Regulated DC Power Supply Basics

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Questions and Answers

In a circuit with a capacitor filter, what is the primary function of the output capacitor?

Acts as a line filter to improve transient response

What is the purpose of the input capacitor in a circuit with a voltage regulator?

To prevent unwanted oscillations

What IC series is commonly used for fixed negative voltage regulation?

The 7900 series

What is the output voltage of a 7912 IC regulator?

<p>-12 V</p> Signup and view all the answers

What are the three terminals of the LM317 adjustable voltage regulator?

<p>Input terminal, output terminal, and adjustment terminal</p> Signup and view all the answers

How is the output voltage of the LM317 adjustable voltage regulator changed?

<p>By using an external voltage divider</p> Signup and view all the answers

What is the adjustable output voltage range of the LM317 regulator?

<p>1.25V to 37V</p> Signup and view all the answers

Besides voltage regulation, what is another specification that is important for the LM317?

<p>The load current it can supply: 1.5A</p> Signup and view all the answers

A power supply has a change in output voltage of 0.5V when the load current changes by 1A. What is the output impedance?

<p>0.5 Ω</p> Signup and view all the answers

A power supply has an output impedance of 0.5 ohms. If the load current decreases, and the output voltage when loaded is 10.5V, what is the no-load voltage?

<p>11V</p> Signup and view all the answers

A power supply with an output impedance of 0.01 Ω has a load current that varies between 0.5A and 1A. What is the peak to peak change in output voltage?

<p>5 mV</p> Signup and view all the answers

For a power supply, is it better to have a higher or lower output impedance?

<p>Lower</p> Signup and view all the answers

A voltage regulator experiences a 10 μV change in its output voltage when its input voltage changes by 5V. What is the line regulation?

<p>2 μV/V</p> Signup and view all the answers

A power supply's output impedance is calculated by dividing the change in what value, by the change in load current?

<p>Output voltage</p> Signup and view all the answers

When specifying a power supply, what is an important consideration related to its output characteristics?

<p>Output impedance</p> Signup and view all the answers

In electronic systems with rapidly changing loads, how does the output impedance of a power supply typically behave with respect to the frequency of the load change?

<p>It varies with frequency</p> Signup and view all the answers

What does a line regulation of 2 μV/V for a voltage regulator signify?

<p>It means that for every 1V change in the regulator's input voltage, the output voltage will change by 2 μV.</p> Signup and view all the answers

What is the primary function of a regulated DC power supply?

<p>To maintain a constant output voltage irrespective of fluctuations in the AC mains or variations in the load.</p> Signup and view all the answers

List the basic components of a regulated power supply.

<p>A regulated power supply consists of an ordinary power supply and a voltage regulating device.</p> Signup and view all the answers

What are the three main circuits comprising the regulated power supply described?

<p>The regulated power supply described consists of a bridge rectifier, a capacitor filter, and a zener voltage regulator.</p> Signup and view all the answers

What is the purpose of the bridge rectifier in a regulated power supply?

<p>The bridge rectifier converts the transformer secondary AC voltage into a pulsating DC voltage.</p> Signup and view all the answers

What role does the capacitor filter play in a regulated power supply?

<p>The capacitor filter reduces the pulsations in the rectifier's DC output voltage.</p> Signup and view all the answers

What are the two main functions of the zener voltage regulator?

<p>It reduces the variations in the filtered output voltage and keeps the output voltage nearly constant.</p> Signup and view all the answers

What is a primary disadvantage of using a simple zener regulator?

<p>The main disadvantage is that the zener wastes a significant amount of power.</p> Signup and view all the answers

In a zener diode voltage regulator, what limits the minimum value of the series resistor ($R_S$)?

<p>The maximum current the zener diode can handle.</p> Signup and view all the answers

In a series feedback voltage regulator, what does the feedback fraction (m) represent?

<p>The feedback fraction represents the ratio of the feedback voltage to the output voltage.</p> Signup and view all the answers

In the context of Example 17.11, what is the minimum current through the zener diode ($I_Z(min)$) when calculating the maximum value of $R_S$?

<p>3 mA</p> Signup and view all the answers

How is the closed-loop voltage gain (ACL) related to the feedback fraction (m) in a series voltage regulator?

<p>The closed-loop voltage gain is the inverse of the feedback fraction (m), so $ACL = 1/m$.</p> Signup and view all the answers

In a transistor series voltage regulator, what component provides the reference voltage?

<p>A zener diode.</p> Signup and view all the answers

What two voltage components that determine the output voltage of a series voltage regulator?

