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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi kilovolt,
Yes OMGPIX plz!
The way to implement the OCD is to use two 1k resistors in series with your gate drive inputs, and two small mosfets which clamp the inputs to ground (which inturn are controlled by NE555; see Steve Ward's DRSSTC 1 schematic)
I've also found out that current tends to settle to almost constant value once the spark loading has commenced. I'm wondering what kind of consequences this might have - so doubling the voltage may do more like doubling the power output than quadrupling it... which means I might not even be able to reach the ultimate power level I wanted using a half bridge - we'll see.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
This thread inspired me to bring my QCW back to life after staying dormant for a couple months. I'm going to need a larger primary so that I can use my 10nF MICA. The 25nF teflon cap is currently the only thing that will tune with the 14 turn primary. The secondary is being driven at 320KHz upper pole.
I have it all set up so anything to try out on it while I have it setup do suggest
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Edit: Goodchild posted while I was typing. Nice picture, the sparks look very similar to a VTTC. My sparks branch a bit...
Note that this is all temporary proof-of-concept test setup, for the final version everything will be rebuilt and a new secondary will be wound.
So this is the mess. The PC PSU case contains a breadboard with the controller on it, and a power supply. The tank capacitor was rewired for 20.5 nF instead of the calculated 57 nF, because the frequency had to be increased from 80 to 125 kHz, higher voltage also allows higher Q. One CT is for current measurement, the "cascade" is for feedback.
The bridge. Heatsinking is inadequate but ok for short runs, a fan blows some air through the bridge. The bypass caps are 5uF 400V each, connected 2x2. Two transistors and one diode on each heatsink.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Eric, some questions -
Firstly, I've heard that those russian teflon caps suck heavily. How long did you find yours to last in a QCW or CW duty, and how much current can they stand without overheating?
Secondly, I'm highly puzzled now how can you get any output with such large primary impedance, compared to what I observed. Have you atempted to run your system true CW without modifications, and what was the power drawn by the coil, compared to the peak power of cour QCW bang?
Marko
PS. kilovolt nice work :) No sparkpics though?
I guess you might be getting away with lower primary impedance because of low coupling provided by your huge, tall secondary. I think you might actually und up dissapoint with power level/sparks if you don't switch to full bridge or use a lower primary reactance.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Marko wrote ...
Eric, some questions -
Firstly, I've heard that those russian teflon caps suck heavily. How long did you find yours to last in a QCW or CW duty, and how much current can they stand without overheating?
Secondly, I'm highly puzzled now how can you get any output with such large primary impedance, compared to what I observed. Have you atempted to run your system true CW without modifications, and what was the power drawn by the coil, compared to the peak power of cour QCW bang?
Marko
PS. kilovolt nice work :) No sparkpics though?
I guess you might be getting away with lower primary impedance because of low coupling provided by your huge, tall secondary. I think you might actually und up dissapoint with power level/sparks if you don't switch to full bridge or use a lower primary reactance.
Marko
hehe the secondary is actually rather short! only 9 inches tall. The coupling as of now is at 0.369k and the peak primary current is low, only about 75Apeak. The OCD is set at 100Apk but is not tripping.
The Russian teflon caps do indeed suck hard! Running at about 10pps with the on time about 10mS the cap heats up to about 100C in about 10 mins. I have a supper awesome MICA that is just begging to be used but the performance is sucky because I can only tune the primary to about 350KHz rather than the 320KHz that the secondary is at.
The QCW is not about peak current! That's what a lot of people don't get. The spark is built on top of it's self and and grown or stretched out over a long time. In order to make a spark in a very short amount of time like a regular DR you need a LOT of voltage to do it, which means high tank currents! But with the QCW you build the sparks over a long period of time using very little top volts <50KV in the sparks pic shown above. This equates to low tank currents and a high Z tank. That is why it's not flashing over with the supper high coupling.
The highest Ive run the QCW at is about 80% duty cycle. During this run the power draw was easily 10A from the 120VAC mains and was only making about 1 foot of spark. This was at a relatively low power as those 60N60s can only take so much! In the photo above the coil is at about 40% duty cycle and is pulling about 4A off of the 120VAC mains.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko, by extrapolating the peak current and spark length curves, I am getting close to the "design" values at 560 volt peak input. I know the coupling is low. The new secondary will be fatter and shorter. This is just a proof of concept setup, and I've proved it works
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Dr. Kilovolt wrote ...
Marko, by extrapolating the peak current and spark length curves, I am getting close to the "design" values at 560 volt peak input. I know the coupling is low. The new secondary will be fatter and shorter. This is just a proof of concept setup, and I've proved it works
That looks a lot like the QCW sparks when running it from a strait DC bus rather than the class D modulator! The two key things that make the QCW look like VTTC sparks is the high res freq 300KHz+ and also the slow linear ramp of the power over the burst length.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Goodchild, The coil runs from half wave rectified unfiltered power. There is no interrupter or other modulator. The low switching frequency was chosen because of the IGBTs... I'm not going for anything fancy, just a way to increase inverter efficiency over a normal non-tuned primary, but still not going to the point of "standard" DRSSTCs with long thin sparks and extreme noise.
Note: The coil is running at relatively low power now, I'm going for 2-3 times longer sparks in the final design. This might change the spark appearance too.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Just for giggles I turned up the power a bit. The coil is now running from a 120VAC variac to a doubling transformer so 240VAC in but had it at about 200VAC for the photo. primary current was about 85Apk occasionally hit the 100A limiter once or twice. sparks are about 48 inches. No flash overs so voltage is still less than 60Kv ish.
I also put the 10nF MICA in there for this run, I'm tuned all the way at the bottom of the primary! Event with the coil 20KHz out of tune the thing still manged to push out 48 inches while under 600w
QCW is one awesome coil...
instead of making a new primary I will probably just wind a new secondary. another 9 x 4.5 but with 28 AWG wire rather than 30. This will put the upper pole in the 450KHz+ range and make the primary current tune to an even higher tank impedance. This should straiten out the sparks and also lower the primary current.
Also here is a video of some very close to CW action about 80% duty
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