Re: The "Fat Coil" QCW Tesla Coil
zrg, Sat Nov 15 2014, 10:56PM

Awesome, as always. Hope we'd be able make something comparable to your QCWs one day

Re: The "Fat Coil" QCW Tesla Coil
Sigurthr, Sun Nov 16 2014, 12:03AM

Stunningly beautiful results as always. Perhaps I missed it, but I don't think I ever saw your buck converter schematic on your site. Would love to see it some day.

Re: The "Fat Coil" QCW Tesla Coil
Steve Ward, Sun Nov 16 2014, 02:53AM

Also, a crappy video from my cell phone during a test awhile back:



I'll post more as it shows up.

The loud squealing is a side effect of the current limit protection circuit droping the buck frequency to 10khz. More software tweaks are in the works...

Re: The "Fat Coil" QCW Tesla Coil
Sulaiman, Sun Nov 16 2014, 08:18AM

Your TCs always amaze me ... great stuff.

From reading your post I see that you have a really good understanding of arc impedance and growth, have you documented your thoughts on these in a simple manner for us to learn.

How do you get such short secondaries to shoot out long arcs that do not arc around to the bottom end of the coil or primary and even ground ... awesome.

Re: The "Fat Coil" QCW Tesla Coil
Steve Ward, Sun Nov 16 2014, 07:28PM

Your TCs always amaze me ... great stuff.

From reading your post I see that you have a really good understanding of arc impedance and growth, have you documented your thoughts on these in a simple manner for us to learn.

Thanks! I don't think my understanding is as good as some other forum members here, but I do enjoy building the stuff so I "make it happen". I think i have some practical knowledge on how to chose coil parameters now, mostly from all of the experiments. I do plan on writing something about it someday, when i have a better understanding of it all. For now i go off of experience and "instinct" rather than facts and analysis, so it isn't science yet.

How do you get such short secondaries to shoot out long arcs that do not arc around to the bottom end of the coil or primary and even ground ... awesome.

That's the fun part! High-frequency is key to developing big sparks from less voltage. The voltage is pretty low, otherwise it would arc over where we dont want it to (and it has happened when the electronics malfunctioned and allowed the voltage to go too high). I did a good bit of design work with FEMM to check the field stresses on the coils, which is how i arrived at my primary with a partial torus shape to it. The design should likely fail at 120kV of topload voltage. My estimate on spark growth under these conditions is that it takes 40kV to initiate, and only ~5kV/foot of spark length after that, so I estimate its operating in the 90-100kV range now.

Re: The "Fat Coil" QCW Tesla Coil
Extreme Electronics, Sun Nov 16 2014, 10:33PM

Steve, Stunning Stuff.
Do you have (even a block) schematic. I would love to know the details of your bridge. My attempts at paralleling IGBT's have all ended as bricks.

Re: The "Fat Coil" QCW Tesla Coil
Uspring, Mon Nov 17 2014, 11:37AM

Very impressive work
I did some quick calculations from the specs you provided. It seems, like you didn't use a secondary MMC, which implies (by some more calcs), that arc loading would increase top capacitance by a factor of 2 or 3. How did you deal with that amount of detuning? My guess would be upper pole operation together with some heavy primary low tuning, e.g. 30 or 40%.

Edit: I think I lost a factor of 10 calculating the top load, so likely a secondary MMC was used, decreasing arc capacitance effects strongly.

Comparing arc length to peak power relationships with your previous coil (20kW, 5-6 footers), there seems to be a deviation from the square root law, i.e. arc length ~ sqrt(P). Any idea about this?

Re: The "Fat Coil" QCW Tesla Coil
Steve Conner, Mon Nov 17 2014, 12:31PM

The sqrt(p) thing is only for sparkgap coils and DRSSTCs. The relationship between spark length and power also depends on frequency, break rate (in the limit, for single shot the average power is almost 0, yet the spark length is not 0), burst length, and last but not least the ratio of spark length to coil length. If you want a huge spark from a small coil, you have to make compromises that hurt the efficiency of streamer growth. This coil takes 1kJ to make a spark length that a larger resonator could do with 100J.

