Zener diode threshold voltage

[walid1]

If I have 6.8V 0.5W zener diode, what the minimum value of the applied voltage to make it ON.
thanks.

 

[audioguru2]

Zener diodes aren't switches so don't turn on.
Some zener diodes are designed to be used at 45mA. At 5mA they regulate poorly with a voltage much less than their rating and are almost a resistor.
Other zener diodes are designed for 5mA and up. They reguate well at nearly any current.

I have a book printed in 1968 with datasheets showing voltage vs current. New datasheets don't have those curves so just measure a zener diode yourself.

Here is a comparison of zener diodes:

View attachment 37807

 

[walid1]

Thank you audioguru it ok.