Van De Graaff Generators & High Voltage Stuff
This process is continuous as long as the roller and belt are in motion. Air molecules must also be present for this process to function. This type of charge induction machine is essentially constant current in its nature. In other words, for a given set of materials and motor speed, the charging current which is delivered by the belt to the sphere is essentially constant. In practice, this means that if we vary the load connected to the sphere, we will also vary the output voltage. This is in direct contrast to a battery where its terminal voltage is essentially independent of load, but the load current varies in inverse proportion to the load. It also means that the output voltage of the VDG is extremely sensitive to contamination such as dust, dirt and condensation. Another point to be aware of is the fact that the speed of the drive motor does not control the terminal voltage. The motor speed controls the charging current and hence the rate at which charge can be delivered to the top terminal or sphere. If we reverse the roller materials, i.e. make the PTFE roller the top one and the aluminium roller the bottom one, we will reverse the polarity of the VDG, i.e. the sphere will now charge to a negative voltage.

****SAFETY****

Generally speaking, the VDG generator does not really pose much of a safety threat with the exception of the 240 volt a.c. supply to the drive motor. Normal good wiring practice and earthing procedures should be followed to avoid any accidental contact with the a.c. supply. Although the charging currents within the VDG are very low, the output voltage at the sphere is extremely high. (Presently estimated on my own machine at 250 to 300 thousand volts!) Although the average current when discharged through the human body is very low, it should be remembered that the peak discharge currents are much higher. When subjected to a high voltage d.c. discharge, the muscles within the body will go into contraction which will not only surprise the recipient, but can result in unwanted impact with surrounding structures due to the involuntary limb movement. Also, because the VDG generator is a d.c. source, all discharge currents will flow through the internal tissues and organs of the human body. There will be no skin effect as found in high frequency a.c. sources like the Tesla coil. So be sensible, do not operate these machines in the proximity of anyone with a heart condition, pacemaker or medical implant. Also, on a more general note, do not operate the VDG in the vicinity of any electronic equipment. The extremely high field strengths surrounding a VDG generator and the very high rates of change of voltage gradient during a discharge pulse, are sure death to sensitive electronic equipment.You have been warned!
Having accepted that the Van de Graaff generator is a constant current source, we can now draw the equivalent circuit. Where:-

i = charging current.

C = capacitance of top terminal or sphere.

R = output load including leakage (a variable quantity)

V = output voltage developed across output load.

The charging current is found by measurement, and for the MK3 VDG is 16 ľA.

The capacitance of the sphere can be calculated as follows:-

C = 4 x Pi x eo x r

C= 4 x Pi x 8.85 x 10-12 x 0.241 = 26.8pF

(For a sphere of 19 inches diameter.)

 Where eo = 8.85 x 10-12 (permittivity of a vacuum)

and r = radius of sphere in metres.

Last update 10/02/03