3-40 Figure 3-43.—Basic VR tube regulator. Since the operating limits of a VR tube are determined by its maximum and minimum currents, circuits using such tubes should be designed to allow maximum variations in current above and below the normal point of operation. The normal point of operation, which allows maximum variation in current, must be midway between the current limits of the tube. This median current is called I_{mean}. and can be calculated by the use of the following equation: We can determine the mean current for the VR90-40 as shown in figure 3-44 by using the following values: To calculate the value of series dropping resistance R_{S}, we use the following equation: If the average current flowing through the load of figure 3-44 is 100 milliamperes, we can find the series dropping resistance in the following manner: Figure 3-44.—Simplified VR tube regulator.