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flannelhead

Tue May 07 2013, 08:38PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

After a long break in my HV hobby, I realized I needed a proper SSTC. My previous attempts had been just quick lash-ups that barely worked. But, believe it or not, I finally succeeded! Now say hello to my little friend.

There were several limitations. First, I still don't have a good power supply for my experiments, so I decided to run the coil off a 12v 300mA wall wart (actual output voltage is about 16 volts). Second, I wanted the coil to be as small as possible. I like the idea of genuine portability.

I started with a secondary I had wound on a film canister. I tried several self-resonant and PLL designs with little success, most likely due to my poor work with the board layout and the high resonant frequency (around 6 MHz). Nevertheless, I got a breakout, which encouraged me to keep on experimenting.

Inspired by Richie Burnett's HFSSTC, I decided to give an old school fixed-frequency oscillator a go. I got a 6.144 MHz crystal oscillator chip from a local electronics store and threw a simple driving circuit together on a breadboard. It quickly turned out that the film can secondary's Fres was just a tad too high, so I wound a new secondary, which gave quite a happy breakout after some ten minutes of tuning.

The driver is essentially a pile of transistors, the only ICs being the crystal oscillator chip and a 7812. The crystal oscillator's signal is amplified by a BC546 class A stage, whose output is in turn fed to a 2N2222/2N2907 totem pole. The totem pole drives a single IRFP240, which drives the primary. Pretty primitive, but it works! It certainly isn't as efficient as it could be, though.

There's also a simple interrupter on the board in order to reduce the power consumption, making it a bit easier on the power supply and the gate drive transistors. It's just a plain two transistor astable multivibrator with variable Ton and Toff.

Now I realize there's a plenty of room for improvement. I'm going to run the coil off 24 volts as soon as I get a new power supply. IRFP240 is a bit overkill here, and it could be replaced with a smaller device with a lower Rds_on and smaller gate capacitance. The fixed frequency driver also gives an opportunity to tune the output stage for efficient class E switching. I'm looking forward to borrow my godfather's oscilloscope.

Along with Richie's excellent site, Reaching's 4MHz audio modulated coil and Uzzors' great 4.096MHz Class E SSTC were important sources of inspiration and information for this project. Thank you!

More pictures and improvements to come soon! I'll try to make some kind of chassis for him.

Sigurthr

Wed May 08 2013, 06:45AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Beautiful! I'm amazed you got an IRFP240 to run at such a high frequency without heating issues. I've always wanted to try a fixed frequency coil but have been scared away by the tuning needed.

flannelhead

Wed May 08 2013, 07:02AM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

Sigurthr wrote ...

Beautiful! I'm amazed you got an IRFP240 to run at such a high frequency without heating issues. I've always wanted to try a fixed frequency coil but have been scared away by the tuning needed.

Thank you!

The IRFP240 would definitely heat if it wasn't for the interrupter and low duty cycle. Currently the gate drive transistors heat more than the FET itself.

flannelhead

Mon May 20 2013, 03:36PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

Now things are getting interesting!

I replaced the IRFP240 with a smaller switch, IRF540, which at least doubled the spark length! However, the MOSFET does get pretty hot without a heatsink. Maybe it's time for some Class E tuning.

This is with just 16 volts in! I don't know what do you think, but in my opinion this is quite decent an output, at least for the low voltage. The spark length is about 1-1.5 cm. Now I'm just waiting for the 24V SMPS to arrive.

TwirlyWhirly555

Mon May 20 2013, 04:25PM

Registered Member #4104 Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122

Very nice work ! xD , Love the little SSTC's , specially the HF ones . Look forward to seeing the 24v supply output :D

flannelhead

Wed May 22 2013, 11:05AM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

Well, here you go! I just picked up the little 24V SMPS I ordered.

The sparks are now much brighter and stretch to about 3 cm, which is almost 3/4 of the length of the secondary winding!

It seems to draw more power than I expected, about 40 watts... The power supply might not be enough, except for short runs. The
IRF540N is also running pretty hot now, and I had to install a small heatsink. Let's see if I can figure out a more efficient driving scheme. Maybe I should now get some capacitors to tune the output stage for Class E operation.

ZakWolf

Wed May 22 2013, 10:14PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

Awesome :D I love the size, I would greatly appreciate a schematic i really want to build this sstc. It would be my first solid state...

thanks zak

Thomas W

Wed May 22 2013, 11:24PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Neat!
ive been wondering,
has anyone tried making a SSTC in the Near Ghz range using some of these transistors:

(this is the only one i could find at the second, but there are cheaper ones like that.)

Ben Solon

Fri May 24 2013, 04:29AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

Sorry to jack the thread(sort of), but look here Tom:

flannelhead

Sat May 25 2013, 09:08PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

ZakWolf wrote ...

Awesome :D I love the size, I would greatly appreciate a schematic i really want to build this sstc. It would be my first solid state...

thanks zak

Thanks for the compliments!
I'm afraid I can't provide the schematic, at least not yet. There's still a lot to do to make this coil run properly (mainly more efficient switching). If you want a starting point, the HF oscillator and amplifier part is pretty much the same as in Reaching's coil, GDT and audio modulation left out. I also added a BS170 in parallel with the BC546 and fed the interrupt signal to its gate.

__=|(:3)-|--{__ wrote ...

Neat!
ive been wondering,
has anyone tried making a SSTC in the Near Ghz range using some of these transistors:

(this is the only one i could find at the second, but there are cheaper ones like that.)

Thank you!
I just wonder how small would that kind of a coil actually be! :D It could be a nice idea, though. I suppose at least MOSFETs are out of question there, as I'm already struggling to drive one small FET at just 6 MHz.

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