4 moving pictures show you how Zener diodes work

“To understand Zener diodes, let’s first look at a conventional diode. When the diode blocks reverse current, a large Voltage drop occurs across the diode; when the diode allows current to flow in the forward direction, the voltage drop across the diode is small.

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To understand Zener diodes, let’s first look at a conventional diode. When the diode blocks reverse current, a large voltage drop occurs across the diode; when the diode allows current to flow in the forward direction, the voltage drop across the diode is small.

Now we replace the diode with a Zener diode.

Zener diodes allow current to flow in the forward direction, and in this case, Zener diodes behave like normal diodes. Every Zener diode has a reverse breakdown voltage, and the value of the reverse breakdown voltage is different for different Zener diodes. When a reverse voltage less than the breakdown voltage is applied, the Zener diode blocks current flow like a normal diode, but when the reverse voltage is greater than the breakdown voltage, the Zener diode starts to conduct reversely. When a Zener diode conducts in reverse, the voltage drop across it is almost equal to the reverse breakdown voltage.

Take a look at this circuit consisting of a resistor and a zener diode.

Now we add a light bulb in parallel with a zener diode that prevents the voltage across the bulb from being higher than the breakdown voltage of the zener diode, the bulb represents the output of the circuit. If the input voltage is consistently above the breakdown voltage, then a Zener diode is used as a regulator to ensure that the output voltage remains approximately constant. If the input voltage is consistently below the breakdown voltage, then a Zener diode is used to prevent transient overvoltage conditions that can damage the output.

Suppose we have two zener diodes in series, they point in opposite directions. We can now apply a positive or negative voltage to the output while protecting the output from transient overvoltage conditions in either direction. When protecting against transient high voltage conditions, we need special zener diodes that can react quickly and absorb large amounts of energy in a short period of time.

The combination of two TVS Zener diodes is usually fabricated as a single device and is represented by the symbol shown below.