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5 MHz CW TC with PLL tracking oscillator

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WaveRider

Mon Oct 09 2006, 02:15PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Hi all!
Playing around with class-E tesla coils and amplifiers has tempted me to try a tesla coil that "auto tunes" the oscillator using a NE564 PLL. Hence, I present my first prototype using a PLL tuned oscillator for a tesla coil.

Using the NE564 PLL/VCO running at 4 times the resonant frequency , I use two 74LS74 D flip-flops to divide by 4 (so I can get 0 and 90 deg outputs for trimming the feedback phase more easily).

An IXDD414 driver chip is used with a 2SK2698 MOSFET. Further impedance matching and phase correction is achieved with an LC network that feeds the TC primary (sorta like the DRSSTC).

Some details:

1. Secondary: 24m 1.5mm[sup]2[/sup] pvc coated wire on 90mm diam PVC drain pipe.
2. Primary: 10 turns of 1.0mm diam enameled wire close wound on top of secondary as close to the bottom as possible for max magnetic coupling.
3. PLL feedback used antenna (electric field) pickup.
4. Input phase can be adjusted about 45 deg. Another approx 20 deg variation can be had on output adjustment.
5. Typical running frequency at 80-100 watt input: 4.83MHz. Will vary with power as a result of plasma discharge capacitive loading at the top.

Now, some pix:

The circuit board with PLL/VCO, frequency dividing flip-flops, MOSFET driver, voltage regulators and power mosfet.

The full setup

Sparks! Power input is about 75 watts (30 volts in, 2.5 amps). Bright sparks are from zinc coated wire that I used as a topload.

The MOSFET drain waveform (roughly...there is some ringing from the oscilloscope lead..) Peak drain voltage is about 100V with 30V DC input. This depends heavily on the plasma load.

The plasma "flame" is very quiet and very hot. Startup is reliable and immediate. The PLL tracks changes in frequency as the plasma develops (as the PLL tries to keep the phase difference between the antenna pickup and VCO output at 90 deg...the MOSFET amplifier, matching network and input phase control make up the rest of the required 270 deg phase shift for max output power). Fan cooling of the heatsink for the power transistor must be used for reliable CW operation. My plan is to add PWM voltage control to the MOSFET drain circuit to achieve audio modulation... After seeing some of the spectacular audio modulated tesla coils by Steve Conner, EVR, etc., I just have to try it myself.. Comments, assistance, questions welcome!

Schematics simulations and detailed documentation to come soon.

EDIT 20061011

Adding another turn to the matching network inductor, I get something closer to class-E switching operation. The power MOSFET now gets only slightly warm with 200W operation.

Scope shot of Vds

The discharge (could not get close with a ruler to measure...too hot...)

Current meter shows Idd from 100V DC supply.

Bad news!!! I smell smoke!!! Operating at 200W caused my gray PVC drain-pipe coil form to burn... only on the inside!!!??? The plastic must be slightly lossy or I exceeded the breakdown capacity at some point...

The outside (and coil windings) were unharmed! Sh*t!! I'll have to try a new coil form material (teflon??) or maybe a better quality PVC pipe. For now, I'll rewind the coil on another piece of PVC.

Thankfully, the electronics were not at all harmed. The PLL refused to lock and flames shot out of the top of my PVC coil form along with copious amounts of smoke... Gotta air the place out before the wife returns! What a stink!!!

EDIT 20061120:
In order to reduce the possibility of interference and enhance safety, I mounted the tesla coil driver PCB in a diecast aluminium box.

And using some short lengths of RG58U coax to conect to the resonator in the Faraday cage, EMI can be reduced to very low levels. Before using the coax and enclosure for the driver board, there was considerable detectible field (even in other rooms of the house...because of coupling into the power supply). Now, the power supply decoupling seems quite good. The field strength meter barely deflects.

Close to the faraday cage, there is still an evanescent field close to the wire mesh that is readily detectible.

It helps to keep power supply leads and other 'large' metal objects at least 10-20 cm away from the cage in order to minimise coupling to the resonator field.

Here's another spark pic.

I am presently constructing a PWM modulator based on the TL494 much like the one found on EVR's website.
Hope to have some audio mod working in a few days!!!

ragnar

Mon Oct 09 2006, 08:57PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Wow, neato! I bet those IXDD414s are fun =-)

That's what I love about PLL -- you can tweak it so it starts up at one frequency, and then tunes for the corona loading.

