Figure 4-6 shows how an input to an amplifier not expected to exceed ☑0V can be protected from both high positive and negative voltages. Zener diodes are also used to protect sensitive electronics from high-voltage spikes due to static discharge or incorrectly connected equipment. If it falls below that, the diode will turn off and the Vout will increase again. But wait, since the breakdown voltage is exceeded it will conduct, bringing the Vout down to 5V. The Zener’s resistance will therefore be high and so the voltage across it due to the voltage divider effect of R and the Zener will be higher than the breakdown voltage. To understand how this happens, imagine that the voltage across the Zener diode is less than its 5V breakdown voltage. The output voltage will remain roughly 5V whatever Vin is as long as it’s greater than 5V. These types of diodes have much lower forward voltage and generate less heat. The 1N5819 diode is called a Schottky diode. This is not instantaneous in any diode and in some applications fast switching is needed. The recovery time of a diode refers to how quickly the diode can switch from forward-biased conducting to reverse-biased blocking. The DC-blocking voltage is the reverse-biased voltage that if exceeded may destroy the diode. The forward voltage, often abbreviated as Vf, is the voltage across the diode when forward-biased. Some of the most commonly used rectifier diodes are listed in Table 4-1. Unlike resistors that are bought as a particular value resistor (say, 1kΩ) diodes are identified by the manufacturer’s part number. There are many different types of diodes.
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