Board only built on mostly local parts

Non-kit projects based on the public Gen1 designs

Board only built on mostly local parts

Postby Artlav » Mon Aug 19, 2013 1:21 pm

So, after waiting for the snail mail, hunting exotic components and some interrupter shenanigans, there is finally some light.
It's not the first coil i built, but it's the first that isn't a spark gap one.

I've got the board, the tank capacitor and the current transformer, and have to get the rest locally.

IGBTs are rare - FGA were nonexistant, so i used FGH.

While i was able to find optic fibre, receiver and transmitter, it turned out that the receiver was a total black box - no documentation, not even in Chineese. After some random poking on the pins it apparently died.
The transmitter is just an LED, and i think i'll fit a phototransistor and some glue logic into the receiver to get real control.

But for now the interrupter is replaced by a socket with a attiny85 in it, generating a 10us/990us signal:
Image

The sockets are mostly incompatible - here we have similar parts with dissimilar pinouts.
Power socket fits with bolts at the very edge, the fuse box was of slightly wrong size, DC socket with the right pinout was nonexistant.

The largest capacitors that fit the diametre were only 680uF, i assume that should be enough?

Used a 12cm sphere for the top load - two bowls from Ikea taped together.
Not sure how far out of tune it is - the scope probe in the air and the current transformer output both register something between 250khz and 300khz.

I don't trust the grounding in the sockets here (and extension cords with ground pins are non-existant), so counterpoise ground only.

Image

I've been moving slowly, testing signals, looking for solder problems, etc.
Tried the full voltage without logic on - nothing gone boom.
Tried powering it up to 30V, off a stack of AA batteries, and this gives a 1 cm spark.
I think i've got a GO for mains voltage testing.


So, to sum up the questions before the big test (or boom) tomorrow:
-Am i right to assume that if it produces sparks off the batteries, then the circuitry should be mostly correct, and it won't just blow up?
-Would 680uF power capacitors be enough?
-How far out of tune is too far?

*EDIT: Also, how to get the pictures to scroll?
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Re: Board only built on mostly local parts

Postby Artlav » Mon Aug 19, 2013 2:09 pm

Actually, i didn't wait until tomorrow to test it. :)

Moved everything to a big room, added larger counterpoise and hooked it up to a variac.

Slowly going from 25V towards a 100V (220V location here).
Humming gets louder, corona develops over the breakout point.
Power down, check, repeat.
IGBTs are metal cold, no signs of problems.

A few repeats later later i've added a metal rod next to the coil, and got a 10cm spark going to it at near 90V, but the stand-alone sparks are still vague humming corona a few cm long.

So, i got confident and tired to add more power, without the rod.
Omnious HUMMMMM fills the room, the corona grows... And then starts to shorten! The humming continues to increase with a slight change of pitch.
A notch down - everything back to normal, a notch up - everything going weird.

So i turned it up another notch, to about 130V, and noticed some flickering.
Turns out the (age-worn) secondary was arcing in the middle.

That's it for today, got to wind up a new one.
Questions asked earlier are still valid.
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Re: Board only built on mostly local parts

Postby stegu » Mon Aug 19, 2013 4:32 pm

When you say "large counterpoise", how large exactly do you mean? I seem to have a problem with my secondary being badly out of tune: 320 kHz resonance, too much current is drawn from AC mains on low pulse width settings, and I have serious issues with heat dissipation in the IGBTs and in the primary coil long before I get a good spark. My counterpoise sheet of aluminum foil is 1m by 2m, which was all I could fit on my workshop table.
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Re: Board only built on mostly local parts

Postby Artlav » Mon Aug 19, 2013 5:04 pm

stegu wrote:When you say "large counterpoise", how large exactly do you mean?
Tin foil on the floor, about 1m by 0.5m. Make sure the bottom wire of the coil actually touches the foil (foil is connected to the ground of the coil).

I got about as much sparks as you have in your thread, at 10us/990us signal (no idea what interruptor setting this is, but something low) and half the input voltage, there is no heat anywhere.
I mention this because by my measurements it's also somewhat out of tune, about 270khz primary and 300khz secondary, but i'm not so sure about the measurements.
I measured it directly, at 24V battery power - scope clip to the ground, hang probe in the air=secondary frequency, touch probe to the current transformer output=primary frequency.

Why won't you just lower the secondary frequency? I.e. use a bigger toroid.
Or increase the primary - i.e. spreading the turns on the primary coil.
That's a quick way to tell if it's a problem of tuning or grounding.
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Re: Board only built on mostly local parts

Postby stegu » Tue Aug 20, 2013 5:18 am

Spreading the primary coil windings is not easy with this design. There are brittle glued-on clips holding the six turns tightly together, and those will probably break if I try to remove them. Besides, the coil is so far out of tune that I would have to spread the primary a lot to bring the inductance down to about half its current value (changing the primary resonance frequency from 250 kHz to 330 kHz). I did try using a 2x larger top load by tucking a cylinder on top of the toroid. (I have no larger toroid at hand, and no simple means for making one.) The bigger top load brought the resonance frequency down to 275 kHz, but did nothing much to reduce the abnormal power consumption. It did make the spark somewhat stronger and louder at the same current draw, but not a lot, and I still had 1A current draw from 230V mains at 1/3 power setting with a very weak spark of one, maybe two inches. I have blown several sets of transistors and diodes without ever getting the coil to run reliably at more than 1/2 power, and I have never seen a reasonably good spark except right before the most recent breakdown at 3/4 power.