<p>The zener voltage (VZ) and the base-emitter voltage (VBE).</p> Signup and view all the answers

What happens to the output voltage of a transistor series voltage regulator if the base-emitter voltage ($V_{BE}$) increases?

<p>The output voltage will increase, which will then decrease the base-emitter voltage.</p> Signup and view all the answers

What is the primary drawback of a series voltage regulator related to the pass transistor?

<p>The pass transistor can be destroyed due to excessive load current from a short circuit.</p> Signup and view all the answers

What is the relationship between $V_{out}$, $V_Z$ and $V_{BE}$ in a transistor series voltage regulator?

<p>$V_{out} = V_Z - V_{BE}$</p> Signup and view all the answers

What is the purpose of adding a current limiting circuit to a series voltage regulator?

<p>To protect the pass transistor from damage due to excessive load current by limiting the current.</p> Signup and view all the answers

In the current limiting circuit described, what role (on or off) does transistor Q3 play during normal load current conditions?

<p>Transistor Q3 is <em>off</em> during normal load current conditions.</p> Signup and view all the answers

In a series voltage regulator, why is the transistor labeled as a 'series' transistor?

<p>Because the load current passes through the transistor.</p> Signup and view all the answers

If the output voltage in a transistor series regulator decreases, how does the circuit respond to restore the output voltage?

<p>The base-emitter voltage increases, causing the transistor to conduct more and increase the output voltage.</p> Signup and view all the answers

What happens to transistor Q3 when excessive load current flows in a voltage regulator with current limiting?

<p>Transistor Q3 turns <em>on</em> when excessive load current flows.</p> Signup and view all the answers

How does turning on Q3 in a current limiting circuit prevent the increase in load current?

<p>When Q3 turns on, it decreases the base voltage of pass transistor Q2, thus reducing its conduction.</p> Signup and view all the answers

What is the maximum base current, $I_B (max)$, given that $\beta = 50$ and $I_C(max) = 1A?

<p>20mA</p> Signup and view all the answers

What is the minimum zener current, $I_Z (min)$, used in the calculation of $I_{RS}$?

<p>1mA</p> Signup and view all the answers

What is the total current through $R_S$, denoted as $I_{RS}$, and how is it calculated?

<p>21mA; calculated as the sum of the minimum zener current and the maximum base current</p> Signup and view all the answers

When calculating the value of $R_S$, why is the minimum input voltage ($V_{in}(min)$) used?

<p>It gives the lowest voltage across $R_S$ which leads to the minimum value of current it will need to supply.</p> Signup and view all the answers

Calculate the value of $R_S$ when $V_{in}(min) = 12V$, zener voltage $V_Z = 8.5V$, and $I_{RS} = 21mA$.

<p>166 ohms</p> Signup and view all the answers

Under what conditions does maximum power dissipation in $R_S$ occur?

<p>When the voltage across it is maximum</p> Signup and view all the answers

What is the maximum voltage across $R_S$, denoted as $V_{RS}(max)$, when $V_{in}(max) = 18V$ and $V_Z = 8.5V$?

<p>9.5V</p> Signup and view all the answers

When does the maximum power dissipation in the zener diode occur?

<p>when the input voltage is maximum and the load current is minimum</p> Signup and view all the answers

Flashcards

Output impedance of power supply

The output impedance of a power supply is the opposition it offers to the flow of current from the power supply to the load.

How to calculate output impedance

The output impedance of a power supply can be calculated by dividing the change in output voltage (ΔVL) by the change in load current (ΔIL).

Units of output impedance

A power supply's output impedance is typically expressed in ohms (Ω) and is usually a very small value.

Impact of output impedance

The output impedance of a power supply affects the voltage drop across the load, especially under heavy load conditions.

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Ideal output impedance

The lower the output impedance of a power supply, the better it is. A low output impedance indicates a more stable power supply, able to maintain a consistent voltage under varying load conditions.

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Line regulation

Line regulation refers to the change in a voltage regulator's output voltage due to a change in the input voltage.

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Units of line regulation

Line regulation is typically expressed as a ratio of the change in output voltage to the change in input voltage, usually in units like microvolts per volt (μV/V).

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Ideal line regulation

A lower line regulation value indicates a better voltage regulator, meaning it maintains a more stable output voltage despite variations in the input voltage.

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Regulated DC Power Supply

A power supply that maintains a constant output voltage despite changes in input voltage or load.

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Voltage Regulator

A component that regulates the output voltage of a power supply, keeping it stable despite fluctuations in input voltage or load.

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Zener Voltage Regulator

A simple voltage regulator using a Zener diode to maintain a steady output voltage.