It is very cool though. :)

Re: The "Fat Coil" QCW Tesla Coil
Uspring, Mon Nov 17 2014, 02:29PM

You're right about the fact, that Freaus equation was established with short bursting SGTCs and relates average power input to spark length. So why should it carry over for QCW coils and peak power?

If you think of an SGTC running at various input voltages, average and peak power would be proportional, so a sqrt peak power law would apply also. Varying burst rates will change the average to peak power ratio, which will cause Freaus law to fail drastically at e.g. very low rates, but a relationship between peak power and arc length will not fail to the same extent.

Steve W. QCWs are basically one shot devices (he does generate multiple shots, though, but I think, that the increase of arc length probably is small), so average power is not a very useful concept. Both of his coils run at about the same frequency and have similar burst length, the main difference being peak power.

Peak power for a given arc length depends much on the burst length. The slow arc growth in QCWs creates less branching arcs and thus less energy is wasted in feeding branches. Also longer burst times allow for more heat accumulation in the arc, which will make it more conductive and less power is necessary to carry the voltage to its tip. In terms of bang energy, slow growth is costly, though.

Re: The "Fat Coil" QCW Tesla Coil
Intra, Tue Nov 18 2014, 09:50AM

Just watching this stuff fall me into deep delight! Thanks, Steve that you make this stuff be real. It is absolutely amazing.

Many things are hard to understand yet, but one of them is which mode you are use in H-bridges? Does they drives in phase-shifted mode or in regular like your first qcw with buck?

Look like with new MCu driver many features possibly are done in software (Buck modulator, Buck Snubber driving, bridges driving) and does this software-part could be replaced by analog-based electronics or some features won't properly work without MCu in prototype this high-power kind of QCW?
Simply, if H-bridges work in regular mode, then if I try to make this stuff according to qcwinfo from, for example, Gao site for my home bass fun, then will or won't work it with his(Gao), for example, buck modulator circuit design?

could you please tell something more about driving buck snubber circuit algorithm? It's a little bit difficult to understand how exactly it get stored and get back to bus the energy.

You wrote about duty cycle are 75%. Does 13kVAC RMS MMC are enough for polyphonic operations like bass guitar fun? Did you saw any MMC overheating playing bass?

"8x H-bridge, each drives its own transformer" - means 8 GDT cores, right?

If so, then 2 MCu drivers are works synchronously for multiplying H-bridges gate power or by another reason?

Re: The "Fat Coil" QCW Tesla Coil
Steve Ward, Wed Nov 19 2014, 08:03PM

Uspring:

I did some quick calculations from the specs you provided. It seems, like you didn't use a secondary MMC, which implies (by some more calcs), that arc loading would increase top capacitance by a factor of 2 or 3. How did you deal with that amount of detuning? My guess would be upper pole operation together with some heavy primary low tuning, e.g. 30 or 40%.

I chose to not use a MMC for the secondary due to their limited reliability and cost. I think the Csec = 44pF. My estimate for C of a 11 foot arc (by extrapolating linearly from previous guess of 2.5pF/ft) is 27.5pF which is a bit over half Csec.

It does have upper pole operation, and the primary is detuned, but im not sure its anywhere close to 30-40%. From my FEMM model and Java TC i get a Fpri of 260khz, Fsec of 270khz, K = 0.33, Fupper = 325khz. Im not sure how to define the % of detune, but this looks like just a few %. I wanted to try detuning further but somehow my driver could not cope and the coil would more often than not, end up at the lower pole mode which performed very very poorly (required too much current).

Comparing arc length to peak power relationships with your previous coil (20kW, 5-6 footers), there seems to be a deviation from the square root law, i.e. arc length ~ sqrt(P). Any idea about this?

First off, i mis-calculated the buck power (i did it based off the peak inductor current, but thats not right, it should be average inductor current during a switch cycle). So real max power so far is 460A*350V = 160kW. The tesla gun setup is my other benchmark for the 5-6 footers it uses about 100A*350V = 35kW. So doubling the spark length has called for ~4X the power, which seems to roughly follow a squared/square root law as anticipated, no?

