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Troubleshooting DRSSTC

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Ken M.

Sun Jun 08 2008, 12:25AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Hey Folks,

Just got all the stuff to build my DRSSTC, got it all set up and tried to test fire it With 24VDC on the 1/2bridge nothing happened but the usual of the GDT making noise since there isn't a 100KHz or so Frequency being applied.

My question is, Does the DRSSTC follow the same start up principle as a SSTC where as when the drive electronics are powered up it causes a momentary spike to go threw the GDT and "fire" one of the IGBTs, causing current to flow threw the primary and inducing a signal in the feedback system starting the self oscillations, or does it go by a different set of "mechanics"?

Bennem

Sun Jun 08 2008, 09:08AM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

Hi Ken,

Yes...your interrupter signal initiates the oscillations.

Have you tried reversing the phase of your feedback transformer?
is your primary tuning way off?

My DRSSTC's seem to start up ok at around 30volts on the bridge.

All the best,

Mel

Ken M.

Sun Jun 08 2008, 02:49PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Yes, I thought of reversing the phase after I made the post, tried it and heres the wave forms I get at certain points.

I don't think my tuning is off, calculated up the figures but I might have.

Ken M.

Mon Jun 09 2008, 11:57PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

I realize its not been 48hours, But i think this constitutes an update.

Yea bennem, Switching the Feedback Xformer cleared up that part of the circuit, Cuz now I get Gate waveforms that look decent

Although when I try to get a more detailed look at the gate waveform I get this

Then when I scope across the MMC and the primary I get

Then if I scope across the Feedback Xformer and the sprimary I get this

Then My last shot of the proof that its partialy resonating, the 2nd ch (yellow) is reading 265V(pk-pk)

*~NOTE~*: power applied to 1/2 Bridge is 24VDC from a DC power supply capable of 7A (loaded).

Now I also have some questions, if the OCD trips Just momentary at random intervols, is It working or is it just accidnetaly tripping? A long that same line, when the power was disconnected from the 1/2 bridge and the cap discharges then the GDT Squeals (typical of not getting the correct freq) But also the OCD LED lights completely if I adjust the OCD pot it either gets brighter or at some point it shuts off, Does this mean its Not operating correctly?

Since DRSSTCs are very similar i decided to switch the ends of the primary which didn't turn out too well since it caused the IGBTs to get very hot to the touch, So would this mean that my original phasing is correct or would the phasing that caused the overheating to be correct?

Steve Ward

Tue Jun 10 2008, 02:59AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Something still looks nasty here, those "sine" waves arent very sinusoidal. Im not sure what the issue is, but i dont think its working properly yet.

Primary/secondary phasing of the *Tesla* coil doesnt matter one bit. If one setup made the IGBTs hotter, then that is completely lost on me, it shouldnt matter one bit. What does matter is the phasing of the feedback CT, and thats all.

The OCD should be very solid, not randomly firing, unless of course your primary current is randomly exceeding the limit set. Id say you have noise problems.

Maybe post some pictures of your setup?

Bennem

Tue Jun 10 2008, 05:50AM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

Something dosn't sound right with your OCD.
When the OCD pot is turned fully in one one direction,
the OCD light will fully come/ stay on.
When adjusting it after that point, the LED should go off
and it only flickers on when the over current peaks in your
primary reach that desired cut off point.
so....the LED dosn't vary in brightnes...its either on (flickers at cut off)) or off.
As Steve said....perhaps you have noise issues with your OCD circuit?

AS to your swopping Primary connections causing your IGBT's to get hot,
i'm at a loss about that.
Are you using a Half bridge or full bridge of IGBT's?

Mel.

Tom540

Tue Jun 10 2008, 03:29PM

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

How about post a pic of your boards and layout?

Steve Ward

Tue Jun 10 2008, 06:11PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Looking again, the second scope shot you show looks like you must have the feedback backwards because the gate transition is LEADING the red trace (which im assuming is maybe the primary current?). This would be caused by negative feedback, whereas you need positive feedback. What is the frequency of the switching in the second scope shot? I bet its far higher than you may have anticipated? It also would explain any severe IGBT heating.

Ken M.

Tue Jun 10 2008, 09:01PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

So yea looks you wer right Steve, i switched the feedback and decided to be stupid and try and see if I could get the secondary to arc to my finger...It produces HV but maybe only 1k if that.

Here is a wide shot of my setup ATM, the primary is all over the former because its the way I was able to measure 200some volts pk-pk on the secondary, But I tightend the primary and moved it to this position(right).

I'm using a 1/2 bridge right now since I only have enough gator clamps for 2 igbts, and since fixing the Feedback to positive feedback..the igbts get hot and in a hurry so I mounted them to some heatsinks till I get his to pump out some decent arcs, anys i got a better scope shot of the primary, The red (ch1) is measuring the feedback after teh inverters just as its going into the Fet driver, and the yeelow (Ch2) is measuring the point where the 2 igbts connect and go to the mmc and the primary.
Also just a breadboard shot.

I realize that the original design said the primary was below the secondary, so I thought perhaps if I moved it down, to the top of the duct tape it might allow the primary to resonate and the secondary produce more voltage, but it doesn't it just decreases arc output.
So Anyone still got any ideas as to how to adjust this to push out more power without stepping up voltage, or any other helpful advice?

Steve Ward

Tue Jun 10 2008, 11:41PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Build your bridge onto a circuit board. You have so much extra inductance and resistance everywhere that it will never give much performance in its current state.

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