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Rectification

The process of converting alternating current (AC) to pulsating direct current (DC).

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Capacitor Filter

A circuit that smooths out the pulsations in a DC signal, removing voltage variations and producing a more stable output.

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Input Voltage (Vin)

The input voltage to a power supply, typically in a changing or fluctuating form.

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Output Voltage (Vout)

The output voltage of a power supply, typically in a more stable and regulated form.

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Maximum Rs Value

The maximum allowable RS value is determined by the total current flowing in the circuit. It ensures the zener diode receives a minimum current (IZ(min)) when the load draws the maximum current.

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RS (max) formula

The maximum value of RS is determined using the formula: RS (max) = (Vin - VZ) / (IL (max) + IZ (min)).

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Minimum Zener Current

The minimum zener current (IZ (min)) is the lowest current the zener diode needs to operate correctly.

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Maximum Load Current

The maximum load current (IL (max)) is the highest current the attached load can draw from the circuit.

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Series Voltage Regulator

A series voltage regulator uses a transistor and a zener diode to stabilize the output voltage, ensuring a steady supply to the load.

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Transistor Q1's Role

Transistor Q1 acts as a variable resistor, increasing its conductivity to raise the output voltage and decreasing conductivity to lower it, ensuring the output voltage remains constant.

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Constant Base Voltage

The base voltage of the transistor Q1 is held constant by the zener diode's reference voltage.

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Output Voltage Equation

The output voltage is determined by the zener diode's reference voltage (VZ) and the base-emitter voltage of the transistor (VBE).

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Feedback Fraction (m)

The ratio of feedback voltage (VF) to the output voltage (Vout). It determines how much of the output voltage is fed back to the input.

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Closed-Loop Voltage Gain (ACL)

The gain of the entire regulator circuit, taking into account the feedback network.

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Regulated Output Voltage

The voltage across the zener diode (VZ) plus the voltage across the base-emitter junction of the pass transistor (VBE).

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Short-Circuit Protection

A circuit that limits the current flowing through the pass transistor to prevent damage from excessive load currents.

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Pass Transistor (Q2)

A transistor used to control the flow of current through the load. It is often called the 'pass transistor'.

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Current Limiting Transistor (Q3)

A transistor used as part of the current limiting circuit. It turns on when the load current exceeds a certain limit, reducing the current flow through the pass transistor.

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Current Limiting Resistor (R4)

A resistor in the current limiting circuit. When the load current is excessive, the voltage across this resistor causes the current limiting transistor to turn on.

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Feedback Resistor (R1)

A resistor in the feedback network. It's connected in series with R2.

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Output Capacitor (C2)

A component used in power supplies to filter out unwanted voltage variations, improving the transient response.

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Input Capacitor (C1)

A capacitor used in power supplies to prevent unwanted oscillations, ensuring stable operation.

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7900 Series IC Regulators

A series of integrated circuits (ICs) designed for providing fixed negative voltage regulation, with output voltages ranging from -5V to -24V.

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Adjustable Voltage Regulator

A type of voltage regulator that can be adjusted to provide any desired DC output voltage within its specified limits.

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LM317

A popular three-terminal adjustable voltage regulator IC known for its versatility and high current capability, providing an output voltage range of 1.25V to 37V.

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External Voltage Divider (R2)

A component used in adjustable voltage regulators to set the output voltage. Changing its resistance alters the output voltage.

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IN Terminal

The input terminal of a voltage regulator IC, where the unregulated input voltage is applied.

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OUT Terminal

The output terminal of a voltage regulator IC, providing the regulated DC output voltage.

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Maximum Base Current (IB) in a Zener Regulator

The maximum value of base current (IB) flowing through the transistor in a zener regulator circuit. This current is calculated using the maximum value allowed for the base current and the transistor's beta (β).

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Maximum Current through RS

The maximum current that the series resistor (RS) needs to be able to handle in a zener regulator circuit. This current is the sum of the minimum zener current (IZ) and the maximum base current (IB).

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Calculating Series Resistor (RS)

The value of the series resistor (RS) needed in a zener regulator circuit. It's calculated to ensure the zener diode operates correctly under all input voltage variations. This calculation considers the minimum input voltage, zener voltage, and the maximum current through RS.

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Maximum Power Dissipation in RS

The maximum power that the series resistor (RS) can dissipate in a zener regulator circuit. This occurs when the voltage across RS is at its maximum.

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Maximum Power Dissipation in Zener Diode

The maximum power that the zener diode can dissipate in a zener regulator circuit. This occurs when the current through it is maximum, which happens when the input voltage is maximum and the load current is minimum.

