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I have this 1N4742 Zener diode which is specified to have 12V nominal Zener voltage. What will happen when I apply a 20V as a source voltage for circuit containing the mentioned Zener diode?

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    \$\begingroup\$ It's not voltage that will kill it. It's current. If you put a 12V zener on a 20V supply, you almost surely will kill it because there will be nothing limiting the current. To use a zener properly, you need to select an appropriate series resistor such that the current is limited to a safe level. Do you need help doing this, or is it just a general curiosity question???? \$\endgroup\$
    – Kyle B
    Mar 23 at 20:49
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    \$\begingroup\$ Mr. Zener gets a capital 'Z' as it's his name. What happens depends on the direction of the Zener diode. There's a schematic button on the editor toolbar and it has a Zener diode amongst its components. \$\endgroup\$
    – Transistor
    Mar 23 at 20:52
  • \$\begingroup\$ Datasheet will give you a target current to ensure the zener effect occurs (Sorry @Transistor ... Zener) Use ohms law .... E= I * R ... In your case it's R=E/I ... E=(20-12) and I= whatever the datasheet says. \$\endgroup\$
    – Kyle B
    Mar 23 at 20:55
  • \$\begingroup\$ @Transistor Dr Zener at least. PhD from Harvard and subsequently professor at various institutions. \$\endgroup\$
    – Graham Nye
    Mar 23 at 22:57
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If you do this, it will go "bang!":

schematic

simulate this circuit – Schematic created using CircuitLab

The Zener diode will act like a short circuit, and try to pass all the current the supply can deliver.

If you do this, it will be fine:

schematic

simulate this circuit

The difference between "bang!" and OK is that resistor. It limits the current through the Zener diode and allows it to safely regulate the voltage.

In the circuit as shown, the current through the resistor is \$I = \frac {E}{R} = \frac {V_{Supply}-V_{Zener}}{R1} = \frac {20V - 12V}{1000 ohm} = \frac {8V}{1000 ohm} = 8mA\$.

Since the resistor and the Zener diode are in series, the current through the diode is the same as the current through the resistor.

As long as the Zener current is below the maximum rated current and the product of the current and the Zener voltage is less than the power rating for the diode, then the diode will be fine.

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If you connect a 20 volt source to a 12 volt zener diode without a series resistor to limit the current, the Zener diode will do its best to pull the 20 volt supply down to 12 Volts, while the supply will do its best to hold its output voltage at 20 volts. Either the Zener diode or the power supply (or maybe both) will sacrifice their lives in the attempt.

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  • \$\begingroup\$ I think I got it now! Thanks \$\endgroup\$
    – Nazfull
    Mar 23 at 20:56

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