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CT feedback problem

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Dosinski

Thu May 20 2010, 06:13PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
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I'm building a DRSSTC with secondary bottom feedback. I was testing out the coil last week but i had to stop because i blew up my igbts after a few minutes and didn't have any spare ones lying around. Yesterday, i solder in the new igbts and hook up all the circuits to the oscilloscope and everything is running perfectly expect my CT. My CT has a turn ratio of 1:150. If i wind my secondary through it, i get no signal from the driver. If i wind one turn on it and hook it up to my signal generator, the driver gives me the proper signal.

I tried doing primary feedback, but my driver starts acting weird and outputs a sine wave O_o. Any suggestions?

ScotchTapeLord

Thu May 20 2010, 06:49PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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What's wrong with a sine wave? What exactly do you mean by "driver"? The current through the primary should be a sine wave due to its resonant nature in a DRSSTC, and so should the unclamped output of the CT. Does the coil have a startup oscillator or does it run exclusively off feedback?

1:150 probably won't reduce the current enough from the primary...

Any other information would be helpful. I think you're onto something with the primary feedback.

Dosinski

Thu May 20 2010, 07:19PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
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Posts: 45

By "driver", i mean the circuit that takes in the resonant frequency signal via CT and the interrupt signal, and outputs a square wave to the GDT. The driver runs exclusively off feedback. It ran fine last week (albeit very low power (15 volts).

If i use the primary feedback, the driver outputs a sine wave instead of a square wave. If i increase the power going into my tesla coil, the sine wave from the GDT drops from +-20V down to +-5V

ScotchTapeLord

Thu May 20 2010, 08:50PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

Could you explain the topology for your GDT driver, perhaps post or link a schematic? Most I've seen are only capable of producing a square wave... and the problem usually lies with one of the ferrites. Have you checked the waveform before and after the GDT, as well as the signal going directly into the driver? All of this information should hopefully give light to where the problem is.

The fact that increasing power changes the GDT driver voltage means there may be a serious issue going on... I would recommend perfecting your control circuitry before applying any voltage to the power circuit, but... that just isn't possible when you need the feedback.

Unfortunately since this is such a unique problem, it's going to be hard to analyze without knowing pretty much your entire setup, values, etc...

If you ask me, it sounds like there might be a problem with some silicon somewhere along the line. I don't know what kind of amps are circulating in your primary, but that current is being divided by 150 (ideally) by your CT, and then completely bled off by your burden resistor and clamping diodes. Usually people use signal diodes for this, and if you have too much current then you can damage them and possibly overvolt your GDT driver and create strange problems. But this all assumes that you haven't chosen a proper value burden resistor based on your primary current, which I cannot say, not knowing either your primary peak current or burden resistor value.

Dosinski

Thu May 20 2010, 10:26PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
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Posts: 45

Using secondary feedback, i hooked up my CT to my scope and only got 1.2 volts peak to peak which is much too low
Here's the schematic: (

1274394330 2680 FT89502 Shemativdrsstc6logic

ScotchTapeLord

Thu May 20 2010, 11:02PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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UCCs should never output a sine wave...
I noticed that you used polarized capacitors in series with your GDTs. This is an AC circuit, so you should probably switch them out. Most people insist on a certain type but I've gotten away with ceramic capacitors. But that probably won't solve the problem.

The 1.2V peak to peak means there isn't much power in the secondary...
I would say you should be getting better response to the primary because it won't depend as much on tuning, but I know you've had issues with it.

I've had trouble myself with feedback systems since running the system relies on the system running... stupid tail-chasing tesla coils.
Maybe someone with more experience with testing this Steve Ward-esque feedback system can give you some advice.

Dosinski

Fri May 21 2010, 02:40AM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
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Posts: 45

i made a new CT with around 150 turns. I used primary feedback and it almost works. The main problem is that i need to input a signal to get it started :*(

Goodchild

Fri May 21 2010, 05:21AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

This circuit is very similar to Steve Ward's early driver I would suggest using his newer driver it provides a solution to almost all of your problems.

Here is a link:

hope this helps

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