DEVELOPMENT OF THERMOELECTRIC DEVICE CHARGER: Week 10 : The Voltage Regulator, Heat Sink and DC Brushless Fan Motor

Objective:
To identify and research about the voltage regulator, heat sink and DC brushless fan motor.

Analysis/Procedure:
1. VOLTAGE REGULATOR:

Figure 6: Sample of voltage regulator

A voltage regulator is designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. In automobile alternators and central power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line.

Transistor regulator

In the simplest case a common collector transistor (emitter follower) is used with the base of the regulating transistor connected directly to the voltage reference:

Figure 7: Transistor regulator circuit

A simple transistor regulator will provide a relatively constant output voltage, Uout, for changes in the voltage of the power source, Uin, and for changes in load, RL, provided that Uin exceeds Uout by a sufficient margin, and that the power handling capacity of the transistor is not exceeded.

The output voltage of the stabilizer is equal to the zener diode voltage less the base–emitter voltage of the transistor, UZ − UBE, where UBE is usually about 0.7 V for a silicon transistor, depending on the load current. If the output voltage drops for any external reason, such as an increase in the current drawn by the load (causing a decrease in the Collector-Emitter junction voltage to observe KVL), the transistor's base–emitter voltage (UBE) increases, turning the transistor on further and delivering more current to increase the load voltage again.

Rv provides a bias current for both the zener diode and the transistor. The current in the diode is minimum when the load current is maximum. The circuit designer must choose a minimum voltage that can be tolerated across Rv, bearing in mind that the higher this voltage requirement is, the higher the required input voltage, Uin, and hence the lower the efficiency of the regulator. On the other hand, lower values of Rv lead to higher power dissipation in the diode and to inferior regulator characteristics.

where VR min is the minimum voltage to be maintained across Rv
ID min is the minimum current to be maintained through the zener diode
IL max is the maximum design load current
hFE is the forward current gain of the transistor, ICollector / IBase

For more detail about voltage regulator there is video at down below.

2. HEAT SINK

Figure 8: Sketch of a heat sink in a duct used to calculate the governing equations from conservation of energy and Newton’s law of cooling

Heat transfer principle

A heat sink transfers thermal energy from a higher temperature device to a lower temperature fluid medium. The fluid medium is frequently air, but can also be water, refrigerants or oil. If the fluid medium is water, the heat sink is frequently called a cold plate. In thermodynamics a heat sink is a heat reservoir that can absorb an arbitrary amount of heat without significantly changing temperature. Practical heat sinks for electronic devices must have a temperature higher than the surroundings to transfer heat by convection, radiation, and conduction. The power supplies of electronics are not 100% efficient, so extra heat is produced that may be detrimental to the function of the device. As such, a heat sink is included in the design to disperse heat to improve efficient energy use.

To understand the principle of a heat sink, consider Fourier's law of heat conduction. Fourier's law of heat conduction, simplified to a one-dimensional form in the x-direction, shows that when there is a temperature gradient in a body, heat will be transferred from the higher temperature region to the lower temperature region. The rate at which heat is transferred by conduction,

, is proportional to the product of the temperature gradient and the cross-sectional area through which heat is transferred.

q_k = -k A \frac{dT}{dx}

Consider a heat sink in a duct, where air flows through the duct, as shown in Figure 8. It is assumed that the heat sink base is higher in temperature than the air. Applying the conservation of energy, for steady-state conditions, and Newton’s law of cooling to the temperature nodes shown in Figure 8 gives the following set of equations.

\dot{Q} = \dot{m}c_{p,in}(T_{air,out} - T_{air,in})

(1)

\dot{Q} = \frac{T_{hs} - T_{air,av}}{R_{hs}}

(2)

where

T_{air,av} = \frac{T_{air,in} + T_{air,out}}{2}

(3)

Using the mean air temperature is an assumption that is valid for relatively short heat sinks. When compact heat exchangers are calculated, the logarithmic mean air temperature is used.

is the air mass flow rate in kg/s.

