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Why are diodes non-ohmic conductors?


I do not believe I have heard the term "non-ohmic conductor" in about 12 years, the simpler term "semi-conductor" seems to suffice. Simply put, unlike a standard resistor, such as a long piece of wire or a heavy trace of pencil lead on paper, the resistance of a diode is not a fixed value.

With normal conductors, such as this 2200 ohm resistor sitting on my desk for no apparent reason, the amount of current that will pass through it depends directly on the amount of volatge applied. More voltage equals more current. More importantly, if I link several resistors together in series, the amount of voltage 'dropped' across each one will be proportional to its portion of the whole. (Really large resistors in value will expend most of the voltage, while relatively small ones will expend a relatively small amount of the total voltage)

With a diode however, the voltage dropped acrossed it is NOT a factor of its resistance compared to the whole. It is a constant in itself. The two main types of diodes are Silicon and Germanium. A Silicon Diode will drop 0.7 volts, regardless of the current, while a Germanium diode will drop 0.3. This means that if I wired up a silicon and germanium diode in series with a resistor, there would always be a total of 1 volt acrossed the two diodes, regardless of how big the resistor is, or what the applied voltage is. (Well, provided at least one volt is applied.)

Ryan Belscamper

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