Solar PV Theory.pdf

Figure 4 schematic symbol and physical representation

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Figure 4. Schematic symbol and physical representation of a diode. The most common kind of diodes is the semiconductor diode. Semiconductors are materials that have a certain resistance at room temperature. Therefore, they are neither good conductors nor good insulators. However, applying a voltage to a semiconductor greatly varies its resistance. With certain voltage values, the resistance decreases dramatically and the semiconductor becomes a good conductor. Conversely, with certain other voltage values, the resistance increases dramatically and the semiconductor becomes an excellent insulator. This explains why these materials are referred to as semiconductors. Silicon and germanium are the two most commonly used semiconductor materials, silicon being the prevalent material used in modern electronics components. 2 . The Diode D ISCUSSION O UTLINE D ISCUSSION The direction of the diode symbol’s “arrowhead” points against the direction of elec- tron flow. Scientists of the 17 th century arbitrarily decided that current flows from the positive terminal to the nega- tive terminal. This so-called conventional current direction is still used today, and is the accepted direction of current flow, but it is worth noting that the actual direction of electron flow is opposite to the con- ventional current direction. Anode Cathode Schematic symbol Physical representation Band
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The Diode Discussion 4 Solar Power Operating principles of a diode Although it has no moving parts, a diode acts like a high-speed switch whose contacts open and close according to the following rules: When no voltage is applied across a diode, it acts as an open switch, and no current flows between A and K. See Figure 5. Figure 5. When no voltage is applied across a diode, it acts as an open switch. When a reverse voltage E R is applied across the diode, so that the anode is negative with respect to the cathode, the diode continues to act as an open switch. In this case, the diode is said to be reverse biased. See Figure 6. Figure 6. When the anode is negative with respect to the cathode, the diode acts as an open switch. When a forward voltage E F is applied across the diode, so that the anode is positive with respect to the cathode, the terminals (or electrodes) become short-circuited. The diode acts as a closed switch and a current I immediately flows from the anode to the cathode. In this case, the diode is said to be forward biased. See Figure 7. Figure 7. When the anode is positive with respect to the cathode, the diode acts as a closed switch. As long as current I flows between the anode and the cathode, the diode acts as a closed switch. When current I stops flowing, the diode returns to its original open state. See Figure 8.
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