Constant (Fixed) Load

Connect the load resistor, R, and a current monitor, I, as shown in Figure 1a to apply a constant load to the power supply. One end of the resistor is connected to the high voltage unshielded tip of the output cable by an appropriate lead. The other end is connected to the current monitor. The load resistor must have voltage and wattage ratings sufficient to withstand the highest voltage to be applied and to avoid excessive heating, which could cause the load resistor to drift in value. In practice, this means that the load resistor must be constructed from a series of smaller resistors connected in series to limit the voltage drop across any one resistor to a safe level. Care must be taken to space these resistors far enough apart to avoid arcing. Notice that the current monitor is connected in series with the low voltage return path, keeping it at a low and safe potential.

Changing Load

To apply a load that changes in value, refer to Figure 1b. Load resistor R1 is selected to load the supply with the minimum desired load. If it is desired to establish a no-load condition, R1 should be infinite, or an open circuit. One end of R1 is connected to the output cable tip by an appropriate lead. The other end of R1 is connected to the current monitor, which again is in the low voltage return path. Load resistor R2 is selected to achieve the desired maximum load. The equivalent maximum load, R3, is equal to RI x R2/R1 + R2. Both R1 and R2 must be capable of withstanding the maximum voltage and wattage to be applied. Because the resistance of R2 is generally smaller than that of R1, it will dissipate the greatest power. Again, in practice, the load resistors must be constructed from a series of smaller resistors.

One end of R2 is connected to the junction of R1 and the current monitor. The other end of R2 is attached to one end of an insulated rod. This rod should be Plexiglas or other acrylic material. A 2-foot rod is adequate for voltages below l00 kV. This rod, held at the opposite end from R2, is used temporarily to  make contact with the output cable tip and increase the load to the supply. Both ends are at a low and safe potential when the rod is withdrawn from the cable tip.

Figure 1a

HVTP* = high-voltage terminal point

Figure 1b

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