Sound and its Application

Fireworks are usually used for new-year celebration. Many types of fireworks are launched and explode in the sky. These things give us interesting performance. If we watch fireworks explode from far away, we will find that light from explosion and sound are not at the same time. This also happen when light storm blitz, its sound is left. These Phenomena is caused by the speed of sound and light is different. As we know that speed of sound is about 340 m/s, while light speed is 300.000 km/s. Since the speeds are known, so we can determine how far the distance from the fireworks to us is. It can be assumed that light from firework is seen at the same time when fireworks explodes and emits the sound because light speed is very high. Then after we watch fireworks explode, we can measure the time required until we hear the sound. The distance between us and fireworks is speed of sound multiplied by time (s=v x t).

Sound also can be used to determine the depth of sea. This system is called sonar system. Sonar system consists of transmitter and receiver. Function of transmitter is to produce sound in certain frequency. This sound will be reflected of by object (it can be coral reef or seabed). This reflected sound will be received by receiver. If the speed of sound in sea water is known, the depth of sea can be obtained.

Based on principle of sonar system, sound travels from transmitter to seabed and then it is reflected to the receiver. It means time required since sound is produced by transmitter until received by receiver is twice times of time needed of sound to reach seabed. So the deep of sea is half of time required multiplied by speed of sound in sea water (s = 0.5 x v x t).

The frequency used is ultrasonic, that is above the range of human hearing. The reason is because this frequency is inaudible. This frequency is shorter, so that the beam spread less and smaller objects can be detected.

Chiller and water cooling system

Chiller and water cooling system is usually used in commercial building as a part of air conditioner system. Basically, chiller system consists refrigeration cycle which is used at common air-conditioner, it may be vapor compression or absorption. The difference between chiller and common air conditioner is what the machine cools. In common air conditioner, heat transfer occurs from air to evaporator (refrigeration system). But in chiller system, air is not cooled by evaporator. We can imagine how much refrigerant needed if we just use common air conditioner and how much power consumption for compressor. Chiller system uses refrigeration to cool water first, then cooled water will be pumped to Air handling unit (AHU).

In chiller system, there is heat exchanger which can be used to transfer heat from liquid to liquid, in this case between refrigerant to water. The common heat exchanger is shell and tube heat exchanger. Water temperature at outlet is colder than inlet because heat is transferred from water to refrigerant. This cooled water is pumped to Air Handling Unit (AHU). Heat transferred from room air occurs at this AHU. Air from AHU is cold and will be distributed by ducting to the room. 

Similar process is also used is water cooling system. In refrigeration cycle, condenser needs to release heat to ambient. Common air conditioner transfers heat from condenser to air. Condenser temperature is higher than outdoor temperature, thus heat transfer from condenser to the air can occur. This is why condenser is put outside. Basically we can put condenser inside, but it will make higher heat gain and it will be more difficult to cool the air.

In water cooling, condenser transfer heat to the water, Temperature of water at outlet is higher than inside. Outlet water from chiller (condenser) is pumped to cooling tower to be cooled, then this water flow to chiller again (condenser).  

Spreading thermal resistance and Thermal interface material

In electronics cooling, usually heat is transferred from chip to ambient. Heat sink is common to be attached at chip as cooling system. Surface area of heat sink usually larger than chip, this condition makes spreading heat transfer.

What is spreading Thermal resistance?

Thermal resistance is the ratio of temperature difference to the corresponding power. The difference temperature here refers to the place or point. At a processor with heat sink, heat dissipation is started from ship to heat sink. So obviously chip temperature is higher than heat sink. The temperature difference in this case is the difference of chip temperature and the heat sink, so we called chip to heat sink thermal resistance. It also can be determined, thermal resistance of other point. The total thermal resistance is from the source to the ambient.  

Spreading thermal resistance occurs when heat source is smaller than cooling system, such as heat sink. That geometry doesn’t distribute uniformly through the heat sink, but centralized on the middle where the chip is attached.

To analyzed and design cooling system in electronic, usually uses one dimension analysis. Therefore spreading thermal resistance is one of the interests of researcher and electronics cooling engineer.

