Van de Graaff Generator

I am trying to build a small Van de Graaff generator according to these directions scitoys.com/scitoys/scitoys/electro/electro6.html
It looks simple enough, but mine just does not work at all. I am fairly handy with building stuff like this, but I just cant seem to make it work.

If anyone has some more directions that may work better or some suggestions or things for me to try please let me know.

I would really like to see this work.

Thanks so much

 

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well, what have you built so far?

 

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Van De Graaff

Hi there,

I'm Julie, sadly I don't have any advice or good websites for you to refer to but I was interested in why you are making one of these machines? are you a science teacher?

Cheers, Jules

 

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I'm shocked that your are having problems. Could you please illuminate me as to your short circut?

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I have never seen a kit, but can guess what it is like. You may need a high voltage meter* or make a "Lyden jar" capacitor to do some of the following.

Divide and conquor problem:

Confirm you are putting charge (electrons) on the (rubber?) belt.
If OK confirm you move them to top (charge not leaking off to rollers etc.) (Real Van de Graqphs usually run belt in vaccum to prevent air discharge .)**
If OK then confirm you are gettting them off at top (metal shell removed)via the "metal comb."
If OK, does your "comb collector" electrically connect well to inside of the metal shell? (You have a metal shell and belt runs inside it plastic support tube I assume.)
--------------

**Does yours have at least hand pump to reduce pressure? If yes, you know that many kV can make X-ray with air absent where electons fall thru this voltage and stop in metal, I hope.

*Make one with 2mm wide 2cm long thin AL foil folded over to make inverted V hanging on modified paper clip, which passes thru rubber stopper. Inverted V is inside glass jar to protect from air currents - note a mutual deflection of two halves of the foil due to mutual repulsion of the foils which are initially close. It will not be large, but observable. Some desicant in the jar (especailly if summer) may be needed to keep air dry - may not work for hour after it has been opened - test your meter with rubber comb & silk etc.

 

Thanks so much for all the responces. I will try to answer all the question as best I can.

I am making it just because I like to make stuff, I like electricity and would like to spend time with my kids by making things that are cool. Sadly it is not cool because it does not work.

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The last post is a bit over my head, but I will try to answer as best I can.
The belt is rubber, it is a rubber band from office depot. I dont not have any way to create a vacume.
The thing that makes me so frustrated is that I have used the same plans a couple of years ago to make it and it worked. Also, it worked a little at first, but then stopped working all together. I dont know what changed between the time it kinda worked and when it completely stopped.
Can some one help me go step by step to "Divide and Conquer" the problem?
1. how can I "Confirm you are putting charge (electrons) on the (rubber?) belt."
2. how do I "confirm you move them to top (charge not leaking off to rollers etc.) "
3. how do I "confirm you are gettting them off at top"

Thanks so much!!

 

Small neon lamps (NE-2 type) can indicate the presence of an electrostatic field quite well. They are available virtually anywhere, cost almost nothing, and begin to break down at around 90 volts, so they will flash brightly when exposed to high electrostatic potentials such as the front of a CRT monitor (upwards of 20kV). They look like this:

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This one has a 33k series resistor on one lead for use in a 120V circuit; it is unnecessary for your experiment but will not interfere with it either.

Head down to your local Radio Shack and pick up a few. Hold one lead in your hand and bend the other one outward away from you to act as a probe. With the lights down low, drag the probe lead across the CRT face and observe the lamp flickering. Now touch it to the applicator comb in the base of the insulating column and see if it glows - this will indicate if you're putting a charge on the belt. Now try it at the comb at the top. Eventually, you will be able to isolate at what step in the transmission process your charge is leaking away to ground.

 

To Echo3Romeo

That is a good suggestion, but it may fail to glow if his current production is as low as I think it must be. (If his currnet is not almost nill, he should be getting voltage on the dome as every electron placed inside will go to surface.)

The electrostatic foil volt meter I suggested may fail too for essentially the same reason, but it takes almost zero current.

