Author Topic: A few questions and calculations about DRSSTC  (Read 3986 times)

Offline ged_mago

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A few questions and calculations about DRSSTC
« on: April 23, 2021, 05:46:38 PM »
Hi everybody , i’m from Argentina, sorry for mi english. I maked some calculations for kaizerpowerelectronics's (Mads Barnkob) DRSSTC 1 and i have a few questions:

Primary capacitor: 450 nF
F0: 65 Khz
Peak primary current: 500A:
XC (sourge impedance): 5.4 ohm
Primary voltage for 500A: 2.7 Kv
Energy in primary circuit: 1.64 Joule
Voltage gain (supposing a total secondary capacity  28 pF): 127
Output voltage: 343 Kv

The first question is how is possible make sparks of 1.5 meters with only 343 Kv?
There are some relation between the spark length vs. BPS?
How i can calculate the máximun spark length for a specific design?

I maked a small DRSSTC with the following parameters:

600 V bus DC
Primary capacitor: 28.6nF
F0 primary: 80 Khz
F0 secondary: 100 Khz
Peak primary current: 125 A:
XC (source impedance): 69 ohm
Primary voltage for 125A: 8.625 Kv
Energy in primary circuit: 1 Joule
Voltage gain (Top load around 12.3 pF): 48
Output voltage: 414 Kv
BPS:30
Half bridge of two IGBT IRG4PH50UD

This DRSSTC produce spark of around 40 centimeters. Never i pushed it beyond of 30 BPS.
Whitout secondary coil, the primary current rise to 125 A in about 140 µS (that is the rigth rise time that i calculated for the primary source impedance)  , but when i put the secondary coil, the primary current rise to 125 A in about 250µS, and i have de máximun transference in 400 µS , when the coil produce your maximun output.
Due to long duration of interruptor (400 µS), is that i never pushed it beyond 30 BPS.

What do you think of all of this?

Can you explain me a little about the "Magic 50k"  impedance of secondary coil?

Best regards for all !!!
« Last Edit: April 23, 2021, 05:50:42 PM by ged_mago »

Online AstRii

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Re: A few questions and calculations about DRSSTC
« Reply #1 on: April 24, 2021, 06:39:04 AM »
Hello!

1.) Measuring the exact voltage at these high frequencies and amplitudes and only short pulse duration is extremely difficult and what you usually see is more of an estimate than a measured/calculated value. It is true that Tesla coils usually do not reach nowhere near voltages like a million Volts or higher. You can create a huge spark with "only" a few kV. This is thanks to the "spark loading", basically it takes time before the voltage reaches the maximum amplitude and the spark which formed during this voltage rise is already acting as a very low impedance path. So as your voltage is rising, the more and more air gets ionized and acts almost like if you had a conductive wire stretching longer and longer. Check out "QCW-DRSSTC" topology.

2.) Usually with higher BPS you get longer sparks, that's because at higher BPS the arcs are coming out of the secondary before the air cools down and deionize. Basically after you fire a spark, the air takes some time to settle back to it's non-conductive state.

3.) That's a very though calculation, I don't think many people are even trying to calculate this, as it's very hard to estimate that. Tesla coils are pretty high-Q systems, which means they need precise tuning. A slight change of tuning can totally affect your output and it's very hard to include every variable into your calculation. I call DRSSTC a success when the sparks are at least 2x the height of the secondary winding, which is not as hard to do even with bad tuning. For a normal DRSSTC, I would say you can expect 1.5x-3x the height of the secondary. Of course you can really push your transistors and/or use pulse-skipping for even bigger sparks.

Your coil seems to run with very high primary impedance, thus you have pretty low primary current even with 250us on-time. 28.6nF is a very low capacity for a coil of this size. My recommendation there is to make your MMC at least 4x bigger (in capacitance) and use at least 300A primary current. Then you will be able to operate it with more common interrupter values (like 0-100us on-time / 0-1000BPS+).

Without the secondary coil, the energy in the primary coil doesn't have anywhere to go, and it will reach the peak current much sooner. It's never really a good idea to run the coil at full power without the secondary in place or something else to draw energy from the primary circuit as you are pushing your transistors even further than with the secondary in place.