Intra:

Many things are hard to understand yet, but one of them is which mode you are use in H-bridges? Does they drives in phase-shifted mode or in regular like your first qcw with buck?

The bridges are ZCS only, no phase shift modulation.

Look like with new MCu driver many features possibly are done in software (Buck modulator, Buck Snubber driving, bridges driving) and does this software-part could be replaced by analog-based electronics or some features won't properly work without MCu in prototype this high-power kind of QCW?
Simply, if H-bridges work in regular mode, then if I try to make this stuff according to qcwinfo from, for example, Gao site for my home bass fun, then will or won't work it with his(Gao), for example, buck modulator circuit design?

I chose to do things digitally, but the same performance could come from a bunch of TTL chips and analog circuits. I think you have the right idea in mind.

could you please tell something more about driving buck snubber circuit algorithm? It's a little bit difficult to understand how exactly it get stored and get back to bus the energy.

The snubber consists of a diode feeding into a capacitor bank (2x250V lytics in series). This bank has a boost circuit (series inductor, IGBT to bus -, diode to bus +) that will take the 475V-500V level of the snubber caps, and send that energy back to the 700V main bus via boost circuit. The voltage on the snubber capacitor is monitored by the MCU, if the level exceeds 490V the buck converter is forced to shut down. If the voltage is > 475V, the booster circuit is enabled to bring the snubber voltage down. Fairly simple stuff to implement in a MCU, which is why i prefer this system, but the start up time to design it is much higher.

You wrote about duty cycle are 75%. Does 13kVAC RMS MMC are enough for polyphonic operations like bass guitar fun? Did you saw any MMC overheating playing bass?

"8x H-bridge, each drives its own transformer" - means 8 GDT cores, right?

If so, then 2 MCu drivers are works synchronously for multiplying H-bridges gate power or by another reason?

When playing music through the coil, the peak power is usually limited because the average power is much higher and so the silicon is running hotter. The MMC has been very impressive, i havent noticed it getting warm at all after many minutes of making huge arcs .

The transformers are on the output of the H-bridges, this is not typical, but unique to my design for current sharing among all IGBTs.

The IGBTs are still GDT driven. Technically you only need 1 GDT with 32 secondary windings to drive all the IGBTs, but for ease of construction i used 4 GDTs total.

The 2 MCU-based controllers/drivers control the buck stage and the tesla coil stage separately, but share communication over rs-422. I could likely make 1 controller do all of this instead of 2, but this was the fastest way forward for now. There is a fiber optic link from the buck driver to the human controller, which generates the modulation signal.

Re: The "Fat Coil" QCW Tesla Coil
Uspring, Thu Nov 20 2014, 11:39AM

Thank you for the additional values.

From Cpri * Lpri = Csec * Lsec, Lsec should be around 8 mH. For some reason Javatc gave me a Lsec of about 3 mH, so I assumed Csec to be a lot higher, than your value of 44pF, which made be believe in a secondary MMC.

My estimate for C of a 11 foot arc (by extrapolating linearly from previous guess of 2.5pF/ft) is 27.5pF which is a bit over half Csec.

I've estimated arc capacitance differently, i.e. by its power consumption. In the corresponding TCML thread, you mentioned an estimate of 100kVac top voltage. I assumed this to be peak voltage. From this and 160kW power, the arc resistive load would be 0.5 * V^2 / P = 31kohm. From my measurements, I've found Conners hungry streamer model to be approximately accurate, which states, that capacitive and resistive loads are similar, i.e. R = 1 / (2*pi*f*Carc). Carc would then be about 20pF. That's a bit lower than your estimate, but the same ballpark.

So doubling the spark length has called for ~4X the power, which seems to roughly follow a squared/square root law as anticipated, no?

Great, that's pretty much what I expected

Re: The "Fat Coil" QCW Tesla Coil
BSVi, Fri Nov 21 2014, 06:05AM

Why don't you just shut down buck in overcurrent condition and let groundstrike consume ~40 J of energy that is left in LC filter?

I dont think that could harm bridge. From my experementation, such an long streamer has considerable impedance and primary current rises by only ~100А. So I think it's safe to disable buck and let system rampdown by itself.

