A Zener diode is a silicon semiconductor that can allow current to flow in a forward or reverse direction. The symbol for this is as follows:
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Zener diodes are almost always used in the reverse direction. In the reverse direction, they will not allow any current to pass through acting as an open circuit until a certain voltage (called the reverse breakdown voltage) is reached. Once that voltage is reached, the diode will let current through, but also keep the same voltage. The following graph shows this behavior:
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Since a Zener is used in the reverse direction, we focus on the third quadrant (bottom left). Notice how the Zener does not let any current through until it reaches the “specified voltage”. After it reaches the specified voltage, it will start letting current through and keep the same voltage.
A Zener in the forward direction will as a regular diode, usually with a small turn-on voltage of 0.3~0.7 V (voltage required to let current thought).
Here are some important specifications to keep in mind when choosing a Zener:
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Here is a common example of a Zener:
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Notice that the Zener will make sure that the V output will never exceed 5.1V since if the input power source ever crosses 5.1V, the Zener will just let the current through but will “clamp” the voltage. If the voltage across the Zener is less than 5.1V, then Vout will have whatever voltage the Zener has.