Besides spreading thermal resistance, Thermal contact interface also influences heat transfer. When chip is attached to heat sink, there is contact between two surfaces.  To optimize heat transfer, there are several ways to be applied in thermal interface material. Those are: grease, elastomeric pads, thermal tapes, phase change materials, gels, thermally conductive epoxy, solder, and soft metal.

White LED

Light Emitting Diode or LED is one of the light sources. Basically, LED is diode which consists of two types semiconductor, those are n-type and p-type. These semiconductors are connected at a junction. As a diode, LED can be a conductor if forward current flow through the LED.  How can LED emit light??

Electrons are majority charge carrier at n-type semiconductor, while holes are majority charge carrier at p-type semiconductor. When forward voltage is applied to the LED holes and electrons are pushed to the junction, then they meet followed emitting light. This light depends on band gap energy between conductance and valence band.

Basically, LED can emit only one color, but now white LED which consist many colors has been introduced to be applied in technology such as monitor. Now, White LED has begun to replace incandescent lamp or fluorescent lamp as a lighting lamp. There are two primary ways to produce white LED, RGB system and Phosphor-based system.

In RGB System, three basic colors (Red, Green, and Blue) are mixed resulting white LED. The second way is similar with fluorescent lamp which use phosphor to create white light. How can Phosphor create white light??

Stokes Shift is the key how to produce white light in Phosphor based system. Light from junction could be blue light or UV light then the energy from this wavelength will be is absorbed by phosphor. Then phosphor emits the light with lower energy than it absorbs.

LED has many advantages compared with another light source such as incandescent and fluorescent. Long life, shock resistance, compact, high efficiency are the advantages. However, the problem that challenging now is how to maintain junction temperature. LED performance largely depends on the junction temperature. Increasing junction temperature will decrease LED performance. So that thermal management in LED is very important.  

Magnetohydrodynamic (MHD) Micropump

Magnetohydrodynamic (MHD) Micropump

There are two types of micropump, mechanical and non mechanical micropump. In mechanical micropump, moving parts is involved to create fluid flow. The principles of this pump use several phenomenon such us piezoelectric, electrostatic, Thermopneumatic, and magnetic which are used to drive membrane to generate pumping action.

In non mechanical micropump, there are no moving parts involved. One of non mechanical micropump is magnetohydrodynamic micropump. This pump uses Lorentz force to generate pumping action.

Figure 1 Right Hand Rule (source figure: http://en.wikipedia.org/wiki/Lorentz_force)

When current is applied in magnetic field, there will be a force which the direction is follow right hand rule. In figure, magnetic field is represented by B, current is represented by I, and Force is represented by F.

Figure 2. Simple MHD pump

Figure 2. describes how MHD principle is applied as micropump. Magnet results magnetic field from N pole to S pole. Electrical circuit is arranged like at the figure. When there is fluid in channel, current will occur from top to the bottom resulting electrical loop. Then, Magnetic field and current will generate Lorentz force and resulting fluid flows.

Fluid in MHD should be a conducting fluid. Pure water is not conducting fluid, thus it cannot be used to MHD pump. But, if water consist material such as NaCl or Iron, it will be ionic solution and becomes conducting fluid.

Due to its principle which is not involved moving part and can generate continuous flow, therefore Microhydrodynamic pump is very suitable is biological applications.

Heat pipes

Heat pipes are often used for cooling system. Heat pipe is defined as a vapor-liquid phase-change device that transfers heat from a hot reservoir to a cold reservoir using capillary forces generated by a wick or porous material and a working fluid (The CRC Handbook of Thermal Engineering). How does heat pipe work?

Heat pipe has three basic components, those are container, working fluid, and capillary structure. When heat pipe receives heat, temperature will increase including working fluid. If working fluid has already saturated, it will change its phase to be vapor. This vapor will through capillary structure and transfer heat to the wall of heat pipe. Then after release heat, working fluid becomes liquid and goes to the heat source again. This phenomenon is repeated again.

Heat pipes has many advantages, such as high heat transfer capacity, precise isothermal control, functional independence of evaporator and condenser, quick thermal response, remote applications, high reliability, small size and light weight. Because of its advantages, heat pipe is often used in cooling system such as electronic cooling system.

Besides electronic cooling system, heat pipe also is used for water heater using solar energy. Because it has very high transfer capacity, heat radiation from sun is received and used to heat water. This hot water is used for residential needs.

