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First Drsstc!

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Andrea87

Sat Nov 11 2006, 01:07PM

Registered Member #494 Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31

After having built a big VTTC, and several little zvs oscillators, its time for a small but tremendous coil, a DRSSTC.

I've made a small secondary out of a 2" (50mm) tubing, wound with 1400 turns of 30AWG (0.254mm) wire up to a length of 35cm (13.6"), for a 1/4Î» top-loaded frequency of 365KHz.
this is the tiny coil:

I've made a controller circuit based on Steve Ward's tutorial, exactly that, thanks Steve! Atm I'm dropping it into the FeCl3 to etch it.
By now I've ready there the secondary (still needs to be coated in resin) the little 4" spheric topload made from old cds and tape, and a little 1:300 current transformer I'll use to sync the oscillator to the sine wave of the primary circuit:

I've got here a bunch of HGT1N40N60A4D and some other igbts, I still need to choose wich ones to use. I'm gonna make a Half bridge, the gdt is ready, and the Ucc's are expected to come here in a week

As primary, I'll go on copper tubing I think, still need to find it. The mmc will be made of a ton of 0.015ÂµF 1,5kV polipropylene caps, got some hundreds of these some time ago for 1c each.

Does anyone have any suggestion on this coil? I think when finished will look good and crazy!

Marko

Sat Nov 11 2006, 01:21PM

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

I'v also been warned: for your first DRSSTC, you should make it HUGE.

This will lower your Fres down, help IGBT's stayng cool and sparks staying big.

I would also go for a fullbridge from start as I was somewhat dissapointed with my first halfbridge results. (but do as you want).

What cores are those used in your CT?

Andrea87

Sat Nov 11 2006, 01:30PM

Registered Member #494 Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31

Thanks for the reply Firkragg.

The two cores are (I think) a pair of ferrite powder cores, got these at a local ham radio fair.

I think I'll build a 80mm x 60cm coil with the 0.25 wire, I should stay much under the 200KHz range.

I've done some calculations by now, with a 2-turns 18cm (7") primary (approx 4 ÂµH) I can get the Fres of about 360KHz with only 0.047Âµ of caps, what about it? by now I have here only the secondary of the VTTCBIG, that is 160mm x 65cm, wound with 0,72mm wire, for a self res of about 330KHz, big but high freq too. I'll try to build this coil, and if the drsstc setup doesn't give the best results I'll change it into a simple OLTC, leaving the Drsstc bridge for a bigger coil.

Marko

Sat Nov 11 2006, 01:49PM

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

Drsstc's usually like lower frequencies, around 100kHz because of slow switching speed of IGBT's.

Lots of people, although, made theirs work at several hundreds of kHz, but IGBT's die more easily there.

For my coil i switched from 400kHz secondary to 150 with good results.

Regarding cores; ferrite powder seems to have low permeability to normal cores, although I dunno if it's going to be a problem with CT's.

I would rather pick some ferrite toroids just to be sure, if you have them at hand.
(to eliminate CT's from list of possible problems).

I also happen to be rebuilding my drsstc in new box recently, and I can post some pics in new thread if you and others are interested.

Cap was my biggest problem as I couldn't get away from poliester caps, but it seems that you solved it!

Lots of small paralell caps is actually a very good approach because they will share much smaller portions of current and be more happy.

Just make sure that they are not 'normal' MKP (they must be film/foil, film/foil+metalisation or at least double metalised).

Good luck with the coil

Andrea87

Sat Nov 11 2006, 02:42PM

Registered Member #494 Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31

For the gate driver xfmr I've used a certainly ferrite core, for the current feedback by now I've these, next fair I'll pick up some diff cores.

Maybe just as test I'll buildup a simple zvs driver for that coil, the common royer circuit that works allways and in any config with good results.

The caps are good, metallized film, KP, tested on the VTTC under severe load and worked well. I've got also a hundred of 2u2 400V MKP caps, but they look like to be a lil oversized for a such high freq.

on monday I'll go get a new tube, and start to wind it, to ged a lower freq.

shall I change this drsstc into an OLTC?

Marko

Sat Nov 11 2006, 02:53PM

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

Definitely not. OLTC's are proven to work relatively worse than DRSSTC's, need operating frequency <50kHz for good results and need more expensive siliocn for same spark.

'KP' is not metallised film but rather film and real metal foil instead.
These caps are proven best for high-current DRSSTC duty.

Andrea87

Sat Nov 11 2006, 03:22PM

Registered Member #494 Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31

Firkragg wrote ...

'KP' is not metallised film but rather film and real metal foil instead.
These caps are proven best for high-current DRSSTC duty.

well!

I have about 300 of these here around, marked for 0.015uF 1500V.
As the secondary coil, I'll wind a 100mm x 50cm coil with the 0.25 wire, and get a good 121KHz 1/4 wave fres.

now I'm gonna try the little coil with the mazzilli's circuit... let's see what occurs!

Edit: made a littler coil since the zvs got easily to 1.2MHz, tomorrow I'll see how it goes...

Andrea87

Mon Nov 13 2006, 04:57PM

Registered Member #494 Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31

Today I've done some shopping... a serious pvc tube for the tc.

the tube is 4" outer diameter (10cm), I'll wind it with a 0.25mm wire for 50cm, leaving 2-3 cm free on every side. With a serious topload (expected to be a 30cm sphere - 12") I'll get a resonant frequency at 1/4Î» of about 120KHz, much more easier to work with than the other coil I've wound, that, still with a huge topload, would do 1/4Î» at a mad 350KHz. I've also there a good amount of 0.17mm wire, but I don't know if it will work... I'll go tomorrow to my old school and ask them to use their lathing machine to wind the coil XD.

As the driver, I'll use steve's one, it's a lil complex but should run well.

I would like to ask a thing to the most experienced drsstc coilers about the feedback, on the schematic the sync to the resonation is taken from the secondary, is this better than taking it directly on the primary coil?

Do anyone know where I can get some good copper tubing? I've searched a lil here in my town, but without any effort :(... by now I'm planning to use an alluminium primary, made with 2 turns of a 2x25mm bar

edit: wound the gate driver, done it with a 36mm (1.4") ferrite toroid, 20 turns of quadrifilar 0.4mm as secondary and 10T of 0.6mmq primary winding, to get an 1:2 ratio, and about 30V on the gates at 15V of power supply. pic here:

It's made to drive either a full or half bridge, by now I think I'll go for the second option for a simpler setup and less igbts involved :D

Andrea

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