Musical solid state Tesla coil theory questions

[skysurf76]

I am a senior in E.E. in the U.S. and am taking the summer off school and don't want to waste the time.  For years I have wanted to build a solid state Tesla coil capable of playing music and I think my education is about to the point where I can attempt the project, but I want to make sure my understanding of the theory is correct.

I understand that a Tesla coil is basically a transformer.  The primary coil is powered with a sinusoidal input, and thus the secondary is powered through magnetic coupling with the primary.  To make sparks with the Tesla coil the primary must be energized at a certain frequency so that energy is continually added to the secondary as the energy in it "bounces" back and forth between ground and air until sufficient energy has been added so that the voltage is sufficient to ionize the air and cause a spark.

On a scope trace of the secondary it is my understanding that the voltage will climb with each sinusoidal period of the primary until enough voltage is present in the secondary so that it ionizes the air and discharges as a spark.  As the secondary discharges through the air it will look like a heavily damped sinusoid because the high voltage will initially ionize the air bleeding off the energy until the air begins to resist the current again.  This frequency is called the "excitation frequency".

The excitation frequency is dependent upon the physical construction of the coil (size, turns ratio, wire gauge) and the power input to the primary, as well as environmental conditions...composition of the air, humidity, etc.  Because of this I plan to build some type of feedback to achieve maximum voltage.

Because the sparks are happening so fast (lets call the excitation frequency 1Mhz), if we can control how often they occur then we can make the spark happen at specific frequencies.  If we can control their frequency with a music input then the sparks should make music.

So if we observe on an oscilloscope that it takes lets say 5 pulses of the primary at a primary frequency of 1 megahertz to achieve 1 spark with 5 cycles after for the secondary to settle, then if we wanted to make a 20 kilohertz sound we would send 5 pulses to the primary (computer controlled) every 1/10,000 of a second and the sparks should happen at 20kilohertz.  This should allow a sound resolution of [(1 megahertz/5 pulses per spark (with 5 pulses for secondary discharge))=100kHz] maximum spark frequency down to any arbitrary minimum which includes the entire human auditory range.  (5 primary periods to spark and 5 periods for secondary to fully discharge are arbitrary and can be controlled by the physical design/feedback mechanism)

I just want to make sure my fundamentals are in place before I begin any design.



Thank you in advance for your time for anyone who responds.

[Richard Crowley]

As you say, a Tesla coil is based on resonance to efficiently couple input power from the primary winding over into the secondary coil. So a "Tesla Coil" is basically just a large, resonant transformer.  "Musical Tesla coils" appear to be based on amplitude modulating the carrier frequency (the resonant frequency of the transformer).

Here in the 21st century (and even in the later parts of the 20th century) higher-power amplitude modulation (AM) is done (as for radio broadcast transmitters, for example) by PWM, Pulse Width Modulation.  That would be my first line of research for making a musical Tesla coil.  There should be more than adequate information out there from broadcast engineering and amateur (ham) radio sources about PWM for AM.

[skysurf76]

As you say, a Tesla coil is based on resonance to efficiently couple input power from the primary winding over into the secondary coil. So a "Tesla Coil" is basically just a large, resonant transformer.  "Musical Tesla coils" appear to be based on amplitude modulating the carrier frequency (the resonant frequency of the transformer).

Here in the 21st century (and even in the later parts of the 20th century) higher-power amplitude modulation (AM) is done (as for radio broadcast transmitters, for example) by PWM, Pulse Width Modulation.  That would be my first line of research for making a musical Tesla coil.  There should be more than adequate information out there from broadcast engineering and amateur (ham) radio sources about PWM for AM.

Thank you for your response.  I really appreciate it.  Unfortunately you have confused me a bit.  Not about PWM or the Tesla coil, but about AM (amplitude modulation).  I just took signals and systems and we studied AM radio a fair bit.  What you are doing in AM is using the trigonometric identity of cos(wmt)*cos(wct) =0.5 cos [(wc+wm)t]+cos[(wc-wm)t] to move the signal up in the frequency spectrum (Wc=carrier frequency, Wm=modulated signal on top of carrier).

 You take an input cosinusoid and multiply it by a carrier cosinusoid to get the input signal at the lower and upper side of the carrier frequency.   What it ends up doing is sending the signal "on top of" the carrier frequency as amplitude modulation of the carrier frequency.  I'm not sure where pulse width modulation, which is digital in nature, would apply to AM. 

[Kappes Buur]

What would we do without Youtube ....

[skysurf76]

Kappes Buur I have searched for every conceivable combination of words I can think of on youtube over the years for musical solid state Tesla coil and never came across that video.

You're youtubefu is strong.

I do still need my question answered about the spark though.  Does the energy build up in the secondary through several periods of drive signal from the primary before it sparks, or does one pulse from the primary cause one pulse, and a spark, in the secondary?

Mathematically I'm sure you could physically build your coil so that one period of the primary signal could cause a spark in the secondary, but I'm not sure if that how its usually done.