The simplest heat pipes that can be made is using pipe (copper) and water as working fluid. To adjust with the requirement, the pressure inside heat pipes should be decreased using vacuum pump. So, boiling temperature of water will be decreased and can be used for below 100 Celsius degree. Other working fluid can be used for low temperature are acetone, ammonia, methanol.

For high temperature, heat should use another working fluid. Based on operating temperature, heat pipes usually use cesium, potassium, sodium, Lithium. Cesium is used in range 300 to 600 Celsius. Potassium is used in range 400 to 1000 Celsius. Sodium is used in range 500 to 1200 Celsius. And Lithium is used in range 900 to 1700 Celsius.

Phase and Temperature Change

Phase change of substance can be obtained by heating or cooling. When we heat the substance the temperature will rise until melting point or boil temperature. For example, when we heat ice at below 0^oC (let’s say -5^oC), it will not melt immediately, but temperature will rise until melting point temperature (0^oC) first. Then, ice will melt while the temperature stays at  0^oC. After all of water melt, then temperature will rise again until boiled temperature (100 ^oC). Water temperature will stays at 100 ^oC, when water is boiled. Then water phase will change to vapor. After all become vapor the temperature will increase again.

For simplifying these phenomena:

•           Rising temperature until melting point temperature. The phase is solid
•           Phase change from solid to liquid. This phenomenon doesn’t change temperature
•           Rising temperature from melting point temperature to boiling point temperature. The phase is liquid
•           Phase change from liquid to gas. This phenomenon doesn’t change temperature.
•           Rising temperature of gas.

The explanation above is heating process. Cooling process is similar but the opposite.

For calculate heat needed to removed, the general formula are:

•           Every Rising temperature: Q = m.c.∆T
•       Where Q: heat, m: specific heat of the substance, ∆T: temperature differences
•          Change phase from solid to liquid: Q=m.L
•       Where L: heat fusion of the substance
•          Change phase from liquid to gas: Q = m.U 
•       Where U: heat of vaporization

These conditions are approach, in actual we may find that water will evaporate although the temperature is not at boiling temperature. Also the other phenomena such as melting.

Photometry Quantities

Photometry is defined as a science that study about light which is perceived by human eyes. It is different with radiometry which measure all of radiant energy including visible light. Photometry focus radiant energy which is sensitive to human eyes, that is wavelength range from 380 nm to 750 nm.

IEC said light is energy radiation which can be evaluated visually. Gigahertz-Optic defined light as part of electromagnetic spectrum radiation which can be seen. The point of these definitions is same but in different words. There are several parameters in Photometry: Luminous flux, Luminous Intensity, illuminance and Luminance.

Luminous flux is the total power electromagnetic radiation emitted by source which the spectral is can be sensed by human eye. In other words, it is on in range of visible light (380 nm -750 nm). Unit of this parameter is Lumen. 

Luminous Intensity is total luminous flux per unit solid angle that emitted in certain angle. Unit of this parameter is Lumen/steradian or Candela (cd).

Illumninance is the total luminous flux which is received by a certain surface area. Unit of this parameter is Lumen/m² or Lux.

Luminance is total luminous intensity which is received by a certain surface area. Unit of this parameter is candela/m².

These parameters are similar with radiometry. The difference is photometry measure only electromagnetic radiation than can be sensed by human eye.

Source of light emits both of visible light and non visible light. The ratio of power in visible light range to the total radiant power is luminous efficacy which has unit Lumen/Watt.

Luminous efficiency is also known to determine the amount of visible light power in unit watt which emitted by total radiant power (also) in unit watt. This can be represented in percent (%)

Decibel in sound level

The frequency of vibration can be heard if the frequency is in range of human hearing, that is 20 Hz – 20 kHz. If the frequency below 20 Hz, it is called infra sonic. Otherwise, if the frequency more than 20 kHz, it is called ultrasonic.

How can we hear the sound?

Sound can be heard because sound vibrates our eardrum. Source of sound which is vibration will create wave propagation in medium (such as air). Then it will vibrate eardrum. The propagation of sound has Intensity. Sound Intensity is defined as sound power in a surface area or in mathematic equation:

I = P/A.

Human hearing can hear sound if the Intensity of sound at least 10^-12 W/m².

How to calculate Sound intensity level?

