Concept:

A Zener diode regulator maintains a constant output voltage (V_Z) across the load resistor (R_L) as long as the Zener diode current is greater than or equal to its knee current (minimum operating current).

To maintain regulation, the Zener must conduct at least the knee current (I_ZK = 5 mA).

Given:

Input Voltage (V_in) = 14.5 V

Zener Voltage (V_Z) = 7 V

Series Resistance R_s = 100 Ω

Zener Knee Current I_ZK = 5 mA

Calculation:

Total current through R_s:

\( I_{total} = \frac{V_{in} - V_Z}{R_s} = \frac{14.5 - 7}{100} = 0.075~A = 75~mA \)

Minimum current required by Zener = 5 mA

So, maximum load current:

\( I_L = I_{total} - I_{ZK} = 75 - 5 = 70~mA \)

Using Ohm’s law to find minimum R_L:

\( R_L = \frac{V_Z}{I_L} = \frac{7}{0.07} = 100~\Omega \)

Final Answer:

Option 3) 100 Ω 

Zener Diode in Reverse Bias Condition

Definition: A Zener diode is a special type of diode designed to reliably allow current to flow "backward" when a specific, predetermined reverse voltage, known as the Zener breakdown voltage, is reached. In the reverse bias condition, the Zener diode remains non-conductive until the supply voltage exceeds the Zener breakdown voltage. Once the breakdown voltage is reached, the diode conducts a significant current while maintaining a nearly constant voltage.

Working Principle:

When a Zener diode is reverse-biased, it initially behaves like a standard diode, with negligible reverse current flowing. However, as the reverse voltage increases and reaches the Zener breakdown voltage, the diode begins to conduct in the reverse direction. The Zener breakdown occurs due to quantum mechanical tunneling of electrons across the narrow depletion region, and the diode enters a stable breakdown region where the voltage across it remains constant, regardless of further increases in current.

Correct Option Analysis:

The statement under consideration is:

"For a Zener diode working in reverse bias condition, the Zener current is _________ for supply voltage _________ the Reverse Zener breakdown voltage."

The correct option is:

Option 2: Close to zero; less than

Explanation:

• In the reverse bias condition, when the supply voltage is less than the Zener breakdown voltage, the Zener diode does not conduct significantly. The current flowing through the diode is the reverse saturation current, which is extremely small (close to zero).
• As the supply voltage is below the breakdown voltage, the diode remains in the reverse blocking region. The depletion region prevents significant current flow, and the Zener diode behaves like an open circuit.
• Thus, the Zener current is "close to zero" when the supply voltage is "less than" the Zener breakdown voltage.

Important Information:

To better understand the context and eliminate confusion, let’s evaluate the other options:

Option 1: Close to zero; greater than

This option is incorrect. When the supply voltage exceeds the Zener breakdown voltage, the diode enters the breakdown region and begins to conduct significant current. The current is not "close to zero" in this scenario.

Option 3: Close to infinity; greater than

This option is incorrect. While the Zener diode conducts significant current in the breakdown region, the current is not "close to infinity." The diode stabilizes the voltage across its terminals, and the current depends on the external circuit (like the series resistor), ensuring it remains finite and within safe operating limits.

Option 4: Close to infinity; less than

This option is also incorrect. For supply voltages less than the Zener breakdown voltage, the Zener diode remains in reverse bias, and the current is close to zero, not "close to infinity."

Conclusion:

The behavior of a Zener diode in reverse bias is a fundamental concept in electronics. When the supply voltage is less than the Zener breakdown voltage, the current through the diode is negligible, and the diode does not conduct significantly. Once the breakdown voltage is reached, the diode maintains a stable voltage across its terminals and conducts a controlled current. This property makes Zener diodes widely used in voltage regulation applications. The correct option, therefore, is Option 2: "Close to zero; less than."

From the circuit diagram,

⇒ 5i = 20 / 400

⇒ 5i = 1 / 20

⇒ i = 1 / 100 = 0.01 A = 10 mA (Load current)

Given load voltage, V_L = 5 V

⇒ Load resistance, R_L = V_L / i

⇒ R_L = 5 / (10 × 10^-3) = 500 Ω

∴ Load current i = 10 mA and Load resistance R_L = 500 Ω

IC 723 as a Voltage Regulator

The IC 723 is primarily used as a voltage regulator. It is a versatile and reliable integrated circuit (IC) that is designed to provide stable and regulated voltage output. The IC 723 is widely used in various electronic applications due to its excellent performance characteristics and ease of use.

