How to construct a resistive load.

No special precautions are necessary to prevent voltage breakdown for loads designed to work up to 10 kV, or perhaps as high as 20 kV. Instead, the emphasis must be on mounting the resistors in such a way that an adequate flow of air is provided to dissipate the heat generated.

Shown below is a photograph of a typical configuration. Sheets of phenolic insulation are separated by drilled and tapped Delrin spacers. The resistors, in this case 50 watt wire wound resistors, are connected in series on the first sheet, as shown in the inset photograph, and then repeated sheet-by-sheet until the required number of resistors are installed. A freestanding fan can be positioned to draw air through the vertical sheets to cool such a load.

Figure 6

The construction of a resistive load designed to operate at a voltage higher than 10 kV is more complicated.  Here the dual problems of heat dissipation and possible voltage breakdown must be addressed. Shown below is a photograph of a typical helical construction that solves both of these problems.

A framework of flame-resistant thermoplastic acrylic insulation, such as Lexan, is made by butt gluing two vertical pieces of plastic at right angles to and along the centerline of a main vertical piece to form a vertical cross of symmetrical cross section. This structure can be stiffened by gluing a series of small blocks of plastic where the surfaces butt together. Holes in the outer edges of the four vertical members will allow for the support of the resistor leads.

First, it is necessary to determine the size of this framework. Let's consider the earlier example where we designed a 100 kV load consisting of 200 individual 50 k ohm, 10 W resistors. If we plan on 12 resistors for each turn of the helix, we will need 16.7 turns to accommodate all 200 resistors. This means that the voltage difference between similar points on any two adjacent turns will be 100 kV/16.7 or 6 kV. If we use a conservative safety factor of 1 inch of clearance for each 5 kV, the minimum vertical spacing between adjacent turns necessary to avoid voltage breakdown is 6/5 or 1.2 inches. This means that the total height of the framework must be at least 1.2 x 16.7 or 20 inches. The width of the framework will be determined by the physical size of the resistors, remembering that each turn will contain 12 resistors, or 3 resistors between each adjacent vertical surface.

As shown in the inset photograph, we have used a small crimped sleeve to join each pair of resistor leads before soldering. When soldering, be sure to avoid sharp projections. Each joint should consist of smoothly rounded surfaces to prevent any concentration of electric fields.

Figure 7

The entire framework can be mounted on an elevated base in which a fan is installed or, as in the example shown in the photograph, the fan frame itself can form the base of the load. To more efficiently direct the air flow from the fan through the helix, a sheet of mylar can be formed into a cylinder, using tape to seal the junction, and dropped over the helix.

If necessary to prevent corona, an aluminum toroid can be connected to the top of the load to provide an equipotential surface at the point of highest voltage.

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