Audio modulation is with these reasonably silent coils is very rewarding, and I might mention that a little bit of FM is surprisingly effective so long as you sit on the F > Fres side of the peak and don't modulate too deeply. Although I always found the band of good class-E operation to be very narrow on my coils, there's enough slope to the curve to make frequency modulation worthwhile (and surprisingly easy if one is using a permutation of the 4046 chip)

By the way, if I didn't know better I'd suggest your shunt capacitor looks a wee bit too big; Vds crashing down (discharging into the channel) every cycle..?

When you get round to it, for the sake of audio clarity, try this:

I found an even distribution of corona around the thin copper wire provided a little less volume but greater clarity (of high and low frequencies) than audio reproduced by a plasma from a single point.

If you're looking for a challenge, try running the primary remotely through a metre and a half of coaxial cable. This was one of my challenges for my quadraphonic system -- they all ran from a central control unit, but the 'speakers' could be moved around individually.

Ohh that coax was fun.

Marko

Mon Oct 09 2006, 09:06PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yep, cap looks somewhat too big.
Legendary class E operation looks like almost perfect halfwawe-sine, no ringing at bottom as all energy from cap is supposed to be wasted at that time.

With my present class E coil I put the cap few more cm away from mosfet and little ringing showed up at peak of waveform. It doesn't bother me into getting cap closer as it would be difficult, and I'l always have some leakage inductance.

How does your gate waveform look =) ?

One ''extra'' :) question to waverider if I may: If I put a ferrite toroid/bead over both leads (wich are passing close together, current in opposite diretions) can I reduce leakage inductance that way?

ragnar

Tue Oct 10 2006, 09:29AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

I saw your nice fix in the attachments thread =-)

Here are my waveforms (tuned slightly > Fres prior to modulation, hence the overshoot)

Top wave is Vgs, bottom wave is Vprobe-hanging-near-coil

WaveRider

Tue Oct 10 2006, 10:26AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Hi BP, Firkragg

Yes, the drain cap is too big when the coil is run at low power (75 watts, VDD=30V)
Here, I am running the coil at nearly 250W (VDD=115V). The peak voltage is about 250V on the scope. The plasma loading effects reduce the coil input impedance and the matching network behaves more like it should (giving almost class-E operation).

This waveform is already pretty good in that the power mosfet runs relatively cool. Fan cooling of the heatsink is almost not necessary (the driver chip and voltage regs get hotter than the MOSFET). I plan to modify the phase lead network at the PLL sense antenna input. I should be able to "dial up" class-E...I almost can do it now...just need a few more degrees of phase...

One ''extra'' :) question to waverider if I may: If I put a ferrite toroid/bead over both leads (wich are passing close together, current in opposite diretions) can I reduce leakage inductance that way?

Firkragg: A ferrite bead around the primary input leads should not change primary leakage inductance much. A bead on the leads will reduce common mode radiation (ground-loops, power-supply contamination with RF, etc..). In my case, I use an air-core variable capacitor in the matching network to tune out primary leakage inductance. All other capacitors in the matching network are silver-mica type RF caps. They can handle large RF currents without heating up.

An amusing sidenote: I used my normal alligator-clip test leads to connect the air-variable cap across the TC primary.. after 10 seconds, I began to smell overheating vinyl insulation... The leads were overheating badly!!

I estimate about 10A of reactive RF current was flowing.. My small test leads could not cope... There's a lesson in this.

Now I use nice fat leads...

The gate waveform is pretty much a square wave, given that the gate is driven directly by the output of the IXDD414 (a bipolar totem pole circuit). No resonant drive, just a big fat on-off gate signal that resembles (thankfully) the output of the flip-flop Q signal with about 30ns of delay...

BP: I have cooked up a simple PWM circuit with some 555s, a couple of transistors and the usual caps and resistors to get 0-90% duty cycle modulation using a p-ch mosfet. Pulse freq us about 50kHz. The RF amplifier RF choke and decoupling capacitor should comfortably accomodate 15kHz audio bandwidth for modulating the drain circuit. I'll let you know how it goes.

Modulating the frequency of the VCO is not so easy in my case, because the PLL loop bandwidth is about 10 kHz.. Freqs lower than 10kHz would be greatly attenuated because of the PLL trying to correct the frequency.

I believe you when you say that the coax created problems... Improper matching will cause ringing at harmonics of the fundamental as well as transmission line transformer effects which complicate the design of matching networks. Moreover, if PLL tuning is used, phase shifts introduced by transmission line delays must be compensated for if good performance is to be had. Small cables can also introduce losses, reducing spark size, but short runs with good cable shouldn't be very lossy, if system is well matched.

