Van de Graaff help

[Blaze1212]

TL;DR Summary

Van de Graaff

Hello, Ill start off by saying, I don’t have an engineering degree and only play with electronics for fun, so prepare to laugh at my questions.

I've been making this generator for over 6 months now, but only because I always have other projects going on has taken so long. I learned about Van de Graaff’s a couple years ago and instantly wanted to make one. I usually jump into projects with very little knowledge and that usually makes me figure out a bunch of ways not to make something which is still fun for me and helps me usually have better understanding how something works.

I wanted to make the structure out of 3d printed parts and make it nice looking. So started designing everything in cad and started printing and got it all complete and then tried it and it didn't create any sparks and that's when I looked up most designs and it always showed the upper comb inside the dome, So I scraped my whole design and made a new one and finally have it all done but it only produces about a 1 cm spark.

I bought a cheap Van de Graaff from used electronics store and fired it up with my belt and dome and it puts out a 3-4'' spark. So, something is wrong in my build either the rollers I used (probably) or the 3d printed structure (PETG) is shorting out somehow. I see most structures are made of acrylic because a good insulator I assumed 3d Filament would be a good insulator as well, maybe not. The rollers I used were also printed upper is nylon and the lower is Poly carbonate; I used these filaments because I saw they were on opposite sides of the triboelectric series and I had them on hand. Seeing if you guys have any suggestions or ideas on what is wrong?

I also have some other questions. Its my understanding that van de graff are relatively safe because its static electricity and the high voltage can produce enough miliamps to hurt you but since it discharges for a very short time its relatively harmless. And when you add in a capacitor to store the charge is when bad things can happen because the capacitor can discharge over a longer period of time. Is this correct? When does the Voltage get high enough its dangerous (lightening) can kill you?

Also, if capacitors are close to the generator could the capacitor be charging, without being attached to the dome? And if its 1000’s of volts would the capacitor get damaged if it was a small voltage capacitor like 50 volt capacitor? Also, can other things in my office act as a capacitor?

Attached are pics of the original design where the dome would sit above upper comb which didn’t work at all. Scrapped

And the new design that only makes about a 1cm arc

 

[Averagesupernova]

It seems you have the basics about what to watch out for. I question if your 3D printed parts are not a little bit leaky. I found that the closer the brushes are to the belt, yet not touching, the better. The generator I've worked with worked better with a conductive cover over the bottom as well. It was similar to the top although squared up a bit but with round edges.

 

[Baluncore]

Welcome to PF.

3D printed polymers always absorb a few percent of moisture from the atmosphere, so they do not make good insulators. Some black plastic also contains carbon black, as a pigment and a UV stabiliser.

 

[Tom.G]

I built one of those (with Dad's help) in High School.
Both @Averagesupernova and @Baluncore brought up excellent points - in fact the same problems and solutions I had.

For the vertical structure, mine had hardwood strips that had been dried in the kitchen oven and then shellac applied. Not enough! I had to cut a section out of the uprights and insert a couple inches of polycarbonate plastic to reduce the leakage current.

Additionally:
Move those brushes them away from the pulleys and get them, much closer to the belt.

The lower brush should face the rising belt just above the lower pulley.
The upper brush should also face the rising belt just below the upper pulley.

The pulleys were hardwood turned on my uncles lathe.
I think the belt I used was polyester ribbon used for wrapping gifts; had to edge-glue two lengths to get sufficient width.

The lower brush gets connected to an Earth Ground.

Another enhancement (that may or may not help):

Place a metal strip behind the belt where each of the brushes are.
(That's two metal strips, one for each brush.)

Place two more brushes facing the descending length of belt, at approximately the same height as the active brushes. Then wire each brush to the correspondig metal plate you just added.

[Edit] Another potential problem is high humidity. [/edit]

Please keep us updated on your project -- we like to learn too!

Cheers,
Tom

 

[hutchphd]

I had a REMCO toys Van de Graaf generator kit as a kid. It was a fabulous toy.
I am, in retrospect, probably lucky to have not caused myself harm in my meanderings through high voltage. I did make Leiden jars (capacitors) which I am now certain could have been lethal. I recommend extreme care. The energy stored in a capacitor goes like $\frac {V^2} 2$ so you can get in trouble pretty easilly. Not a good idea. But educational!
As I recall mine worked better when the base was well grounded (and best in the Midwestern indoor winter dry air. Enjoy.

 

[tech99]

I have used brushes made from a thin copper sheet which is slitted with scissors into a comb, and then positioned so it just touches. I also notice your brush assembly is very spiky, which might be losing charge. As others have mentioned, I think the problem with your VdG is likely to be leakage. I have received many shocks from one of these machines without any problem, but then I am acclimatized to a certain level of electric shocks by having worked with vacuum tubes all my life.