Additional Information
 Applications of IC 723

The IC 723 is used in a wide range of electronic applications where a stable and regulated voltage is required. Some common applications include:

• Power supply circuits: The IC 723 is used in power supply circuits to provide a stable and regulated voltage output for various electronic devices and systems.
• Battery chargers: The IC 723 is used in battery charger circuits to regulate the charging voltage and ensure safe and efficient charging of batteries.
• Voltage reference circuits: The IC 723 can be used as a voltage reference in precision measurement and instrumentation applications.
• Voltage regulators for microcontrollers: The IC 723 is used to provide a stable and regulated voltage supply for microcontrollers and other digital circuits.

Conclusion

In conclusion, the IC 723 is primarily used as a voltage regulator. Its versatility, stability, and ease of use make it an ideal choice for a wide range of electronic applications. The IC 723 provides a stable and regulated output voltage, making it suitable for power supply circuits, battery chargers, voltage reference circuits, and voltage regulators for microcontrollers.

Explanation:

To determine the correctness of the given statements about the IC 723 voltage regulator, we need to understand the specifications and characteristics of the IC 723.

IC 723 Overview:

The IC 723 is a versatile voltage regulator IC used for both positive and negative voltage regulation. It can operate over a wide range of input voltages and can be configured for different output voltage ranges and current outputs. The IC 723 is known for its reliability and flexibility in various power supply designs.

Specifications of IC 723:

• Output Voltage Range: The IC 723 can be configured to provide an output voltage range from 2 volts to 37 volts. This is achieved by adjusting the external components connected to the IC, such as resistors and capacitors.
• Output Current: The IC 723 can deliver an output current of up to 150 mA when used in its basic configuration. However, with the addition of an external pass transistor, the output current can be increased significantly, depending on the specifications of the external transistor.

Analysis of Statements:

• S1: The output voltage ranges from 2 to 37 volts.
• S2: The output current is up to 150 mA.

Based on the specifications of the IC 723, we can analyze the correctness of the statements:

• S1: This statement is correct because the IC 723 can indeed provide an output voltage range from 2 to 37 volts by adjusting the external components.
• S2: This statement is also correct because the IC 723 can deliver an output current of up to 150 mA in its basic configuration. The output current can be increased with the use of an external pass transistor, but the statement specifies the basic configuration.

Conclusion:

Since both statements S1 and S2 are correct, the correct answer is option 1, which states that neither S1 nor S2 is incorrect.

Important Information:

The IC 723 is highly versatile and can be used in various voltage regulation applications. Here are some additional points to consider:

• The IC 723 includes an internal reference voltage source, error amplifier, series pass transistor, and a current limiting circuit.
• It can be used to design both linear and switching regulators.
• The output voltage can be adjusted using external resistors connected to the voltage adjustment pins of the IC.
• The IC 723 can operate with input voltages up to 40 volts, making it suitable for a wide range of power supply designs.
• For higher current applications, an external pass transistor can be added to boost the output current capability of the IC.
• Thermal protection and current limiting features enhance the reliability and safety of the IC 723 in various applications.

In conclusion, the IC 723 is a reliable and flexible voltage regulator IC suitable for a wide range of applications, offering an output voltage range of 2 to 37 volts and an output current of up to 150 mA in its basic configuration. Both statements S1 and S2 are correct, making option 1 the correct choice.

Concept: 

The working of the Zener diode is explained in the below figures.

Calculation:

Given,

Zener voltage Vz = 10 V

E_in = 6 V ⇒ E_in < V_z

Hence zener will be reverse biased and get open-circuited.

Output voltage E₀ = 0 V

Concept

The value of Zener current is given by:

\(I_Z=I_S-I_L\)

\(I_Z={V_S-V_Z\over R_S}-{V_Z\over R_L}\)

where, Iz = Zener current

V_z = Zener voltage

V_s = Source voltage

R_s = Source resistance

R_L = Load resistance

Calculation

Given, Vz = 20 V

Vs = 30 V

Rs = 100 Ω = 0.1 kΩ 

RL = 1 kΩ

\(I_Z={30-20\over 0.1}-{20\over 1}\)

I_Z = 100 - 20 mA = 0.08 A

Zener Diode:

The Zener diode operates just like the normal diode when in the forward-bias mode, and has a turn-on voltage of between 0.3 and 0.7 V. However when connected in the reverse mode, which is usual in most of its applications, a small leakage current may flow and the voltage across it is kept constant.

V - I characteristic of Zener diode

Zener as a Voltage Limiter:

A resistor, R₁ is connected in series with the Zener diode to limit the amount of current flowing through the diode and the input voltage V_in (Which must be greater than the Zener voltage) is connected across as shown in the image, and the output voltage V_out is taken across the Zener diode with V_out = V_z (Zener Voltage).

They are used as Voltage limiter and voltage regulator.

