wanna know about how this circuit works

[micro_man]

hi guys
i wanna know the working of a 30v zener diode here.how will this zener works and what will be the effect on the output side

thanks

 

[andre_luis]

Makes no sense placed on input, not referenced to ground.
Generally would be placed at output side, near to closed-loop net R2/R3.


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[davoud]

I think it will be effect on output at input more than 30-40volt according to R1 & R2 amounts. Vr2 = Vout-Vz thus we have approximately Vout-Vz + 1.25 at output

 

[FvM]

The z-diode has been placed to pass overvoltages to the output, trying to prevent breakdown of the regulator. The effect depends on the output load characteristic. If the load can tolerate surges, the circuit may be helpful. Otherwise it reduces the safe input voltage.

 

[andre_luis]

So, why didn´t polarized the Zenner diode with a separated resistor, instead to share R3 ?
The effect is to produce a positive interference on output.


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[godfreyl]

Similar to FvM, I suspect it's for protection, but to protect against low output voltage rather than high input voltage.

Imagine we have a circuit with unregulated input = 60V, regulated output = 40V. That should be OK because there's only 20V across the LM317.

However if there is a capacitor between output and ground, then there is a problem at switch-on. Without the zener, the voltage across the LM317 will initially be 60V, before the capacitor charges up.

 

[FvM]

it's for protection, but to protect against low output voltage rather than high input voltage

Hard to distinguish, I think.

However if there is a capacitor between output and ground, then there is a problem at switch-on. Without the zener, the voltage across the LM317 will initially be 60V, before the capacitor charges up.

Describes a case of possible failure. The initial capacitor voltage will be still zero, forcing the LM317 into current limit. Pulling the sense terminal up by a zener diode can destroy LM317 and zener diode immediately.

 

[godfreyl]

A bit of an odd circuit - the LM317 output is connected to the output via R1, not directly, so there'd be some current limiting. I wonder about HF stability if there's a cap at the output though.