Could any body tell what's the difference to use zener diode in series or parallel?
https://drive.google.com/open?id=0B1OI4U7RW3YyYWVSRHdkaTFKcXM

 

Could any body tell what's the difference to use zener diode in series or parallel?
https://drive.google.com/open?id=0B1OI4U7RW3YyYWVSRHdkaTFKcXM

Yes and yes, depending on what you want to accomplish. In your top drawing the voltage regulator will not start working until the input voltage gets greater than VZ + V Reg. In your lower drawing the Zener protects the voltage regulator from input voltages higher than the Zener diode voltage.

 

In your drawings, the Zeners are like switches, they close when its rated voltage is reached,

so in the Top Diagram the regulator wont work until it reaches 35V.

in the Bottom Diagram the regulator works upto 35V input, then the zener holds the voltage at 35V.

 

The bottom schematic is ok for explanation, but not in reality. In the present setup, at input > 35 Volts, that Zener will be a dead short. If the input supply can limit the current to less than the Zener specs, no harm done. Else you have a shorted stub that was once a Zener.

 

And the top schematic is probably not what would be intended because the note is specifically that Vin is 10 V, indicating that the circuit is supposed to function properly when that is the input voltage. As it is, the regulator won't function until the input voltage is somewhere in the vicinity of 39 V or so (35 V for the zener, 3.3 V for the regulator output, and whatever the dropout voltage happens to be).