Copy edited.
Peter Mortensen
• 1.7k
• 3
• 17
• 23

ANeither a power supply nor zenera Zener diode is ideal. A power supply has a very low output impedance and a zenerZener diode has a non-zero impedance. Without a series resistor this means:

1. The Zener diode current will getbe very high
2. The voltage across the zenerZener diode will be higher than spec'dspecified due to its internal resistance
3. Due to the large current that may flow the power dissipation, ($$\P_{dissipated}=V_{zener}×I_{zener}\$$), will be huge. Your average zenerdiodeZener diode will have a 400mW400 mW or 500mW500 mW power rating.
jippie
• 32.2k
• 15
• 88
• 153

A power supply nor zener is ideal. A power supply has a very low output impedance and a zener has a non-zero impedance. Without a series resistor this means:

1. Zener current will get very high
2. The voltage across the zener will be higher than spec'd due to its internal resistance
3. Due to the large current that may flow the power dissipation ($$\P_{dissipated}=V_{zener}×I_{zener}\$$) will be huge. Your average zenerdiode will have a 400mW or 500mW power rating.