Introduction: 120,000 Volt Van De Graaff Generator
It's Frankenstein time. Who doesn't enjoy the smell of ozone and the snap of a big fat spark?
The dome is rotating due to vibration
Although a Van De Graff generator has recently been posted I decided to post mine based on it's substantial differences.
Step 1: Supplies
Two matching mixing bowls
3" PVC pipe
1/2" pvc pipe and end caps
assorted nuts and bolts
A 540 size electric motor
copper wire for brushes
and a pair of pantyhose ;-)
Step 2: Making the Sphere
Start with your mixing bowls, BIGGER is BETTER. Measure how tall one bowl is and write that down somewhere I wrote it inside the bowl. Cut a 3.25" dia. hole in the bottom of one bowl, a sheet metal nibbler makes this easy. Now tape the two bowls together with electrical tape, and then tape around the nibbled opening. We use electrical tape not because it's insulating, it's not at these voltages, but because it smooths and reduces corona discharge.
Step 3: The Motive System
The motive system consists of a roller made of pvc pipe attached to a 540 size motor. To build the roller carefully drill a hole smaller than the motor shaft diameter through the center of each end cap. Heat the motor shaft with a lighter and press one end cap onto the motor shaft squarely. Now cut a short length of pipe and assemble the roller. The bearing for the end opposite the motor is made from a piece of hardened steel rod epoxied to the head of a bolt. The top roller is made similarly to the bottom roller only it rides on simple brass bolts and is covered in aluminum tape.
Step 4: The Belt
This is where you need the pantyhose. Cut the legs off at the crotch and cut the toes off the legs. We'll use the legs for our belt, depending on the height of our generator you;ll make adjustments to total belt length, It should be stretched but not taut, or taut but not tight ;-). Anyways, overlap the ends of the leg and hold it in place with some scotch tape and wick in some superglue, hit it with some accelerator or let it cure overnight. If you use accelerator wash with alcohol after to remove residue.
Step 5: The Base and Column
The base is an old laundry bucket , inverted. Lay the power roller and motor on top of the bucket and mark the location of the mounting holes and roller location. Using those markings locate the closet flange and mark it's mounting holes and the area inside to be cut out. Cut out the hole to match the closet flange and drill out all the locations for assembly bolts.
The column is easy, simply mark down one end a distance equal to the height of one mixing bowl, now mark four points equidistant around the pipe at that location. Drill a 2" hole through the pipe through two opposite points and a pair of 1/8" holes through the other two points.
Step 6: Gross Assembly
The column is mounted to the bucket with the closet flange. The power roller is mounted inside the bucket and the mounting manipulated to make roller perpendicular to the column. The belt is slipped on BEFORE adjusting the roller. The belt is then pulled through the column with a scrap of wire and the top roller installed using small bolts threaded into the PVC with nuts on the inside, as axles.
At this point apply a few volts to the motor and check to see the belt runs true, if it runs to one end, the end of that roller needs to be moved OUT, where out is away from the middle of the machine.
Step 7: The Brushes and Tuning
The brushes are multi strand copper wire fanned out and mounted approx. 1/4" inch from the belt roller combo at each end. The top brush is connected to the inside of the sphere and the bottom brush is connected to ground, whether an earth ground or power supply ground seems to be irrelevant.
At this point slip the dome on and apply about 6 volts to the motor. You should feel a charge around the dome.
Tuning. Don't allow the brushes to rub against the belt, the deposited copper will make the belt conductive and the generator will stop working. If it fails to work at all adjust the brushes and try again. If all else fails degrease the rollers and belt with rubbing alcohol, grease is death to triboelectric charge separation.
Step 8: What's the Voltage Kenneth?
The breakdown voltage for dry air at sea level is commonly given as 3 million volts/meter. So for every centimeter of spark you have 30,000 volts. So far my longest recorded spark is 40mm or 120,000 volts.
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I am making a Van de Graaff generator for an exhibition in my college. Even the school students will visit the exhibition and so the generator should be fun and safe as well. So what should be the input voltage for the generator?