Varactor Diode is a type of diode whose internal capacitance varies with respect to the reverse voltage.
It is used in frequency modulation for FM transmitters and phase-locked loops.

Tunnel diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. They areused in oscillator circuits, and in FM receivers.

PIN diode is a diode where an intrinsic layer of high resistivity is sandwiched between the P and N-region of semiconductor material. They are used in attenuators, fast switches, photodetectors

Concept:

Zener diode as voltage regulator:

• Zener diodes are used in circuits to maintain a fixed voltage across a load.
• It breaks down at a specific voltage when it is reverse biased. This means that a Zener diode will stop a reverse current from flowing through it until the reverse voltage applied across it reaches a fixed value known as the breakdown voltage (V_z).
• It maintains the voltage at its breakdown voltage as long as the supply voltage is above this value.

Zener Knee Current:

The reverse current flows through the Zener diode at the break-down voltage.

Calculations:

\(\rm \frac{{\left( {20 - 18} \right)}}{{100}} = {I_Z} + {I_R}\)

\(\rm \frac{2}{{100}} = {I_{zmin}} + {I_{R\left( {max} \right)}}\)

\(\rm 2 \times {10^{ - 2}} = 2 \times {10^{ - 3}} + \frac{18}{{{R_{min}}}}\)

\(\rm {R_{min}} = \frac{18}{{18 \times {{10}^{ - 3}}}} = 1000\ \Omega\)

Symbols of various diodes:

1.) PN junction diode

2.) Zener diode

3.) Tunnel diode

4.) Schottky diode

5.) Varactor diode

6.) Photodiode

Concept:

A Zener diode is a silicon semiconductor device that permits current to flow in either a forward or reverse direction. The diode consists of a special, heavily doped p-n junction, designed to conduct in the reverse direction when a certain specified voltage is reached.

Voltage Vz: The Zener voltage refers to the reverse breakdown voltage.

Current Iz (max.): Maximum current at the rated Zener voltage Vz.

Current Iz (min.): Minimum current required for the diode to break down.

Calculation:

Given;

V_z = 10 V

Open circuit the Zener diode to calculate the voltage appearing across the diode.

V_x = 16 × 4.2/(4.2+1) = 12.92 V

As, V_x > V_z → Zener diode will be in the breakdown region.

V_L = V_Z = 10 V

Concept:

If load Resistance is varying then it is known as load Regulation.

Under load Regulation:

I_{L max} = I_S – I_{Z min}

I_{L min} = I_S – I_{Z max}

Calculation:

V_Z = 6 V

I_{Z min} = 5 mA

Since we want a constant 6 V across R

\({I_S} = \frac{{10 - 6}}{{50}} = \frac{4}{{50}} = 80\;mA\)

I_{L Max} = I_S – I_{Z min} = 80 – 5

= 75 mA

\({I_L} = \frac{6}{R}\)

\(R = \frac{6}{{75 \;\times\; {{10}^{ - 3}}}} = 80\;{\rm{\Omega }}\)

Zener diode:

• A Zener diode is a heavily doped semiconductor device that is designed to operate in the reverse direction.
• When the voltage across the terminals of a Zener diode is reversed and the potential reaches the Zener Voltage (knee voltage), the junction breaks down and the current flows in the reverse direction. This effect is known as the Zener Effect.
• The Zener diode acts as a voltage regulator.

The device used for the voltage regulator is a Zener diode

Voltage regulator:

It is used to regulate the voltage level. It generates a fixed output voltage that remains constant for any changes in load conditions or input voltage.

Zener diode:

• It is PN junction (silicon PN junction) diode and has a low specified reverse voltage which takes advantage of any reverse voltage applied to it.
• Unlike a normal diode (Schottky diode, varactor diode, tunnel diode) that blocks any flow of current through itself when reverse biased.
• From the I-V characteristics curve above voltage across the diode in the breakdown, the region is almost constant.
• In the breakdown region, it can be used as a voltage regulator applications.

Schottky Diode:

• The Schottky diode is a semiconductor formed by the junction of a semiconductor with metal. It has a low forward voltage drop and near-zero reverse recovery time hence used in very fast switching circuits.
• Schottky barrier diode can be used as a low noise amplifier due to metal-semiconductor junction.

Tunnel diode:

• A tunnel diode is a highly doped semiconductor diode.
• The tunnel diode shows negative resistance when the voltage value increases by decreasing the flow of current. Tunnel diode works based on the tunnel Effect.

Tunnel diode is represented by the symbol

Varactor Diode:

• Varactor diode refers to the variable Capacitor diode, which means the capacitance of the diode varies linearly with the applied voltage when it is reversed biased.
• Varactor diode is commonly used for Electronic tuning.

It is represented by a symbol of diode terminated in the variable capacitor as shown below: