Zener diode

1. Mar 27, 2013

asitiaf

How do Zener diode keep voltage constant if input voltage is less than Zener voltage

2. Mar 27, 2013

Staff: Mentor

It can't.

3. Mar 27, 2013

asitiaf

I'm rather confused about Zener diode.
Can someone explain in detail about it.
As per my opinion when the input voltage is less than Zener diode, the diode acts like an open switch and no current flow through the circuit. But while I'm using it as a voltage regulator, we connect the load across Zener and then let the Zener act like an open switch, the circuit is completed through the load and current will flow. So this is creating a contradiction. Please clarify. The more I'm trying to know, the more I'm getting confused.

4. Mar 27, 2013

Staff: Mentor

The zener conducts when the applied voltage exceeds the zener voltage. Usually there is a series resistance "upstream" of the zener and load, so that resistor drops the "excess" voltage.

If you want a crude model of the zener that you can analyze for the basic behavior, think of it as an ideal diode in series with an ideal voltage source, where the potential of the voltage source is the zener voltage.

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5. Mar 27, 2013

asitiaf

I understand that excess voltage will be dropped across Rs, when input voltage is greater than Zener voltage.
But I'm asking will there be any current through RL, if input voltage is less than Zener voltage.

6. Mar 27, 2013

Staff: Mentor

Well, the circuit is simple enough. You should be able to analyze it and draw the conclusion. If the input voltage is less than the zener voltage, will the diode be conducting or not?

7. Mar 27, 2013

asitiaf

The diode will not be conducting if input voltage is less than diode voltage.
Vin = (RL+RS)*I
Voltage across Rs = Rs*I
Suppose, first case
Vz = 5V
Vin = 4V
Rs = 2ohm
RL = 6ohm
Now,
I = 0.5 A
Voltage across Rs = 0.5*2 = 1V
And voltage across RL = 0.5*6 = 3V

Second case,
Vin = 5V
Rest all same.
Now
Voltage across RL = 5V because of the Zener diode
But what will be voltage drop across Rs?
Third case
Vin = 6V
Rest all same
Now
Voltage across RL = 5V
And voltage drop across Rs = 1V

This concludes me that voltage across RL remains 3V in first case and 5V in second and third case.
What type of voltage regulator this Zener diode is, if voltage across load keeps varying.
I am sorry for asking fundamental doubts.

8. Mar 27, 2013

Staff: Mentor

Okay.
No, that's not true. Rs and RL form a voltage divider, so the voltage across RL will be less than 5V.

The input voltage must exceed some minimum threshold voltage before the zener can begin to participate in the action.
Nope again. The voltage divider situation still exists for the Vin = 6V case.
The voltage across the load will be regulated to Vz provided that Vin is maintained higher than a certain threshold voltage.

9. Mar 28, 2013

asitiaf

Thanks a lot for clarification.