When the primary is active and pumping energy into the secondary, the frequency is 250 kHz, the resonance of the primary circuit, and when the secondary is left to ring down by itself, its frequency is 330 kHz with the recommended topload, 275 kHz with the large extra topload I tried. With the frequencies more in tune, the only real difference I see, apart from a marginally better but still bad performance, is that there is some interference happening during the pumping stage, making the rise of the amplitude more uneven. See the attached images.

What puzzles me is why I am so badly out of tune. I have built this from the kit exactly to specifications, using the secondary coil wound by the guys at oneTesla and a toroid of the recommended dimensions, 8" by 2". The connection from the bottom of the secondary to my counterpoise is short, straight and well connected, so that should not be the problem.
Attachments
recommended_topload.jpg
Recommended top load. Badly out of tune. 230W power draw (1A RMS from 230V mains) at this fairly short pulse length.
(161.8 KiB) Downloaded 777 times
larger_topload.jpg
Larger topload, almost 2x the capacitance. More in tune, but still bad performance. 230W power draw (1A RMS from 230V mains) at this slightly longer pulse length.
(135.92 KiB) Downloaded 777 times
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Re: Board only built on mostly local parts

Postby Artlav » Tue Aug 20, 2013 1:27 pm

Meanwhile, got a new secondary winded up, tweaked the primary to fit it, and on to a power test.
Image
Image

Approximately 30% power (30us on pulse), producing over a foot-long sparks at full voltage.

Unfortunately, the video does not do it justice, and a "better" full-HD camera produced even worse one.
http://www.youtube.com/watch?v=g7HKtZPbHm0

I throttled the input voltage with the variac at the start of the tune, so it goes down a bit.
There, at half voltage, you can feel the variac shudder, so i suspect that at full voltage that sharp start would just burn something out.
My MIDI implementation is quite unbalanced, it seems.

There isn't much heating at the IGBTs after this type of a run, so it could probably work at a higher power, but i don't trust it to handle more in such an scattered state.
The primary seems to be too close to the secondary - there are hints of arcing there.

Other than that, i'll call it a win. :)
Nice design, people, i don't regret investing in you.
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Re: Board only built on mostly local parts

Postby stegu » Tue Aug 20, 2013 3:46 pm

Congratulations! Yes, your primary is way closer than what you would get if you built this with the oneTesla kit. Their primary coil is six turns on a separate 9 cm acrylic tube, with quite a bit of spacing to the secondary, 1800 turns on a 6 cm ceramic tube.

I got my coil to work tonight as well, but only after adding a disproportionally large extra top load to bring the secondary into tune with the 250 kHz primary. When the coil is properly tuned, it draws less current, produces considerably less heat and throws way longer sparks.
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Re: Board only built on mostly local parts

Postby Bayley » Wed Aug 21, 2013 5:05 am

Wow, I am very impressed :) I love that socketed ATMega interrupter.
What pulse repetition frequencies are you running at 30uS? The "official" interrupter does 20uS at 2KHz, depending on how brave you are (watch out for noise, that micro might not be too happy!) you should be able to push further.
A word on noise - if you are not careful, noise getting into the interrupter will cause the whole thing to latch up, possibly with the output high - and DRSSTC's really don't like their interrupter inputs getting stuck high...
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Re: Board only built on mostly local parts

Postby Artlav » Wed Aug 21, 2013 8:18 am

Yes, the interrupter is charmingly straightforward - attiny, capacitor, programming plug, signal plug:
Image

The idea i had in mind is the old school video game console, which had the games on cartridges.
So, this thing is a music cartridge - about 1 kb of overhead and 7 kb for the song. Flight takes about 3kb, so you can fit a few minutes in there easily.
Advantages in simplicity and lack of need for a ton of EMI-suceptible equipment like a laptop and USB-MIDI converter.
Disadvantage is in lack of interactivity - no keyboard playing, no power control other than by a variac.

I'm only worried about the reset line - got to add a tie-up resistor to it.
After that, wrap it into duct tape, then grounded aluminium tape, and nothing should get to it.


Considering power consumption, measured on the variac's 220V input.
At 150V input it draws an average 0f 0.2A on low notes, and 0.6A on high notes, with peaks up to 3.5A.

I didn't try to go higher yet - at 150V the high notes of the flight of the bumblebee makes the variac shudder - i'm not quite sure the coil can stand it.
I have 30us on time fixed for all notes, so the high ones would probably be over the limit at full power.

Anyway, i'll try to record the whole song at full power once the sun goes down enough to see anything.
If it's going to go out with a bang, might as well catch it on film. :)
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Re: Board only built on mostly local parts

Postby Artlav » Wed Aug 21, 2013 11:40 am

Bayley wrote:A word on noise - if you are not careful, noise getting into the interrupter will cause the whole thing to latch up, possibly with the output high - and DRSSTC's really don't like their interrupter inputs getting stuck high...
Speak of the devil.

http://www.youtube.com/watch?v=iKsD_yyKSdQ

I was testing the camera setup to record the action (missed the focus), turned the power up a bit to get a clear spark, then FOOOM!
It went into a huge-spark-FOOOM-sound mode.
I turned the power off at once, checked everything - the bridge is barely warm, no signs of failures.
Turned it on again a little - everything works.

What was that?

It happened at about half input voltage, the ammeter wasn't on.
I thought a latch-up to CW mode was instant board death - some luck coupled with low input voltage and small capacitors?

I guess i'll try again with a well insulated interrupter cartridge. And camera in focus in case it's not the last glitch for the day.
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