Primary Current Bounce

[alan sailer]

I've been playing with my Mini DRSSTC and getting much better results. Last night I tried increasing
 the coupling of the coil and got much larger sparks. However the current waveform starting showing
an interesting shape. It bounces ie has a lower frequency modulation of the basic waveform.



I have tried several keywords to try and search for other posts that might discuss this but have had no luck.
I doubt that it is anything new in the world.

JavaTC shows that my increased coupling should put me at 0.152. The old coupling was 0.134. I didn't see
this "bounce" at the lower coupling but I also could not reach 200Vbuss because the OCD would trip solid
above 150V.

Cheers.

[futurist]

Check Richie Burnett's website
http://www.richieburnett.co.uk/operatn2.html
Be sure to disable SSL or pictures won't load

I remember there was some discussion about coil tuning and current notching, try searching my DRSSTC build

[alan sailer]

Thank for the link. I am trying to understand it. I also checked out your DRSSTC build and copied the part about wave forms.

At a first pass through the Richie B. site the explanation is geared towards SGTCs. I know that there are many things
about SGTCs that apply to DRSSTCs but may have some problem figuring out the differences.

In addition I went through the whole upper and lower poles thing when I was trying to tune my two QCW coils. I even
used my scope and a swept signal generator to plot the poles and se how they moved with different tuning. At the time
(and this could be my fault) I found none of that information helped me to tune the coils. It was darn entertaining but
the bottom line was, I just had move something (coupling or primary taps) and see if the sparks get longer or shorter.

Other than just sort of thrashing about with a soldering iron (changing primary taps) or spacers (changing coupling)
I never got any systematic idea on how to tune a coil.

Either I'm missing something or it really is just cut and try.

Cheers.

[davekni]

The current waveform is classic coupled-resonator.  The initial current ramp-up may be slowed a bit by the arc forming.

The best way to understand and explore is modeling with an analog circuit simulator.  My favorite (and free) one is LTSpice.  The only hard part to model is arc impedance (resistance and capacitance).  There are some discussions of arc modeling here, and I have a few variations.  For starting, a fixed resistor/capacitor is simplest.  Doesn't model arc growth.

Here is a simple simulation of my DRSSTC showing similar current behavior:


If you use (or want to learn) LTSpice, here is the simulation schematic in that format, zipped to fit with this forum:
t0.zip

This sort of coupled-resonator waveform is common when running a coil at low power with relatively little detuning.  Indicates to me that your coil needs to be pushed a bit harder - either higher bus voltage or higher repetition frequency.  That will increase arc size.  Larger arcs will add secondary damping, minimizing any remaining energy to transfer back to the primary and cause such "bouncing".

Of course, watch for other issues such as racing sparks when increasing primary voltage.

[alan sailer]

David,

Thanks for the input. I did some simulations back when I was having troubles
with a QCW buck circuit and I believe they were of some help in understanding
what was going on. So I may give your LTSpice circuit a try and see if a light-bulb
goes off.

My constant bugaboo so far in the DRSSTC learning curve is hitting the OCD trip
point. Initially, when I had the secondary frequency too high it was because the
primary was tapped too low in frequency. I was essentially running the coil without
a secondary. Raising the primary frequency wasn't an option because my number
of turns got too small. I was limited to about 100 Vbuss.

With the new secondary the turns number (six) is more reasonable. I quickly
went to 150 Vbuss and now 200. This is where I started seeing the notching. I tried
210 at one point but the coil hit OCD hard.

I am backing off for a few days to massage the set-up. I want to be able to tap
the primary in finer increments. I also want to add a fan. Everything is stone cold
right now but might not be in the future.

I had a scare yesterday when the coil died at high rep rates. I was watching the
coil not the scope and didn't see anything strange. My fear was IGBJT death, the
 reality was a 10 amp fuse. Since I am using a doubler I will need at least a 15
amp Variac.

Cheers.