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I know that IZT is the test current at which the zener's reverse voltage is established. But what is IZ? (So I can enter the characteristics of one in CircuitLab.

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  • \$\begingroup\$ Izt is the test current which is used to measure the parameters and fill the datasheet of the zener diode. Iz is the actual zener current. Think of it as a bias current. Suppose input voltage is 9V, zener voltage is 5.1V and you are using a 390R resistor connected in series. Iz=(9-5.1)/390=10mA is the zener current. \$\endgroup\$ – Rohat Kılıç May 5 '17 at 10:32
  • \$\begingroup\$ The zener current is a parameter independent of the implementation. \$\endgroup\$ – Simon May 5 '17 at 12:50
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Your question refers specifically to CircuitLab. Here is the dialog in question:

enter image description here

It is clear here that what they call I_z is actually Izt. They call the dynamic impedance of the Zener R_S, on the datasheet it will likely be called Zzt, and it should be the value at Izt.

The datasheet will probably give you a maximum value for Zzt, which is likely much higher than the 'typical'. SPICE models usually use the typical values. In the particular case of the 1N4742, the datasheet maximum is 9 ohms, which is almost an order of magnitude higher than the typical.

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By looking at the usage of I_Z in these datasheets

https://www.fairchildsemi.com/datasheets/1N/1N4733A.pdf http://www.vishay.com/docs/85816/1n4728a.pdf

I assume that I_Z is the nominal zener current. Meaning the current which typically causes the rated zener voltage. Less current will cause a lower zener voltage, more current will cause a higher zener voltage.

In case of the type 1N4728A from the first datasheet you would have to set U_Z to 3.3 V and I_Z to 76 mA to model the typical characteristic within CircuitLab.

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  • \$\begingroup\$ Err, isn't a Zener diode's raison d'être that the breakdown voltage is constant, no matter the current (within reason)? That's why they're used for voltage regulation. \$\endgroup\$ – Paul Uszak May 5 '17 at 11:25
  • \$\begingroup\$ Have a look at the characteristic of zener diodes and their output impedance. It is not zero and therefor the zener diode has a nominal zener current. \$\endgroup\$ – Simon May 5 '17 at 12:55

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