Sound Intensity level is a logarithmic measure of sound intensity comparing with the reference Intensity. In this case, reference Intensity defined as the minimum intensity can be heard by human, that is 10^-12 W/m². Sound Intensity level can be calculated by:

Where I₀ is reference Intensity. For example if the intensity of sound is 10^-6 W/m², then we can calculate sound Intensity level using that equation. That is L = 10 log (10^-6/10^-12) = 60 Decibel.

Infrared thermometer

Temperature can be measured by several methods. Each of them has advantages and disadvantages. For example, mercury thermometer which uses fluid to convert heat in to expansion, thus we can see rise of height when temperature increase. This thermometer is very simple.

Thermocouple is inexpensive sensor temperature. This sensor consists of two different materials and both of them are connected by junction at one of their edge of. When temperature of junction is getting higher this junction produces more voltage which can be converted to temperature value. The disadvantage of this sensor is we need interface that convert from voltage to temperature.

The other thermometer is infra red thermometer. Every molecules move if their temperature is above absolute zero. Higher temperature causes faster movement of molecules. When the molecules move, they emit infrared radiation. If they move faster, more infrared radiation emitted. Sometimes if the temperature so high, they emit visible light. For example, metal will glow red when it is heated until very high temperature.

This phenomenon is called black body radiation and this is the principle of infrared thermometer. Infrared radiation can be focused, absorbed or reflected. Infrared thermometer has lens to focus this radiation and it will be received by detector/sensor. This sensor is called thermopile. Thermopile absorbs infrared radiation and converts it into electricity, then this electricity is converted to show the temperature of the object.

Infrared thermometer is usually completed with laser or LED that can emit beam from that thermometer. It is just for show what you are measuring at. The advantage of this thermometer is noncontact measurement, so this thermometer will not affect the system want to be measured. Furthermore, it can be used from the distance.

Why is air in air-conditioned room dry?

When we are in the room or car which is air conditioned, we feel air surrounding us is dry. How come?

Air contains several kinds of gases such as nitrogen, oxygen, carbon dioxide, etc. Besides that air also contains moisture. If moisture in the air is high, then it makes air becomes more humid. The amount of moister in the air is called humidity. Humidity can be quantified as relative humidity and humidity ratio.

 

Latent heat is heat which doesn’t change the temperature. This heat only change phase. Moister inside air causes latent heat becomes high. More moisture contained in the air will cause higher latent heat because heat removed from air is not only used to decrease temperature, but also condense moisture.

Air-conditioner cool the air and it causes air temperature decrease until dew point temperature, if the air is still cooled, temperature will decrease followed condensation of moisture. Dew point is the temperature when moisture is saturated.

Room air is sucked to the air conditioner and blown again to the room. Before it is blown again, air is cooled by air-conditioner. In other words air is circulated and cooled by air-conditioner. The component of air-conditioner which cool the air is evaporator, thus air which is close to evaporator is colder than other and achieve dew point temperature earlier, then start to produce condensate. That is why water is often (condensate) found at air conditioner.

Since moisture in the air condenses at evaporator, the amount moisture is being reduced and resulting dry air. 

Simple Heat calculation and Black Principle

Temperature of matter can be increased or decreased by heat or cool it. If a matter is heated, it means amount of energy is received by the matter, then the temperature increase. For example, when we heat water on the stove, heat energy is transferred from stove to water, then increase water temperature. The question is:  How much energy is required to increase temperature?

Heat energy required to increase the temperature can be calculated by this equation:

Q = m.c.∆T

Where:

Q: Heat energy

m: Mass

c: specific heat

∆T: Temperature difference

Each matter has different specific heat value, such as water has specific heat 4200 J/kgC. It means we need 4200 Joules of energy to increase 1 Celsius for 1 kg of water.

For example, we want to heat 1 kg of water from 20 Celsius to 50 Celsius. Heat energy we need is:

Q = m.c.∆T

Q= 1 kg . 4200 J/kgC . (50-20) = 126000 Joules

How about calculation of mixing temperature of matter?

Assume we mix 1 kg water at 20 Celsius and 2 kg water at 80 Celsius, How is the temperature of the mixing water?

