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smaller coils (and a 12MHz target)

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LAN_403

WaveRider

Wed Nov 15 2006, 08:53AM

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

BP.. It's outstanding work!

Perhaps if you made all the wiring as compact as possible (short leads, no "big" loops where your primary connects to PCB, etc.) and shielded everything, your bench instruments should not go crazy (I noticed this too before I put my coil in a faraday cage.) You are no doubt radiating significant harmonic power as well as possibly inductively coupling into power-supply leads... Just a thought....

EMC...it's not just for breakfast anymore!

Cheers,
Bill

Avalanche

Wed Nov 15 2006, 09:01AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Amazing work, love it!

That breakout is scary, looks like there's a lot of rf current going into the streamer

ragnar

Wed Nov 15 2006, 09:32AM

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

Thanks Peter, Avalanche, WaveRider =)

In these pics here, you can see the output board from close up:

My primary is connected straight to the drain, i.e.:
+VE >> PRIMARY >> MOSFET >> GND

On that circuit board, the DRAIN and +VE are the big pads on the left and right of the FET, respectively. I've snuggled the decoupling cap in there as best as I can, and you can see where the big negative pad connects straight to SOURCE.

I could connect the primary straight to the heatsink underneath the positive pad (not using insulators, for now, which greatly helps with removing heat from the TO-220 package).

It's interesting to note I'm getting ~100V peaks on the drain with a 32V supply. The IRF730 is a 400V FET, so if I can keep it well enough in tune, I'd consider tripling the supply voltage =D You guys can flame me later.

I should also play around with the turns and coupling on the primary -- I'm sure that two turns isn't optimal.

Keep the ideas coming, thanks again guys =)

Steve Conner

Wed Nov 15 2006, 11:00AM

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

Wow, excellent work BP!

ragnar

Sat Nov 18 2006, 12:10PM

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

[Forgive the double post -- I hit 'submit' twice -- my apologies.

]

ragnar

Sat Nov 18 2006, 12:11PM

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

I raised the secondary by putting two tiny C-shaped aluminium heatsinks over the primary to sit it on, and now he resonates at 12.7MHz. I also get a higher voltage (or at least, longer breakout) at the top of the coil. The plasma appearance is just breathtaking at such high frequencies -- it looks less and less like an electrical discharge and more and more like a tiny blowtorch flame.

The wire forming my secondary (0.5mm, or 0.02in)gets ridiculously hot after a few minutes of operation... considering the skin depth (or lack of!) in copper at 12+MHz, I shouldn't be surprised.. If I had thicker wire, I'd like to rewind it with more turns and a smaller diameter.

I will also adjust primary turns and consider enclosing in a cage.

Avalanche

Sat Nov 18 2006, 01:24PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Nice pictures! How does the discharge compare to your 'lower' frequency class-e coils (in terms of appearance/impressiveness more than size)? I'm seriously considering cutting my frequency in half with my current setup, as it should be easy to get 6Mhz at quite a bit of power with this lot.

Marko

Sat Nov 18 2006, 01:48PM

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

How long is that spark BP?

When I lower my power (1,5Mhz) to few watts I can get a 1cm spark looking like that;

My problem is that I usually don't use enough filter caps so spark buzzes, but if power is very low it just hisses faintly.

ragnar

Sat Nov 18 2006, 01:55PM

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

The angle of the photo doesn't help, but that spark is ~9-10mm in length.

When the signal generator is stable, the arc makes NO noise. In still air it does NOT even appear to move.

If you didn't already see them, the pics from my 6.5MHz coil can be found here:
http://www.infringingwebsite.com/matt/6.5mhz/
Notice how in all the pics, although the edges of the discharge are a very fuzzy plasma glow, it *still* branches out into 'streamers' of some description.

The 6.5MHz discharge still retains a lightning-like appearance that causes hissing/rippling noises, which is why I set my aspirations at 13.56MHz.

As I coax more and more power from the coil (I need to get my Variac operational for this), things will get more interesting, and I think the rippling noises will start again.

Marko

Sat Nov 18 2006, 02:58PM

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

When the signal generator is stable, the arc makes NO noise. In still air it does NOT even appear to move.

Yeah, exactly same thing for me at 1,5Mhz

It looks like a small sword and doesn't move around.

Except I could never take so good pics of it

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