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The Tesla coil is my junk coil running on half of a 12kV 30mA NST. It has a few ceramic caps and a 3 segment static gap. Primary is 15 turns and secondary is 260 turns in 11 inches. There is usually no toroid but I used one to intensify the sparks by putting an old tin on top.
The motor and first surface mirror is as shown and rotates at about 2900 RPM. It is a synchronous motor with ground flats that doesnt quite sync at 3000 RPM with the extra load.
Mirror to spark distance is 38cm which means that the image moves at about 100m/s. The picture represents about 2cm width and 4cm height ie vertical scale is 40us. (just over 100ns/pixel)
So what do we see and how to interpret it? There is a ladder of sparks with each spark being fairly discrete and without any obvious parallel sparks. All sparks seem complete and there are no discontinuities. Almost all sparks are bright at the ends but less bright in the centre third. This also corresponds with what you see when it is running. I am not sure what it means, however, if each spark is a single cycle then the negative one third may brighter each half cycle, leaving the centre dim.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
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Could you explain in more detail how the mirror works and how you are capturing the arcs image in a brief period of time? With a rotating mirror, i can't see how that is occuring especially considering that the minimum shutter duration for your camera is only 125us and you are actually using a shutter speed of 1/2 second.
I would think you would only want to rotate the mirror once extremely fast to capture a brief period of time as continously spinning the mirror will just integrate the image over the full 1/2 second as opposed to capturing say only a 5us fragment.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
EastVoltResearc wrote ...
Could you explain in more detail how the mirror works and how you are capturing the arcs image in a brief period of time? With a rotating mirror, i can't see how that is occuring especially considering that the minimum shutter duration for your camera is only 125us and you are actually using a shutter speed of 1/2 second.
I would think you would only want to rotate the mirror once extremely fast to capture a brief period of time as continously spinning the mirror will just integrate the image over the full 1/2 second as opposed to capturing say only a 5us fragment.
The mirror spreads the image over time and due to the mirrors speed, it will only show a 4cm section out of each circumference of 2*pi*38cm = 238cm. This means that only 4/238 = 1.7% of the time is the mirror aligned to allow a spark image to the camera. It is however travelling at 238cm*2900RPM = 238*48cm/s=11500cm/s. It will do this repeatedly over the duration that the shutter is open. Typically with camera shutter speeds of perhaps 1/10 sec you only get to see single sparks. This is 4.8 mirror rotations only with a 1.7% chance of hitting a given spark. About even odds for a 10Hz spark. I did do some 1/400 shutter speeds and never saw a spark once in about 10 shots. Each spark image will be overlaid but if there are few enough, then they won't overlap too much.
So no, this isnt a high speed shutter but a way of introducing a high speed time axis to events. If it were looking at stepped leaders and other lightning stuff, it would be very interesting as the spark crosses from left to right in a stepwise manner.
I need to try this on a bigger TC to show the banjo effect, and perhaps on a DC spark through an inductor to show the spark develop into an arc. As it is these small TC sparks are too small and fast to show structure on a microsecond timescale.
Pic shows a typical single spark seen with a 1/10 exposure and slow TC firing rate.
This is like the "streak" cameras used to see high speed arcs, but your pictures are 1000X better than other pictures like this I have seen!! Probably due to them being old film based things developed by people that had no idea what they were doing :o) The very high speed arc makes the pictures very nice to as opposed to things that a streak camera really blures out.
Apparently, old laser printers also have a very nice eight sided front surface mirror on a stepper motor. It scans the laser beam across the paper. I guess you could just mount the camera so the mirror would reflect the coil's arcs... With rechargable digital cameras, one does not have to worry about wasting 10,000 pictures now
I was going to try it once but it all seemed to messy. However, you have shown it to be easy after all!! It would be very cool to see streamer growth with this method!!
Dan - Imagine if the camera were spinning and you took a picture of the arc. The fast arcs would be recorded one after another next to each other on the film. Since the arc is "moving", the small sub oscillations are recorded next to each other in a row. Since the arcs are sharp bright and fast, they each show up clearly as opposed to being a long blur. I tried to find a nice explanation on the net, but they are all goofy high level crap... In out case we don't need a slit or anything like that.
The pictures are normally only good for a "thin line" of sight smeared arcoss the film. But the sparks take care of that for us in this special case
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Terry Fritz wrote ...
This is like the "streak" cameras used to see high speed arcs, but your pictures are 1000X better than other pictures like this I have seen!! Probably due to them being old film based things developed by people that had no idea what they were doing :o) The very high speed arc makes the pictures very nice to as opposed to things that a streak camera really blures out.