I don't know how about others but for me this "Magic 50k" impedance is just a rumor. I've seen coils running just fine way outside this 50k zone. But what do I know :D

Good luck with your coil and don't forget to share some pictures/videos with us =)
Bc. Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

Offline ged_mago

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Re: A few questions and calculations about DRSSTC
« Reply #2 on: April 26, 2021, 04:06:28 PM »
AstRii, thanks for your answer!!
When i designed the small DRSSTC, i do it thinking that the maximum spark or streamer length was proportionally to the output voltage. I maked a secondary coil with large inductance (around of 204 mH) and a top load with small capacitance (around of 14 pF) for bigger voltage gain.
Now, if the spark length is not depending of voltage output, i must take another start point to my design. I think that the spark length must be relation with the energy in the sistem.
The bang energy en the primary circuit is determinating by the capacitance and voltage over the capacitor:
Energy [Joule]=1/2 C . V^2

The power given to the sistem is:
power [Watt]= Energy . BPS

So, more energy and high BPS, more power to the sistem.
Are you agree?
« Last Edit: April 26, 2021, 07:38:02 PM by ged_mago »

Offline davekni

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Re: A few questions and calculations about DRSSTC
« Reply #3 on: April 26, 2021, 08:04:23 PM »
I think the "magic" 50k-ohm secondary impedance is empirical, roughly what successful coils ended up with.  My DRSSTC is about 38k.  If secondary impedance is too low, too much energy is required to get a spark started.  If secondary impedance is too high, even a small arc is too much load, dropping the voltage.

Another way to look at this is by considering capacitance of the arc itself compared to top-load capacitance.  If top-load is small (high secondary impedance), then a small arc can double capacitance, dropping secondary frequency below primary, preventing further energy transfer.  With a large top-load (lower impedance), a longer arc is possible before secondary frequency drops too far.

300A may be a bit too high for those IGBTs.  250A should be possible with phase-lead adjusted properly (half-bridge output switching slightly before current zero-crossing).  The IGBTs are rated for 180A peak, so you could go at least that high.
David Knierim

Offline ged_mago

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Re: A few questions and calculations about DRSSTC
« Reply #4 on: April 26, 2021, 10:00:53 PM »
I think the "magic" 50k-ohm secondary impedance is empirical, roughly what successful coils ended up with.  My DRSSTC is about 38k.  If secondary impedance is too low, too much energy is required to get a spark started.  If secondary impedance is too high, even a small arc is too much load, dropping the voltage.

Another way to look at this is by considering capacitance of the arc itself compared to top-load capacitance.  If top-load is small (high secondary impedance), then a small arc can double capacitance, dropping secondary frequency below primary, preventing further energy transfer.  With a large top-load (lower impedance), a longer arc is possible before secondary frequency drops too far.

300A may be a bit too high for those IGBTs.  250A should be possible with phase-lead adjusted properly (half-bridge output switching slightly before current zero-crossing).  The IGBTs are rated for 180A peak, so you could go at least that high.

Great explanation Davekni!
I think that first i will make a bigger top load for more secondary capacity.
I was reading about the coupling between primary and cecondary coil and the transference of energy ...
I read that in the first notch of primary current is the complete transference of energy to the seconadary coil.
In my case, this notch apear at 400 µS, is possible that mi coupling be too small?

I blowed up the IGBT's in my latest tests. I`m not using phase lead circuit, my primary frecuency is about 80Khz.
Can do you recomend me some phase lead circuit?

Thanks!!!
« Last Edit: April 26, 2021, 10:06:11 PM by ged_mago »

Offline davekni

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Re: A few questions and calculations about DRSSTC
« Reply #5 on: April 27, 2021, 04:39:32 AM »
Yes, increased coupling may help.  Even though it is aimed at spark-gap coils, JavaTC is a great tool for calculating coil parameters including coupling.

UD2.7 is the most common driver here that includes phase-lead.  What driver are you using now?  Older drivers such as UD1.3 and many of the Chinese ones available on EBay do not include phase-lead.  Phase lead is not a necessity, but allows IGBTs to run closer to their limits before frying.
David Knierim

Offline ged_mago

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Re: A few questions and calculations about DRSSTC
« Reply #6 on: April 27, 2021, 03:19:18 PM »
Yes, increased coupling may help.  Even though it is aimed at spark-gap coils, JavaTC is a great tool for calculating coil parameters including coupling.

