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THE DIODE TUBE The diode vacuum tube we are about to study is really Edison's old incandescent bulb with the plate in it. Diode means two elements or two electrodes, and refers to the two parts within the glass container that make up the tube. We have called them filament and plate. More formally, they are called CATHODE and PLATE, respectively. Sometimes the filament is called a HEATER , for obvious reasons-more on this later. Within a few years after the discovery of the Edison effect, scientists had learned a great deal more than Edison knew at the time of his discovery. By the early 1900s, J.J. Thomson in England had discovered the electron. Marconi, in Italy and England, had demonstrated the wireless, which was to become the radio. The theoretical knowledge of the nature of electricity and things electrical was increasing at a rapid rate. J.A. Fleming, an English scientist, was trying to improve on Marconi's relatively crude wireless receiver when his mind went back to Edison's earlier work. His subsequent experiments resulted in what became known as the FLEMING VALVE (the diode), the first major step on the way to electronics. OPERATION OF THE DIODE TUBE Before learning about Fleming's valve, the forerunner of the modern diode, let's look at Edison's original circuit. This time, however, we'll draw it as a schematic diagram, using the symbol for a diode instead of a cartoon-like picture. The schematic is shown in figure 1-3. Figure 1-3. - Schematic of Edison's experimental circuit.
Note that this is really two series circuits. The filament battery and the filament itself form a series circuit. This circuit is known as the filament circuit. The path of the second series circuit is from one side of the filament, across the space to the plate, through the ammeter and battery, then back to the filament. This circuit is known as the plate circuit. You will note that a part of the filament circuit is also common to the plate circuit. This part enables the electrons boiled from the filament to return to the filament. No electron could flow anywhere if this return path were not completed. The electron flow measured by the ammeter is known as plate current. The voltage applied between the filament and plate is known as plate voltage. You will become familiar with these terms and with others that are commonly used with diodes and diode circuits as we progress. Diode Operation with a Positive Plate Fleming started with a two-element tube (diode) similar to Edison's and at first duplicated Edison's experiment. The results are worth repeating here. Look at figure 1-3 again. With the plate POSITIVE relative to the filament, the filament hot, and the circuit completed as shown, the ammeter detected a current flowing in the plate circuit. Because current is the same in all parts of a series circuit, we know that the same current must flow across the space between filament and plate. We know now that the electrons boiled from the heated filament are NEGATIVE and are attracted to the POSITIVE plate because UNLIKE CHARGES ATTRACT. Diode Operation with a Negative Plate Fleming's next step was to use a similar circuit but to reverse the plate battery. The circuit is shown in figure 1-4. Figure 1-4. - Diode with a negative plate.
With the plate NEGATIVE relative to the filament, the filament hot, and the circuit completed as shown, the ammeter indicated that ZERO current was flowing in the plate circuit. Fleming found that the NEGATIVE charge on the plate, relative to the filament, CUT OFF the flow of plate current as effectively as if a VALVE were used to stop the flow of water in a pipe. You have all of the facts available that Fleming had. Can you give an explanation of why the diode cuts off current when the plate is negative? Let's put the facts together. The filament is hot and electrons boil from its surface. Because the filament is the only heated element in the diode, it is the ONLY source of electrons within the space between filament and plate. However, because the plate is NEGATIVE and the electrons are NEGATIVE, the electrons are repelled back to the filament. Remember that LIKE CHARGES REPEL. If electrons cannot flow across the space, then no electrons can flow anywhere in the plate circuit. The ammeter therefore indicates ZERO. It might seem to you that electrons flow from the negative plate to the positive filament under these conditions. This is NOT the case. Remember that it takes a heated element to emit electrons and that the filament is the only heated element in the diode. The plate is cold. Therefore, electrons cannot leave the plate, and plate-to-filament current cannot exist. The following is a summary of diode operation as we have covered it to this point:
Measuring Diode Voltages As you know, it is impossible to have a voltage at one point, because voltage is defined as a DIFFERENCE of POTENTIAL between two points. In our explanation above we referred to plate voltage. To be exactly right, we should refer to plate voltage as the VOLTAGE BETWEEN PLATE and FILAMENT. Plate voltages, and others that you will learn about soon, are often referred to as if they appear at one point. This should not confuse you if you remember your definition of voltage and realize that voltage is always measured between two points. M1 and M2 in figure 1-5 measure plate voltage and filament voltage, respectively. Figure 1-5. - Alternating voltage on the plate.
The reference point in diode and other tube circuits is usually a common point between the individual circuits within the tube. The reference point (common) in figure 1-5 is the conductor between the bottom of the transformer secondary and the negative side of the filament battery. Note that one side of each voltmeter is connected to this point. Q.3 Name the two series circuits that exist in a diode circuit. |
 
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