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QCW - how to make sparks straight?

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LAN_403

zzz_julian_zzz

Fri Jul 15 2016, 07:07AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hi All,

It's been a while since I tried to make my bucked - qcw work well, here are the specs:

Sec Fres Alone: ~350 kHz
Pri Fres Alone: ~330 kHz

Total Fres: ~400 kHz
I pri: 165 Amps (in the photo, but I limit it at 180 amps- OCD kicks in)
V bus Charged to: 430V
V ramp peak: 400 V

Breakout point to ceiling: 74 inches

I don't know why spark branches.. I want to make it sword like..
Ramp is smooth as in the photo, but branching is always there :(
can anyone help me to remove or at least reduce its branches?

Here is the Vid:

another thing, is when I try to change tap / or reduce capacitance of Pri Caps to higher primary frequency, Bus / and Primary Current waveform is being distorted? looks like it's having a flat at the middle of the ramp.. causing branches.. but in the photo below, I tapped the primary whereas the distortion doesn't occur.

Thanks in advance in any help!

BR,
Julian

Shrad

Fri Jul 15 2016, 09:46AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

breakout material?

zzz_julian_zzz

Fri Jul 15 2016, 01:52PM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Copper 12 awg 3 inches.. Does it affect the straightness of the spark?

Shrad

Fri Jul 15 2016, 03:24PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

I don't know, but I would say that it it becomes too hot it could modify the spark form... that's not my specialty though

Myke

Fri Jul 15 2016, 10:28PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I haven't messed with a QCW coil before but what's the effect of shortening the ramp duration (for matching the streamer propagation)?

Uspring

Sat Jul 16 2016, 07:56PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

another thing, is when I try to change tap / or reduce capacitance of Pri Caps to higher primary frequency, Bus / and Primary Current waveform is being distorted? looks like it's having a flat at the middle of the ramp..

The primary current does not follow bus voltage in a simple way, since the primary current also depends on arc loading. The capacitance of the arc changes the secondary resonant frequency and that will change the tuning. This again determines how much power is transferred from the primary to the secondary. If there is much power transfer, then the primary current might drop due to the energy drawn or remain flat, although bus voltage increases. A too rapid increase of power transfer to the secondary might also cause the arc to branch.
Perhaps it will help to play around with the tuning. The point of straightest sparks might not coincide with the one of longest ones. Another approach would be to change the shape of the bus voltage ramp.

Steve Ward

Sun Jul 17 2016, 07:05PM

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

I can't say i know what to do to make the sparks less branchy, but some things i would look at are:

1) Truncate the ramp/pulse window to see roughly when the sparks tend to branch. Sometimes it happens early, sometimes late in the evolution of the spark.

2) I used a 4-segment piece-wise-linear ramp generator that allowed some fine adjustments of the ramp rate at different parts of the burst, this did seem to help on the coils that just didn't naturally produce the straightest sparks.

3) Try different tunings. I found that tuning the primary below secondary Fres seemed to help, but my first system didn't have this option to begin with, and it still made pretty straight sparks under the right conditions. I believe tuning acts as its own modifier to the "ramp" as the effective impedance of the system changes with spark load.

However, i can think of instances that contradict what i just said about tuning, so i really don't know. Every QCW coil i ever made behaved differently and some would never give nice branchless sparks. I note that i got the longest, straightest sparks when using a 4"x13" toroid, or a 3"x16" toroid.

As to breakout points, i always found that too-long is no good as the sparks would tend to branch early. I've used sharp ones and blunt ones and it didn't seem to matter.

I also note that it generally seems hard to make really long sparks that dont branch because the really long sparks require higher voltages which will promote the branch formation on an existing "non branched" spark. The non-branched spark is just really unnatural

. If you aim for shorter spark lengths (say 1/2 - 3/4 of max) can you more easily find the right settings for no-branch? That was often the case for me. The pulse length should be roughly 1/2 to 3/4 of the length of the longest sparks. I find an ideal rate was about 3-4mS per foot of spark.

zzz_julian_zzz

Tue Jul 26 2016, 10:45AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hi Everyone,

Ok, first of all I would like to thank you for your suggestions.

The day after I post the first video (more spark branch) I found the issues..

1. V ramp input is not firm (it causes distortion in the actual /following arbitrary DC voltage in the inverter side of the coil)
2. My Ramp generator is analog (it is actually made by 555 timers, some PNP transistors and PWM generator)
3. Ramp generator is connected via 5 ft RCA cable
4. Tuning on the Primary (I moved to 20 kHz higher)

After doing adjustments base on the above points, I recorded another video.. It seems it improved compared to the first vid in my post above

To Upsring: Yes, but I think Current with respect to V should be proportional by bigger percentage; loading could affect current but still just a bit. I truly agree on the point whereas rapid transfer will cause branch. I observed the current while coil is working. I can see if there are even mild spike in current, spark will branch regardless if it is at the beginning, middle or peak of the ramp. My current set up is :
charge the Filter Cap (20mF) to 430V via boost converter then I can control the peak of the ramp( up to 400v) by pots on my ramp generator.
I can also change the magnitude of initial dc bias realtime via pots (~50v dc) before ramp slope begins; I can also change realtime the length of the ramp 15ms to 25ms. If the peak is too high (~400v) this means that the SLOPE creates bigger angle and this causes sudden curve in the ramp (I will show pics soon on this).. Now I am still planning to reduce the tank impedance since I want more current with less voltage (this reduces the angle of slope in the ramp).. until now, my setup is ~2.5 ohms (400v/160A) effective impedance.. I will try to reduce it to 2.25 or 2 ohms by Tap change and Cap change.

To Steve:

On your points:

1. TRUE
2. I don't have access to it - I am working all analog (my QCW is I think the first one not using any micro nor fibers :) )
3. TRUE with exceptions; big torroids means greater spark control; Breakout point - i don't know if this is true (this is in the area of 2 to 3 inches)

"If you aim for shorter spark lengths (say 1/2 - 3/4 of max) can you more easily find the right settings for no-branch?" - Same in my observation, my thinking rationale are the following: a. Ramp Slope is less abrupt b. Cap run out of juice (?) c. interference is less

"The pulse length should be roughly 1/2 to 3/4 of the length of the longest sparks. I find an ideal rate was about 3-4mS per foot of spark"
-I am in these regions too, producing more than 6 ft sparks @ 160 Amps @ 15ms is where I am at., and I think increasing the length of the ramp would cause less angle in the slope, I tried this but not successful. I think I am hitting the length vs Current magnitude relation whereas too long for less Current could cause brushy fat sparks.

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