UD2.7 is the most common driver here that includes phase-lead.  What driver are you using now?  Older drivers such as UD1.3 and many of the Chinese ones available on EBay do not include phase-lead.  Phase lead is not a necessity, but allows IGBTs to run closer to their limits before frying.

Ok, Davekni, I'm using an old version of Steve Ward's driver adapted to local components, but it work fine. I will update it with a phase lead.
Thanks !!!!

Offline Uspring

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Re: A few questions and calculations about DRSSTC
« Reply #7 on: April 29, 2021, 01:28:07 PM »
Quote
Can you explain me a little about the "Magic 50k"  impedance of secondary coil?

Here's an attempt of a derivation:

For a given number of amp-turns of the primary tank the maximum amount of power transfer to the secondary can be achieved if this equation holds approximately:

Zsec = k * Rarc,

where Zsec is the secondary impedance, k the coupling and Rarc the loading resistance of the arc. This holds only for tuned coils, i.e. ones, where primary and secondary resonance frequencies match. The equation can be seen as an impedance matching condition. It is surely possible to run a coil with this condition not met, but then some more amp-turns are required to keep the power level up. More amps usually aren't available, since the driving bridges are often operated at their limit. More turns imply more resistance in the primary tank, which causes losses. The larger inductance of more turns also requires longer bursts, since current build up in the primary tank takes longer.

From a measurement that Hydron made, I've calculated Rarc to be about 210 kohm, when the arc was full size (about 2 m). https://highvoltageforum.net/index.php?topic=117.msg1831#msg1831
With e.g. k=0.15 we get an optimum Zsec = 31.5k. That is a bit below the magic 50k, but there are 2 issues to observe:

1. The measurement of 210k arc loading was at the time of max arc length, i.e. strongest loading. During most of the burst time the load was much less, i.e. the loading resistance Rarc was much bigger. So the higher magic Zsec is a compromise between different arc loadings during the burst.
2. The coil is almost never really in tune due to the varying resonance frequency of the secondary as the arc grows. So there is some advantage to choosing a higher Zsec, since it will lower the secondary Q, which widens the secondary resonance curve. Thus the coil is somewhat less susceptible to arc detuning.

So why is 50k magic for other coils besides Hydrons?
Empirically Rarc seems to be inversely proportional to arc length, so an arc of twice the length has half of the resistance. Also there seems to be a similar relationship for arc frequency and resistance, i.e. doubling the frequency will also halve the resistance. Putting this together we get:

Rarc = const/(frequency * length)

for some value of const. Bigger coils with longer arcs often operate at lower frequencies. So the product of length and frequency might not vary very much. The magic 50k value is more a guideline, than a strict requirement. It might not be appropriate for particular designs.

Offline ged_mago

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Re: A few questions and calculations about DRSSTC
« Reply #8 on: April 30, 2021, 08:16:51 PM »
Quote
Can you explain me a little about the "Magic 50k"  impedance of secondary coil?

Here's an attempt of a derivation:

For a given number of amp-turns of the primary tank the maximum amount of power transfer to the secondary can be achieved if this equation holds approximately:

Zsec = k * Rarc,

where Zsec is the secondary impedance, k the coupling and Rarc the loading resistance of the arc. This holds only for tuned coils, i.e. ones, where primary and secondary resonance frequencies match. The equation can be seen as an impedance matching condition. It is surely possible to run a coil with this condition not met, but then some more amp-turns are required to keep the power level up. More amps usually aren't available, since the driving bridges are often operated at their limit. More turns imply more resistance in the primary tank, which causes losses. The larger inductance of more turns also requires longer bursts, since current build up in the primary tank takes longer.