[Gyro]

I don't know if you can get any inspiration by looking at the various Plasma Tweeter designs on the web?

[skysurf76]

I don't know if you can get any inspiration by looking at the various Plasma Tweeter designs on the web?

Kappes video really helped.  I think the only question I have left is do you design your Tesla coil so that a single primary input pulse causes a spark, or can the energy in the secondary "bounce" between air and ground so that you can add energy to the secondary on every primary pulse until the secondary sparks.  After making this post and thinking about it I suspect you can design it to do either, but I"m not going to build anything until I'm sure.

After looking at the schematic in Keppes youtube video some more I think that the excitation frequency coming into the negative (inverting) input to the U3 Op amp is probably a much higher frequency then the input sound signal across J2 so that what you are in essence doing on U3 is letting through the excitation frequency whenever there is a sound signal.

[Richard Crowley]

... What you are doing in AM is ... move the signal up in the frequency spectrum...  You take an input cosinusoid and multiply it by a carrier sinusoid to get the two signals at the lower and upper side of the carrier frequency.

What it ends up doing is sending the signal "on top of" the carrier frequency as amplitude modulation of the carrier frequency.  I'm not sure where pulse width modulation, which is digital in nature, would apply to AM.

I'm not really following what you mean by "move the signal up in the frequency spectrum".  But, as you say, the end result is simply modulating the amplitude of the carrier wave by the audio signal.

Traditional methods of Amplitude Modulation required large amounts of power. For example it took a 25,000 watt audio amplifier to modulate a 50,000 watt transmitter.  Not to mention a modulation transformer approaching the size of a small car.  And thousands of watts more power just to operate the gigantic vacuum tubes that this was all implemented with.

But now that we have high-power switching transistors, we can simply take the carrier frequency and pulse-width modulate it quite easily (and efficiently and inexpensively). You are correct the the output of the PWM is a digital-like square-wave.  However, you are not including the effect of the resonant "tank" of the Tesla Coil itself.  (Or the large L/C resonant filter in the case of a broadcast transmitter.)  The end effect is that the amplitude of the carrier wave varies depending on how much power is pumped into the resonant circuit. And the power is determined by the width of the pulse. 

People who make broadcast transmitters switched to this method several decades ago and even high-power equipment uses essentially digital techniques right up to the final output stages.  Modern ATSC digital video is also transmitted in AM, but restricted to 8 different amplitudes so that each cycle represents 3 binary bits of information.  This is called 8VSB and it is also pulse-width modulated. Even FM broadcasting uses similar digital techniques in modern times.

[skysurf76]

... What you are doing in AM is ... move the signal up in the frequency spectrum...  You take an input cosinusoid and multiply it by a carrier sinusoid to get the two signals at the lower and upper side of the carrier frequency.

What it ends up doing is sending the signal "on top of" the carrier frequency as amplitude modulation of the carrier frequency.  I'm not sure where pulse width modulation, which is digital in nature, would apply to AM.

I'm not really following what you mean by "move the signal up in the frequency spectrum".  But, as you say, the end result is simply modulating the amplitude of the carrier wave by the audio signal.

Traditional methods of Amplitude Modulation required large amounts of power. For example it took a 25,000 watt audio amplifier to modulate a 50,000 watt transmitter.  Not to mention a modulation transformer approaching the size of a small car.  And thousands of watts more power just to operate the gigantic vacuum tubes that this was all implemented with.

But now that we have high-power switching transistors, we can simply take the carrier frequency and pulse-width modulate it quite easily (and efficiently and inexpensively). You are correct the the output of the PWM is a digital-like square-wave.  However, you are not including the effect of the resonant "tank" of the Tesla Coil itself.  (Or the large L/C resonant filter in the case of a broadcast transmitter.)  The end effect is that the amplitude of the carrier wave varies depending on how much power is pumped into the resonant circuit. And the power is determined by the width of the pulse. 

People who make broadcast transmitters switched to this method several decades ago and even high-power equipment uses essentially digital techniques right up to the final output stages.  Modern ATSC digital video is also transmitted in AM, but restricted to 8 different amplitudes so that each cycle represents 3 binary bits of information.  This is called 8VSB and it is also pulse-width modulated. Even FM broadcasting uses similar digital techniques in modern times.

Richard, because lower frequencies require larger antennas it is beneficial to transmit signals at higher frequencies.  What AM allows you to do is take a low frequency signal (a signal human ears can hear as an example) and place it "on top" of a much higher frequency "carrier" signal.  This can be done by using a trigonometric identity of multiplying the varying cosine signal (sound) with a constant frequency cosine carrier signal.  This results is the signal showing up at the carrier signal with it mirrored at half power on both sides of the carrier signal. 

I'm attaching my signals and systems AM notes for reference.

