When a diode blocks current in the reverse direction, the diode can have a large voltage drop across it.

When a diode allows current to flow in the forward direction,

the diode has a very small voltage drop across it.

Let's now replace the diode with a Zener diode.

The Zener diode also allows current to flow in the forward direction,

in which case the Zener diode behaves just like a regular diode.

Each Zener diode has a “Reverse Breakdown Voltage”,

and the value of this “Reverse Breakdown Voltage” is different for different Zener diodes.

When a reverse voltage smaller than the breakdown voltage is applied,

the Zener diode blocks current just like a normal diode.

But, when a reverse voltage larger than the breakdown voltage is present,

the Zener Diode will start to conduct in the reverse direction.

When a Zener diode conducts in the reverse direction,

the voltage drop across it stays approximately equal to the reverse breakdown voltage.

Consider this circuit consisting of a resistor and a Zener diode.

Let's now add a light bulb in parallel with the Zener diode.

The Zener diode prevents the voltage across the light bulb from going higher than the Zener's breakdown voltage.

The light bulb represents the output of our circuit.

If the input voltage is fluctuating but is always higher than the breakdown voltage,

then the Zener diode can be used as a voltage regulator

ensuring that the output voltage stays approximately constant.

If the intended input voltage is always lower than the breakdown voltage,

then the Zener diode can be used to protect against transient overvoltage conditions

that could damage the output.

Suppose we use two Zener diodes in series pointed in opposite directions.

We can now apply a positive or negative voltage to the output,

while protecting the output against transient overvoltage conditions in either direction.

When protecting against transient high voltage conditions,

we need special Zener diodes that are able to react very quickly

and that are able to absorb a very large amount of energy for brief periods of time.

This combination of two transient suppressing Zener diodes is often manufactured as a single device,

and is represented with the following symbol.

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