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\$?
Answer
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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