I hadn't heard of "streak" cameras before. Doing this with film would have been a real pain. As it is. my mirror is too small to allow my camera to autofocus and manual focus is not accurate enough. I would like to increase spark intensity and length and push the boundaries in regard to speed. My motor is awfully unbalanced with the mirror despite a small counterweight so I have to physically hold it down to prevent it going out of alignment while it picks up speed.
Terry Fritz wrote ...
Apparently, old laser printers also have a very nice eight sided front surface mirror on a stepper motor.
I used on of those in a laser scanner project just recently. The mirror is only a few mm wide but they are accurate first surface mirrors and you get 8 times the chance to see a spark And you dont have to hold them down...
Terry Fritz wrote ...
I was going to try it once but it all seemed to messy. However, you have shown it to be easy after all!! It would be very cool to see streamer growth with this method!! The pictures are normally only good for a "thin line" of sight smeared arcoss the film. But the sparks take care of that for us in this special case
I would like to increase to speed/distance of the spark from the mirror although I need to maintain distance between the spark and the camera so I might need to change the setup to a vertical one. so the mirror will reflect at 90 degrees rather than only 20 or so.
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Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Sulaiman wrote ...
.. secrets of the arc must remain secret stop now before it's too late!
LOL But there will always be the dark gap to make sure this remains a Dark Art....
I have now done a LED calibration which shows my geometry is a bit out. Nevertheless with the motor in sync at 3000RPM (better balancing), and two LED's at 2.9m running at 100kHz, this is the result. The green LED is brightest and a white LED beside it is an older type and not very bright. The 100kHz is a square wave but the op amp gain drops off markedly so it might be a bit sinusoidal. I need to check it on a CRO.
Nevertheless there are 10 flashes in 1100 pixels at 100kHz. ie One cycle of 110 pixels occurs in 10us. Each pixel represents about 100ns.
Focus is an interesting thought. I am thinking that (for a Tesla coil) the distance would be say 5 to 10 feet so that helps. I will have to look at my cameras to see if they have any other settings besides "auto"
An 8 sided mirror would be nice since the probablitlity of the thing being in frame are 8 times higher. But your right about the very thin mirrors (nice scanner project you did BTW ).
A am thinking of taking a small block and drilling and pressing a shaft through it. Then one could accurately mill off four flats on it in relation to the shaft. Then one could slobber glue all over it to hold the mirrors on and mess up the ballance But the block could also be easily drilled for setscrews for ballancing weight. Then you would have a pretty easy to make four sided mirror that would be pretty well ballanced.
Then, you could hook it to a high speed motor somehow and spin it really fast till the mirrors fly off into the camera lens to win one of those photography versions of the Darwin Awards Ok, it would need a lexan shield too
I don't think vibration would be a giant problem since everything happens so fast. It would be easy to put it all on a frame the could mount the camera too and then it would all go on a tripod. The motor could just be a plain DC one to make it simple since the speed would not be very critical at all. Some parts would have to be a bit massive while others are light.
I am not sure how fast it should or could turn. It would be cool to catch each cycle of a bang and see how the leader forms. "Theory" is that they form very quickly in relation to to top voltage of that cycle.
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Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Well there was a break in the weather so I go to set everything up to run the TC. The TC is my 4 inch one. I'll look up the details later but it was set up for 4 then 6 inch sparks between pointed electrodes to a grounded object. Power was 4 MOT's and current draw about 10A 250 V so enough to have a reasonable power arc rise in the centre if it got going.
The distance from camera lens to mirror was 30cm and from mirror to TC 140 cm. First two pics show the setup (obviously taken with my older camera).
Third pic shows the through the non-rotating mirror showing the toroid on the left with the reversed image.
Fourth pic shows a single spark with a series of up to 5 parallel sparks. Each space between sparks is 50 pixels which going by the photos before is 5us period or 200kHz. This implies a 100kHz waveform if there are two sparks per sine wave. Seems in the ballpark.
Note that this is not the Banjo effect seen on a windy day which is just the spark gap firing rate of 100/120Hz for a synch gap (or 1100Hz with my fast asynch gap which was running flat out as I didn't have a third variac setup). This is 100 - 1000 times faster.
I've taken a few hundred shots in the last few days (and lots of blanks) so there is a lot more there. I have not investigated streamers yet.
As far as I know, these are the first high speed streamer pictures like this!!! We have often wished, but never have "had" them before!!
I PDFed some stuff about streak cameras and using them for high voltage arcs from Bazelyan and Raizer here (3MB):
But they are using slits and all so their stuff is sort of fuzzy (the pictures are that bad ). But yours are beautiful!! I am working on getting parts for mine
This is probably the most important tool to come along in years for Tesla coil studies!!!
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High speed Tesla spark photos
And you dont have to hold them down...
). But yours are beautiful!! I am working on getting parts for mine 