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SSTC - New Secondary = Problems

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MRacerxdl

Sun Oct 25 2009, 05:20PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Hi all, I restarted my work with SSTC again, but now with a fullbridge of IRFP250N.
For the first time I got a Fullbridge working =D

The circuit is like always, 74HC14->74HC08->TC4426->FullBridge IRF540/9540 -> GDT-> FullBridge IRFP250N
I tested with a flyback and a signal generator, and good results with, and I thought that was good to try a tesla coil.

The first SSTC that I made was that:

Its a 4'' x 30cm secondary with 30 AWG wire. It works perfectly with all SSTC's circuits that I ever tested. But I dont have this secondary here now (its with my friend), and I am trying to make a new secondary work.

The new secondary I made, its same for one of my future DRSSTC, I maintan the aspect 3:1 and it has 6'' diameter (In brazil all PVC tubes are in CM, but sold by inches. So its 15cm diameter), and 45cm tall, wound with 30AWG Wire too.

I put it on the fullbridge with a 4 turn primary, tested with 40V and gave me about 3cm sparks. That is without a toroid, if I put a small toroid in the top, the sparks decrease, if I put a large toroid, it decreases more. But the bridge wont fail until 80V ( I didnt test it more)

What can be the cause of that? GDT isnt good to switch that frequency? I am using a old blue core toroid

from a IBM SMPS (I think it was current transformer), it worked very well for my Fullbridge 1.9Mhz SSTC

and I think it doesnt work in the frequency of the coil (that is below than 150Khz, with the big toroid I get 90kHz~)
The other SSTC coil works perfect with that core, and its about 250kHz Fres.
The GDT is 1:1:1:1:1.

Thanks =D

loveHV

Sun Oct 25 2009, 07:09PM

Registered Member #1854 Joined: Wed Dec 10 2008, 06:09PM
Location: West-Flanders Belgium
Posts: 88

are u sure the circuit locks into resonant frequency, i had some problems like yours with my first SSTC.

without toroid nice sparks and with it the sparks were gone. because the toroid made my resonant frequency drop and my driver didn't lock anymore because it was set on the resonant frequency without the toroid.

hope this helps

Dr. Dark Current

Sun Oct 25 2009, 07:36PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Do you use the antenna feedback?
If so, you probably have wrong phasing. Try to swap your primary connections if it makes a difference.

raff

Mon Oct 26 2009, 03:41AM

Registered Member #2315 Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161

placing a topload lowers the Fres... with PLL+VCO osc, I have to replace C1 cap with higher cap value with topload compared to w/o topload.. of course, its better "testing" the values with lower PS ratings (like 30-50Vdc) so as not to kill precious MOSFETs

MRacerxdl

Mon Oct 26 2009, 06:43PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Here I'm not using a PLL, because I never got it to work =P (Without a scope, its not easy to make a pll work)

About the primary phasing, if I invert it, the coil doesnt work, and doesnt oscillate.

About the out of tune, I tested here... Measured with my Frequency Meter, HF Oscillator, and Tesladownunder Fres Finder (I made one board with all those things =P), and get the Fres frequency of each mode.
First, without toroid, the fres is arround 160kHz

Here is the coil:

With the first toroid (the small one) I got 139kHz:

Here is the coil:

With the second toroid (the big one) I got 115kHz:

Here is the coil:

Ok, that is what I measured, now in the running circuit, I tested with my Multimeter Counter, that is more precise, but as the Multimeter is not the same thing that I used to measure the Fres, some differences may ocur, and so, the fres finder is not so precise.
Ok Running I got those values:
For the first, I got 158.4kHz (near than 160khz):

For the second I got 144.5kHz (near than 139kHz):

And for the last one, I got 118.9kHz (near than 115kHz):

The first I got 2cm to the air, second 0,5cm, the last I didnt get any sparks. All tests running at very smooth CW.
So, what can be the problem? GDT is not switching good the mosfet and mess all it up? Or something else?

Thanks =D
(The Links are from the project source system of my site, that is because its not direct a image link =) Here are the site:

)

raff

Mon Oct 26 2009, 10:41PM

Registered Member #2315 Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161

hmmmm THAT is wierd.. your problem looks like your osc is coming from a fixxed freq ..where placing a torroid *will* change the Fres...

last night, I even compared the streamer -outputs- between an antenna (PLL + VCO) versus CT feedback 4046(internal buffer + OR)..this means no VCO nor PLL, just the sensitive input because I cant get oscillation with HC14... the later circuit ran on CW, the former was ran in interrupted mode (high BPS, high duty cycle) ... well, they looked kinda similar to me...

one thing I noticed with CT feedback circuit is, I have to power up FIRST the halfbrdige driver, THEN the feedback osc, to get oscillations.. took about 100mS(I guess) before I get streamers... unlike the PPL+VCO once powered it up, it immediately fires up due to the VCO always running...

with 50Vdc supply, I could get about 2cm streamers in CW mode and 3.5cm in interrupted mode..

MRacerxdl

Tue Oct 27 2009, 04:07PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Yeah, but I the frequencies are almost exact that is need to oscillate, so its VERY strange thing.

Other thing, if I put the 10x30cm secondary, it works normally. with 10cm or more sparks at same voltage.

The aspect is the same, but the frequency is lower. (Much lower)

MRacerxdl

Wed Oct 28 2009, 02:31PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Sorry for double post.
Noticed that If I put a flyback on the place of the primary. And put a fixed oscillator on the Antenna Input, the flyback only works with a good output at more than 150kHz, less than that I get less than 1cm sparks from 50V input. I think for fullbridge its too litle.

Any Ideas?

vasil

Wed Oct 28 2009, 05:11PM

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

Maibe the new secondary has an opposite winding direction? I remember that was the cause of failure on Conner SSTC at Derby teslathon in 2004.

Arcstarter

Wed Oct 28 2009, 05:16PM

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

Have you scoped the gates? Maybe the GDT is saturating.

Increase or decrease the windings (keeping the same ratio) to better suit your secondary resonant frequency.

Other than that, i have no clue!

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