Do you know the current required to sustain a Ne bulb glow?

I happen to know they fire at about 90V and go out at about 70V on the leads, because I once worked under QRC-65 (Quick Reaction Contract), helping to make TWO demonstartion/test fighter-to-fighter communications jammers for SAC bombers trying to penetrate into the USSR. We knew that USSR's high power ground based units could fry the control logic if it used transisters. (All had 3 leads coming out of their individual TO-5 cans back then.) So Ne glow lambs were the "two stable states" of our simple control computer and its memory. It ran on 100V (DC as I recall).

Job was quite an experience as SAC wanted it "yesterday" in cost plus contract and gave huge bonus if delivired in 3 months. I was a hired summer student and did only the power supplies and cable harnesses etc. The more important designers (all five were just-hired temp-enginneers from a NYC "body shop") could not tell what currents they would need etc. and the lay-out guy did not know where the components would be (length of wire runs). Some MIL-Spec wire orders could take a couple of months to deliver so in first week I asked boss what should I do.

Answer effectively was: Order at least 20 times more of every color, every size, every type of wire any engineer can even imagine we will need and assume the harness go all around the outside of the plane's rack space SAC is giving us. - So I did. We used a less than one precent of the wire I ordered, but the tiny company's stock room was happy to take the 99+% left over.

The contract came to this tiny (<15 employees) because it had a unique inductor for the "tank coil" of the final transmitter stage. It had a DC winding on the ferite core that could even saturate it and thus inductavely tune or "jump frequencies" very fast.

The "electronic tunning" reciever associated (not part of our QRC-65 contract) would listen for 10 ms or so., ground its antenna, and tell our computer what frequencies were active so we could efficiently put 50W narrow band power transmitted on more than a dozen different selected frequences for 10-15 ms each in a sequence and then we shut transmitter down for the reciever to get us a new frequency list to jam. I.e. made at least 7 such cycles every second.

Seven or more strong narrow RF band noise bursts in the fighter's recievers every second made voice coordination amoung even relatively distant fighters impossible. SAC's broad band jammers could not block USSR's frequency hopping communicarion system. We expected the closer in receivers would just stay saturated, if following a frequency hopping transmitter. I returned to school a few days before the bonus dead line and never learned if we made it. If they did, the bonus alone was more than the company earned the prior year.

 

A NE-2 with a 33k series resistor will have a glow discharge current of about 3.6mA at 120V. Those tend to be rated for higher brightness. Some will remain lit with a 47k resistor, and a discharge current of 2.5mA. I'm not sure what current can be continuously supplied from the face of a CRT. The secondary anode supply (the suction cup on the funnel of the tube's back side) is usually capable of supplying +25kV at 5-6mA, which is somewhere around where the beam current of the electron gun(s) are and therefore enough to neutralize most of the negative charge that would otherwise build up on the front of the picture tube during operation, at least in theory. From simple observation we can conclude that the current that can be collected from the tube face is much, much less, as anything more than perhaps 50 microamperes would present an electrical and radiological hazard to consumers - remember those old TV sets that needed leaded glass and shielding due to the X-rays produced inside the CRT?

An electroscope has the advantage of being able to detect electrostatic fields easier. It registers coloumb electrostatic force rather than the flow of current, so it is capable of measuring and maintaining a steady state. However, a neon lamp will work in a pinch and is easier to get a hold of. There should be plenty of current to get intermittent activity inside the lamp; enough to tell our friend whether or not ions are collecting on the test surface. It will not remain lit continuously, but when the probe lead comes into contact with an area where significant ionization has built up, the rush of current through the lamp will produce a quick flash of light.

Amazing what can be done when cost is not an issue. It reminds me of something more recent: C-RAM (that crappy web page is all I could find about it) is a USN CIWS mounted on a flatbed and loaded with self-destructing explosive rounds. Instead of antiship missiles and low-flying aircraft, it can track and engage indirect fires incoming to its position (mortar, rocket, artillery) and shoot them out of the sky. A few of them were slapped together in 2005 as part of a crash program to deal with the artillery threat.

