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DRSSTC Ground Strike and Interrupter Problems

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Daniel Kramnik

Sat Oct 01 2011, 04:39AM

Registered Member #3885 Joined: Sun May 15 2011, 12:47AM
Location: Newton, Massachusetts, United States
Posts: 94

Hello again, 4hv!

As some of you may know, I've been working on building my first DRSSTC since the end of my sophomore year of high school, (I'm now a junior) and a couple days ago, I finally did it thanks to the helpful people at MITERS!

It's driven by a half bridge of TO-247 30N60 IGBTs and Steve Ward's basic primary feedback DRSSTC controller with 75nF at 8kV of tank capacitance and the primary current just barely reaches 200A (measured with an oscilloscope and CT) at 400Vbus. Sparks are 2 - 3 feet long and I'm currently working on a full bridge design to increase this figure.

Unfortunately, however, I'm having some issues with my coil not being able to ground strike properly. Whenever I bring a ground stick close to the discharge, it momentarily strikes it and then stops oscillating but the bus capacitor remains charged and no transistors fail. I need to unplug and reconnect the controller (not the interrupter, though) for it to work again, and it can keep going from where it ended; I do not need to soft start it with a variac.

I first suspected a fault in my OCD circuit, but the red indicator LED does not flash (although this could be a bad LED) and when I disconnected the LM311 comparator (not the 555, to avoid leaving the MOSFET's gate floating), the problem persisted. I've never needed to use OCD for anything (the coil's surge impedance effectively limits primary current to 200A), so I'm not sure if it even works, but I think it's basically irrelevant in this problem. Am I correct?

I'm also having a slight issue with my interrupter; it doesn't prevent me from running my coil, but it is a bit annoying. Essentially, whenever I turn the BPS rate past a certain threshold, the whole interrupter stops oscillating. It only started happening after I installed a 7805 voltage regulator in series with the interrupter's 9V battery power source to bring down the output pulses to 5V, as opposed to the 9V pulses that were killing my 74HC14. I scoped the interrupter's output and found that it works perfectly fine until I turn the BPS too high. It's only 2 555 timers, so I'm a little lost about what could be wrong. You can see this problem in the video when I'm twiddling the interrupter controls and the coil suddenly goes out and then comes back on again as I sweep past the threshold frequency and then back again.

Can anyone help?

Thanks!

-Daniel Kramnik

Goodchild

Sat Oct 01 2011, 06:27AM

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

Hey I want to start off by saying, that's a great little DRSSTC you have built and I commend you on getting your first one working!

I will start with your ground strike problem. Surge impedance of your primary circuit will only limit the current so much. When you get the ground strike the Q can change creating larger currents in the primary. I would suggest ditching the 555 and MOSFET all together. I'm assuming you are using one of Steve Ward's older drivers. I would use a flip flop like in his later design for the OCD. You basically use the output of the LM311 to set the clear on the D flip flop. This disables the Q output until the next rising edge of the interrupter (going into the clk input of the same D flip flop). Schematic here:

This will make it so that once the flip flop has been tripped by the LM311 it keeps the system disabled until the next burst resets it. This is a much cleaner and safer way to implement the OCD.

As for the interrupter, I know the glitch you speak of well! I have several interrupters one of which is a 555 based one with an LM3805 powering it DOH

Basically when the resistance on the pot gets to low and also when the 555 is being powered with a lower supply voltage this can put the 555 into a nasty state were it can no longer trigger and latches until you bring the resistance back into a suitable range.

I can't remember specifically what I did to fix this problem as I have not used that interrupter for some years now. But I recall adding a series resistor with the pot to limit the absolute min or max resistance. Play around with this and you should find a solution it's not a supper big problem, more of an annoyance!

Killa-X

Sat Oct 01 2011, 04:56PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

No idea if this is helpful or not, You said it completly stops oscillating. Does this happen when its at a lower voltage? Or does it happen when you ramp the power up? On My coil, I had a time where it was shooting sparks. As I ramped up, it suddenly stopped. Then i turned the variac down, and it started again. For me, it was because I was 7ft from the coil, and my interrupter was picking up RF...The RF was 50khz, and caused the system to be responceless. Else, Like Goodchild said, too low resistance. Good practice to always put a resistor in series so there's always something

Example, Steve Ward's Mini-SSTC uses 2.2K in series with the potentiometers

Otherwise, Nice coil, looks like it preformed very nicely =)

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