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Registered Member #61375
Joined: Sat Dec 17 2016, 05:45PM
Location: Norway Aalesund
Posts: 13
Hi. I am teacher in electrics in a Norwegian highschool. I am planning on making something to demonstrate corona discharge for the students during their high voltage part of the semester. I am interested in suggestions on how to get a good visible demonstration of the corona discharge. To get high voltage i have several MOTs, some OBITs and also several zvs drivers with flyback transformers. I think maybe the zvs + flyback gives the safest approach, but it might not provide the nessesary power?
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
MOT's would be dangerous in a classroom setting because of the high current involved. Also, you need to understand that the circuit breakers in your classroom might trip as a result of the high primary side current being drawing during a demonstration.
For corona displays, metal foils attached to glass work very well. The Ozone generation apparatus works entirely on this principle. The details are on the web.
A flyback as a source works well and has a lot less current then the MOT, so discharges are much safer.
You would want to use some metal objects, one tapered to a point, a small sphere (which will still corona) and a large sphere (less discharge per unit area), and a plate of glass with different shapes of metal foil attached. The metal foil could have a triangle with two rounded edges and one point, a square with sharp corners and radius corners, etc.
Registered Member #61375
Joined: Sat Dec 17 2016, 05:45PM
Location: Norway Aalesund
Posts: 13
Thanks for reply Hazmatt! So, if i understand corectly, i should attach one of the lines from the flyback to one small sphere, and the other to a larger sphere and then place the glassplate with metalfoil attached to between the two spheres?
Registered Member #61375
Joined: Sat Dec 17 2016, 05:45PM
Location: Norway Aalesund
Posts: 13
I have run two MOTs in series to do some wood burning with bakingsoda solution with good results, so the breakers holds the starting peak current fine. But I agree that flyback is the safer way to go.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
No.
See with the flyback the potential is high frequency, so it tends to spray every which way, which is exactly what you want for your demonstration.
The high voltage return at the base of the flyback is grounded well.
The high voltage lead is what you are using for demonstrations of corona, and is fixed some 10" above the flyback to a needle or what ever you like. Then you can add a sphere to the needle and show how the discharge is only corona and not a spray of electrons. You can position the needle to charge a plate of glass with the different metal foils, etc.
One of the fun things I liked was the HV electrode as a sphere at the center and a grounded hoop surrounding the sphere, you really see corona that way, but again you have to suspend the sphere or position it with an articulating arm.
You can make an articulating arm of fiberglass rod, and that way you can position your demonstrations easier, and have the high voltage lead attached to the end of a screw. You will need to cover the areas of the screw that you don't want exposed or spraying with silicone.
Registered Member #61375
Joined: Sat Dec 17 2016, 05:45PM
Location: Norway Aalesund
Posts: 13
Thanks again Hazmatt!
Do you know of any illustration of this setup i can find on the net? I am woundering on the dimmensions of the different parts, glassplate thickness, size of sphere etc. When you say "position the needle to charge a plate of glass", do you meen i should put the needle at the glassplate? Should the negative lead be attatced on the opposite side of the plate then? Sorry if my questions seems silly. It would be of great help to see a simular setup picture or video. Regards. Martin
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Yea I know, a picture is worth a thousand words, unfortunately I do not have a setup for demonstration purposes.
I was making demos for kids a long time ago, but i didn't think of making a corona demo, otherwise I would have one.
So basically your flyback has a hot lead, so that connects to some kind of electrode, threaded, or a needle, or what ever you like. That will demonstrate infinite charge density at the tip causing a spray of electrons. (surface area becomes very small, charge per unit volume becomes nearly infinite)
If this part is attached to an insulator that articulates, you can position it for demonstration. There are swivels available in the Fisher Sci. catalog that you can purchase or borrow from the Chemistry department, they are standard apparatus.
I think Fisher has insulated rod as well, you could use Acrylic but it tends to flex and it will burn, which is why I like fiberglass, it tends to burn less.
The end electrode "needle" can be positioned to a suspended or held piece of glass with various shapes of metal foil, and the needle electrode touching the metal on the glass. The glass can be standard window pane glass, thicker is better because it's more mechanically stable. Tempered might be the way to go, you will have to experiment with what you can and can not do here.
When stressing glass with high voltage on one side, and a grounded surface on the other side, standard glass windows tend to shatter. That's why we try to avoid stacks of glass plate capacitors in Tesla coil operation, the losses are high, they are hard to compress, and they tend to go off like a bomb, its just bad all around.
Spheres can be anything. Pith balls, ping pong balls with Al-foil, spun metal balls, its whatever you like. The idea is they are different sizes. Larger sizes have more surface area so it takes more potential before you see corona. A small 1" ball will have small tendrils of discharge, whereas a 4" sphere may just have a purple glow about it.
The grounded ring. A wand can be a length of wood or PVC with a hoop of refrigerator tubing that is maybe 4" ID and placed around the 1" charged sphere, that gives you a corona ring.
I might have to spend some time drawing these up, if I can find some time. I don't have CAD.
Registered Member #61375
Joined: Sat Dec 17 2016, 05:45PM
Location: Norway Aalesund
Posts: 13
Thanks again Hazzmatt! Realy appreciate your help here. So, the spheres and the glassplate + foil are two different experiments..? I got confused at first, but i think i got the picture now. A drawing would be of much help tough, but when i start to experiment i hopefully will find out what works best.
Again, thank you for input. Greatly appreciated! Regards. Martin
Registered Member #61449
Joined: Fri Jan 27 2017, 06:55PM
Location:
Posts: 2
Sulaiman wrote ...
a nice ion wind spinner (I bought one of these, it works ok)
I tried to buy exactly that, a fortnight ago - and was told " the P904 Electric whirl is out of stock and we won't be getting any more in. Unfortunately we don't have an alternative product in stock"
which is a shame seeing as it was so cheap. Easy enough to make something DIY though.
Another demo which I thought was very good is shown in a Youtube video "DIY Volta's Hailstorm Experiment(Electrostatic indicator)" by Thomas Kim which is nice to watch and might be a useful topic to show.
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