Thats, at least, what i did in my coil:

Re: The "Fat Coil" QCW Tesla Coil
Intra, Fri Nov 21 2014, 02:16PM

Steve, thank you clarify all the details. It very helpful for me.

Re: The "Fat Coil" QCW Tesla Coil
Steve Ward, Sun Nov 23 2014, 10:18PM

Why don't you just shut down buck in overcurrent condition and let groundstrike consume ~40 J of energy that is left in LC filter?

That is the main mode of operation for my system, except the buck tries to maintain constant output current during the ground strike instead of simply shutting down. This gives much more spectacular ground arcs, especially during high rep-rate operation.

Overcurrent on the bridge is set pretty high to avoid ever tripping it, but some ground strikes have done it, and i see my "snubber over voltage" indicator come on when it does. It would be nice to have less complexity and not need the snubber, but i think there should always be some over-voltage protection in place since you have no guarantee the tesla coil driver might not suddenly shut off for any number of reasons, allowing the buck output to over-voltage.

Re: The "Fat Coil" QCW Tesla Coil
Weston, Sun Nov 23 2014, 11:35PM

Very interesting coil.

Does the micro controller form part of the feedback path? If so, does it sense the tank current with a comparator on the zero crossings or an ADC?

If clocked logic or a micro controller is used in the feedback path it seems like you would get a lot of jitter in the gate drive waveforms and fall into hard switching some of the time. Does this not end up being an issue?

Re: The "Fat Coil" QCW Tesla Coil
Gregory, Mon Nov 24 2014, 06:23AM

Hello guys!

That is the main mode of operation for my system, except the buck tries to maintain constant output current during the ground strike instead of simply shutting down. This gives much more spectacular ground arcs, especially during high rep-rate operation.

Can you explain it better?
When over current is tripped the coil driver start to skip cycles and the buck goes in a "constant current mode" to help the cycle skiping thing to work better?

Re: The "Fat Coil" QCW Tesla Coil
teravolt, Mon Nov 24 2014, 05:15PM

do you have problems with EMI in your electronics running that much power

Re: The "Fat Coil" QCW Tesla Coil
Intra, Thu Nov 27 2014, 05:48PM


Steve, is it works this way?

Re: The "Fat Coil" QCW Tesla Coil
Steve Ward, Sun Nov 30 2014, 08:36PM

Can you explain it better?
When over current is tripped the coil driver start to skip cycles and the buck goes in a "constant current mode" to help the cycle skiping thing to work better?

There's a lot going on, i'll try to re-explain with more detail. The buck converter has a current limiter that momentarily disables the drive signal to the IGBTs. Its supposed to act as a "cycle by cycle" peak current limit, however, due to bandwidth limitations on my sensor there is significant delay in the current signal. This delay causes the over-current event to be detected after the PWM cycle has completed (in the undesirable case), which causes the buck to skip PWM cycles (unintentionally). This causes the high-frequency sounds that you hear, as the buck is switching at just 10khz, somewhat chaotically.

At the same time, the tesla coil driver has an over-current protection that will cause the H-bridge to skip a driving cycle. This is a good method for most systems, but when supplied by a buck converter it causes un-wanted effects (the voltage at the buck output will suddenly rise as the tesla bridge stops drawing current for a moment). If this happens, the Buck often shuts down from an over-voltage fault condition.

I do plan on improving the current limit behavior of the Buck, and once that is proven reliable, the tesla coil current limit will be set very high so that it should never engage unless some other fault condition exists.

Does the micro controller form part of the feedback path? If so, does it sense the tank current with a comparator on the zero crossings or an ADC?

If clocked logic or a micro controller is used in the feedback path it seems like you would get a lot of jitter in the gate drive waveforms and fall into hard switching some of the time. Does this not end up being an issue?

