Van De Graaffs are Fickle

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Jason C Wells
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Van De Graaffs are Fickle

Post by Jason C Wells »

I have a nice big Van De Graaff. What a fickle machine! Dead simple, and robust in so far as I can always count on it to run. It's output varies from day to day even with a bit of care. This isn't anything I haven't already read, but I'm obstinate that way. I can always make machines work. Sometimes it's not worth the effort.

My equipment that I've aggregated to date has served it's purpose. I think I've done all that I can with what I have. I think I'll be refactoring my entire set up with better controls and better instrumentation. But A-number-one improvement I think would be to swap out the VDG for a big Cockroft-Walton ladder.

I could probably spend a bit of time tuning up my VDG to get more reliable output from it. Bah!

Regards,
Jason
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Richard Hull
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Re: Van De Graaffs are Fickle

Post by Richard Hull »

Getting a VDG to do what you want it to do, when you want it to do it is like telling a cat..."come here boy!"

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jason C Wells
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Re: Van De Graaffs are Fickle

Post by Jason C Wells »

That is the most perfect description of a VanDeGraaff.
Jerry Biehler
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Re: Van De Graaffs are Fickle

Post by Jerry Biehler »

Jason C Wells wrote: I could probably spend a bit of time tuning up my VDG to get more reliable output from it. Bah!

Regards,
Jason

You would probably have to come up with a way to control the humidity around the belt and pickups. Humidity has a huge effect on these things.
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Dennis P Brown
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Re: Van De Graaffs are Fickle

Post by Dennis P Brown »

Humidity is a major issue - one could use a de-humidifier for the room; also, corona rings are impotent. Further, are the drive rollers metal? or plastic? Do you use a high voltage spray on the belt? Using a drying agent in the housing will help, as well but dust is an issue. So, be careful with drying agents. Belt speed is important and many commercial units tend to be on the slow side. Belt width is critical and frankly, commercial units use very narrow belts. Just some random thoughts on the subject.
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Re: Van De Graaffs are Fickle

Post by John Futter »

Jason
I do not know what Dennis is trying to say
Commercial VDG's tend to run @ around 10-15 meter per sec belt speed and the belts used are 24 to 36 inches wide all this as per our High Voltage Engineering KN VDG @ work.

Dennis did you mean commercial hobbyist machines??
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Richard Hull
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Re: Van De Graaffs are Fickle

Post by Richard Hull »

Of course, many really large commercial units run in a rigidly controlled, sealed, tanked environment. They never run and would never run in the normal open air to any design spec that would be repeatable for reliable experiment. The very best these fusor.net guys can do is try and control the humidity, the number one enemy of any static machine after coronal leakage to air.

The only giant air machine I know of, and it is of no value in current experiment, is the one half part of the zepplin hanger van de graff of the 1930's. It was or is at the Boston Science museum as a large lightning bolt snap-and-pop exhibit.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Van De Graaffs are Fickle

Post by Dennis P Brown »

LOL about scientific units that, as you and Richard pointed out, are housed in special chambers and work very well.

I was, of course, referencing Jason's own commercial unit (the cheap ones online - not scientific grade that cost $$$$!) - these online cheap commercial units have tiny (width wise) belts. Yes, units (some) reach 36 " high but having at best a belt just an inch and a half wide produce very little current - length is ok for getting higher voltages but for better current, belt width and dome size matter a great deal. Hope that addresses your concerns. Any VdG works best with as close to zero humidity as possible and I want to make that very clear - any method used to get the unit (VdG) working in as dry an environment as possible will raise the current but I doubt that alone is enough. Using a faster belt speed helps since this approximates a wider belt. Likewise, adding a charging unit helps (only a little I found.)

I am experimenting on bigger units (that anyone can build) to see if this route is worth taking (relative to cost; of course if one has enough money the problem is reduced.) Jason is struggling with his small on-line commercial unit and I am offering easy fixes to help his unit provide more current with a view to being simple "fixes".

Any post about VdG's really aren't appropriate for the "Fusor Input Power" thread. Rather, I'd think the "Ion Gun" thread is better (or more appropriate.)

A Cockroft-Walton ladder is no simple device and can be dangerous if one uses large caps or really fast power supplies. So, if you go that route, be careful - these are not VdG's. These "simple" voltage multipliers are good for the 100 kV range but oil is essential in my opinion - otherwise, air leakage is really bad. I am using a NST as the base power supply (with all its down side) but I have big caps and diodes; got deals on both so still cheap but that can be an issue, too.

You appear to be following my path so check out my designs but better still, look at the one by Rapp Instruments in the "Ion Gun" thread. That is a good unit!
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Jason C Wells
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Re: Van De Graaffs are Fickle

Post by Jason C Wells »

I've been away for a while since I finished up my program. There always has to be that one guy who doesn't quite fit into the established rubric. My VDG is analogous to the power supply of a fusor, so I landed here. I definitely have an eye on Dennis' ladder project. For right now I have a huge pile of stuff to get cleaned up and some drywall to hang.
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