From a measurement that Hydron made, I've calculated Rarc to be about 210 kohm, when the arc was full size (about 2 m). https://highvoltageforum.net/index.php?topic=117.msg1831#msg1831
With e.g. k=0.15 we get an optimum Zsec = 31.5k. That is a bit below the magic 50k, but there are 2 issues to observe:

1. The measurement of 210k arc loading was at the time of max arc length, i.e. strongest loading. During most of the burst time the load was much less, i.e. the loading resistance Rarc was much bigger. So the higher magic Zsec is a compromise between different arc loadings during the burst.
2. The coil is almost never really in tune due to the varying resonance frequency of the secondary as the arc grows. So there is some advantage to choosing a higher Zsec, since it will lower the secondary Q, which widens the secondary resonance curve. Thus the coil is somewhat less susceptible to arc detuning.

So why is 50k magic for other coils besides Hydrons?
Empirically Rarc seems to be inversely proportional to arc length, so an arc of twice the length has half of the resistance. Also there seems to be a similar relationship for arc frequency and resistance, i.e. doubling the frequency will also halve the resistance. Putting this together we get:

Rarc = const/(frequency * length)

for some value of const. Bigger coils with longer arcs often operate at lower frequencies. So the product of length and frequency might not vary very much. The magic 50k value is more a guideline, than a strict requirement. It might not be appropriate for particular designs.

Thanks Uspring !!!

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Re: A few questions and calculations about DRSSTC
« Reply #8 on: April 30, 2021, 08:16:51 PM »

 


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June 02, 2024, 08:25:37 PM
post Re: Steve Ward SSTC5 build
[Solid State Tesla Coils (SSTC)]
Michelle_
June 02, 2024, 04:51:58 PM
post Re: RDRSSTC - Project Build
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Michelle_
June 02, 2024, 04:44:30 PM
post Re: My completed 14-stage Cockroft-Walton voltage multiplier
[Voltage Multipliers]
MRMILSTAR
June 02, 2024, 03:25:11 PM
post APC Smart-UPS 1500VA Teardown
[Electronic Circuits]
Mads Barnkob
June 02, 2024, 01:36:11 PM
post Re: My completed 14-stage Cockroft-Walton voltage multiplier
[Voltage Multipliers]
pete_dl
June 02, 2024, 11:43:40 AM
post Re: RDRSSTC - Project Build
[Dual Resonant Solid State Tesla coils (DRSSTC)]
ZakW
June 02, 2024, 08:49:11 AM
post Re: RDRSSTC - Project Build
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Benjamin Lockhart
June 02, 2024, 08:36:07 AM
post Re: RDRSSTC - Project Build
[Dual Resonant Solid State Tesla coils (DRSSTC)]
ZakW
June 02, 2024, 12:41:50 AM
post Re: Steve Ward SSTC5 build
[Solid State Tesla Coils (SSTC)]
Benjamin Lockhart
June 01, 2024, 08:24:35 PM
post Pearson current monitor model 1330 teardown (100kA)
[Laboratories, Equipment and Tools]
Mads Barnkob
June 01, 2024, 08:22:28 PM
post Re: Steve Ward SSTC5 build
[Solid State Tesla Coils (SSTC)]
Michelle_
June 01, 2024, 03:56:53 AM
post Re: Racing sparks
[Spark Gap Tesla Coils (SGTC)]
Terry
June 01, 2024, 12:03:50 AM
post Re: First DRSSTC questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Hi-Deff
May 31, 2024, 11:46:20 PM
post Re: Steve Ward SSTC5 build
[Solid State Tesla Coils (SSTC)]
Michelle_
May 31, 2024, 06:13:59 PM
post Re: First DRSSTC questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 31, 2024, 01:55:22 AM
post Re: First DRSSTC questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
May 30, 2024, 11:57:49 PM
post Re: First DRSSTC questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Benjamin Lockhart
May 30, 2024, 11:50:06 PM
post First DRSSTC questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Hi-Deff
May 30, 2024, 11:23:35 PM
post Re: Steve Ward SSTC5 build
[Solid State Tesla Coils (SSTC)]
davekni
May 30, 2024, 10:43:06 PM
post Re: Benjamin's DRSSTC3
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 30, 2024, 10:24:35 PM
post Re: SSTC tripping circuit breaker
[Solid State Tesla Coils (SSTC)]
NyaaX_X
May 30, 2024, 06:10:04 PM

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