These techniques are used to create special-purpose diodes that perform many different functions. Until the 1950s, vacuum diodes were used difference between diode and zener diode pdf frequently in radios because the early point-contact semiconductor diodes were less stable. The same did not apply to a negatively charged electroscope, indicating that the current flow was only possible in one direction. At the time, he was investigating why the filaments of his carbon-filament light bulbs nearly always burned out at the positive-connected end.
He had a special bulb made with a metal plate sealed into the glass envelope. Edison was awarded a patent for this invention in 1884. Braun patented the crystal rectifier in 1899. 1903 and received a patent for it on November 20, 1906. Other substances offered slightly better performance, but galena was most widely used because it had the advantage of being cheap and easy to obtain. T used these in their microwave towers that criss-crossed the United States starting in the late 1940s, carrying telephone and network television signals.
The symbol for an indirect heated vacuum-tube diode. From top to bottom, the components are the anode, the cathode, and the heater filament. The alternating voltage to be rectified is applied between the cathode and the concentric plate electrode. However, when the polarity is reversed and the plate has a negative voltage, no current flows, because the cathode electrons are not attracted to it. The plate, being unheated, does not emit any electrons. So electrons can only flow through the tube in one direction, from the cathode to the anode plate.
Some types of smaller thermionic rectifiers had mercury vapor fill to reduce their forward voltage drop and to increase current rating over thermionic hard-vacuum devices. Throughout the vacuum tube era, valve diodes were used in analog signal applications and as rectifiers in DC power supplies in consumer electronics such as radios, televisions, and sound systems. Today they are still used in a few high power applications where their ability to withstand transient voltages and their robustness gives them an advantage over semiconductor devices. There are alternative symbols for some types of diodes, though the differences are minor.
The triangle in the symbols points to the forward direction, i. Typical diode packages in same alignment as diode symbol. A galena cat’s-whisker detector, a point-contact diode. A pointed metal wire is placed in contact with an n-type semiconductor. Some metal migrates into the semiconductor to make a small p-type region around the contact.