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schematic

simulate this circuit – Schematic created using CircuitLab

I'm going to use 1N4740A as a voltage reference in a similar way you see in the schematics above. In textbooks, a zener property called "minimum zener current \$(I_{z,min})\$" is thought, but it is almost never mentioned in the actual datasheets of zener products.

How do I calculate minimum current of a zener diode so that I can calculate highest value of the pull-up resistor \$R\$?

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  • \$\begingroup\$ Just a reminder ... Don't forget to account for the current through the load. \$\endgroup\$ – Tut Aug 26 '13 at 12:41
  • \$\begingroup\$ The load will be a FET input opamp (TL082), so I will just ignore it. \$\endgroup\$ – hkBattousai Aug 26 '13 at 13:08
  • \$\begingroup\$ I figured you had it covered. Just wanted to mention it for others referencing this. \$\endgroup\$ – Tut Aug 26 '13 at 13:17
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The minimum current for reasonable regulation is Izk, but this parameter is not always shown in the datasheet. It is the current in the zener at the corner where it is just starting to break over.

For the 1N4740A from Fairchild, that is listed as 250 microamps.

There is an excellent reference on zener diodes available. (Search for Izk).

Edit: Updated part number for correct suffix. Made note on what Vz to use.

When calculating the maximum resistance, I use the maximum zener voltage (although it is unlikely to be this high at Izk).

HTH

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  • \$\begingroup\$ I've got a BZX 85 24 volt zener running at what I think is 14nA and still regulates around 24V. It throws out a hell of a lot of noise though but are specified as only 5% accurate anyway, so... \$\endgroup\$ – Paul Uszak Sep 28 '16 at 13:09
  • \$\begingroup\$ Paul: 14nA is below max leakage current. electronics.stackexchange.com/questions/242750/… says max leakage current is hard to characterize. Are leakage currents typically orders of magnitude less than the specified maximum? So that such use as yours is atypical, but in practice doable? I want to clamp a solar cell having currents in 10uA range, don't want leakage to be much. \$\endgroup\$ – bootchk Dec 2 '16 at 14:46
  • \$\begingroup\$ The reference link doesn't work anymore. \$\endgroup\$ – vrleboss May 5 '17 at 10:17
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A zener diode datasheet will almost invariably specify the nominal zener current or the maximum zener current, rather than the minimum:

  • Nominal (i.e. ideal) value may be specified as the zener test current, such as in the case of the 1n4740A: 25 mA, table on Page 2 of datasheet
  • Maximum, as a function of the maximum power dissipation for the particular Zener diode family: Izmax = Pdis / Vzener

Note that the second method depends on the maximum power dissipation, which needs to be derated for actual junction temperature, as per a derating value also typically provided by the datasheet.

The minimum current one can use is determined by how much precision is required. As the current is reduced, at one point the voltage graph for the device will stray outside the acceptable error. Therefore, for regulation purposes, the nominal value if available, or half the maximum value, would be good practices to follow.

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