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Forum: Analog Circuits Zener diode-to protect_BJT


Author: Daniel-p (Guest)
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Could any body please help me with the following problem:
As shown in the attached picture I have 2 networks, network-1 and 
network-2. Network-2 should takes current out from the capacitor(charge 
it to negative voltage levels).
Transistor must be off during the charge period.


Problem:The zener diode I used to protect the transistor provides a 
current path while the voltage level in the capacitor becomes smaller 
than zero, therefore I can't charge the capacitor!!

How can I solve this problem?

thanks a lot in advance

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Author: oszi40 (Guest)
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Author: Daniel-p (Guest)
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Sorry I think I should modify the question:

As shown in the attached picture I have 2 networks, network-1 and 
network-2. Network-1 charges the capacitor to positive voltage levels 
and network-2 charges it to negative voltage levels.
the network-1 must be seprated from the capacitor for the time in which 
the network-2 is working. I have selected a BJT as a switch to break the 
path when network-2 is working.
(Transistor must be off during the negative charge period)

Question:
1-since the capacitor is charged to positive voltages is a protecting 
zener diode necessary?when yes how should it be connected?
2- Or a zener diode is required because of negative voltages on the 
capacitor?when yes how should it be connected?

Note: care must be taken that the zener diode doesn't provide an 
unwanted current path while charging to negative voltages(no current 
path through this BJT).

How can I solve this problem? or any other idea?

thanks a lot in advance

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