If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
I've used some data provided by Steve Ward to show the effect of the 'tuning trick' he described. The diagrams below show the primary current of a simulation. In the first diagram the coil is made to run at the upper pole. In the second, the coil is run 'conventionally' at the lower pole.
In lower pole operation there is a surge of primary current due to the fact, that the coil is strongly out of tune before arc loading begins. Afterwards the arc pulls the coil into tune, so that current drops low. In upper pole operation that is pretty much reversed. The coil is close to tune initially but then gets somewhat out of tune later. This is desirable, since otherwise there would not be much primary current available to lengthen arcs in QCW operation.
Below are 2 diagrams, which show the real (blue) and imaginary (red) parts of the primary impedance (ohms) as a function of frequency. These diagrams show the behaviour when the currents have settled to their final states. Zero current switching occurs, if primary current and input voltage are in phase, i.e. when the red curve crosses 0.
The difference between the 2 diagrams is caused by the heavier arc loading in upper pole operation. There the coil runs at about 330kHz. Primary impedance is about 2 ohms. In lower pole operation the frequency is 270kHz and the primary impedance about 4 ohms. This difference in impedance causes higher primary currents in upper pole mode.
The ultimate tuning trick, which keeps primary currents as low as possible, would be to measure the phase between primary and secondary current and keep it at 90 degrees by adjusting frequency. That involves hard switching, though.
I've been wondering, if the initial surge of primary current is bad, when working with non QCW coils. The primary tank accumulates a lot of energy, which leads to a power surge to the secondary at the point, when the secondary is pulled into tune and then empties the primary tank of its energy. This can easily double peak power output compared to what the input from the bus provides. This effect can be enhanced by using large primary inductances since the energy stored in an inductance is 0.5 * L * I^2. Max transistor currents could stay limited, but burst length would have to be increased due to the slower rampup of primary current. One could e.g. double the primary inductance, halve the MMC, double the burst length and then would have twice the energy in the primary tank.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Uspring wrote ...
I've been wondering, if the initial surge of primary current is bad, when working with non QCW coils. The primary tank accumulates a lot of energy, which leads to a power surge to the secondary at the point, when the secondary is pulled into tune and then empties the primary tank of its energy. This can easily double peak power output compared to what the input from the bus provides. This effect can be enhanced by using large primary inductances since the energy stored in an inductance is 0.5 * L * I^2. Max transistor currents could stay limited, but burst length would have to be increased due to the slower rampup of primary current. One could e.g. double the primary inductance, halve the MMC, double the burst length and then would have twice the energy in the primary tank.
Has anybody tried that?
Yes, since I used a PLL driver on my DRSSTC, I could select which pole to use and even swap between them during a run to compare the effects.
The effects I saw were consistent with your explanation. In particular, operating on the lower pole frequency gave trouble with racing sparks and flashovers, as you might expect from a sudden dump of energy into the secondary.
I settled on the upper pole frequency for all my DRSSTCs because it seemed to give smoother and more controllable operation, not to mention much more spark output before the flashovers started. The sudden energy dump seemed to be more of a liability than an asset.
I always argued that a DRSSTC should be designed to transfer energy straight from the electrolytic bus caps to the streamer load, with the coils just operating as a matching network. It gives more spark length per dollar than a design that charges up the primary tank circuit like a classic spark-gap coil and then dumps it into the secondary, because electrolytic caps are cheaper per joule stored than film/foil ones. It also allows increasing the burst length without limit to get apocalyptic super-bangs.
As an aside: For a musical coil you want a smooth correspondence between burst length and spark loudness. Upper pole operation seems to help with this too. On the lower pole, as the burst length is increased the output jumps suddenly from a tiny fizzle to a big bang.
My spark model seems to indicate, that for spark length it is advantageous to spend the bang energy in a short power surge instead of a longer more even release. Lower pole mode would just do that. The simulation might be wrong, though.
If I tune my primary only a little bit low, the simulations show a reduced primary current peak and a larger current thereafter. It looks similar to upper pole mode. I've tried that on my coil but spark output is less this way. Possibly I need much longer bursts in that case.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
This might well be the case if you're trying to optimise spark length per watt of input. However if you are looking to optimise spark length per inch of secondary coil, you have to lower the peak power because a small secondary can't stand a high voltage, and then lengthen the burst to compensate.
The two design goals are conflicting and every Tesla coil is a different compromise, from John Freau's spark-gap coils at one extreme to Steve Ward's QCW at the other.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi Uspring, you say
Uspring wrote ...
The ultimate tuning trick, which keeps primary currents as low as possible, would be to measure the phase between primary and secondary current and keep it at 90 degrees by adjusting frequency. That involves hard switching, though.
