First Sparks - Holy Crap!

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Re: First Sparks - Holy Crap!

Postby junktronix » Mon Sep 23, 2013 8:38 pm

Looks like the rectifier diodes and the IGBT's are dead. I just ordered replacements from Digikey. Once I get the board repaired, I'll set things up exactly like I had them in the video (except with the breakout pointed away from the power cords) and do some measurements.
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Re: First Sparks - Holy Crap!

Postby DJKOR » Mon Sep 23, 2013 8:44 pm

junktronix wrote:Looks like the rectifier diodes and the IGBT's are dead. I just ordered replacements from Digikey. Once I get the board repaired, I'll set things up exactly like I had them in the video (except with the breakout pointed away from the power cords) and do some measurements.


Hmmmm. I wonder if this was due to any of those strikes to the power lead. Were you running your coil at full power?

You can still measure the secondary resonance however if you needed something to do in the meantime :D
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Re: First Sparks - Holy Crap!

Postby stegu » Tue Sep 24, 2013 5:45 am

The breakdown happened when you pointed the breakout point upwards and had flashovers, right? Then the flashovers are most likely the culprit. They can do all sorts of bad things to the circuit. Ground strikes to the power cord are not healthy, but from your video it is apparent that the circuit survives them, at least for a while.
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Re: First Sparks - Holy Crap!

Postby Bayley » Wed Sep 25, 2013 2:17 pm

Striking to the power cord should be fine as long as you have a good ground.
Flashover is not healthy, it detunes the secondary and causes power to go into all the wrong places.
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Re: First Sparks - Holy Crap!

Postby junktronix » Wed Sep 25, 2013 2:34 pm

It was definitely flashover, likely caused by the position of the breakout point. I bent one end of the brass rod so that I could hook it under the thumb screw and then bent the rod up at a 45 degree angle. The arcs were nice and high and didn't hit the power cords, but I could see tiny arcs along the length of the rod and had a couple of significant flashovers before it died. In the video, I have the rod laying on top of the toroid with one end barely protruding. It wants to roll off though so I'll modify my (already modified) rod to protrude the same length past the toroid while still being attached at the thumb screw so it won't roll away.

New parts should be here tomorrow. I hope to have time this weekend to work on it some more.
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Re: First Sparks - Holy Crap!

Postby Alex » Wed Sep 25, 2013 5:21 pm

junktronix wrote:I bent one end of the brass rod so that I could hook it under the thumb screw and then bent the rod up at a 45 degree angle.


I do the same except I bend it over the toroid so it comes out horizontal. It only protrudes about half an inch from the top of the toroid.

I've had plenty of nasty flashovers but my coil seems to be fine :|
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Re: First Sparks - Holy Crap!

Postby stegu » Thu Sep 26, 2013 4:06 am

Seeing that your coil has the same resistance as the replacement coil I received, and that your inductance is very likely the same as well, I would be very, very interested in seeing either an inductance value from your secondary or a measurement of the secondary resonance frequency. One way of doing it is as shown in a previous post in this thread, and another way that might give better results when using a low cost signal generator is described in my tuning guide, currently on page 4:

http://www.itn.liu.se/~stegu/onetesla/o ... _draft.pdf

If your signal generator is not happy with a shorted output, you might want to add a small resistor in series with the primary. (Note: in series, not in parallel.) 100 ohms should be enough. Commercial signal generators are designed to be robust against short circuits, so they should be fine, but if you build your own oscillator, it might need that resistor on its output to behave well and not risk taking damage to its output.

If your secondary coil is in fact similar to my replacement coil, it means that your secondary is tuned somewhere around 315 kHz, 25% higher than the primary frequency of nominally 250 kHz. It would be very interesting to learn that the strongest spark comes from a coil that is 25% out of tune at min power. This would mean that a "perfectly tuned" oneTesla (tuned for maximum spark length) is just on the verge of breaking from detuning at medium power, but when it hits full power it gets into tune. AFAIK, that strong of a shift in frequency in the presence of a streamer would be unheard of in coiling circles, but the oneTesla is an unusually strong DRSSTC for its size.

If you can find a capacitance meter, could you measure the tank capacitor as well? (Disconnect the primary from its terminals before doing this, or the rest of the circuit will disturb the measurement.) Capacitors tend to have a rather high tolerance, and yours might not have the nominal value. People's primary coils are bound to be very similar, so the tank capacitor is what determines the primary frequency the most.

An oscilloscope trace from a running coil at min power is also a very good way of measuring both the primary and the secondary resonance frequencies. See pages 2-3 in the tuning guide above. If you make this kind of measurement with an oscilloscope, all other measurements are in fact optional. Informative, but optional.

I am sorry to request all this work from you, but we are all very interested in getting more data on tuning for the oneTesla, and you would be doing a great service for the community.
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Re: First Sparks - Holy Crap!

Postby junktronix » Fri Oct 11, 2013 11:03 pm

Work has been crazy and I just found the time to repair my coil tonight. Unfortunately, I have bad news. I swapped the rectifier diodes and IGBTs (and of course, the fuse). It ran OK for a minute or so. Hard to say for sure, but it seemed like significantly shorter arcs than before, but it blew the fuse before I really had a chance to compare. This time only the IGBTs were bad so I swapped them and fired up one more time. I was careful not to run at full power, but it died again after only 10 to 20 seconds and with the power at 60% or so. There was no flashover. It did seem to get slightly louder for a fraction of a second before it died.

I only ordered 2 pairs of the IGBTs so I'm dead in the water again. Should I have proactively replaced any of the other components?

I'll try to get static measurements of the windings and capacitor, but it seems something has changed and my coil is weak and unreliable now :(
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Re: First Sparks - Holy Crap!

Postby Bayley » Fri Oct 11, 2013 11:22 pm

junktronix wrote:Work has been crazy and I just found the time to repair my coil tonight. Unfortunately, I have bad news. I swapped the rectifier diodes and IGBTs (and of course, the fuse). It ran OK for a minute or so. Hard to say for sure, but it seemed like significantly shorter arcs than before, but it blew the fuse before I really had a chance to compare. This time only the IGBTs were bad so I swapped them and fired up one more time. I was careful not to run at full power, but it died again after only 10 to 20 seconds and with the power at 60% or so. There was no flashover. It did seem to get slightly louder for a fraction of a second before it died.

I only ordered 2 pairs of the IGBTs so I'm dead in the water again. Should I have proactively replaced any of the other components?

I'll try to get static measurements of the windings and capacitor, but it seems something has changed and my coil is weak and unreliable now :(


Check the soldering on the top side of the board around the bridge - we've had issues after bridge replacements where the top-side connections weren't quite soldered down right.
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