I use a Cypress PSoC 5LP, which handles the IGBT switching, and it does the phase-lead stuff too. For the most part this circuitry is built into hardware, which is controlled by the MCU to provide proper tracking. Its clocked at 64MHz, and the actual gate timing can be off by 1-2 clocks typically (~15-30nS) from the ideal switch time.

do you have problems with EMI in your electronics running that much power

I dont think its any worse than a small coil running at similar supply voltage. Most of the noise comes from dV/dt, so going bigger doesn't necessarily make things worse, sometimes its better because its slower. I do have a lot of filtering on all of the control-board I/O, it is a tesla coil afterall...

Intra, almost got it, but you didnt get the extra IGBT/diode boost circuit correct. It follows a standard boost converter scheme, i think you can fix your schematic .

Re: The "Fat Coil" QCW Tesla Coil
Gregory, Mon Dec 01 2014, 04:58AM

Thanks.

I use a Cypress PSoC 5LP, which handles the IGBT switching, and it does the phase-lead stuff too.

Can you explain what method did you use to make the phase-lead. I was thinking to use a FPGA in my QCW controller but I didn't get a good and reliable idea for phase leading.

Re: The "Fat Coil" QCW Tesla Coil
Shrad, Mon Dec 01 2014, 10:45AM

couldn't phase lead be deduced from T-1 times a correction factor? thus no fancy computation needed...

Re: The "Fat Coil" QCW Tesla Coil
Gregory, Tue Dec 02 2014, 12:52AM

The simple idea I can see is to put the feedback signal trought a FIFO to delay the signal by 180º - phase lead but this method will be frequency dependent

Re: The "Fat Coil" QCW Tesla Coil
Shrad, Tue Dec 02 2014, 08:10AM

is T-1 really that frequency dependent? you'll never be that precise due to field constraints anyway, and it would be the simplest approximation

the situation will not change completely over a period, and even if it does over five or ten periods, isn't this enough?

I'd be curious about how a PID would behave in this field, as you could play with overshoot

Re: The "Fat Coil" QCW Tesla Coil
Steve Conner, Tue Dec 02 2014, 10:41AM

There are two fundamental ways of doing phase lead.

You can estimate when to make the next switching transition based on how far out you were in trying to hit the previous ones, in which case you have a PLL. The PLL loop filter determines the weighting given to different times in the error history. "T-1", if I understand correctly, is a digital PLL with a 1st order FIR loop filter, that works on the single previous switching transition and forgets the others. If you use a PID controller on the T-1 error signal, it becomes a classical PLL, as the I term remembers previous values of T-1, therefore T-2, T-3 etc.

Or, you can extrapolate the current waveform by assuming it is sinusoidal, and guess what it will do next based on its first derivative. This is the phase lead driver aka "Predikter".

The PLL is slower to respond to changes in streamer loading, because it is using information at least one half cycle old. But the phase lead driver is more susceptible to noise because the extrapolation process involves differentiation, which amplifies high frequencies. In control engineering classes we were taught never to differentiate anything, let alone the output of a Tesla coil. Nevertheless, both methods seem to work well.

Common sense would recommend phase lead for DRSSTCs with short bursts, where you want to hit each and every switching transition even though they may be unevenly spaced. And PLL for CW and QCW SSTCs where you want to be able to select a particular pole frequency. But there are examples of both methods being used with both types of coil.

Re: The "Fat Coil" QCW Tesla Coil
Goodchild, Tue Dec 02 2014, 04:37PM

In my opinion the FIR is the way to go, particularly if you can get 3 or 4 taps built into hardware. It's noise immunity is superior to anything I've used in the past.

The downside however, is the vast amount of hardware you need to implement it. I've done it in Xillinx FPGAs, but something like a mico probably could't handle the vast amount math required in a timely manner.

Re: The "Fat Coil" QCW Tesla Coil
Gregory, Tue Dec 02 2014, 08:08PM

You can estimate when to make the next switching transition based on how far out you were in trying to hit the previous ones

How can I calculate the error ? I will need a precise and real time reference of the bridge output to calculate the T-1 error by measuring edges time diference.

In my opinion the FIR is the way to go, particularly if you can get 3 or 4 taps built into hardware. It's noise immunity is superior to anything I've used in the past.

Do you mean a FIR with a know phase response at the particulary ressonant frequency?

