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High speed Tesla spark photos

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Tesladownunder

Thu Sept 21 2006, 01:11AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Terry Fritz wrote ...

... will "guess" two LEDs a resistor and a MOV. The strike current might be 2000A for a few nS which will kill most TVSs and such. It really helps to keep the device voltage low so the instant power is lower too. A capacitor might also work if one could guess the right value.

I was surprised that fast devices fared worse than LED's. Interestingly only my two red LED's have survived and I am really pushing them very hard. Blue and green worked on my lower power ignition coil only but not on the TC even when treated very gently.
Two LED's and a resistor OK, but not a MOV.

The photos are interesting. Remember a negative strike lights the left LED. From what I have seen they show that the first strike can be positive or negative but there is a lot of structure and chaotic events in the few microseconds either side of a strike. There is often a small streamer hit of opposite polarity just before but both polarities are often seen within the strike. Perhaps much higher frequency oscillation>1Mhz.

Peter

1158801088 10 FT15766 Hvrotmirrorpolaritydetailnegstrike

1158801088 10 FT15766 Hvrotmirrorpolaritydetail

1158801088 10 FT15766 Hvrotmirrorpolaritydetailposstrike

1158801088 10 FT15766 Hvrotmirrorpolaritystreamerstrike

Terry Fritz

Thu Sept 21 2006, 01:58AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

One of my high speed pictures caught something unusual. It was just a normal power arc:

But when I pumped up the brightness and contrast to see the lead up pulses, I see there is like a "cloud" of ions or something after the strike (time is increasing upward). 1MB picture since I did not want to loose detail.

There is a little air streamer super imposed on the lower image while the shutter was open. Note the cloud that almost looks like smoke from the main strike. They are fairly bright and fuzzy so I assume some type of ionized cloud or something.

A similar strike does not show the effect:

A blank frame seems like the situation is clear of abnormalities:

So I think the cloud or whatever was real.

Cheers,

Terry

Tesladownunder

Thu Sept 21 2006, 02:43AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Terry Fritz wrote ...

One of my high speed pictures caught something unusual. ... I see there is like a "cloud" of ions or something .... so I assume some type of ionized cloud or something....
So I think the cloud or whatever was real.

..Who do you call

Terry Fritz

Thu Sept 21 2006, 09:40PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi Peter,

Simply, it reads the polarity of a spark. The led closest to the spark is negative ie the spark from the toroid on the left is a negative leader.
They then alternate polarity far further than one can see as a spark.

I made one too but I have not tested it with the camera yet. Mine uses a CT, 2 resistors, and the two LEDs:

The CT is just in the secondary base lead:

I just made a little wood block to hold the LEDs:

My coil hits about 6 amps peak on the secondary which puts about 32mA into the LED with this circuit.

My camera is undergoing the table saw, glue, wood, clamps and black paint now for REV-2

Once the paint is dry and it is back together, I will have a much wider veiwing area. I gave up on adding another mirror since the physics did not work out. So I just fixed all the issues with the one I had. Wonder if I'll still see the ghosts

Cheers,

Terry

Tesladownunder

Fri Sept 22 2006, 12:50AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

The circuit diagram mystery of the polarity detectng LED's is very simple. Just two 10 ohm 1 watt resistors and the two red LED's.
One of the resistors is connected to the ground electrode on one end and the spark hits the other end of the resistor. So if 2000 A passes then a 10 ohm non inductive resistor should develop 20,000 V across it, which is enough to spark right across it on the outside. I have seen this happen with higher value resistors.
The other 10 ohm resistor is in series with the two LED's which are in parallel but of opposite polarity (=anti-parallel). This then goes across the first 10 ohm resistor. So why should this work when theoretically the LED's should be receiving half of the 2000A. ie 1000A in each LED in each cycle. We know that we can push current limits in an IGBT to perhaps 100 times the continuous current rating, but this is 50,000 times overload (1000A vs 20mA).
So what is happenning? I suspect that the actual arc current is a lot lower. Added to that is the higher inductance of the loop made by the LED and secondary 10 ohm resistor compared with the main 10 ohm one alone.

I knew I wouldn't be able to work out real performance with out some trial and error so that's how I came up with this. I did try the resistors separately to see if they would flash over due to inductance issues but they were OK. The values seem bizarre but work. Note that you can't run the LED's off DC without putting about 0.4A through the 10 ohm resistor (=4W) and burning it up rapidly.
It really is bright considering the fact that it is only pulsing in several microsecond pulses for brief periods. I think something is different about the red LED's other than just the lower forward voltage (compared with the blue/green ones) that allows them to survive.

