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This is a back to basics question. More specifically how to calculate the series resistor to for the zener diode if one is for example making a series pass regulator with a transistor or a CCS.

Experimenting with zeners led me to the conclusion that a resistor value calculated for a 15V 1W zener will differ to one calculated for a 15V 400mW zener.

The 1W needs a higher min Iz to keep regulation. But what is the min Iz needed? I know datasheets sometimes state this, but sometimes one has an unknown brand zener. What then?

Anyone have a quick fail safe method?

Experimenting with zeners led me to the conclusion that a resistor value calculated for a 15V 1W zener will differ to one calculated for a 15V 400mW zener.

The 1W needs a higher min Iz to keep regulation. But what is the min Iz needed? I know datasheets sometimes state this, but sometimes one has an unknown brand zener. What then?

Anyone have a quick fail safe method?