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Zener Diode


Zener Diode

1. Zener Diode in Reverse Bias

  • A Zener diode is designed to operate in reverse bias.
  • When the reverse voltage reaches the Zener voltage (VZ):
    • The diode does not get damaged (unlike a regular diode).
    • The voltage across the diode stays almost constant at VZ.
    • Any extra current flows through the diode without significantly increasing the voltage.

2. Behavior

  • Voltage: Approximately constant at VZ
  • Current: Can vary widely, limited only by external series resistance.
  • This is why Zener diodes are used as voltage regulators.
IZ = (Vin - VZ) / Rs
  • Vin = supply voltage
  • Rs = series resistor
  • IZ = current through the Zener diode

Voltage remains roughly VZ as long as IZ is within the diode’s max rated current.

3. Intuition

Think of it like a water dam with a spillway:

  • Once the water reaches a certain level (Zener voltage), extra water (current) just spills over, keeping the level (voltage) constant.

Summary

  • Zener diode limits voltage to VZ
  • Allows extra current to flow safely through it
  • Used for voltage regulation and protecting circuits from overvoltage

Zener Diode at Exact Zener Voltage (VZ)

Suppose the Zener diode is connected in reverse bias across a series resistor Rs and a supply voltage Vin.

Voltage across diode: VZ (by definition, at breakdown)

Current through Zener diode: depends entirely on the series resistor and supply voltage:

IZ = (Vin - VZ) / Rs

Key point: There’s no “magic current” just because it hits VZ. The Zener voltage is just the voltage clamp; the current is controlled by the rest of the circuit.

Special Case: Vin = VZ

IZ = (Vin - VZ) / Rs = 0
  • Current through Zener diode is zero in this ideal case.
  • The diode is at the threshold of breakdown, but no current flows because there’s no excess voltage.
  • Practically, a tiny leakage current exists (ฮผA range), but essentially 0.

Intuition

Think of it like a valve that opens only when the pressure exceeds a threshold:

  • If pressure = threshold → valve just about ready to open → no flow
  • If pressure > threshold → flow starts and increases as pressure rises

 

Further Reading

  1.  
๐Ÿ“Š MATLAB & ONLINE SIMULATIONS (Web-Based)
๐ŸŽ“ GATE & UGC - NET SOLUTIONS
๐Ÿ“ก WIRELESS & SIGNAL PROCESSING


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