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.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
The worst 'drawback' of class E I found is it's sensitivity to change in load impedance (and reactance).
If I didn't let out a breakout on my little coil topload peak coltage would become enormous and coil would start drawing insane currents, burning most of power on the mosfet due to mismatch (heavy over-dampness). I would have to drastiically retune the coupling to make coil work like that and then I would get puny output with breakout point.
Winding HF coils like 'normal' coils probably isn't best practice at all (I had over 1k unloaded impedance), but that's what we got...
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Hi Firkragg! 1k input impedance should not create problems, unless you risk "overvolting" your MOSFET. (BTW-how did you measure the impedance?) The problem is if you happen to start up near the series resonant frequency of the coupled coil...and particularly if your MOSFET is seeing a capacitive load reactance (which switching circuits do not like very much). Large currents will result. (I've smoked a few MOSFETs verifying this theory! )
I've done some computations using a rigorous model that indicate that the highest fields are attained at the top of your coil between the series and parallel resonances. At the peak voltage point, the primary feedpoint will likely exhibit a fair bit of capacitive reactance...which, for the health of your MOSFET switch, is best to tune out using a matching network. See the bottom of my PLL tesla coil page where I try to continually add stuff..... (I couldn't resist giving my coil a name from Norse mythology...given the trend here at 4HV! )
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
How can you compute impedance from skin depth and dc resistance for a non-arcing coil? (Skin depth can give you the AC surface resistance of the windings, but nothing else..) You are modeling an inductively coupled resonant circuit.
If the input impedance is dominated by inductance (just below the parallel resonance), little current is drawn from the DC supply... If, on the other hand, you excite the series resonance, the low feedpoint impedance can cause excessive current to flow in the MOSFET drain...thereby smoking it. Somewhere in between generates the highest voltage in the coil.. I discuss in my PLL Tesla coil page that it seems that the "maximum spark" is obtained where the feedpoint impedance exhibits significant capacitive reactance (which puts strain on switching elements). Of course, the loading of the primary with any capacitance (MOSFET drain capacitance, parallel drain capacitor, for example) causes more capacitive reactance to appear as well as introducing a second resonance that can possibly kill mosfets.. I am putting together some computations that show the effects of this..
To stabilise things, some series inductance seems to be needed in the primary circuit.. As you see in my driver circuit, a variable parallel capacitance can be used in conjunction with a bit of extra series inductance to adjust for ideal match while the all-pass phase shift network can correct phase errors introduced by the matching network. I can freely dial up class E or class C operation and anywhere in between..
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Oh, stupid me ^ I see I nowhere mentioned that I used prsimary itself as a charging 'choke' instead of separate class E stage.
With big enogh decoupling cap it seems to work just as well.
AFAIK (sccording to r. burnett) when coil is tuned into resonance it's unloaded base impedance *is* just the AC resistance of windings, and it grows bigger when sparks load the coil.
I never had any serious 'out-of-tune' problems since coil seems to draw relatively little current no matter if it is higher or lower than resonance. The 'capacitiveness' thing you are mentioning doesn't seem to be a problem at all as capacitance is very small and I don't think it will look like soemthing more than few hundred pf on the mosfet.
When I decide approximate number of primary turns the fun begins - lots of moving up-down and changing number of turns, until best mach and nicest class E waveform is acheived. (toast for a 'scientific' approach ^^)
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Oh, stupid me ^ I see I nowhere mentioned that I used prsimary itself as a charging 'choke' instead of separate class E stage.
I figured I was missing a bit of info..
AFAIK (sccording to r. burnett) when coil is tuned into resonance it's unloaded base impedance *is* just the AC resistance of windings, and it grows bigger when sparks load the coil.
You are quite right...as long as we are talking about series-like resonance. Add in the effect of the coupling (primary) coil on secondary resonant frequencies and things become more complicated ..even more when we add capacitors/inductors to the primary circuit..
I never had any serious 'out-of-tune' problems since coil seems to draw relatively little current no matter if it is higher or lower than resonance. The 'capacitiveness' thing you are mentioning doesn't seem to be a problem at all as capacitance is very small and I don't think it will look like soemthing more than few hundred pf on the mosfet.
The effect I am describing may or may not be a problem... Depends on many things (coupling, losses and loading, etc.) I just found it interesting to model and discover that the max voltage at the top of the coil may not necessarily occur at the "resonant" frequencies seen at the primary coil input terminals.
When I decide approximate number of primary turns the fun begins - lots of moving up-down and changing number of turns, until best mach and nicest class E waveform is acheived. (toast for a 'scientific' approach ^^)
Yours is a perfectly good way to handle the coupling problem. As the coupling goes down, the capacitance effect decreases (as primary self-inductance begins to dominate). I designed my matching circuit as an attempt to transfer maximum power to the secondary while maintaining something close to class-E transistor switching (using a variable capacitor to vary the primary coupling). It seems to work... Yesterday, I ran the coil for two hours. The transistor was barely warm to the touch (I was using fan cooling.) The base of my coil did get a little bit warm, tho' not enough to hurt the PVC insulation or coil form...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
You are quite right...as long as we are talking about series-like resonance. Add in the effect of the coupling (primary) coil on secondary resonant frequencies and things become more complicated ..even more when we add capacitors/inductors to the primary circuit..
It's a base impedance, considered when secondary is unloaded and without the primary mess.
The base of your coil is getting warm because secondary is coupling capacitively to primary, causing heavy dielectric losses in PVC.
I melted the base of my first secondary that way, so I use a wide former-less primary wich is set higher to keep coupling same.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Hi Firkragg and "balsamplack"
The lower windings of the coil get hot because the current near the base of the coil is maximum (can be amps to 10s of amps depending on power level, loading, etc..)
These HF coils behave a lot like a 1/4 wave transmission line which is open at the end and shorted at the other. Current is high at the shorted end (where the coil is connected to a "ground-plane" or faraday cage) and voltage is high at the open end..
I did some simulations here when I was attempting to get my Royer/Armstrong coils to work. (The primary geometry is a little bit different, but the resonator behaviour should be the same..) This plot shows the normalised coil current along the helix for a coil with no topload and the primary wound right at the base. It is highest at the base (0m) and small at the top (25m), therefore copper (ohmic) losses from the current are highest here...so the base gets hot!
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.
5 MHz CW TC with PLL tracking oscillator
)
)