The above equations show that

When the air flow through the heat sink decreases, this results in an increase in the average air temperature. This in turn increases the heat sink base temperature. And additionally, the thermal resistance of the heat sink will also increase. The net result is a higher heat sink base temperature.
The increase in heat sink thermal resistance with decrease in flow rate will be shown later in this article.
The inlet air temperature relates strongly with the heat sink base temperature. For example, if there is recirculation of air in a product, the inlet air temperature is not the ambient air temperature. The inlet air temperature of the heat sink is therefore higher, which also results in a higher heat sink base temperature.
If there is no air flow around the heat sink, energy cannot be transferred.
A heat sink is not a device with the "magical ability to absorb heat like a sponge and send it off to a parallel universe".
Natural convection requires free flow of air over the heat sink. If fins are not aligned vertically, or if fins are too close together to allow sufficient air flow between them, the efficiency of the heat sink will decline

Heatsink Design

What characteristics make a heatsink a good one? There's a number of factors to consider:

High heatsink surface. It's at the surface of the heatsink where the thermal transfer takes place. Therefore, heatsinks should be designed to have a large surface; this goal can be reached by using a large amount of fine fins, or by increasing the size of the heatsink itself.

Good aerodynamics. Heatsinks must be designed in a way that air can easily and quickly float through the cooler, and reach all cooling fins. Especially heatsinks having a very large amount of fine fins, with small distances between the fins may not allow good air flow. A compromise between high surface (many fins with small gaps between them) and good aerodynamics must be found. This also depends on the fan the heatsink is used with: A powerful fan can force air even through a heatsink with lots of fine fins with only small gaps for air flow - whereas on a passive heatsink, there should be fewer cooling fins with more space between them. Therefore, simply adding a fan to a large heatsink designed for fanless usage doesn't necessarily result in a good cooler.

Good thermal transfer within the heatsink. Large cooling fins are pointless if the heat can't reach them, so the heatsink must be designed to allow good thermal transfer from the heat source to the fins. Thicker fins have better thermal conductivity; so again, a compromise between high surface (many thin fins) and good thermal transfer (thicker fins) must be found. Of course, the material used has a major influence on thermal transfer within the heatsink. Sometimes, heat pipes are used to lead the heat from the heat source to the parts of the fins that are further away from the heat source.

Perfect flatness of the contact area. The part of the heatsink that is in contact with the heat source must be perfectly flat. A flat contact area allows you to use a thinner layer of thermal compound, which will reduce the thermal resistance between heatsink and heat source.

For more detail on heat sink click the video down below.

3. DC BRUSHLESS FAN MOTOR

Figure 9: Example of brushless DC electric motor

Brushless DC electric motor (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) are synchronous motors that are powered by a DC electric source via an integrated inverter/switching power supply, which produces an AC electric signal to drive the motor. In this context, AC, alternating current, does not imply a sinusoidal waveform, but rather a bi-directional current with no restriction on waveform. Additional sensors and electronics control the inverter output amplitude and waveform (and therefore percent of DC bus usage/efficiency) and frequency (i.e. rotor speed).

The rotor part of a brushless motor is often a permanent magnet synchronous motor, but can also be a switched reluctance motor, or induction motor[citation needed].

Brushless motors may be described as stepper motors; however, the term stepper motor tends to be used for motors that are designed specifically to be operated in a mode where they are frequently stopped with the rotor in a defined angular position. This page describes more general brushless motor principles, though there is overlap.

Two key performance parameters of brushless DC motors are the motor constants Kv and Km

The video below will explain more on brushless DC electric motor.

Conclusion:

In conclusion, the heat sink with DC brushless motor fan are the important role of thermoelectric generator because they can create a constant temperature of cold side thermoelectric module and the voltage regulator are play important role to give a constant output (5V) but there are several problems which I will be facing:

How low temperature can be generate using heat sink with DC brushless motor fan at cold side thermoelectric module?
How much load current can be produced when applied voltage regulator?

Reference:

DEVELOPMENT OF THERMOELECTRIC DEVICE CHARGER

Friday, 21 November 2014

Week 10 : The Voltage Regulator, Heat Sink and DC Brushless Fan Motor

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