[Richard Crowley]

"Moving the frequency up" is not really a description of either AM or FM modulation.  In both cases we are transmitting the carrier wave (for example 1 MHz in the AM radio band, or 100 MHz in the FM radio band, or 500 MHz in the TV band) which can be propagated by various physical methods for 10s or 100s or even thousands of miles. But an unmodulated carrier wave is of very limited value.  Clearly, we can't send "baseband" audio or video signals through the air for any useful distance. So we modulate the signal of interest onto the carrier wave.  I've never before heard of that as "moving the frequency up", so that is why I was confused.

I have to admire the work of the pioneers to derive all the mathematical models describing how traditional modulation schemes worked. But I am a bit surprised that they don't mention any of the modern methodology or technology used today.

[skysurf76]

Sorry Richard.  I think we were victims of terminology. 

For some reason in my signals and systems class  in my mind I saw AM as "moving a signal up" from a low frequency to a high frequency.  When you take the signal cosine and multiply it by the carrier cosine you get the original signal showing up at half power on both sides of the carrier cosine, so I always thought of amplitude modulation as moving the original signal up in frequency on top of the carrier.  After I learned about Fourier transforms I started thinking about everything in terms of frequency.

[rob77]

apart from describing AM modulation as "moving frequency up" and trying to explain that higher frequencies are beneficial because of antenna sizes while you was asked about something else...

how exactly would you make that feedback in your tesla coil to achieve maximum voltage ? (it will have to be a extremely fast feedback loop handling very high voltages)

[alsetalokin4017]

The SSTC is not "just" a transformer, it is an air-core 1/4-wave resonator coupled to a low-inductance primary, that achieves high voltage by standing wave resonance. You use a phase-locked loop (PLL) system to keep the coil in resonance at the higher frequency, which will generally be in the 0.5 - 2 MHz range, depending on the physical and electrical dimensions of the coil. You use a PWM or chopper to interrupt the drive to the primary at audio frequencies. The PLL can be implemented with the CD4046BE PLL chip and the modulation can be done with a simple 555 circuit.

So the OP's original post actually describes the situation fairly well as far as the audio modulation is concerned.

Here's an illustration of a very basic PLL + chopper-modulated SSTC in development, using just a single mosfet to drive the primary:

More sophisticated designs will use a full H-bridge to drive the primary but the basic idea is the same.

[skysurf76]

apart from describing AM modulation as "moving frequency up" and trying to explain that higher frequencies are beneficial because of antenna sizes while you was asked about something else...

how exactly would you make that feedback in your tesla coil to achieve maximum voltage ? (it will have to be a extremely fast feedback loop handling very high voltages)

I was thinking of using a small coil with just a few windings placed near the Tesla coil and connected to a microcontroller of some type that would control the frequency of the coil.  I was thinking using the ADC on the micro to take voltage readings on the coil and make adjustments to the Tesla coil frequency.

I'm sure what I just said is incredibly simplistic and will have all kinds of problems but it was my planned starting point.

[Richard Crowley]

I was thinking of using a small coil with just a few windings placed near the Tesla coil and connected to a microcontroller of some type that would control the frequency of the coil.  I was thinking using the ADC on the micro to take voltage readings on the coil and make adjustments to the Tesla coil frequency.

It is not clear by what method you expect to influence either the frequency or the voltage of a Tesla coil with "a small coil with just a few windings placed near the Tesla coil"?  Do you have any examples of how people do that?

And it is not clear why you need to "make adjustments to the Tesla coil frequency"?  Don't you already know what frequency you want to produce?  The description of your plan seems incomplete.

[alsetalokin4017]

The resonant frequency of a Tesla coil depends to some extent on the surroundings, like changing setup locations or people walking by or drawing arcs from the top of the secondary. If there is no mechanism to keep the coil exactly in resonance as the surroundings change, its output will not be as great as it could be. Hence the use of the Phase Locked Loop to keep the coil in resonance by varying the drive frequency as necessary. And/or, a couple of CMOS gates can be used with an antenna pickup to assure that the coil is always driven at its resonant frequency. Some architectures just use CMOS gates to feed the mosfet driver chip(s) directly without a separate PLL chip, some use CMOS gates to feed the PLL chip, some just use a pickup antenna or coil to feed the PLL chip directly.

CMOS feedback controlled SSTC without separate PLL chip:
http://www.stevehv.4hv.org/SSTC5/miniSSTCfnlsch.JPG

Audio-modulated PLL SSTCs:
http://uzzors2k.4hv.org/index.php?page=pllsstc2
http://esbuzz.net/trends/video/my-pll-audio-modulated-sstc-again
http://www.lessinger.net/_/AUDIO_SSTC.html

et cetera....

[Grzegorz2121]

I didn't recommend to build TC from electroboom channel.
Why? Because real musical TC needs feedback from secondary to work- putting fixed freq. to coil can
destroy you mosfets (because resonant freq. will change and this depends on capacitance of secondary)
So you need to provide resonant frequency from antenna that will pick that freq.

I recommend one of these designs:
http://www.stevehv.4hv.org/SSTCindex.htm