This may be a very simple question with a straight-forward answer, but I cannot get a clear answer on this.

Consider the following Zener diode. The table on page 2 lists a column for Zener voltage \$V_Z\$ and also a column for Reverse voltage \$V_R\$, with \$V_R\$ < \$V_Z\$ for all the device variants.

What is the difference between these two parameters? If I want to use the Zener diode to clamp the maximum input voltage to a circuit to 24 V (in case of a transient*), do I choose the device based on \$V_Z\$ = 24 V or \$V_R\$ = 24 V?

NOTE * - This will actually form part of a circuit where a TVS diode is used, but I cannot find a TVS diode that will clamp at a maximum of 24 V with a maximum working voltage of about 19 V (the actual operating voltage is 12-18 V). I am therefore resorting to choosing a TVS with a maximum clamping voltage higher than 24 V and want to use the Zener (placed in parallel, after the TVS) to clamp the input voltage to 24 V in case of a transient.


1 Answer 1


First off: TVS diodes are Zener diodes.

You should pick Vz = 24 V

Pick part number: 3SMAJ5934B

Vr is the latest voltage at which the diode is not yet entered the Zener region and draws a small current called "maximum reverse current".

3SMAJ5934B maximum reverse current is 1 μA

Once entered the Zener region the diode will draw much more current than 1 μA. See the test data in the following table.

enter image description here

  • 3
    \$\begingroup\$ Once entered the Zener region the diode will draw 15.6 mA Not really, when a test current of 15.6 mA is applied to the device, a zener voltage \$V_Z\$ of typically 24 V will develop across the diode. Generally "test" currents and voltages are applied to the device rather than that this value is measured. The test current is a test condition. \$\endgroup\$ Commented Apr 9, 2021 at 9:45
  • 1
    \$\begingroup\$ Thanks Enrico and Bimpelrekkie for your answer and comment. \$\endgroup\$
    – wave.jaco
    Commented Apr 9, 2021 at 11:20

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