**Rectifier Diodes**:Commonly used for converting AC voltage to DC in power supplies.

**Beyond Rectification**:Diodes serve in various applications beyond rectification.

**Zener Diodes**:Optimized for breakdown properties.

Essential for

**voltage regulation**in circuits.

**Other Diodes Covered**:**Optoelectronic Diodes**(e.g., LEDs)**Schottky Diodes**(known for low forward voltage drop)**Varactor Diodes**(used for variable capacitance)Additional special-purpose diodes for specific applications.

**Operation**:Unlike small-signal and rectifier diodes, Zener diodes are designed to operate in the

**breakdown region**.Backbone of

**voltage regulators**, maintaining a nearly constant load voltage despite fluctuations.

**Schematic Symbol**:Two variations of Zener diode symbols resemble a

**“z”**for "Zener."

**Breakdown Voltages**:Vary from

**2V to over 1000V**, depending on doping levels.

**Operating Regions**:**Forward Region**:Conducts at \(\sim0.7~\mathrm{V}\), like a regular silicon diode.

**Leakage Region**:Small reverse current between zero and breakdown.

**Breakdown Region**:Sharp knee followed by a near-vertical increase in current.

Voltage stays

**approximately constant**\((V_Z)\) in the breakdown region.

**Safe Operation**:Maximum reverse current: \(I_{ZM}\).

Exceeding \(I_{ZM}\) leads to diode destruction.

A

**current-limiting resistor**is essential to protect the diode.

**Third Approximation of a Diode**:Forward voltage =

**Knee voltage + Voltage across bulk resistance**Reverse voltage =

**Breakdown voltage + Voltage across bulk (Zener) resistance**

**Zener Resistance**:Defined as the

**inverse slope**of the I-V curve in the breakdown region.More

**vertical breakdown region**=**smaller Zener resistance**

**Effect on Reverse Current**:Increase in reverse current leads to a

**slight increase in reverse voltage**.Typically, this voltage increase is only

**a few tenths of a volt**.

**Design Considerations**:Zener resistance matters in design but is often ignored in

**troubleshooting**or**preliminary analysis**.

**Visual Representation**:**Fig.**shows typical Zener diodes.

**Constant Output Voltage**:Zener diode maintains a constant output voltage even as current changes.

Requires

**reverse bias**for normal operation.

**Operating Conditions**:Source voltage (\(V_s\)) must be greater than Zener breakdown voltage (\(V_Z\)).

A series resistor (\(R_s\)) is used to limit current and prevent burnout.

**Voltage Measurement**:Measure voltage across \(R_s\) by taking readings at each end with respect to ground.

Subtract voltages to find the voltage across the series resistor.

**Zener Voltage Regulator**:A

**Zener regulator**provides a DC output voltage that is**less than the power supply output**.Commonly used for

**voltage regulation**in circuits.

**Current through Series Resistor**:Voltage across the series resistor (\(R_s\)) = Source voltage \((V_S)\) - Zener voltage \((V_Z)\).

Current through the resistor: \[I_S = \frac{V_S - V_Z}{R_S}\]

\(I_s\) is equal to the Zener current since the circuit is in series.

Important: \(I_s\)

**must be less than**\(I_{ZM}\) to prevent damage.

**Ideal Zener Diode**:For

**troubleshooting and analysis**, the breakdown region can be approximated as vertical.**Voltage is constant**even with varying current (ignoring Zener resistance).

**Circuit Simplification**:A Zener diode in the breakdown region acts like a

**constant voltage source**\((V_Z)\) .This simplifies analysis as you can mentally replace the Zener diode with a

**battery**of voltage \(V_Z\).

The breakdown voltage of the zener diode shown in the Fig. is 10 V. What are the minimum and maximum zener currents?

Applied voltage \(20-40~\mathrm{V}\) (vary)

Zener diode replace with a battery of \(10~\mathrm{V} \Rightarrow\) output voltage is \(10~\mathrm{V}\) (fixed)

\[\begin{aligned} \text{Min. current}~I_{S}&=\frac{(20-10)~ \mathrm{V}}{820 \Omega}=12.2 \mathrm{mA} \\ \text{Max. current}~I_{S}&=\frac{(40-10)~ \mathrm{V}}{820 \Omega}=36.6 \mathrm{mA} \end{aligned}\]

**Output Voltage Stability:**Held constant at**10 V**.**Source Voltage Variation:**Changes from**20 V**to**40 V**.**Zener Diode Behavior:**Increased source voltage leads to more

**Zener current**.Output voltage remains

**rock-solid**at**10 V**.

**Conclusion:** Zener diode ensures stable output
voltage despite changes in input voltage.

**Operation**:Zener diode holds

**load voltage constant**in the breakdown region.**Load voltage**\((V_L)\) stays fixed at the**Zener voltage**\((V_Z)\) despite changes in source voltage or load resistance.

**Voltage Divider & Thevenin Voltage**:Thevenin voltage (\(V_{TH}\)) facing the diode: \[V_{TH} = \frac{R_{L}}{R_{S} + R_{L}}V_{S}\]

Breakdown occurs only if \(V_{TH} > V_Z\).

**Series Current**\((I_S)\): \[I_S = \frac{V_S - V_Z}{R_S}\]\(I_S\) remains the same whether or not the load resistor is connected.

**Load Voltage**\((V_L)\):Ideally, \(V_L = V_Z\) because the load resistor is in parallel with the Zener diode.

**Load Current**\((IL)\): \[I_L = \frac{V_L}{R_L}\]

**Total Current**\((I_S)\): \[I_S = I_Z + I_L\]The sum of Zener current and load current equals the series current (Kirchhoff’s Current Law).

**Zener Current**\((I_Z)\): \[I_Z = I_S - I_L\]\(I_Z\) no longer equals \(I_S\) as in an unloaded regulator; it’s reduced by the load current.

**Calculate Series Current**\((I_S)\): \[I_S = \frac{V_S - V_Z}{R_S}\]**Determine Load Voltage**\((V_L)\): \[V_L = V_Z\]**Calculate Load Current**\((I_L)\): \[I_L = \frac{V_L}{R_L}\]**Find Zener Current**\((I_Z)\): \[I_Z = I_S - I_L\]