What is also interesting is to use spark gap arrestors which also show the really fast stuff without burning out. Interestingly, focally they are brighter than the arc itself and a bit more of a point source. In the end I used the LED's but certainly worth thinking of the arrestors for > 1MHz fast stuff if you could find a way to rectify them. A knowledge of microwave type design and components is something I don't have.

Peter

1158886233 10 FT15766 Hvrotmirrorpolarityledresistors

Terry Fritz

Fri Sept 22 2006, 03:19AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi Peter,

I made a different version.

Mine works off secondary base current. Sort of 90 degrees out of phase, but easy if you know what's going on

) It seems to give very sharp points of light to the streak cam. Maybe good or bad depending on how one looks at it. Positive is closest to the spark per your standard

Mine looks like this to cam V-2.0.

One will notice the "1/4 wave" leader there

))

StreakCam V-2.0 is "here":

The mirror was spinning at 7500 RPM in those photos but "cheap cam" made it look "still"... I have been pushing higher RPMs now to separate the lead up leaders... If anything gives way, I get to go buy a "new" camera

)

I think the polarity indicator needs to be moved to the top terminal point. I think we both can do that...

Cheers,

Terry

UPDATE:

This high-speed streak cam photo pretty much sums it all up!

Cheers,

Terry

Steve Conner

Fri Sept 22 2006, 09:12AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Wow! I never would have thought that secondary base current would spike during a strike. The secondary is a big inductor after all: it shouldn't be possible to force sudden sharp pulses of current through it.

By this line of reasoning, I always assumed that the high currents in a ground arc were supplied from the capacitance between the toroid and grounded surfaces in the room, and therefore they would return through the main ground connection, not the secondary base. I guess the secondary has self-capacitance too though, that looks roughly like a capacitor between the toroid and the secondary base connection. So that could explain it.

Could you try putting your indicator CT in the wire that grounds the whole apparatus to whatever earth you use? (and maybe try the strike target wire too for comparison) It would be interesting to see the relative magnitudes and timings of the secondary base spike and the "surroundings current" spike.

Tesladownunder

Fri Sept 22 2006, 03:38PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Terry Fritz wrote ...

Mine works off secondary base current. Sort of 90 degrees out of phase, but easy if you know what's going on

) It seems to give very sharp points of light to the streak cam. Maybe good or bad depending on how one looks at it. Positive is closest to the spark per your standard

The LEDs do give a sharp point although my pics were out of focus. Mine really shows ring down well with perhaps 24 LED flashes and yours shows ring up well with several LED flashes. I have pondered this and I suspect that mine is a very hot close spark running 10 inches when it is able to jump 4 feet. It certainly looks more like a Jacobs ladder when that close.
BTW the LED nearest the spark has the cathode end towards the spark (the big internal support) so I figured the toroid is at a negative potential.

Terry Fritz wrote ...

I think the polarity indicator needs to be moved to the top terminal point. I think we both can do that...

I think I can find a crocodile clip somewhere...

Terry Fritz wrote ...

This high-speed streak cam photo pretty much sums it all up!

It's a great photo and I vote it for number one Tesla streak camera photo of the week.
...but I haven't finished yet. I am about to glue a bigger mirror with which I will be able to catch a full width spark with full lens aperture and screen resolution. I should see more of the streamers connecting as you do. Hopefully some shots tomorrow if I can get it set up and it doesn't rain again like today.

Photo shows a negative strike with a brief initial flash of positive but it remains negative for a full cycle after that. Hard to explain. (Time is down, strike is followed by a ring down at about 100kHz.)

Peter

1158939531 10 FT15766 Hvrotmirrorpolaritydetailnegstrikelong

Terry Fritz

Sat Sept 23 2006, 08:43PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Steve Conner Wrote:

Wow! I never would have thought that secondary base current would spike during a strike. The secondary is a big inductor after all: it shouldn't be possible to force sudden sharp pulses of current through it.

Have to get the scope in there too look around. Ground strikes apparently discharge the top terminal in 10's of nS so current probes don't show it we'll.

The arc actually continues on until the energy stored as current in the secondary is also grounded out which takes far longer (but at far lower current) and is probably the "wash out" we see in the main arc. Of course there are all kinds of space charges and such going on too... One could probably use a larger number of Peter's probes to get a better idea. The fast discharge oscilates too with the ground leads and all so maybe that is some of the fast reverse polarity Peter sees. My probe reacts too slow for such things.

