Re: Corona discharge
Hazmatt_(The Underdog), Fri Feb 03 2017, 07:12PM

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.

Re: Corona discharge
Martin Slinning, Fri Feb 03 2017, 08:09PM

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?

Re: Corona discharge
Martin Slinning, Fri Feb 03 2017, 08:14PM

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.

Re: Corona discharge
Hazmatt_(The Underdog), Fri Feb 03 2017, 11:57PM

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.

Re: Corona discharge
Martin Slinning, Sat Feb 04 2017, 04:07PM

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

Re: Corona discharge
Hazmatt_(The Underdog), Sat Feb 04 2017, 06:34PM

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.

Hopefully the description makes sense.

Re: Corona discharge
Martin Slinning, Sat Feb 04 2017, 07:19PM

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

Re: Corona discharge
Sulaiman, Sun Feb 05 2017, 08:13AM

the conductive shapes;
.
.

a nice ion wind spinner (I bought one of these, it works ok)
.

Re: Corona discharge
Electronic Battle, Sun Feb 05 2017, 12:03PM



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.

Re: Corona discharge
Wastrel, Sun Feb 05 2017, 04:53PM

Some ozone generators use the corona effect from a wire, but the real things use dielectric barrier discharge, which isn't the same as corona.

To demonstrate high voltages to high school children I think you'd be mad to use anything other than a Van de Graaf. It's the standard in school safe high voltage.

Re: Corona discharge
Hazmatt_(The Underdog), Sun Feb 05 2017, 08:33PM

I think Wastrel brings up a good point, as much as I dislike it because it's an example of "watered down" science these days.

Ultimately, you will need to talk with your safety officer and get your "system" approved, or you could easily loose your job.

I was trying to help my college science center get a Crooke's tube working again, the one with the Maltese cross, and those are pretty much banned these days for fear of X-ray generation.
Which is ludicrous, but that is the world we live in now, and that was almost a decade ago.

I cannot commit to anything because of work and people line-up to demand I do things for them (so I never get to my projects in time) but I may be able to machine a spinner for you. That's not a promise because I'm busy, and exhausted, but I can see what I can do.

You should also see what Fisher sci. has available, and do searches on old CENCO equipment, it's really good and has lasted 100 years. (I do like CENCO stuff, it's pretty neat)

Re: Corona discharge
Martin Slinning, Mon Feb 06 2017, 08:29AM

Yes, safety is always focus nr1! We follow national guidlines with at least two safety barriers. I will point out that the students themself will not handle this equipment at any time. It will be demonstrated by me with safe distance to students. However i also agree with Hazmatt that we tend to move to "watered down" science to day. When kids need helmets just to go outside it has gone to far..

Re: Corona discharge
DekuTree64, Thu Feb 09 2017, 07:21AM

Build a Bonetti electrostatic machine :) And perhaps a Wimshurst to get it started with, because the sectorless style is tough to get started if the humidity isn't low enough. But sectorless is a much better educational tool. With 15-20" diameter discs you can really feel the resistance on the crank, whereas a Wimshurst is so inefficient you can't feel anything beyond the friction of the mechanics. And the discs glow with corona where the positive charge is bleeding off into the air, bright enough to be visible to the whole class in a dark room. And you can make long sparks. And demonstrate how smooth surfaces can contain higher electrical pressure, with corona discharge being sort of like a leaky pipe :) And as long as you don't put any capacitors on it, it's safe to play with and stick your hands in it. The sparks sting a bit, but no harm done. Getting sprayed with corona and then shocking people with your static charge is also fun :)

Re: Corona discharge
Dr. Slack, Thu Feb 09 2017, 09:55AM

No need to use a Whimshurst to 'boot up' a Bonetti. A glass rod recently rubbed with a silk cloth, held close to one disk opposite a comb, is all that's needed to get it started.

Re: Corona discharge
Erlend^SE, Thu Feb 09 2017, 08:45PM

Use a flyback driver and TV transformer, also take the output via >10 (suggested 100) MOhm HV resistor.

ZVS does current well, you want voltage (thus flyback driver, 555+FET would be the crude form)

That way, it should be hard to get a dangerous output current.

Re: Corona discharge
Martin Slinning, Wed Mar 08 2017, 07:49PM

Hi again. Thanks for all replies.

My goal is to make corona discharge between two (or three ) small lines of wire to demonstrate corona discharge in a powerdistibution net line in the most safe way for the students.(Zvs + TV transformer?) I was maybe a bit unspesific in my first post. Sorry for that.
Any suggestions?

Martin

Re: Corona discharge
Martin Slinning, Wed Mar 08 2017, 08:15PM

I was of course interested in all cind of corona demonstrations, but for these studendts, the high voltage powerline distribution corona discharge are the most interesting to be awere of.

Re: Corona discharge
DerAlbi, Wed Mar 08 2017, 10:26PM

So from university teacher to university teacher
Use a dark room. Use very thin wire, (the thinner, the earlier it glows)
You can easily explain that to the students and even reference that to every day technology.. like HV-Lines made out of 4 thiner strands.. not to carry more current, but to increase the effective wire diameter without sparing that much material so the electric field is not peaking so high.
Then you can explain why different voltage lines will have different geometry, like the common 2 strands, 4 parallel wires, or even 8 on really high voltage lines.
You can even demonstrate the same voltage on different wire diameters and show that they start to glow on different voltage levels in that manner. Then you can make small notches in one thicker wire and show that those imperfect areas break down easier. Then make references to rain drops or dirt on insulators and stuff.
I think for the practical understanding of the phenomenon and with respect to power transmission, this would be the most relevant stuff.
For this i would suggest even DC voltage or a flyback.
I think this is a visible thing. Touch or interaction is not important.
Turn on, show, turn of, change wire, turn on... and so on. Safety is only a concern for you, students should not be able to interact with the stuff.

If you cant make sure of safety, then use high frequency like 1MHz++. This will glow somewhat like DC but when you touch it you dont get electrocuted, it will just leave a burn mark that will heal. Its safe due to skin effect.

Re: Corona discharge
Newton Brawn, Thu Mar 09 2017, 02:34AM

LOOK here

Re: Corona discharge
Martin Slinning, Mon Mar 13 2017, 08:38PM

DerAlbi.

Thanks a lot for your reply!! This was exactly what i was after. I will try this out, and hopefully post some pictures of the setup.
Brilliant!