Thermoelectric Effects and Devices

Thermoelectric Voltage

• The thermoelectric voltage is dependent on the temperature difference between the junction and the free ends, and the combination of metals.
• Some common metal combinations and their voltage outputs per 100°C:
  • Chromel - Alumel: 4.2 mV
  • Bismuth - Antimony: 10.0 mV
  • Constantan - Copper: 4.2 mV
  • Platinum - Platinum-Rhodium: 1.0 mV

Thermocouples

• Thermocouples measure temperature ranging from 200°C to 2300°C.
• In aviation, they are primarily used to measure Exhaust Gas Temperatures (EGT) of engines.
• Chromel and Alumel are frequently used in modern engines.
• The thermocouple part is called the “Hot Junction” and the indicator part is called the “Cold Junction.”
• A balancing resistor is used to compensate for differences in length and resistance.
• Multiple thermocouples can be connected in parallel in a housing referred to as a “Junction Box.”

Photocells

• Photocells convert light into electrical energy.
• Voltage is generated when light hits a photocell.

Galvanic Cells

• Electrochemical reactions can generate electrical voltage.
• Two dissimilar metals immersed in an electrically conducting liquid (e.g., water with sulfuric acid) create a voltage difference.
• Galvanic cells are categorized as Primary Cells (batteries) and Secondary Cells (rechargeable power supplies).

Primary Cells

• Zinc-Carbon is a common battery material combination.
• Primary cells are not rechargeable.

Secondary Cells

• The rated voltage of a lead battery cell is 2 V.
• Lead batteries are discharged when the cell voltage reaches 1.83 V.
• Their capacity is measured in Ampere Hours (Ah) (e.g., 50 Ah is 5 A * 10 Hours).
• Aircraft commonly use Nickel-Cadmium batteries.

Static Electricity

• When a body is charged, the charges repel each other, causing an even distribution across the body's surface.

Coulomb’s Law

• Describes the force between electrical charges.
• Formula: F = 1 / 4πε) (Q1 * Q2 / d2)
• F = Force
• Q = Charge
• ε = Permittivity
• d = Distance between the Charges
• π = pi (3.14)
• Permittivity represents the ability of a medium (e.g., air) to conduct electric fields.

Electrical Conductivity

• Metals have free electrons that allow for electrical conductivity.
• Electrons flow from the negative to the positive poles when a voltage is applied.

Electrolysis

• The process of decomposing a liquid by passing an electric current through it.
• Electrolysis is used in various applications, including galvanizing and battery charging.
• The electrically conducting liquid is called an electrolyte.
• Electrons move at a speed of a few millimeters per second.

Electrical Engineering & Gases

• Gases are used in various electrical engineering applications.
• Nitrogen, Argon, or Krypton are used to prevent filament burnout, oxidation, and vaporization in light bulbs (> 60 W).
• Halogen bulbs also incorporate halogens (e.g., Iodine or Bromine).

Gas Discharge

• The process of electrical conduction through a gas.

Oscillographs/Oscilloscopes

• Visualize rapidly changing electrical processes.
• Electron beam oscilloscopes are widely used for their ease of operation, versatility, and shock resistance.

Electron Beam Tube

• Invented by K.F. Braun, a German physicist.
• Also known as the "Braun" tube.

Electron Gun

• Developed by A.R. Wehnelt, a German physicist.
• Bundled electrons for various applications, including television tubes.

Voltage Units

• 1 V = 1000 mV
• 1 mV = 0.001 V
• 1 kV = 1000 V

Common Voltages

• Radio Aerial - 0.1 uV... 5 mV
• Telephone - 1 mV... 800 mV
• Single Cell Battery - 1.5 V
• Mains Voltage - 230 V... 400 V
• Power Line - 6 kV... 380 kV
• Lightning - Several MV

Hazardous Voltages

• 50 V AC or 120 V DC

Direct Current (DC)

• Current flows constantly in one direction.
• The direction of electron flow depends on the polarity of the applied voltage.

Alternating Current (AC)

• Current periodically changes direction.

Conventional/Technical Current Flow

• Current flows from the positive pole to the negative pole.

Electron Flow

• Current flows from the negative pole to the positive pole.

Frequency

• Measured in Hertz (Hz) or Cycles Per Second (CPS)
• Represents how quickly a voltage changes direction.

Aircraft Electrical Systems

• Standard Voltage: 115 V and 200 V
• Frequency: 400 Hz

Voltage Generation Methods

• Friction
• Pressure
• Heat
• Light
• Chemical Action
• Induction

Friction

• Early Greeks observed the "Triboelectric Effect" when rubbing amber with wool or leather cloth.
• Generating static electricity through friction with insulators causing charge separation.
• Can reach high voltages in the kV range.

Pressure

• Deforming piezo crystals creates voltage.
• This is known as the "Piezoelectric Effect."

Heat

• Thermionic emission: Heat causes electrons to be emitted from a metal, resulting in voltage generation.
• This method is used to measure Exhaust Gas Temperature (EGT) in aircraft turbines.