# Electronic Devices and Circuits Questions and Answers – Transistor Case and Heat Sinks

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Transistor Case and Heat Sinks”.

1. Why does the heat sink has fins?
a) to provide cooling to the processor
b) to provide airflow to the radiator
c) to preserve the energy
d) to provide radiator to the airflow

Explanation: The heat sink has a thermal conductor that carries heat away from the CPU into fins that provide a large surface area for the heat to dissipate throughout the rest of the computer, thus cooling both the heat sink and processor.

2. Heat sinks work through the process of _______
a) resistive heat transfer
b) conductive and convection heat transfer
c) active process
d) no heat transfer

Explanation: The main purpose of a heat sink is to expel heat from a generating source. Heat sinks work through the process of conductive and convection heat transfer. Heat sinks are a passive form of cooling, as they have no moving parts and require no power. In most cases, heat sinks are used in conjunction with fans.

3. Copper has around twice the thermal conductivity of aluminum.
a) True
b) False

Explanation: Copper has around twice the thermal conductivity of aluminum, around 400 W/M K for pure copper. Its main applications are in industrial facilities, power plants, solar thermal water systems, HVAC systems, gas water heaters, forced air heating and cooling systems, geothermal heating and cooling, and electronic systems.

4. Fin efficiency is increased by_______
a) Using insulating material
b) Decreasing the fin aspect ratio
c) Using more conductive material
d) Decreasing the fin aspect ratio & Using more conductive material

Explanation: Fin efficiency is one of the parameters which formulate a higher thermal conductivity material significant. A fin of a heat sink may be measured to be a flat plate with heat flow in one end and being dissipated into the surrounding fluid as it travels to the other. As heat flows through the fin, the grouping of the thermal resistance of the heat sink impeding the flow and the heat lost due to convection, the temperature of the fin and, consequently, the heat transfer to the fluid, will decrease from the bottom to the end of the fin. Fin efficiency is defined as the actual heat transferred by the fin, separated by the heat transfer were the fin to be isothermal.

5. How can one decrease the spreading resistance in the base of the heat sink?
a) Increase the base thickness.
b) Decrease the base thickness.
c) Choosing a material with less conductivity.
d) There is no possible way.

Explanation: Spreading resistance occurs when thermal energy is transferred from a minute area to a larger area in a substance with finite thermal conductivity. In a heat sink, this means that heat does not distribute uniformly through the heat sink base. The spreading resistance occurrence is shown by how the heat travels from the heat source location and causes a large temperature gradient between the heat source and the edges of the heat sink. This means that some fins are at a lower temperature than if the heat source were uniform from corner to corner of the base of the heat sink.
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6. How can a pin fin heat sink be classified on the basis of fin arrangements?
a) Pin, straight pin, flared pin
b) Pin, circular pin, cylindrical pin
c) Straight pin, circular pin, isotopic pin
d) No classification

Explanation: In general pin fins are classified as straight pin and flared pin. For the straight fin it was 44 °C or 6 °C better than the pin fin. Pin fin heat sink performance is significantly better than straight fins when used in their intended application where the fluid flows axially along the pins rather than only tangentially across the pins.

7. _______are usually utilized to extract heat from a variety of heat generating bodies to a heat sink.
a) square fins
b) cylindrical fins
c) cavities
d) no option

Explanation: Cavities (inverted fins) embedded in a heat source are the regions formed between adjacent fins that stand for the essential promoters of nucleate boiling or condensation. These cavities are usually utilized to extract heat from a variety of heat generating bodies to a heat sink.

8. Which of the following is not an application of transistor heat sinks?
a) Soldering
b) Light Emitting Diode Lamps
c) Microprocessor Cooling
d) Environment Monitoring Systems

Explanation: Heat dissipation is an unavoidable by-product of electronic devices and circuits. LED performance and lifetime are strong functions of their temperature. Effective cooling is therefore essential. A case study of a LED based down lighter shows an example of the calculations done in order to calculate the required heat sink necessary for the effective cooling of lighting system. Temporary heat sinks are sometimes used while soldering circuit boards, preventing excessive heat from damaging sensitive nearby electronics.

9. The epoxy bond between the heat sink and component is __________
a) Temporary
b) Loosened
c) Permanent/semi-permanent
d) Highend

Explanation: The epoxy bond between the heat sink and component is semi-permanent/permanent. This is done to make the re-work very difficult and at times impossible. The most typical damage caused by rework is the separation of the component die heat spreader from its package.

10. Which of the following characteristics makes passive heat sinks differently from active heat sinks?
a) It possess mechanical components
b) It possess electrical components
c) Does not possess mechanical components
d) It possess electrical & mechanical components

Explanation: Unlike active heat sink passive heat sink do not possess any mechanical component and are made of aluminum finned radiator.

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