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SSTC does not buzz!?

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brtaman

Mon Aug 17 2009, 08:23PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Hello everyone,

I have run into a peculiar problem with my SSTC. It is based off Steve Wards Mini-SSTC, pretty much exactly as far as the drive side is concerned.

Only differences are:
-It is running full-wave rectified (220VAC).
-uses 1n34a diode on the schmitt trigger

Secondary is around 500kHz
Primary is @9 turns

The problem I am having a lack of buzz that can be seen in all SSTC videos I have come across. Instead all I get is a shriek that is not only bad for the FETs but also plenty annoying. Performance is quite good actually (2-3 inches with low duty), though atm it is only running with a 75W bulb in series. Apart from the shrieking and the fact I just burned my last fet, it is actually working quite good. ATM I have no o-scope (I am eying a decent one on e-bay atm), so I cannot provide any waveforms.

So my questions to the guys who have had this strange problem, how did you get rid of it?

The GDT is 16 turns of CAT5 cable on a core with excellent properties for a GDT. Its Ui of (4300) and AL (7850) are not the highest from the ferrite torroids and materials I have come to posses, but the materials very low losses all the way up to 1MHz should make it ideal for this relateivelly high frequency SSTC.

This problem seems to not have been documented much, the only reference to it that I found was on the forum archives in which Steve Ward suggested moving the antenna around and informed a forum member that it occurs due to improper feedback, though in my case moving the antenna has produced no results, it basically doesn't do anything. I am getting some fets tomorrow, have a good electronics shop on the commute from work (460s are "only" 2.45 EURO's a pop), I am thinking of making the antenna shorter down to 15cm, hopefully this will help.

Any help would be greatly appreciated.

Thanks
brtaman

Mads Barnkob

Mon Aug 17 2009, 09:09PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

It could be a combination of breakrate and your rather high resonant frequency.

Have you tried with a larger topload to lower the frequency?

The buzzing sound will almost only come from a continues wave fed sstc, when its interrupted its more like bangs.

I saw a video of a sstc in a kitchen that screamed quite awful, but can't remember much more than that, it was on youtube.

brtaman

Mon Aug 17 2009, 11:09PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Hey madskaizer,

I have tried different toploads does not change the sound too much.

BTW found a MiniSSTC based TC making the exact same sounds on youtube:

Here is another one, sound is a bit clearer on this one:

Judging by the power of the first one, it is not as big of a problem as I imagined?

Arcstarter

Tue Aug 18 2009, 12:46AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Hey, Brtaman.

The shrilling on those videos is just because of the high interrupt rate.

I have ran the mini sstc circuit to 1.4mhz. The toroid was not right, and i did not add any more turns like i should have to avoid saturation of the GDT, but the UCCs would run for about 30 seconds before having any real heat. So 500khz is not too much.

I might suggest trying a CT though... It helped bring the mosfet deaths and heating down, and there is no risk of antenna arcovers, corona, or the antenna touching a surrounding metal object and killing the feedback. Just get a good toroid and something like 1:60 or 1:100 or something is suitable. Ground one side, and the other goes to a 1kohm resistor and a .1uf capacitor. That is how i did it, and it worked great for the mini sstc and the PLL audio mod.

brtaman

Mon Aug 24 2009, 11:32PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Hey,

Thanks for the suggestion arcstarter, I will look into a using a CT, I have already wound one with 30 AWG magnet wire, ~90 turns. Should give some good results as you said. However, how did the TC act when you drew an arc? Did it keep in tune?

I am putting this project on hold for now, there seems to be something inherently wrong with my GDT which I don't understand. The torroid material is PERFECT for the application, I couldn't find anything better searching the internet.

(It is from an OEM company they only sell in the 10's of thousands to all over the world, luckily their headquarters is only 20 miles from my house, I managed to get some samples of great materials, one of them is perfect for DRSSTC this one should be perfect for an SSTC with extremely small losses up to 1 MHz)

I have tried everything but there is still something going on with the GDT, I suspect ringing, however higher values of gate resistors don't help etc. I am on the hunt for an o-scope atm. ebay.co.uk has some pretty nice deals coming from time to time, just have to snag the right one.

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