In fact, my first HF coil used a coax feed from my class-E RF amplifier. I used an SWR meter to see when I had a good match (small reflected power). Worked reasonably well, but was a bit bulky and was not well suited to the PLL tuning idea...

EDIT Hi again BP... Just saw your new message. Looks great! You said the top waveform is Vgs... Is this a typo? It looks a lot like Vds..
I love the "fiery loop" pic. Question: With sustained operation, does your secondary begin to heat up a bit (particularly near the base of the coil)? Mine does when I crank it to 250W, but since I used fairly thick gauge windings, it never gets extremely hot...

Marko

Tue Oct 10 2006, 12:42PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

A ferrite bead around the primary input leads should not change primary leakage inductance much.

Oh, no, I didn't think it for primary, but for leads between mosfet and shunt cap wich seem to cause some extra ringing. I wondered If I can 'couple' them together with a ferrite bead (both of them trough same bead) to kill leakage inductance at that few cm of wires.
(those long leads at top of the pic)
I guess it's not important anyway.

With my coil I got sparks up to about 10cm long with interrupter, and some 6cm in CW, at about 80W input power (power of supply transformer, I don't know for matching and other stuff).

This is how my hard-drive gate waveform looks at 2Mhz.
It could go even faster without the 1 ohm resistor but I don't want to cook the UCC unneedingly.

Single UCC is as close as pssible to mosfet with gate reistor.

Some sparks at unknown <100W (nothing impressive but just for your measure), about 7cm long streamers, coil powered from a triac dimmer.

On the right pic you can see the coil sucessfully opening a small portal to another plane of existence, but I had to close it before nasties started coming trough.

Woops, I spammed the thread enough. I guess It's a frustration after my own class E thread got repeatedly ignored

Steve Conner

Tue Oct 10 2006, 01:17PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Firkragg: I'll let you away with the threadjacking this time, since your pic of the "portal" was neat.

The ferrite bead would increase the common-mode inductance, but it's the differential mode inductance that causes your problem, and the bead won't change this any more than binding the two wires together tightly or shortening then.

WaveRider

Tue Oct 10 2006, 01:43PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Firkragg: The ring of fire is great! How did you do it?

I get about 5 cm of fat hot flame with a couple of cms of hairy streamers at the tip with 220W of DC power on the drain. My point-and-click digital camera is not to good with close-ups...nonetheless, I'll try to get a photo with a ruler scale in the picture.. My main interest is in audio mod, not streamer length, tho...

Yeah..SteveC is right. The ferrite bead in your case will not do much. Can you make PCBs? Use a double-sided board with one side unetched (as a ground plane) and put the components on the etched side...RF microstrip style. This cuts trace inductance and reduces radiation and coupling into adjacent conductors massively. (This is what I've done.) With over 200V RF signal in the MOSFET drain, I see a little over 0.1V RF on the power supply rails. This is good for preventing EMI (keeps thos angry hams away!)

Firkragg... I lurked lots in your and BPs Class-E threads. You guys are doing lots of interesting stuff! Sorry for not being more interactive... I've been burdened with lots of work/personal stuff lately. My little bit of free time was spent on "projecting" rather than HV forum posting...sorry for being a bit absent!

Steve Conner

Tue Oct 10 2006, 02:31PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

WideRaver wrote ...
My little bit of free time was spent on "projecting" rather than HV forum posting

What were you thinking! The whole point of forums is to use up any free time that you might spend on outdated 20th century pursuits like actually building things.

ragnar

Tue Oct 10 2006, 03:04PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

RiveWader, the ring of fire is an oldie but a goodie -- if you've a secondary made from thin enamelled wire, simply unwind a top turn, power up, and watch him spray around -- noisy... until he melts into a ball, that is. =-)

I've got a couple of good "vortex" pics on my other PC.

For the sake of audio, I found my 6.5MHz coil _aaalmost_ totally silent... not a hissing or spraying noise to be heard anywhere near it... but there was still the slightest 'rippling' noise as the plasma flailed in the air. Multiple discharge points to reduce the size of the plasma (and disturbances from airflow, etc) will also reduce the noise if you can cause them to share the corona load evenly.

The plasma appearance at > 6MHz is just amazing... which is why I'm so keen on getting my 13.56MHz coil going...

**strokes new gatedrivers**

I'm quite close, I'll let you know how it goes. I've found fixed-frequency pursuits to be a rewarding challenge... because then I never have an excuse to complain about retuning all my resonant circuits because I fiddled the freq

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