I am looking into the LM3914 thing too...

Peter Wrote:

(Time is down, strike is followed by a ring down at about 100kHz.)

"I" have never seen the ring "down" "after" the strike... Apparently "you" have never seen the ring "up" "before" the strike... I suspect one of us has something backwards or upside down

I'll try to check mine out...

I will also try to get you indicator going here.

Cheers,

Terry

UPDATE:

I built Peter's LED thing and it works fine! I put the LEDs in anti-parallel where I think he put them in series. I also wonder if the 10 ohm resistor in series with the LEDs should be a higher value since the LEDs seem "too" bright.

So I set it up like this:

Time is from bottom to top (I checked it again). The files are big to presever detail (~400k).

My base current sensor is on the ground strike point to the right while Peter's LED thing is on the arc wire to the left. The distance is 22 inches. I also uped the speed of the mirrors to 10,000RPM now.

This is a pretty typical arc. Note the well placed breif opposite current indicated on peter's sensor as the arc stikes.

This one shows the leaders:

This one shows that odd 2.5uS second harmonic or whatever. This leader just barely began to strike.

This arc has lots of preglow whish may be related to the 2.5uS thing...

Peter's sensor measures a very long current for the strike here:

This one shows the ringup and back down without a strike:

This one has a lot going on:

Another near miss:

My base current sensor records "two" negative pulses here:

Cheers,

Terry

Tesladownunder

Sun Sept 24 2006, 12:51AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Terry Fritz wrote ...

"I" have never seen the ring "down" "after" the strike... Apparently "you" have never seen the ring "up" "before" the strike... I suspect one of us has something backwards or upside down

Terry, try reducing your spark distance to 6 inches and I bet you will see the ring down. There is less streamer action at that distance though as they connect a lot faster.
I have a photo with ring down and ring up in the same photo. Your ring up is OK. Streamers ring up and sparks ring down (from TC spark photography 101

)

Terry Fritz wrote ...

I built Peter's LED thing and it works fine! I put the LEDs in anti-parallel where I think he put them in series. I also wonder if the 10 ohm resistor in series with the LEDs should be a higher value since the LEDs seem "too" bright.

My LED's are in anti parallel. They work amazingly well. It is quite remarkable to see huge arcs playing around them. What is even more remarkable is that I wired one set in parallel by mistake and they didn't blow. They must avalanche well, at least the red ones. I will start a new thread on the LED topic as I have some other info as well.
I thought that 10 ohms would be too little but with a true adventuring spirit I kept going down. Perhaps I should try 1 ohm or nothing. I have plenty of LED's. It is all a bit non-intuitive for our usual electronics.

Terry Fritz wrote ...

My base current sensor is on the ground strike point to the right while Peter's LED thing is on the arc wire to the left. The distance is 22 inches. I also uped the speed of the mirrors to 10,000RPM now.

You are getting great results. Why don't you use two of my LED sensor things

They are brighter and have a ? faster response by the look than your base current transformer. Also no phasing issues

So back to my progress or lack of it.
My larger mirror was a conventional rear silvered mirror of dubious parentage. I was hoping it wouldn't matter. I was wrong (I think). I can't get my focus to work properly when it is running. If I can't get around that I will need to get a decent first surface mirror that is halfway flat.

Here are the pics . Note that this is is in the USA format with time up and toroid on left.
(oops this one isn't the one that shows my incorrectly wired diodes on the right giving a single pulse only.)
The first shot shows a 25inch spark with lots of ringdown on the LED's but none visible. The lack of visibility may be partly due to the poor focus. (I used to be able to see them OK).
Second pic is of streamers connecting. It would be interesting to see how each side (toroid and ground) would respond in this case. They may not be identical.

Peter

Update:
I have tried a new focusing technique which helped but didnt fix the focus issue so there is likely a residual mirror effect. I will consider a repair with a better mirror.
Nevertheless I got better shots now I have wired my LEDs better. The LED issue is even more interesting and I will post interesting stuff when I get a reply to the LED post.
I have got some streamers ring up shots and spark ringdown shots, but never the two together.
Will post more pics with next reply or check the website.
I've taken 500 photos in the last two days with a combination of the Tesla streak cam shots, new improved vortex generator and the LED experiments. Takes time to collate them all.

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