# Zener diode characteristics question

1. Jul 20, 2014

### ranju

As I have studied that when the reverse voltage across zener diode is less thsan the zener voltage it is replaced by open circuit..now in the given fig. (input voltage 100V & zener voltage=50V) we are not knowing the voltage across the reverse voltage across the diode..but in its solution it is replaced by open circuit..why so?? what is the logic behind replacing it by open circuit.. it is stated that we first have to detrmine the state of zener diode..
but in other questions the state is not determined..still in the solutions there was no suc step..they solved accordingly..

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2. Jul 20, 2014

### Simon Bridge

Imagine you start at an applied voltage of 10V and slowly increase the voltage - the voltage across the zener depends on the two resistors. You will agree that the zener is not conducting for these low applied voltages.

Now consider:
At what applied voltage does the zener begin to conduct?
How did you work that out?

BTW: I don't see the analysis so I cannot comment on what they have done.

3. Jul 21, 2014

### ranju

In the solution of this ques. they open circuited the zener diode and stated that to determine the state of zener diode..
but why they open circuited it??

4. Jul 21, 2014

### Simon Bridge

If the zener starts short-circuited, what is the voltage across it?
What voltage is required to make the zener conduct?

5. Jul 22, 2014

### Staff: Mentor

That has the same effect on the remainder of the circuit as if you were to remove the zener from the picture alltogether. That's how I'd suggest that you approach the problem---first remove the zener from the circuit and work out the voltage that would exist between the two points in the circuit where the zener had been.

Then you can deduce things ....

6. Jul 22, 2014

### ranju

So does removing the diode means open-circuiting it.. why can't we shortcircuit it n then solve .I am not saying for this particular case but for other circuits..!!

7. Jul 22, 2014

### Staff: Mentor

You can do whatever you like. So long as your short-circuit exists only on paper then nothing will be harmed.

Nowhere in its behaviour does a zener diode act like a DC short-circuit, so representing it as a short-circuit won't get you far. But over part of its characteristic the zener does resemble an open-circuit, so representing it as an open-circuit can help with your analysis and understanding of the circuit's operation.