“Black” principle said heat received is equal heat given. So the equation is:

Q₁ = Q₂   (Q₁ means heat given and Q₂ means heat received)

This condition resulting heat transfer from higher temperature to lower temperature, so water at 80 Celsius gives heat to water at 20 Celsius. Let’s say T_mix is final temperature after mixing water, then

m₁.c₁.∆T₁ = m₂.c₂.∆T₂

2 . 4200. (80-T_mix)= 1 . 4200 . (T_mix-20)

8400 . (80-T_mix) = 4200 . (T_mix-20)

672000-8400 T_mix=4200 T_mix-84000

Then we can get T_mix= 60 Celsius.

Heat transfer

What is Heat transfer??

Talk about heat transfer, there are two words heat and transfer. The definition of heat is the form of energy that can be transferred from higher temperature to lower temperature. So, Heat transfer is heat energy transfer from one system to another system due to temperature difference.

Heat can be transferred by three ways, conduction, convection, and radiation. What is the difference?

Conduction

Conduction is kind of heat transfer without any movement of molecules. When we burn rod of metal at one of its edge, after a while we will feel the other edge is also hot. This is conduction phenomena. Heat transfer by this phenomena is affected by several parameters, those are surface area, thermal conductivity coefficient, temperature difference and length. As increasing surface area, thermal conductivity coefficient and temperature difference, heat transfer will also increase. As increase of length, heat transfer will decrease. We can say heat transfer by conduction is directly proportional to surface area, thermal conductivity, and temperature difference, But inversely to the length. Mathematically we can write

Q=(k . A. ∆T) / L

Convection

Convection is kind of heat transfer that involves movement of molecule. For example, when we heat water on a pan by stove, the highest temperature is on the bottom of pan. Water temperature at bottom is also higher than at the top. Density of matter is lower if the temperature is high. So, density of water at bottom lower than at the top, thus water from the bottom moves up and water from top moves down.

Convection heat transfer is affected by surface area, convection coefficient, and temperature difference. Mathematically we can write:

Q = h . A .   ∆T

Talk about convection heat transfer, the “game” of this phenomena is at convection coefficient. To determine this coefficient need complicated correlation between fluid mechanics, friction factor, geometry, pressure, etc.

Radiation

Radiation heat transfer is heat transfer without medium. Heat from sun to the earth is transferred by radiation because no atmosphere in outer space. This heat transfer can be written mathematically:

Q = ε . σ . A . T⁴

Where

ε : emissivity coefficient

σ : Stefan-Boltzmann constant

Energy conservation in Building

A huge amount of energy is spent by building. Lighting, motors, electronic equipment, etc are needed and found in building. Moreover HVAC system is the highest energy consumption for commercial building. Energy conservation in building becomes great interest and causes energy audit needed especially for old building.

There are several actions which can be applied to conserve energy for building. Building envelope, Electrical System, and HVAC System are the most common part of it.

Building envelope

Building envelope is related to cooling or heating system because when cooling or heating system is being designed, one of the important thing is material selection for wall, roof, windows, etc. When outdoor and inside temperature are different, heat transfer will occur and add heat gain for the building. Therefore building envelope material selection is very important.

If the building is already built, several actions to improve thermal performance are:

• Thermal insulation attachment for building surface without thermal insulation
• Replace windows with more efficient windows which has high R value, low emisivity glazing, etc
• Reduction of air leakage. Air leakage causes outdoor air enter the inside the building and add heat gain which has sensible and latent heat.

Electrical System

Electrical system involves Lighting, office equipment and motors. In Lighting, the conservation energy actions which can be applied are use energy efficient lamp and ballast. Now, LED technology has begun to replace conventional lighting such incandescent. Besides that, optimize day light is also can be applied.

Office equipment includes computer, printer, fax, copier, etc. Motors can be found in many equipments such as fan, washing machine, etc. For office equipments and motors, the actions which can be applied are choose the energy efficient equipment. Set computers automatically switch to low power also can be applied for it

HVAC

HVAC is the highest energy consumption in building. About 40% of the total energy is consumed in commercial building for HVAC system. Some actions for this are:

•         Set the temperature set point in thermal comfort zone. Turn on the HVAC system only when it is needed.
•        Retrofit of constant air volume system
•              Heat recovery system installation
•              Central cooling plants retrofit.

Source: Hand Book of Thermal Engineering.