Where do you think the transistors will switch, before zero crossing (inductive side) or after it (capacitive)? I was trying to imagine the situation, but I can't make out where the switching transitions will occur.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Uspring wrote ...
The ultimate tuning trick, which keeps primary currents as low as possible, would be to measure the phase between primary and secondary current and keep it at 90 degrees by adjusting frequency. That involves hard switching, though.
Not necessarily, The driver I'm currently working can swing phase anywhere between 90 and 180 and still soft switches. The trick is to use ZVS rather than ZCS (very similar to class E operation). This gives you the ability to use the parasitic capacitance in the switch to commutate the current and control the rise and fall waveform. Another advantage to this driver is that like Steve's PLL I can force the coil to operate at ether pole.
Using this technique you could switch at any point in a half cycle without major losses.
As for your observations, I completely agree that less energy for more time is beneficial to operation. The smooth action of adding less energy over more time is how 90% of my DRSSTCs operate. However rather than doing it with tuning I simply use a higher impedance tank to control the current ring-up. This typically allows for higher coupling without flashing over. In my large DR the coupling is actually almost 0.23k, this kind of coupling wouldn’t be possible with a faster ring-up of the primary current like in most DRSSTC.
Uspring wrote ...
I've been wondering, if the initial surge of primary current is bad, when working with non QCW coils. The primary tank accumulates a lot of energy, which leads to a power surge to the secondary at the point, when the secondary is pulled into tune and then empties the primary tank of its energy. This can easily double peak power output compared to what the input from the bus provides. This effect can be enhanced by using large primary inductances since the energy stored in an inductance is 0.5 * L * I^2. Max transistor currents could stay limited, but burst length would have to be increased due to the slower rampup of primary current. One could e.g. double the primary inductance, halve the MMC, double the burst length and then would have twice the energy in the primary tank.
Has anybody tried that?
Part of the initial surge of current in the QCW is waiting for the voltage on top load to reach a point such that it can break out. In A DRSSTC it's typically not noticeable because this breakout voltage is reached in the first half cycle in most cases. However in the QCW typically 10's to 100's of cycles are needed before this voltage is reached. Until it does break out, the current rings up very fast due to the Q being so high from lack of arc loading.
Also what you are explain using a high impedance tanks with lots of L is how I already run most of my DRs as I was talking about above. I think it's a great method and I have achieved some wonderful results using it.
Where do you think the transistors will switch, before zero crossing (inductive side) or after it (capacitive)? I was trying to imagine the situation, but I can't make out where the switching transitions will occur.
I don't have a precise answer to that. In the lower impedance diagram (light arc load), the red curve (imaginary part of primary impedance) crosses zero at the point where the blue curve is max, which is also the point, where you have 90 degrees phase shift between primary and secondary current. At this point you have ZCS. For heavy arc loads (upper diagram), the primary will look more inductive, if its res frequency is below that of the secondary and more capacitive, if its the other way around. For intermediate loads, the wiggle in the red curve makes the phase between primary voltage and current difficult to predict. The change of secondary res frequency due to arc growth makes that worse.
Eric:
Not necessarily, The driver I'm currently working can swing phase anywhere between 90 and 180 and still soft switches. The trick is to use ZVS rather than ZCS (very similar to class E operation). This gives you the ability to use the parasitic capacitance in the switch to commutate the current and control the rise and fall waveform. Another advantage to this driver is that like Steve's PLL I can force the coil to operate at ether pole.
Now that you mention it, I remember some of your posts about this. It probably slipped my mind, because I never understood it. Does that allow you to safely switch at any time?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Eric, I would think the parasitic capacitances in the switches can't slow down the transitions enough to decrease the turn-off losses, it is still hard switching. You would need to add external capacitance, but then if the tank circuit will look capacitive just for a moment, you are in trouble because the caps discharge right into the transistors.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Yes you are correct in that typically some extra C is added, usually a couple extra nF depending on how slow you want the rise and fall times to be. The system can switching almost anywhere as long as enough current is flowing at the time of turn on or turn off to commutate the voltage. The only time ZVS typically will no fully ZVS is at the very beginning of a burst when the current is not high enough to fully commutate the voltage. This is generally not an issue because the current is usually low at this point anyway and wont constitute much in losses.
Udo, the basic idea is that you use the current flowing in the tank to charge or discharge the capacitors across the switches, when a cap is fully charged or discharged, you have basically forced the voltage across that cap (and the switch) to ether 0V or the +rail voltage (depending on if you are turning on or off). This way the switch turns on or off with no potential across it when it goes to switch, hence zero volts.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Tuning tricks, poles etc.
