2.9 The Zener Diode

One specific type of diode is the Zener diode. Essentially, it has the same characteristic as a regular diode, but the difference is that a Zener diode can withstand breakdown. Even more, a Zener diode is made to be used in the breakdown region. Breakdown voltages are typically lower than in a regular diode and range from 1 or 2 V to 100 V or more. The symbol is shown in Figure 2.29.

zenerdiode_symbolFigure 2.29  Symbol of a Zener diode. In forward bias, the shown current and voltage are positive.

The characteristic of the Zener diode is similar to that of a regular diode (see fig. 2.30). However, since breakdown is “allowed” in a Zener diode, the diode model is extended to include the breakdown region. In this region, the voltage barely changes, while currents can range from near-zero values to relatively large currents. As a result, the zener diode is modeled as a voltage source in this region. Note however that the polarity of the voltage source is reversed with respect to forward bias.

zenerdiode_modelFigure 2.30 Characteristic of the Zener diode. The diode appears to have three operating regions: forward bias, non-conducting and breakdown, and behaves differently in each region.

Diodes - Level 9

A few quizzes will help you to understand how zener diodes behave in circuits, and what their function might be.

Diodes - Level 10

In the next level, you will have to predict the influence of changing parameters in the circuit, e.g. what will happen if a certain resistor value increases. Remember that, if the diode conducts forward, the voltage across the diode equals the knee voltage. If it breaks down, this voltage equals the breakdown voltage.

Diodes - Level 11

The next challenges contain numerical values. You have to calculate voltages or currents. In case of non-integer results, use the decimal dot (.).

Diodes - Level 12a

In these challenges you have to find the I/O characteristic, i.e. the relation between the input voltage vin and vout.

Diodes - Level 12b

A bit more complex now…

Diodes - Level 12c

And a final one.

Go to 2.10 Clippers

11 thoughts on “2.9 The Zener Diode

    • Note the remark on the breakdown voltage: It is larger than the diode’s knee voltage.
      Suppose the diode conducts a very small current.
      Then there is a very small voltage across R2, and the knee voltage across the diode.
      The sum of both is the voltage across the Zener, and this sum might still be below the breakdown voltage.

  1. level 12 a question 2: applying KVL I found Vin – 2.5 – V0 = 0. I thought so because the direction of the current in breakdown. This would hiwever mean that V0 = Vin – 2.5 instead of Vin + 2.5.

  2. the figure on question 12.c.5 doesn’t work anymore. And it would be nice if you putted every circuit drawing with the graphs, i’m thinking here about lvl 6,7 or 8 (not sure about this).

    • Problem with question 12.C.5 is fixed.
      Thanks for letting me know.
      I’ve added the circuit drawings with the graphs in levels 7a, 7b and 7c.

Leave a Reply