1-14 The plate voltage-plate current characteristic for a given diode is a measure of exactly how much plate voltage controls how much plate current. This is often called the E_p - I_p characteristic. The E_p - I_p characteristic for a given diode, is determined by design engineers using mathematical analysis and laboratory experiment. You, as a technician, will never need to do this. However, you will use the results obtained by the engineers. You will also use your knowledge of the diode as you analyze equipment malfunction. Assume that we have the circuit in figure 1-12. (The filament has the proper voltage-even though it isn’t shown on the diagram.) Our purpose is to determine just how a changing voltage on the plate changes (or controls) the plate current. The method is as follows: Figure 1-12.—Determining diode plate characteristic. 1.   Starting with zero volts from our variable dc voltage source, increase the plate voltage (E_p) in steps of 50 volts until you reach 400 volts. 2.   At a each 50-volt step, measure the milliamperes of plate current (I_p) that flow through the meter. Record the I_p meter readings, step by step, so that you may analyze the results. Assume that table 1-2 shows our results. While we could use the table, a more normal procedure is to plot a graph of the values. Such a graph is called an E_p - I_p CURVE and is shown in figure 1-13. Each tube has its own E_p - I_p curve, which is available in commercial tube manuals and in many equipment technical manuals. Each curve will be different in some respects from every other curve. The shapes, however, will be similar. E_p 0 50 100 150 200 250 300 350 400 I_p 0 .002 .005 .010 .020 .030 .040 .042 .045 Table 1-2.—E_p - I_p Values Obtained by Experiment