Cooling Load Estimation

Design and selection HVAC System based on determination cooling load and heating load. There are several methods to determine cooling load and heating load. The most common method is cooling load temperature differences/Solar cooling load/Cooling load factor (CLTD/SCL/CLF) method. This method developed by ASHRAE along ago.

Cooling load is heat that must be removed from space or room or zone want to be cooled. There are several components must be considered.

1. Conduction from building envelope. Heat is transferred from higher temperature to lower temperature. If outside temperature is higher than inside, so spontaneously heat enter inside space. Heat can enter the space through wall, roof, skylight, or windows.
2. Radiation from sun through window and sky light
3. Conduction through ceiling, partition walls and floor.
4. Internal heat gain such as people, light, and equipment in the room.
5.  Heat gain from infiltrating from outside through door, window, etc. 
6. Cooling load calculation also needs to consider the ventilation heat gain and heat generated by fans in system.

There are latent heat and sensible. Sensible heat is heat which causes changes temperature. Besides that, Latent heat is heat which is not makes temperature difference. This heat cannot be measured by thermometer because no difference temperature occurs. This heat only causes phase change. The simple explanation about latent heat is when water is being heated and boiled, its temperature is stay at 100 ^oC although it is still being heated. This heat is used to change phase not to increase temperature.  Inside surrounding air contains moisture, latent in this case is related to phase change of the moisture.

Conduction from building envelope, radiation from sun, are the example of sensible heat and internal heat from people, infiltration is the example of both sensible and latent heat because moisture is involved. 

Thermal comfort for human

Factors that affect human comfort are dry bulb temperature, humidity, air movement, fresh air, clean air, noise level, adequate lighting, proper furniture and work surfaces. However for thermal comfort only consider dry bulb temperature, humidity, and air motion. Therefore the effort to make thermal comfort is how to create those factors are appropriate for the activity level of people in the space.

To make body feel comfortable, the surrounding environmental must be suitable to transfer heat from body to ambient. If surrounding temperature is too high, body feels uncomfortable warm. Moreover if the humidity is also high, it makes body sweating. Also if the temperature is too low, body losses more heat than it produce, and probably it makes body aquiver.

Heat generated by people depends on the people activity. The people who are sitting and doing sport generate different amount of heat. Almost definitely, sport produces more heat than sit. Therefore designing thermal comfort need consider people activity.

For creating environment temperature to be appropriate for the occupancy, cooling or heating system is needed. For example in summer, the temperature is too warm, so air conditioning system (cooling system) is needed. Cooling and heating use heat transfer principle. Those are heat cannot be destroyed, heat flows spontaneously from higher temperature to lower temperature, and heat can be transferred to another substance.

For estimating cooling load and heating load need desired condition. Usually the desired condition is chosen 78^oF (25.6^oC) and 50 % Humidity.  For detailed human thermal comfort, ASHRAE standard 55 Thermal Environmental Conditions for Human Occupancy is made as a standard.

Air conditioning and Ventilation

Air conditioning

Air conditioning is treatment to make temperature, humidity, cleanliness of air meet the requirements of conditioned space. It also includes the distribution of air. Air conditioner can be divided in to several types, those are direct expansion coolers, chilled water system, and evaporative coolers.

 Direct expansion cooler is the kind of usual air conditioner that found in residential house. This air conditioner uses refrigeration cycle as its principle. Air is blown through the cold part (evaporator) and distribute to the room.

Chilled water system doesn’t  cool the air directly. This air conditioner cool water and then water will be flown to the Air Handling Unit (AHU) to cool the air.

Evaporative cooler is usually used in hot and dry climate region. This air conditioner evaporates the water sprayed and resulting lower temperature of air.

Ventilation

Ventilation is the process that removes or supplies air at the room. Every room has function, such as class room, laboratory, etc. Those rooms have different requirements of ventilation. Designing HVAC system consider many aspects of parameter, one of them is how much air must be supplied.  This parameter can be achieved by designing ventilation system.

All parameter HVAC design requirement are treated to make thermal comfort for the occupancy. ASHRAE made a standard about human comfort. (ASHARE STANDARD 55)

Thermal Resistance in Electronics

Thermal resistance is the ratio between temperature difference and power dissipated. In electronics, Thermal resistance is a great interest for engineer. It is due to every electronic equipment produce heat and need to be cooled. If they cannot be cooled properly, it will be harmed because of overheat problem.

