The diode tube

Chapter The diode tube

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • The diode tubeBefore the start of the twentieth century, scientists knew that electrons could carry acurrent through a vacuum. They also knew that hot electrodes would emit electronsmore easily than cool ones. These phenomena were put to use in the first electrontubes, known as diode tubes, for the purpose of rectification.Cathode, filament, plateIn any tube, the electron-emitting electrode is called the cathode. The cathode is usu-ally heated by means of a wire filament, similar to the glowing element in an incandes-cent bulb. The electron-collecting electrode is known as the anode or plate.Directly heated cathodeIn some tubes, the filament also serves as the cathode. This is called a directly heatedcathode. The negative supply voltage is applied directly to the filament. The filamentvoltage for most tubes is 6 V or 12 V. The schematic symbol for a diode tube with a di-rectly heated cathode is shown at A in Fig. 29-1.540 Electron tubes29-1Schematic symbols for diode tubes; C = cathode, F =filament, P = plate. At A, directly heated cathode; at B,indirectly heated cathode; at C, cold cathode.Indirectly heated cathodeIn many tubes, the filament is enclosed within a cylindrical cathode, and the cathodegets hot from infrared radiation. This is known as an indirectly heated cathode. Thecathode itself is grounded. The filament normally receives 6 V or 12 Vac. The schematicsymbol for a diode tube with an indirectly heated cathode is shown at B in Fig. 29-1.In either the directly heated or indirectly heated cathode, electrons are driven offthe element by the heat of the filament. The cathode of a tube is thus somewhat analo-gous to the source of a field-effect transistor, or to the emitter of a bipolar transistor.Because the electron emission in a tube depends on the filament or “heater,” tubesneed a certain amount of time—normally 30 seconds to a few minutes—to “warm up.”