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"Stubby" SSTC

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LAN_403

thedatastream

Mon Dec 04 2006, 02:39PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Finally...

Originally inspired by Steve Ward's Mini SSTC and started about 2 1/2 years ago, I've finally had enough lunchtimes recently to get it sorted.

A 7" high, 4" diameter secondary running at 250kHz (with topload and 1" CW spark) is driven by a half bridge of STE70NM60 ISOTOP MOSFETs mounted on a section of aluminium U-channel. The bridge is laid out on double sided copper clad PCB with the tracks cut out using a scalpel. Control circuit is Steve Conner's DWSSTC PLL driver and GDT is a bifilar secondary with a coaxial primary, 8 turns on a 22mm Epcos N27 toroid. GDT was good enough to use without series gate resistance. It's all now mounted on a bit of board with antennas and stuff and is looking much better than it used to.

Low input voltage performance is OK when correctly tuned using the phase adjust facility, although there is still ringing on the waveforms. However, the peak on the bridge voltage seems to stay pretty constant with increasing voltage so I'm not too worried.

Gate drive

Bridge voltage and current

I ran it up to full operating voltage on a variac this lunchtime in the modle shop at work with a colleague acting as Igor. With interrupter set at about 20% duty it belts out streamers with ground strikes at 6 inches with a really loud buzz. Left it running at maximum input for a few minutes, not much heating of transistors, runs great!

Maximum performance of 6" arcs to ground!

Needless to say, I am well chuffed

, especially after the initial failures with the first layout all those years ago. Next step, less primary turns and see if we can get 7" to ground.

James

vasil

Mon Dec 04 2006, 04:52PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Cool!

Tom540

Tue Dec 05 2006, 06:13AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Very Nice. I was thinking of trying out that PLL chip recently as well. I've had a few laying around for some time but got discouraged after reading the part about how it might not be that great or something on Conners page. I think I'm going to give it a try this weekend.

thedatastream

Tue Dec 05 2006, 01:16PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Reduced number of primary turns from 9 to 6 and ran up to 270 Vac input. Now giving reliable 7" and very occasional 8" strikes to ground.

Sparks still sound a little bit stuttery at some power levels, possibly there is some interaction between interrupter frequency and mains frequency, or perhaps the PLL misses a beat perhaps.

Once I've added an audio modulator then I shall consider this project finished and keep this as my "demo" coil. An arc-to-ground performance greater than the height of the coil is acceptable in my book.

James

coaster_chris

Fri Dec 08 2006, 01:02AM

Registered Member #359 Joined: Sat Apr 01 2006, 09:27AM
Location: Utrecht, The Netherlands
Posts: 16

HA! Looking good!

I'm bussy aswell with a SSTC based on Conner's PLL driver.
I've just got some of philips PLL's and Texas Instruments kindly sent me out some UCC 37321/2 MOSFET driver samples.

I have got the sec wound too. 2000 turns of 0.2 mm wire on a 16cm dia PVC former for a nice low Fres.
Now it's time to start frying some solder, and hopefully I'll get it all working. My hopes are kinda low, but hack... just give it a shot!

Just a question before I start mucking around... what is the way to determine the best values voor de VCO cap and resistor? Just by trial and error?

And err, any other tips 'n trick?

Chris

thedatastream

Fri Dec 08 2006, 08:14AM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

coaster_chris wrote ...
Just a question before I start mucking around... what is the way to determine the best values voor de VCO cap and resistor? Just by trial and error?

From Steve's site

wrote ...
First of all you have to tune the oscillator part of your PLL so that the resonant frequency of your secondary coil lies within the range. The 470pF cap determines the frequency along with the 33 and 47k resistors. The ratio of these resistors determines how far the VCO can be moved by the PLL. I chose a frequency range of 1.5:1 so it could never lock to a harmonic. So it adjusts from (roughly) 150 to 225 kHz. For a higher frequency secondary, you would need a smaller cap, and/or smaller resistors.

I changed the 470p cap to 270p and the 47k to 56k but left everything else as it was on the schematic. That was for a 250kHz coil.

Rgds,
James

thedatastream

Sun Dec 10 2006, 09:05PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Website is now updated with stats, info and pictures -

Stubby also won the xmas decoration competition at work with a wireframe tree decorated with neon lamps and a star emiting streamers on the top! OK, I won by default - no other entries this year - but working for an engineering firm there would have been more than a few votes from the engineers I reckon. Still, won a nice bottle of red for the first prize

I'm going to focus on a smaller coil next, christened "Titch". Expect a project thread at some point.

James

Marko

Sun Dec 10 2006, 09:39PM

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

I don't get why are you using monster 70 ampere mosfets,

but coil looks nice indeed. Any idea of power going into it?

thedatastream

Mon Dec 11 2006, 01:35PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Firkragg wrote ...
I don't get why are you using monster 70 ampere mosfets, but coil looks nice indeed. Any idea of power going into it?

I started off building a bigger SSTC before starting Stubby so I wanted some decent transistors. I had a word with our ST rep and they sponsored me with 25 of those FETs. I must get around to adding a logo or something onto my webpage by way of thanks.

Anyway, large SSTC got dumped - too large to run on desk at work and too much fuss to set up. I decided to re-use the FETs instead of selling them on eBay.

The moving iron meters on top of my variac read approx 270V and 2A when hitting 7" strikes running at about 20% interrupted. I don't know how accurate they are for pulsed current running through them but this equates to about 550W.

James

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