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The hunt for 10 MHz

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Marko

Tue Mar 14 2006, 02:59PM

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

Im not sue about PLL's, if you want FM you could just feed VCO input of single PLL.
It could make some sounds at last driving coil in and out of resonance but simple power rail modulation, especially for such small coil. And I think it would sound much better.

Heatsink is monster, you could use that for 1000W DRSSTC, not miniature SSTC

Probably it would be completly OK even without heatsink.

What voltage are you driving it at?

Plasmaniac

Tue Mar 14 2006, 04:23PM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
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Posts: 72

The heatsink was actually for a bigger coil with this transformator (0,4kW =280...800V)

I ran out of MOSFETs, so I used those on the heatsink.

At the moment, I'm driving it @ 24V and a few 100mA. Later I'll drive it with 12..36V and maybe 2A... I hope to get some 2 or 3cm sparks. The circruit draws just a few 100mA in 1 MOSFET operation because of the choke that I use to eliminate HF spikes (you can see it in the pic from my last post). In full- or halfbridge mode, the circruit allways draws nearly the same current so the choke wont block the current anymore. I also forgot a 2,2ÂµF cap in parallel with the bridge's supply rails which would also allow more current draw.

Just one VCO? I'm too lazy to tune it

Yesterday, I saw how hard the frequenzy changes, if i come closer to the coil with my hand or a screwdriver (I've put my scope's probe ~10cm away from the coils breakout point and played with the coil while watching the perfect sine wave getting shorter and longer again for about 5 minutes, then the driver died

However... I wont build one more non-selftuning coil after this experience

teravolt

Wed Mar 15 2006, 04:22AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

you could use a MAX038 function generator IC that goes to 20Mhz or any ocilator. maby a 10Mhz ocilator IC would work. If you could take that signal and amplify it and drive a RF final tube. You could then adjust your secondary with top load capasitance. At 10Mhz the circuit would behave like transmitter and it might need tank circuits to tune it.

Plasmaniac

Fri Mar 17 2006, 10:22PM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
Location:
Posts: 72

The PLL dirver without audiomodulation works just perfect. I get up to 2cm sparks at 12V and 2A. I am worrying about the coils. Both (secondary AND primary) get very warm (not hot, but warmer, than coils actually should get...) due to skin effect resistance increase. The MOSFETs (now on a homemade 35x40x1,5mm heatsink) get just slightly warm.

Here's a very very bad pic of my prototype driver:

I'll post some better pics as soon as I find my CF-card...

I think I'll go for the audiomodulated design tomorrow. Now, I'm working on the SMD PCB.

Marko

Fri Mar 17 2006, 10:27PM

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

If you look at 'spectacular failures' you can see my molten secondary of small royer coil.
Before it starts to happen to you, pour some poliester or epoxy inside the secondary former

Plasmaniac

Fri Mar 17 2006, 10:44PM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
Location:
Posts: 72

My secondary is filled with hot glue

ragnar

Fri Mar 17 2006, 10:53PM

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

Are you using a 74HC4046?

Some of the nicer Motorola versions can go up to 18MHz.. but most at least 12MHz =)

Marko

Fri Mar 17 2006, 11:00PM

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

HC are usually hard-to-get...

Hot glue is going to melt together with secondary (easier) and it will go bannana, overlap, arc over and get toasted.
My coil gets untouchable hot after few minutes, after half an hour it would be a blob of burning PVC

Thin wire results in large ohmic heating due to skin-effect at high frequencies, so if you cannot cool secondary down you must solidify it somehow...

Plasmaniac

Fri Mar 17 2006, 11:08PM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
Location:
Posts: 72

I'm using a CD4046. IIRC, you can't drive HCs or HCTs at 12V.

The secondary doesn't get that hot, it just gets very warm. The only point, where th hot glue melts is, where I've put in the breakout point which gety very warm because of the hot hf plasma.

ragnar

Fri Mar 17 2006, 11:28PM

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

I thought you might be interested in the 74HC as, even though it requires a 5V supply (shouldn't be a problem?), it can get up to those higher frequencies...

...if you've got a good gate-driver layout and nice FETs, I can't see why it would be too impossible to switch at 10MHz.

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