on: April 02, 2020, 11:46:07 AM »
What about some kind of ballast to limit the power during the first few runs, that's what I normally do. Might have to adjust the spark gap to compensate but at least then any bangs are limited in the event of failure.

A heater in series with the mains could work along with a MOT primary and its secondary side shorted.

Offline Mads Barnkob

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Re: A few (stupidly basic) questions
« Reply #6 on: April 02, 2020, 02:53:24 PM »
Mads,

I'm curious about your line: "I have only built mains powered SSTCs as I find the low voltage versions to be harder to get to work and they also seem to have lower spark-length-efficiency, I am a sucker for long sparks".  If the power levels are equivalent and the part impedances are all scaled appropriately (larger capacitors, lower inductances, etc.), what makes the difference?  The best guess I'd have is that wiring inductance through the primary circuit (H-Bridge to primary coil) isn't reduced enough.  Low impedance would require twisted pairs and tight layout.  Do you have other possible reasons?

Thank you.

I wish I had the time to do the math, so here I just try with some quick figures. http://kaizerpowerelectronics.dk/tesla-coils/sstc-design-guide/

60VAC ~80VDC, around 4 times lower than 320VDC.

Removing 6 turns, so we are down at 2 turns. 25% less inductance and we get the same primary reactanse. 16 Ohm.

But then primary current is still only 5A instead of 20A, since voltage is still lower, so we need to lower the primary reactanse further, but then the frequency rise again. Already being at 1MHz, I think its no longer a optimal route to follow. At least not to expect same performance as with mains powered.


The above was wrong for a SSTC as it assumed secondary circuit to change frequency.

« Last Edit: April 03, 2020, 02:21:56 PM by Mads Barnkob »
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Offline Uspring

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Re: A few (stupidly basic) questions
« Reply #7 on: April 02, 2020, 08:41:49 PM »
Beside higher resistive losses at lower operating voltages, you'll at some point run out of primary turns when trying to keep power levels the same. Turn number is roughly proportional to the input voltage for a given power level.

Offline davekni

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Re: A few (stupidly basic) questions
« Reply #8 on: April 02, 2020, 08:58:12 PM »
Mads,

Isn't SSTC frequency determined by the secondary and top-load?  So the 250kHz example in your design guide would remain constant, not go up to 1MHz.

One point I forgot to mention for scaling voltage: primary geometry.  To scale properly, the primary winding needs to remain the same size.  For scaling from 320V to 80V, the primary in your example needs to be four paralleled 2-turn windings to fill up the same vertical height.  To keep wiring inductance low enough, each two-turn winding would need to route down to the H-bridge separately, ideally as four twisted pairs.

The above primary winding should have 1/16th of the inductance (same geometry, 1/4 turn count), or 635nH, for 1.0 ohms at 250kHz.  That's 80 amps at 80 volts, for the same reactive power as 20 amps at 320V.

For the above comparison, I've used the calculations as they are on your SSTC design guide.  However, the actual peak currents are higher, since square waves have more area than sine waves.  The current through an inductor with square-wave voltage is a triangle wave.  For 250kHz and 320V, the coil sees +320V for 2us.  That ramps the current through a 10uH inductor from -32A to +32A (640uVs).  The peak current is 32A.  Of course, this is with ideal assumptions - no coil resistance and no H-Bridge switching transition time.

Separately, I like the nice picture in your SSTC design guide showing the tight coupling of primary coil to the bottom of the secondary coil.  That prevents that lower secondary portion from building up even higher voltage due to resonant current, since that voltage is tightly coupled to the primary's fixed voltage.  (I had been wondering about how the physical proximity didn't cause breakdown issues, until realizing that it also clamped the lower secondary voltage.)
David Knierim

Offline Mads Barnkob

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Re: A few (stupidly basic) questions
« Reply #9 on: April 03, 2020, 02:20:07 PM »
Mads,

Isn't SSTC frequency determined by the secondary and top-load?  So the 250kHz example in your design guide would remain constant, not go up to 1MHz.

I mistakenly took that as a DRSSTC example with seeing it all to have one resonant frequency. What a nice blunder  ::)

One point I forgot to mention for scaling voltage: primary geometry.  To scale properly, the primary winding needs to remain the same size.  For scaling from 320V to 80V, the primary in your example needs to be four paralleled 2-turn windings to fill up the same vertical height.  To keep wiring inductance low enough, each two-turn winding would need to route down to the H-bridge separately, ideally as four twisted pairs.

The above primary winding should have 1/16th of the inductance (same geometry, 1/4 turn count), or 635nH, for 1.0 ohms at 250kHz.  That's 80 amps at 80 volts, for the same reactive power as 20 amps at 320V.

For the above comparison, I've used the calculations as they are on your SSTC design guide.  However, the actual peak currents are higher, since square waves have more area than sine waves.  The current through an inductor with square-wave voltage is a triangle wave.  For 250kHz and 320V, the coil sees +320V for 2us.  That ramps the current through a 10uH inductor from -32A to +32A (640uVs).  The peak current is 32A.  Of course, this is with ideal assumptions - no coil resistance and no H-Bridge switching transition time.

Separately, I like the nice picture in your SSTC design guide showing the tight coupling of primary coil to the bottom of the secondary coil.  That prevents that lower secondary portion from building up even higher voltage due to resonant current, since that voltage is tightly coupled to the primary's fixed voltage.  (I had been wondering about how the physical proximity didn't cause breakdown issues, until realizing that it also clamped the lower secondary voltage.)

You are right about the, in theory, reactive power that should be the same.

But I have yet to see a low voltage SSTC with the same performance. I am unsure if its down to the, very unscientific saying, "more voltage is more better" when it comes to resonant voltage rise or you can substitute that with longer on-times. Maybe it is because the designs was not built to match the same reactive power due to high current requirements where sourcing the 100A+ becomes more of a hazzle and the power supply/transformer begins to dwarfen the coil it should drive.

One of the most famous 24VDC SSTC is the Skori II mini SSTC: http://skory.gylcomp.hu/tesla/mini_tesla2.html with 100A MOSFETs and forced air cooling on the primary :) Maybe it just gets way too impractical to match a mains fed coil if you try to scale that up.



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Re: A few (stupidly basic) questions
« Reply #9 on: April 03, 2020, 02:20:07 PM »

 


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