What about use a digital PLL with the NCO singnal that goes to the phase comparator pass throught a delay line to generate a constant group delay.
This way we get a frequency independent time-lead, that appear to be better than a phase-lead for long burst coils that change a lot the frequency.

EDIT: I made a simulation of the PLL loop with the FIFO delay line at NCO output and it appears to work very well. I think it can be implemented in simple way with FPGA.

Re: The "Fat Coil" QCW Tesla Coil
Steve Conner, Wed Dec 03 2014, 01:02PM


Most PLL users (myself included) just use the gate drive signal as the reference. The assumption is that the delay from gate drive to bridge output is constant. The effect of making this assumption is that the PLL generates a constant phase lead referred to the feedback signal, that you have to set manually to compensate the delay in the power devices. This is really just the same thing as twiddling the variable inductor in Steve Ward's phase lead driver.

If you used the actual bridge output as your reference, your PLL would be measuring the actual switching delay of the bridge and compensating for it automatically. This could get messy when deadtime is involved, as the IGBTs do not always have control of the switching delay, it sometimes passes to the diodes.

Re: The "Fat Coil" QCW Tesla Coil
Steve Ward, Sun Dec 07 2014, 08:49AM

I made a digital PLL.

There are a few key points about it:

1) the user has to define a starting frequency "Fstart". The driver switches to feedback/pll operation after "N" cycles (set in 0.5 cycle increments), typically i use 0.5-1 cycles for a "transient" type DR, and 4-8 cycles for SSTC/QCW where you need to have a well defined starting frequency/mode). However, it works acceptably with pretty big errors... so its not overly burdening.

2) The user must define the desired phase-lead time "Tlead", which is referenced to the output signals from the PSoC (Conner already explained this perfectly). This lead time is defined as the whole delay time of the system (that includes the feedback CT delay, the processors delay, and IGBT delay).

So how's it work?

First, start driving at "Fstart", during this time, measure the period between zero crossings in the current feedback signal. After "N" cycles at "Fstart", PLL operation can begin. The PLL works as follows:

A one-shot timer is started upon each zero-current-detection of the feedback signal (that is, 2 events per full cycle). The one-shot is set to time out "just before" the next expected zero crossing. When the one-shot times out, the output to the gate driver flips state (as if the feedback signal told it to). If you measured the previous half-cycle's period "Thp", you can predict the next one as Tnext_switch = Thp - Tlead.

If that makes sense, then you understand enough to build your own digital PLL as i did. However... there was a problem with stability if the half-cycle periods were inconsistent, so i added a digital filter that averages the previous 4 half-cycle periods and uses that result for the predictive one-shot timer (knocking it down to 2 half-cycles ought to work too, just havent tried it yet). Conner would probably point out here that there is some dynamic performance loss from this, but thanks to the fact that the one-shot is reset with the feedback input, you can never get too far out of phase with the desired switching command even if the system frequency is changing rather quickly. Oh yeah, and just in case the prediction is too late, the zero-cross detect will command a gate drive transition, making the driver no worse than my old universal driver without phase lead.

Ive used this same driver on a wide variety of tesla coils and its performance is on par with the inductor prediktor and about 1000 times better than the 4046 PLL crap i used to play with. However, getting the whole scheme to work reliably was a considerable challenge and took months of debugging/tweaking to get it to a robust state. Having an FPGA with essentially limitless hardware would have made the task easier, most of my time was spent coaxing the design to fit within my hardware budget on the PSoC. Also, 64MHz clock leaves a bit to be desired at higher TC frequencies.

Re: The "Fat Coil" QCW Tesla Coil
Gregory, Mon Dec 08 2014, 12:12AM

A one-shot timer is started upon each zero-current-detection of the feedback signal (that is, 2 events per full cycle). The one-shot is set to time out "just before" the next expected zero crossing. When the one-shot times out, the output to the gate driver flips state (as if the feedback signal told it to). If you measured the previous half-cycle's period "Thp", you can predict the next one as Tnext_switch = Thp - Tlead.

