Understanding concept of Zener Diodes for voltage

[foobag]

Hey guys

I have a question about how zener diodes usually work as voltage regulators in circuits.

To my understanding, one can place a zener diode in any circuit to stabilize the voltage to the zener diode voltages rating for the whole circuit. This makes sense for one zener diode.

I also understand that multiple zener diodes (2) in series can often be used for clipping AC signals etc, but also can be used to regulate a circuit, for example a schmitt trigger

http://img26.imageshack.us/img26/1348/zenerj.jpg

That is a sample design of a circuit I am curious about. How exactly would the circuit be stabilized and by what voltages?

My initial guess is that when the current flows one way it is (+10 - +5) = 5V, and when the current flows the other way it is (+5 - +10) = -5V, so that the schmitt trigger or Vin of the circuit would be clipped by +/- 5 Volts.

is this logic correct?

 

[vk6kro]

No, a zener has 0.7 volts across it in the forward direction, just like a normal diode, but breaks down at the zener voltage in the reverse direction.

So, your circuit would have 10.7 volts across it if the input was positive and -5.7 volts across it if the input was negative, assuming there was a series resitor to limit the current.

 

[foobag]

assuming ideal zener diodes with no voltage drops, it would be limited by +10v and -5V clipping than?

how does that work though if the voltage is positive and zener diode (+5V) doesn't affect the circuit?

 

[foobag]

for instance:

In this circuit if one zener is +10 V and the other is +5V it would be the same as you had mentioned?

 

[vk6kro]

Yes.

 

[foobag]

ok but from your description let's say an AC signal was going into the circuit

when its positive, the first zener diode of +10V dictates and the 2nd shuts off or doesn't affect the voltage regulation...then when the input is of negative value, why is it -5.7V instead of +5.7 volt?

 

[n.karthick]

The Vout will be negative (as Vin is negative and you are using non-inverting amplifier).
This Vout is clipped by Z2 to -5.7 V

 

[vk6kro]

The Vout will be negative (as Vin is negative and you are using non-inverting amplifier).
This Vout is clipped by Z2 to -5.7 V

Yes, that is right.

Zeners have their Zener voltage across them if the cathode is more positive than the anode by more than the zener voltage.

They become normal diodes if the anode is more positive than the cathode by more than 0.7 volts.

 

[foobag]

ah thanks a lot guys! really helped me a lot!