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RateReducers QCW DRSSTC

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LAN_403

RateReducer

Sun Mar 29 2015, 04:07PM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

Hi.
With great support from loneoceans i managed to build a working QCW DRSSTC.
Some pics of the setup etc.:

Results:

I did some measuring when the coil was running.
Primary current:

-> 297kHz

Probe in the air:

-> 320kHz

Buck converter output + primary current:

And thatÂ´s what i donÂ´t understand...
Why does the current decrease when the voltage on the bridge increases?

I tried to tune the coil to get a better output but unfortunately i never got more than ~70cm with ~300V max. ramp voltage and about 10ms ramp time. When i use a longer ramp e.g. 15ms the sparcs become smaller and thicker.
How can i improve the output of the coil?

Until now i only tried different taps on the primary and changing the MMC capacitance. Now i have ~13nF and tap on the turn 10. With this setup i get the best results as seen on the picture with the spark.

Regards Daniel

DÃ©dÃ©!

Sun Mar 29 2015, 05:52PM

Registered Member #4932 Joined: Thu May 17 2012, 01:42PM
Location:
Posts: 59

Nice little QCW-DRSSTC!
I don't know, but maybe the capacitor on the input of the Buck converter isn't big enough? How big is it? 6800ÂµF?

RateReducer

Sun Mar 29 2015, 06:02PM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

Thanks!

No, the cap is big enough ^^
15.000ÂµF
I would not get a ramp up to 300V (you can see it on the scope picture) if the cap value is too small.

Sigurthr

Sun Mar 29 2015, 11:14PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Looks fantastic!

Mind you, I'm not QCW expert, but my gut instinct is that the primary current drops as voltage increases because the arc is growing longer thus becoming a higher impedance load. Just like with a flyback ZVS if you stretch the arc the current draw drops.

By the way, where did you get such nice bridge heatsinks?

Alex Yuan

Sun Mar 29 2015, 11:30PM

Registered Member #9614 Joined: Mon Jan 14 2013, 10:00PM
Location:
Posts: 44

Sigurthr wrote ...

By the way, where did you get such nice bridge heatsinks?

you can find them from old power supplies for various appliances or you can buy them from

Weston

Mon Mar 30 2015, 12:45AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Impressive coil.

What material is the primary coil form?

Also, it looks like you are using DC rated doorknob capacitors for the tank. Do they have a KVAR rating? How hot do they get?

Kizmo

Mon Mar 30 2015, 06:28AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Other QCW builders came into conclusion that primary circuit resistive losses play significant role in coil performance. Maybe you should beef up our tank circuit? Those wires look kind of small for delivering couple hundred A at 300kHz or so.

Uspring

Mon Mar 30 2015, 10:22AM

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

RateReducer wrote:

Why does the current decrease when the voltage on the bridge increases?

Primary current depends in a complicated way on the choice of primary and secondary tank specs and the secondary loading due to the arc. For secondary Q not too low, it probably can be said, that primary Q, which determines the primary current is about equal to secondary Q, which is determined by arc loading. For larger arcs, arc loading increases, so secondary Q drops and thus primary Q. That implies, that primary current doesn't increase at the rate, that bridge output voltage does. It might even lead to lower currents at higher voltages as in your coil. For the theory minded, this has been discussed in

RateReducer

Mon Mar 30 2015, 11:57AM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

Thanks, Sigurthr.
Alex Yuan is right, i got mine from an old server power supply. Attached the two heat sinks with a threaded rod.

@Weston: the coil form is just a pipe. In Austria it is called "Kanalgrund Rohr" I did not find the english name...

here is a pic.
432px

Those caps are Doorknops. The are nice - i did not notice a heating at all. "20kV 5nF" per piece but actually they have only a bit more than 4nF.
Many people use them in SGTCs.

@Kizmo: I don't think the wires are the problem. They are 2,5Â²mm and kept as short as possible. Other QCWs like Zergs "junky" QCWl and loneoceans QCW do not use thicker wire - i only think so because the wires do not look thicker than mine on the pictures...?

I already talked with loneoceans and he said probably the coil is tuned on the lower pole frequency. That would explain the decreasing current as the growing sparc more and more detunes the coil. I tried to tune to the upper pole but when i use less turns on the primary or less capacitance, increasing the primary frequency the OCD instantly kicks in, even before there is a visible sparc. OCD is set to 100A allowing 70cm sparks with different tuning. But it even kicks in at 150A. When i use a slightly lower frequency strange things happen...
The current looks the same just before the end of the ramp where the current rises really fast (need to take a picture of that) until the OCD trips. That results in a loud bang branching the sparc so it looks like a firework exploding. I think some time ago Steve Ward posted such a picture but i think he said that he increased the slope of the ramp just before the highest point of the ramp to get those branches.

Edit: I forgot that i made a video of the branching sparcs. Took a screenshot:

Unfortunately i did not film the scope...

RateReducer

Thu Apr 02 2015, 03:50PM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

Finally the coil resonates at the upper pole!

Distance breakoutpoint -> ceiling = 80cm.

Need to put the coil on the floor...

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