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Zener Current, Base Current, and Emitter Current Relationship

The relationship between the zener current (IZ), base current (IB), and emitter current (IE) in a zener regulator circuit. When the load current (IL) is minimal, the emitter current (IE) is also minimal. This means that all the current flowing through the series resistor (RS) is flowing directly through the zener diode.

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Minimum Zener Current (IZ) in a Zener Regulator

The minimum zener current required for proper operation of a zener regulator circuit. This current ensures the zener diode operates within its specified voltage range.

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Maximum Load Current in a Zener Regulator

The maximum load current that a zener regulator circuit can handle without affecting its output voltage. This depends on the zener diode's specifications and the circuit's design.

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Study Notes

Regulated D.C. Power Supply

  • Electronic circuits using tubes or transistors require a direct current (DC) power source.
  • Batteries are costly and require frequent replacement.
  • DC power for electronic circuits is obtained from AC mains using rectifier-filter systems (DC power supplies).
  • Ordinary DC power supplies contain a rectifier and filter circuit.
  • Rectifier output is pulsating DC, containing AC components.
  • Filter circuits remove the AC components, providing steady DC voltage.

Limitations of Ordinary DC Power Supplies

  • Output voltage directly varies with input AC voltage.
  • Output voltage decreases with increasing load current.
  • This variation negatively impacts the operation of electronic circuits like oscillators and transmitters.

Important Terms

  • Voltage Regulation: The variation of output voltage with changes in load current. Expressed as a percentage: (Vno-load - Vfull-load) / Vfull-load × 100.
  • Minimum load resistance: A minimum load resistance is necessary for a power supply to deliver the rated output voltage and current.

Operational Concerns

  • Proper operation requires a specific zener current range (Iz).
  • Series resistance (Rs) limits the maximum input current for the circuit's safe operation.
  • There is a minimum load resistance requirement to activate the zener diode's regulation.
  • If maximum power dissipation of a zener is PZ(max) and the zener voltage is Vz, then Iz(max) = PZ(max) / Vz.
  • For a reliable power supply, the voltage variation between no-load and full-load should be minimized.

Types of Voltage Regulators

  • Series Voltage Regulator: Placed in series with the load.
  • Shunt Voltage Regulator: Placed in parallel with the load.

Zener Diode Voltage Regulator

  • Zener diodes maintain a constant voltage across the load, despite changes in current.
  • Suitable for low-voltage applications.
  • Series limiting resistance (Rs) is used to control the current through the diode.

Transistor Series Voltage Regulator

  • Uses transistors and a zener diode for stability.
  • Reference voltage is established by the zener diode.
  • A larger increase in load current will cause lesser current through zener diode.
  • Output is independent of changes in input voltage or load current variation.

Series Feedback Voltage Regulator

  • Uses feedback to regulate output voltage, more stable than a simple zener-based regulator.
  • Pass transistor (Q₂) conducts most of the load current.
  • Sample/adjust circuit using resistors (R₁ and R₂) provides a feedback voltage to control Q₁.

Short-Circuit Protection

  • Added to series regulators to protect them from excessive load current (e.g., shorting).
  • Protects the pass transistor from damage.
  • A current-limiting circuit (using a transistor Q3 and resistor R4) is added.
  • The decrease in base voltage of Q₂ reduces pass transistor conduction and prevents further increase in load current.

Glow-Tube Voltage Regulator

  • Maintains constant voltage despite load changes using a glow tube.
  • Suitable for moderate input voltages.
  • Typically used in places requiring moderate-output voltages.
  • Load variation does not affect output voltage.

Series Triode Voltage Regulator

  • Uses triodes instead of transistors.
  • Output voltage remains constant independent of load current.
  • Maintains regulated voltage despite input voltage variations.
  • Potentially wider input voltage range compared to other regulators.

Series Double Triode Voltage Regulator

  • Uses two triodes for a more stable output than single triode regulators.
  • Provides improved regulation compared to earlier versions, due to negative feedback.

IC Voltage Regulators

  • Integrated circuits (ICs) with built-in functionalities for voltage regulation.
  • Fixed positive regulators (e.g., 7805): Provides a fixed positive output voltage.
  • Fixed negative regulators (e.g., 7905): Provides a fixed negative output voltage.
  • Adjustable regulators (e.g., LM317): Allows adjustable output voltage.
  • Dual-tracking regulators: Provide both positive and negative output voltages using one device.

Additional notes:

  • Different regulator types have varying limitations and applications.
  • Selection of appropriate regulator depends on specific needs.
  • Key components required for creating a regulated power system are:
    • Unregulated DC input
    • Regulator circuit (various components)
    • Load

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