This is the way I first thought to do but I thought it would be very instable. I will give it a chance

Re: The "Fat Coil" QCW Tesla Coil
Goodchild, Mon Dec 08 2014, 03:19PM



Interesting implementation of an ADPLL Steve. I have made a couple of these in FPGA a while ago and I commend you on doing it in something with so few macro cells. From my understanding most ADPLLs need the loop filter in order to work properly anyway, otherwise they suffer from the same instability you descried above.

It's a careful balance getting the loop filter correct, to aggressive and the system acts sluggish, not aggressive enough leads to instability/jitter.

Re: The "Fat Coil" QCW Tesla Coil
Intra, Tue Jun 09 2015, 01:37PM


Hi Steve, could 942C20P15K-F caps be replacement of MMKP for MMC?

Re: The "Fat Coil" QCW Tesla Coil
loneoceans, Tue Jun 09 2015, 05:28PM


The 942 CDE caps have excellent RMS capability and work great. However a similar capacitor bank would require about 90 of those caps (30 x 3) which will be pretty expensive though...

Re: The "Fat Coil" QCW Tesla Coil
Intra, Wed Jun 10 2015, 07:25AM


Thank you Gao!
According to datasheet Irms of 942C20P15K-F is 13.5A and AC is 500V.
Did I made right calculations about if 3pcs 942C20P15K-F in parallel give 20.25kVAC RMS?

The reason why I ask, 104pcs of 942C20P15K-F I already have.

Re: The "Fat Coil" QCW Tesla Coil
Mads Barnkob, Wed Jun 10 2015, 09:23AM


You could use the MMC calculator here to give some quick estimates. 500V is however a pretty conservative rating for these capacitors. But on the other hand they are being punished more in a QCW than in a DRSSTC. Just change the voltage rating in the calculator and the rest of its specifications is inthere.

You simply just need that many to get down in capacitance, these are not suitable to the job, too expensive an option in my opinion.

Re: The "Fat Coil" QCW Tesla Coil
Intra, Wed Jun 10 2015, 10:12AM

Nice calculator.
I made some research about which capacitors will be suitable to the job (for a couple of QCW).
240pcs of MMKP BFC238350273 27nF 1600V will cost $441 (mouser) and 90pcs of 942C20P15K-F will cost $518 (mouser).
But 104pcs of 942C20P15K-F I already have (I didn't bought them, it was a gift), and for MMCs which contains 90pcs of 942C20P15K-F I need only 2*90-104=76pcs more. And 76pcs of 942C20P15K-F will cost $437 only.
Which caps in your opinion will not so expensive for this job? (not in staccato mode)

Re: The "Fat Coil" QCW Tesla Coil
Mads Barnkob, Thu Jun 11 2015, 09:22AM

10 strings of 30 in series of these:

Gives you 15,6nF at 13,2kVAC, uses 300 capacitors that would cost you a total of 36 Euro, however the RMS current handling is lower than the 942C. But it might be total overkill with the 942Cs.

The downside is also you have to do much more soldering with the cheap method :)

Re: The "Fat Coil" QCW Tesla Coil
Intra, Thu Jun 11 2015, 01:00PM

*0*

Thank you very much!

Re: The "Fat Coil" QCW Tesla Coil
Intra, Thu Jun 11 2015, 03:23PM

Look like according to wiki boost circuit schematic, it should be as on this pic, but if so, then I don't understand why you use that white wire.

Does this schematic right?

Re: The "Fat Coil" QCW Tesla Coil
Steve Ward, Wed Nov 25 2015, 04:01AM

I see the snubber circuit is still in question. I have in fact abandoned the active snubber after it failed, quite violently, earlier this year. I'm uncertain whether it was a rouge spark hitting the snubber electronics that triggered the failure, or if i messed up the circuit when "improving" its wiring and mounting to the coil frame (i didnt bench test after re-wiring, maybe i goofed up). Now i've implemented a very simple passive snubber that uses a resistor and TVS diode bank. The schematics should make it pretty clear, i hope.

Re: The "Fat Coil" QCW Tesla Coil
Intra, Mon Nov 30 2015, 08:18AM

Wow! Thank you very much, Steve!

Re: The "Fat Coil" QCW Tesla Coil
Toasty, Sun Jan 17 2016, 05:47PM

wow that coil is insane!