Thermal resistance can be analogized by electrical resistance. Current represent heat flow, voltages represent temperature differences, and Resistor represent thermal resistance. It can be simplified: R_th=∆T/Q is analogized by R=V/I.

There are several kind of thermal resistance, such as conductive thermal resistance, convective thermal resistance, and spreading thermal resistance. Thermal resistance is basically needed to calculate heat transfer from one point/surface to another point or surface.

In electronic, Thermal resistance is the parameter that informs how effective heat dissipated can be transferred to the ambient. Moreover, it can be used to determine heat source temperature, usually chip junction temperature. In other words, heat source temperature can be known if thermal resistance is already known.

Let’s consider a simple electronic device with heat sink. Heat transfer is started from junction to casing and finished at ambient. This construction results several thermal resistances, those are junction to casing thermal resistance, casing to ambient thermal resistance through heat sink. Total thermal resistance of this system is R_total= (T_j-T_amb)/Q= R_JC+R_CA

Power dissipated from electronic device is assumed that all electricity is converted to heat. Therefore heat dissipation is : Q=VxI, where Q represent heat dissipation, V represent Voltage, and I represent Current. For LED heat dissipation, sometimes optical power from LED is considered, so heat dissipation becomes, Q= P_electrical-P_optical = (V*I)-P_optical.

Types of fan

There are several air moving device, those are: fan, blower, and compressor. All of their function is similar, that is to create air flow. Some expert classified them based on their pressure. Fan is the lowest, less than 1000mmAq. Second is blower, 1000-10000mmAq. The highest is compressor, more than 10000mmAq.

Fan is used at many kind of the equipments, especially cooling system equipments. For example, air conditioner, cooling system in electronics, or even just fan for summer. All of them are used to enhance heat transfer from the object to the ambient.

Based on the air flow direction, fan can be divided into two types, centrifugal and axial fan. In centrifugal fans, air comes from the axes direction of rotation and exhaust at periphery. In axial fan, air comes from behind and exhaust to the front.

There are three kinds of centrifugal fan. First is sirocco fan, this fan has forward curved blade, low RPM, Low noise, Low static pressure, and well suited for residential HVAC application. Second is air foil fan, this fan has backward curved blade, high RPM, Low noise, and usually used for clean environment. Third is turbo fan, this fan has backward curved blade, high RPM low noise, high static pressure, and high efficiency.

There are also three kinds of axial fan. First is propeller fan, this fan has propeller only, large flow rate, low static pressure, low efficiency. Second is tube axial fan, it is similar with propeller fan but improved by casing, higher total pressure capability than propeller fan. Third is vane axial fan, this fan same with tube axial fan but have vanes to straighten, this fan has high efficiency.

LED Junction Temperature measurement

Light Emitting Diode or LED has begun to be applied as light source replacing other source light such as incandescent or fluorescent lamp. LED has many advantages such as high efficiency, shock resistance, and long life time. However, LED performance depends on the temperature thus thermal management in LED technology is very important.

Talking about thermal management, the important parameter in LED technology is Junction temperature, but the problem is how to measure it. LED junction is very small and inside the encapsulation, therefore thermocouple or other temperature sensor couldn’t be used to measure it.

There are several methods to measure LED junction temperature, those are forward voltage and shift wavelength method. Both of them based on the dependence temperature, it means the difference temperature causes change of forward voltage and wavelength. LED junction measurement by these methods consists of calibration and actual measurement.

In calibration measurement, LED (including Junction) is heated until certain temperature then injected by pulse current with very low duty cycle. LED junction temperature is assumed same with that certain temperature because heat dissipated from LED is very small due to low duty cycle. Then forward voltage or wavelength is measured. Next, this measurement is repeated at different temperature. By this measurement the relation between forward voltage/wavelength and temperature is got and can be used as reference in actual measurement

In actual measurement, LED is operated in normal condition (in ambient temperature) and current (constant). Junction temperature can be determined by measuring the forward voltage/Wave length. Then data from calibration measurement is used as reference to determined junction temperature.

