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I know that Zener diodes will only let pass a given voltage.Say for example a 3 volt zener diode lets pass 3 volts or higher only(not below).But recently I came across an instructable where a zener diode lets pass voltage when it falls below a range.Back to the 3 volt diode,it allows voltage to flow through it only when its below 3 volts.But how??are they different types of diodes.Please give me a brief idea to clarify my doubts.Hope you get my question

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Post a link to the instructable.
They don't "pass" voltage, they limit the voltage ACROSS them while a current flows THROUGH them.

steveastrouk (author)2013-01-21

The work there is being done by the transistors. The Zener on the left sets the voltage such that the base of the first transistor sees Vcc - 6.2 V relative to the emitter, so it turns on at Vcc=9V and the first LED LIGHTS. When it LIGHTS, the voltage between base and emitter on the SECOND transistor is less than the 0.6 V it needs to turn on, so the RED led is off.

As Vcc FALLS, eventually, the diode on the first transistor means that the relationship Vb= Vcc-6.2 yields a number so low that the first transistor turns OFF. Now the second transistor turns ON.

Thanks everyone!With the relation to water level(iceng's comment),other equations(verence's comment) and of course!all other valuable suggestions I'm getting a better idea about Zener diodes than I did before.I actually messed up the BC547 transistor's function with the diode's.......... AND special thanks to Steve for being the first to give me an answer.

verence (author)2013-01-21

They neither pass the voltage nor limit it. If you want to put it simply, they hold back the rated voltage.

If you put a voltage across a normal (non Zener) diode, you'll measure a voltage around 0.7V (for silicon diodes) if the current is flowing from anode to cathode (anode is more positive than the cathode). This is called the forward voltage. If the voltage is reversed (cathode more positive than the anode) the current doesn't flow, the backward voltage is the applied voltage. That's the rectifying effect. If you increase the backward (reverse) voltage, you may reach the breakdown voltage, where suddenly a current starts to flow in reverse direction through the diode. As the voltage at that point is high, so is the current which means that your poor diode is killed almost instantly. Fortunately that reverse breakdown voltage is normally in the kilovolt area.

Zener diodes in forward direction act like normal diodes. But in reverse direction they show a low breakdown voltage - the Zener voltage they are rated for. If you apply a (reverse) voltage lower than the Zener voltage, no current will flow and you'll measure the applied voltage. If you apply a higher reverse voltage, current starts to flow and the measured voltage across the Zener diode will be the (ta-da:) Zener voltage! (until at some point the diode will by fried again)

For other parts in the circuit it looks as if the Zener diode subtracts the Zener voltage form the applied voltage.

For the circuit you mentioned, that means, that the voltage at the 10k resistor is Vcc-6.2V. So for 9V in there will be about 2.8V. That is enough to open the left BC547 and turn on the green LED. This BC547 will be ON as long as its base is not lower than 0.7V. So, as long as Vcc is higher than 6.9V (6.9V-6.2V=0.7V) the green LED will be on. The right BC547 is just an inverter to turn on the red LED when the green LED is off.

Think of a Zener diode as kind of a hold back for the rated Zener voltage. And for low voltages don't forget that a normal diode acts like a 0.7V Zener  diode.

iceng (author)2013-01-21

Sorry But;

If you put a standard diode in place of a zener
the diode will block the full supply voltage.

In the diode direction they both act identically as you said.

verence (author)2013-01-21

Yep, just as I said: The reverse breakdown voltage of a normal diode is very high. So it will block the full supply voltage (up to some hundreds or even thousands of volts).

The reverse breakdown voltage of a Zener diode is low. It's just the Zener voltage it was built for.

iceng (author)2013-01-21

IF water level is like voltage,
then imagine a water tap slowly pouring water
into a tank with a hole at 5 inches from the tank bottom.

The water level in the tank will stay at a zener level of 5 inches
unless you push too much current into the tank and overwhelm the hole !

A