I have no idea how much it cost, I can only say that it works amazingly well. Here is a video of a C-RAM mount engaging a mortar round in Balad, Iraq (aka Mortaritaville).

 

WOW those were some very interesting posts, over my head, but very interesting.
I will get a few different size neon lamps and see what I can find out.

Thanks so much for the suggestions and help

 

Sure. You could also try a small Xenon flash lamp from a camera. Those break down pretty easily too. The glow is a dark blue/violet and may not be as visible though.

 

Sadily me local Radio Shack did not have any. I will be trying to make it to a local electronics store to find one.

Thanks again!!

 

Then make the electroscope I suggested. (It is fun project with no cost)

 

Using the neon bulb I got absolutely NO flicker or glow or anything at all. I tried the top and bottom of the rubberband. On the top roller I have tried a glass tube (fuse with the ends melted off) and a roller made of scotch tape. The bottom roller is made of electrical tape.
Please help, this is really getting to me.

Thanks

 

Exactly as I expected and predicted in post 8: "That is a good suggestion, but it may fail to glow if his current production is as low as I think it must be."

I have tried. (Told you how to make an electroscope) It will work with voltage only (no current) but you are not listening. -This is the third time: Make an electrocsope - No cost to do so and fun to test with comb and silk etc.

again asking: Where do you live? (I want to know how humid it is where you are trying.)

PS use a foil to "wipe" the rubber band, not just a point contact, when connecting to the electroscope and hold it with CLEAN glass. Either you are not putting charge on the rubber band, or it is leaking away as fast as you do.

 

Billy T, I don't think the bulb is dark for lack of current. The surface of the CRT should have enough capacitance to produce a surge current sufficient to make the lamp flash. It sounds like he doesn't have continuity between the CRT and the generator, or an errant ground somewhere in between. Electrostatic insulation can be tricky to do if you're used to working with lower voltages.

I made a VDG generator in junior high school for a science project on methods of prediction of lightning strikes. For an ion source I used the HVDC supply from a common air cleaner/ionizer, which produced -7.5kV @ 250 microamps. A CRT probably isn't nearly as effective an ion source.

penguinview, try an electroscope as per Billy T's advice. Its threshold of detection is higher, but it indicates a static charge rather than flow of current, as well as the size of the charge. As such it is better suited for this type of work.

 

I missed (or forgot) any ref to CRT. but if that is is charge source it will need some metalic screen wire grid or something like that to collect the charge from entire surface, I would think. Your suggestion of a real HV source - Electrostatic dust collector is better source.

With rotating plastic disk and some rubbing silk etc. he might be able to generate a lot of static electricity to charge his rubber belt with also. (Again assuming his problem is not too high humidity.)

 

There is no crt used. Forget the neon bulbs. No way is there enough current for a visible discharge with this small Van de Graaff. Stick with the glass top roller, as glass/rubber is a very good static combo. Low humidity is best, as mentioned. _Very_ important, must have electrical continuity between top brush and can, and the entire top brush/wire must be enclosed within the volume of the can.

Here's the link to the OP's project:
http://scitoys.com/scitoys/scitoys/electro/electro6.html

 

Yes - the top glass roller MUST be well inside the top can. VDG works by Gauss's law. Same reason why being inside a metal car protects you from lighting. Most think it is the rubber tires that protect -but they have nothing to do with the protection.

 

Guys, it's August and it's summer here in the states. We have had the most humid summer in a long time. A Van de Graaf generator simply isn't going to work in most parts of the states right now. He's either going to have to be in a very air conditioned room or wait until winter.

If you want to figure out whether a Van de Graaf machine will work, rub a balloon against hair or an acrylic or woolen sweater or blanket. If you don't get snapping, crackling, and popping, and if the balloon won't make the hairs of your arm stand up, no electrostatic generator is going to work.