Plate Plate Heater Cathode Cathode Heater Anode Cathode Anode Cathode Diode symbol Zener diode symbol Anode Cathode Anode Cathode Tunnel diode symbol LED symbol Anode Cathode Anode Cathode Gate Photodiode symbol SCR symbol Anode Cathode Anode Cathode Vericap symbol Schottkey diode symbol Output waveform Current flow Plate Heater Cathode Grid Heater Cathode Grid Plate 11-5 Figure 11-8.  A triode has three elements: the cathode, plate, and  a grid.  Figure 11-6.  A vacuum tube diode in a circuit allows current to flow  in one direction only. The output waveform illustrates the lack of  current flow as the AC cycles. Figure 11-7.  Diode symbols.  Figure 11-5.  A vacuum tube diode contains a cathode, heater, and  plate. Note that the arrow formed in the symbol for the heater points  to the direction of electron flow.  is coated with a material whose electrons are excited by the  heat. The excited electrons expand their orbit when heated.  They move close enough to the plate, which is constructed  around the cathode and heater arrangement, that they are  attracted to the positively-charged plate. When the AC  current cycles, the plate becomes negatively charged and  the excited cathode electrons do not flow to the plate. In a  circuit, this causes a check valve effect that allows current to  only flow in one direction, which is the definition of a diode.  [Figure 11-6]  The various symbols used to depict diodes are  shown in  Figure 11-7 . voltage to the grid can influence the cathode’s electrons,  which normally flow to the plate when the cathode is heated.  Changes in the relatively small amount of current that flows  through the grid can greatly impact the flow of electrons from  the cathode to the plate.  [Figure 11-8] Figure 11-9  illustrates a triode in a simple circuit. AC voltage  input is applied to the grid. A high-resistance resistor is used  so that only minimum voltage passes through to the grid. As  this small AC input voltage varies, the amount of DC output  in the cathode-plate circuit also varies. When the input signal  is positive, the grid is positive. This aids in drawing electrons  from the cathode to the plate. However, when the AC input  signal cycles to negative, the grid becomes negatively  A triode is an electron control valve containing three  elements. It is often used to control a large amount of current  with a smaller current flow. In addition to the cathode, plate,  and heater present in the diode, a triode also contains a grid.  The grid is composed of fine wire spiraled between the  cathode and the plate but closer to the cathode. Applying  Triodes