Thermoelectric cooler

Thermoelectric cooler is a device that converts from electricity to temperature difference. This device consists of p-type and n-type materials. When voltage is applied, current flows and result the difference temperature between its surfaces . Hot and cold surfaces can be reversed by reversing the direction of current.

Thermoelectric

Thermoelectric cooler is one of the methods to produce low temperature. Comparing with conventional method (vapor compression), thermoelectric cooler has advantage such as: thermoelectric can be a cooler or heater by change the direction of electrical current, no vibration, quite, reliable, and no refrigerant (environmental friendly)

However, disadvantage of thermoelectric cooler performance is very low. Coefficient of performance of Thermoelectric cooler is only about 0.3 (vapor compression COP : 3-4).

Due to its advantages, thermoelectric cooler can solve the problem that conventional method cannot be applied to it, such as electronics cooling.

The application of thermoelectric has been introduced in many aspects. We can find thermoelectric cooler system at small refrigerator or even thermoelectric air conditioner. In electronics, thermoelectric cooler also is one of the solutions to maintain chip temperature below allowable maximum temperature. Thermoelectric can be attached to the chip and cool the chip directly.

Performance of thermoelectric will decrease if heat dissipation from hot side is not released properly.  Heat sink can be attached to the hot side of thermoelectric cooler to support heat transfer from hot side to the ambient. Moreover, thermoelectric cooler can produce very low temperature by making cascade thermoelectric cooler. It means hot side thermoelectric cooler is attached by another thermoelectric cooler.

Heat Sink in Electronics cooling

Electronics technology has been developing rapidly since the first transistor was invented. Semiconductor technology is the most significant part for electronics technology development. At the first time one transistor with very small size could replace cathode tube. Now, thousand or even million transistors can be operated in one single chip. However, electronics have maximum temperature to be operated properly. A survey showed that the most cause of electronic failure is temperature. Thus electronics cooling cannot be separated in electronics technology.

Many types of cooling system for electronics have been introduced. The most common electronics cooling system is heat sink. Heat sink system can be divided into two parts, forced convection and natural convection.

Natural convection is convection heat transfer without any force applied to heat sink. This convection occurs because of buoyancy force naturally. Fluid (e.g air) has lower density if its temperature is high, this causes air moves up.  In electronics, heat dissipated from chip causes increasing temperature surrounding then air density becomes low, thus air moves up. Since natural convection only uses buoyancy force, usually natural convection heat sink is attached vertically.

Forced convection need additional force to move air flows on heat sink. Fan is usually used to support air flow on heat sink. Heat transfer rate at this heat sink is bigger than natural convection. Many electronic equipments use this type.

Heat sink is attached to heat source to enhance heat transfer rate. When heat dissipated from electronic device cannot be overcome by heat sink, it needs additional system or even different cooling system. Heat pipes or thermoelectric cooler may be an option for additional system in heat sink. 

How to measure pulsed current on electrical circuit??

In some cases, pulsed current generator is needed, such as of LED performance test. In that case, LED need to be injected by pulsed current. Actually, there are some company that produce and sell pulse current generator but it is usually very expensive. To solve this problem we can use pulsed voltage generator instead of it which are cheaper than pulsed current generator.

By using pulsed voltage generator, we may be confused how to measure the current. Pulse current is difficult to measure because of two reasons. First, it is pulsed, so we cannot measure the current by ampere meter or clamp ampere since those are for constant current. Second, it is current which cannot be measured by oscilloscope. So, how to measure it??

To measure pulsed current on electrical circuit, we need a known resistant (such as resistor) to be attached serial on the circuit. Then, activate the circuit and measure voltage at the resistor by oscilloscope. By this method, voltage can be measured and resistance is already known. As known that currents at serial connection have same value, Then by simple equation that current value equals voltage divided by resistance (I=V/R)current can be calculated easily.

Also we can control the current by controlling voltage of pulse generator. If we can’t control the voltage we may use potentiometer. These have the same principle, (I=V/R). When we control voltage of pulse generator it means we change V. And the other hand if we use potentiometer, it means we change the resistance(R).

In using pulsed generator, be careful to set the current (it means set up voltage and selection of the resistor). If current exceed generator limit (see the specification of generator), it may cause damage to it. If it happens, fuse inside generator will be cut. But if fuse is already broken, damage may occur in generator component.