NPN transistor use for zener,How it work?

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alphi

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following fig, use NPN Vebo for over voltage protection,how it work?
 

Most silicon transistors have a max allowed reverse-bias voltage of around 8 – 8.5V and it can be used in the same way as a Zener Diode ..
The bottom transistor adds another 0.7V as it is connected in the normal way ..

If you reverse the voltage, the story is just the opposite: the bottom transistor works as Zener”, the top transistor (normally biased) adds 0.7V ..

All adds up to roughly +/-9V voltage limit ..

IanP
:|
 

how much reverse-bias current is safe ? we can not find this parameter from transistor datasheet.

from fig,Iec=(enlargement factor)*Ieb ?

if i use following sch,does it work well?
 

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