AMEM 10049 - Basic Aviation Electronics - Week 11 Notes

Avionics: Basic Aviation Electronics

• Course: AMEM 10049

• Instructor: Professor Mini Thomas

• Week: 11

• Theme: Future Ready

Overview of Topics

• 1. Diode Applications

• 2. Rectification

• 3. Half Wave and Full Wave Rectifiers

• 4. Filtering

• 5. C, L, LC Filters

• 6. Problems

Peak Detector

• Definition: The peak detector (or peak rectifier) provides a DC output that matches the peak value of the input AC voltage.

• Operation (for 0 to T/2):

  • The diode acts as a short circuit.

  • The capacitor charges quickly, resulting in $V<em>o = V</em>i$ .

• Voltage Behavior:

  • When the input voltage $V_i$ decreases, the voltage across the capacitor cannot decrease because current must flow through the diode in reverse direction.

  • As a result, the diode acts as an open circuit (reverse bias), preventing capacitor discharge and maintaining $V<em>o$ at the peak value of $V</em>i$ .

• Load Influence:

  • Adding a load resistor will allow the capacitor to discharge.

  • However, the time constant $\tau = RC$ can be adjusted to prevent significant discharge of the capacitor.

Diodes in DC Power Supplies

• General Block Diagram: A power supply typically starts with an AC voltage which is then rectified to a DC voltage, filtered to smooth it, and regulated for a stable output.

• Transformers:

  • The AC voltage is connected to a transformer that steps down the voltage to the desired level for DC output.

• Rectification and Filtering:

  • A diode rectifier provides full-wave rectification; initially, a simple capacitor filter is used to produce the DC voltage which may contain ripple.

• Voltage Regulation:

  • A regulator circuit processes this DC input to ensure minimal ripple voltage and a constant DC output despite variations in input voltage or load changes.

  • This regulation is done using specific voltage regulator ICs.

Half Wave Rectifier

• Approximation:

  • Using a diode model:

    • For vS < V{D0} , $V_o = 0$ .

    • If $r_D \text{ (diode internal resistance) is much smaller than } R$:

    • $V<em>o \rightarrow V</em>s - V_{D0}$ .

• Peak Inverse Voltage (PIV):

  • $PIV = V_S$ .

• Problem Example: For $V_s = 12 \text{ Vrms}$ and $R = 100 \text{ ohms}$:

  • Calculate $V_o$ and peak diode current.

Full Wave Rectifier

• Circuit Explanation:

  • Diodes D1 and D2 switch on during the positive cycle; both diodes will be forward biased, allowing current to pass through.

• Peak Inverse Voltage (PIV):

  • PIV experienced by D2:

    • $PIV = 2V<em>S - V</em>D$ .

• Problem Example: For $V_s = 12 \text{ Vrms}$ and $R = 100 \text{ ohms}$:

  • Calculate $V_o$ and peak diode current.

Voltage and Current Waveforms

• Peak Rectifier Circuit:

  • Explain the behavior with C R >> T where T is the time period.

  • Assume the diode is ideal.

• Half-Wave Rectifier + Filter:

  • Formulas:

    • $V_f = C R$

    • $V<em>{dc} = V</em>p - V_r (pp)$ .

• Full-Wave Rectifier + Filter:

  • Formulas:

    • $V<em>{dc} = V</em>p - V<em>r (pp)$ where $V</em>r (pp)$ relates to output ripple voltage.

Limiting Circuits

• General Concept: Application of a sine wave to a (hard) limiter resulting in clipping of the two peaks.

• Characteristics: Soft limiting illustrated.

Types of Special Diodes

• Light Emitting Diode (LED):

  • Overview: Emits visible light when energized.

  • Mechanism: In forward-biased P-N junctions, electrons recombine with holes, emitting energy as photons.

  • Materials: Silicon and germanium primarily emit heat; while gallium arsenide phosphide (GaAsP) and gallium phosphide (GaP) emit visible light.

  • Electroluminescence: The process of light emission due to electrical energy.

• Photodiodes:

  • Similar to regular diodes but designed to be exposed or have an optical connection for light sensitivity.

  • Operation: Photons create electron-hole pairs resulting in photocurrent which depends on light intensity.

Zener Diodes

• Breakdown Characteristics:

  • Enter breakdown when reverse voltage exceeds $V_{ZK}$ (breakdown voltage or Zener knee voltage).

• IV Curve: Rapid current increase with slight voltage change in the breakdown region.

• Application: Voltage regulation providing constant output even under load changes.

• Symbol: Unique circuit symbol for Zener diodes representing their function.

• Notes:

  • Zener diodes have seen decreased popularity due to advancements in IC voltage regulators.

Applications of Diodes in Aircraft

• Transient Voltage Suppression (TVS):

  • Protects sensitive electronics from voltage transients like lightning.

  • Essential in aerospace for applications such as Full Authority Digital Engine Control (FADEC) and avionics computers.

• Key Features:

  • Wide voltage and power ranges, innovative packaging, and high reliability.

• Zener Diodes in Aircraft Cooling:

  • Installed in Aircraft Cooling Unit (ACU) for voltage regulation.

  • Functions by conducting current in both directions once breakdown voltage is reached.

• Ultra Fast Recovery Diodes for Aircrafts:

  • Microsemi's various series (DL, D, DQ, DS, S) designed for high voltage and power applications, with specific features tailored for their use cases.

Electrical Control Units in Aircraft

• Components Included:

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