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CHAPTER 1

INTRODUCTION TO LED

1.1 HISTORY OF LED

For over 30 years, LEDs have been used in various areas of application, whether for industrial

systems, hi-fi equipment, car lights or advertising. LED technical development continues to stride

ahead. In the course of recent years, the white LEDs' luminous efficacy has increased to a startling

130 lumens per watt and more. This is a trend that will continue into the future. In addition, the

physical effect of electroluminescence was discovered more than 100 years ago.

Fig 1.1 Fundamental of Electroluminescence

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Table 1.1 A Short Glance Back Over the History of The LED

1907 The Englishman Henry Joseph Round discovers that inorganic materials

can light up when an electric current is applied. In the same year, he

publishes his discovery in the journal "Electrical World". Since, however,

he was working mainly on a new direction-finding system for marine

transport, this discovery initially is forgotten.

1921 The Russian physicist Oleg Lossew again observes the "Round effect" of

light emission. In the succeeding years, from 1927 to 1942, he examined

and described this phenomenon in greater detail.

1935 The French physicist Georges Destriau discovers light emission in zinc

sulfide. In honor of the Russian physicist, he calls the effect "Lossew

light". Today Georges Destriau is credited as the inventor of

electroluminescence.

1951 The development of a transistor marks a scientific step forward in

semiconductor physics. It is now possible to explain light emission.

1962 The first red luminescence diode (type GaAsP), developed by American

Nick Holon yak, enters the market. This first LED in the visible

wavelength area marks the birth of the industrially-produced LED.

1971 As a result of the development of new semiconductor materials, LEDs are

produced in new colors: green, orange and yellow. The LED's performance

and effectiveness continues to improve.

1993 Japanese Shuji Nakamura develops the first brilliant blue LED and a very

efficient LED in the green spectrum range (InGaN diode). Sometime later

he also designs a white LED.

1995 The first LED with white light from luminescence conversion is presented

and is launched on the market two years later.

2006 The first light-emitting diodes with 100 lumens per watt are produced. This

efficiency can be outmatched only by gas discharge lamps.

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2010 LEDs of a certain color with a gigantic luminous efficacy of 250 lumens

per watt are already being developed under laboratory conditions. Progress

continues to surge ahead. Today, further development towards OLED is

seen as the technology of the future.

1.2 BASIC PRINCIPLE OF A LED (LIGHT EMITTING DIODE)

A light emitting diode consists of multiple layers of semi-conducting material. When the diode

is being used with direct current, light is produced in the active layer. The light produced is

decoupled directly or by reflections. In contrast to incandescent reflector lamps, which emit a

continuous spectrum, an LED emits light in a particular color. The light's color depends on the

semiconductor material used. Two material systems are mainly used, in order to produce LEDs

with a high degree of brightness in all colors from blue to red and, by means of luminescence

conversion, also in white. Different voltages are necessary, to operate the diode in forward bias.

LEDs are semiconductor crystals. Depending on the composition of the crystal compounds, they

emit light in the colors of red, green, yellow or blue, when current flows through them. With the

help of an additional yellowish fluorescent layer, blue LEDs also produce white light

(luminescence conversion). Another method of producing white light consists in mixing red, green

and blue light-emitting diodes (RGB). This is used particularly where the priority is not general

white lighting but rather decorative effects with various rich colors. With the three RGB colors,

any number of color tones may be mixed by varying the proportions of the individual colors. In

this way, the LED lighting can create fascinating worlds of experience.

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Fig 1.2 Cross Section of LED

1.2.1 Advantages of LED technology

LEDs offer numerous advantages over other lighting technologies. Professional users and home

consumers profit to the same extent from limitless design possibilities based on a variety of color,

compact dimensions and flexibility of the LED modules. On the basis of the low energy

consumption, long lifetime and greater service intervals, high economic benefits are produced. In

addition, individual LEDs provide maximum reliability even in difficult environmental conditions.

1. Low power consumption

2. High efficiency level

3. Long lifetime

4. Continuous dimming combined with an ECG

5. Smallest possible dimensions

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6. High resistance to switching cycles

7. Immediate light at switching on

8. Wide operating temperature range

9. High impact and vibration resistance

10. No UV or IR radiation

11. High color saturation level without filtering

12. Mercury-free

Fig 1.3 Comparison of LED with Other Light Sources

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1.2.2 LED Disadvantages

High price:

LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most

conventional lighting technologies. The additional expense partially stems from the relatively low

lumen output and the drive circuitry and power supplies needed. However, when considering the

total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent

or halogen sources and begin to threaten compact fluorescent lamps. Lumina technology uses the

latest high brightness LEDs in an extremely efficient way meaning that less LED are needed and

hence making lights competitive with traditional sources.

Temperature dependence:

LED performance largely depends on the ambient temperature of the operating environment.

Over-driving the LED in high ambient temperatures may result in overheating of the LED package,

eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This

is especially important when considering automotive, medical and military applications, where the

device must operate over a large range of temperatures, and is required to have a low failure rate.

The efficient use of high brightness LEDs means less heat buildup, coupled with efficient thermal

management ensuring optimum performance.

Voltage sensitivity:

LEDs must be supplied with the voltage above the threshold and a current below the rating. This

can involve series resistors or current-regulated power supplies. Lumina never run their products

at more than 350 mA to ensure optimum balance of power consumption, operating temperature

and life expectancy.

Light quality:

Most cool-white LEDs have spectra that differ significantly from a black body radiator like the

sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects

to be perceived differently under cool-white LED illumination than sunlight or incandescent

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sources, due to metamerism; red surfaces being rendered particularly badly by typical phosphor

based cool-white LEDs. However, the color-rendering properties of common fluorescent lamps

are often inferior to what is now available in state-of-art white LEDs. Lumina uses the latest LEDs

and mixes light within the fitting before it comes out of the fitting, this enables us to produce all

variations of color temperature and color rendering.

Area light source:

LEDs do not approximate a “point source” of light, but rather a Lambertian distribution. So LEDs

are difficult to use in applications requiring a spherical light field. LEDs are not capable of

providing divergence below a few degrees. This is contrasted with lasers, which can produce

beams with divergences of 0.2 degrees or less. Lumina have found the solution to this issue and

can produce spherical light fields and can adjust beam and spread angles to suit required

application.

Blue hazard:

There is increasing concern that blue LEDs and cool-white LEDs are now capable of exceeding

safe limits of the so-called blue-light hazard, as defined in eye safety specifications such as

ANSI/IESNA RP-27.1-05: Recommended Practice for Photo biological Safety for Lamp and

Lamp Systems. Lumina’s technology uses LED light indirectly; this avoids contact with the eye

and therefore provides a safe and efficient light output.

1.3 PROJECT BACKGROUND

High power light-emitting diodes (LEDs) can use 350 milliwatts or more in a single LED. Most

of the electricity in an LED becomes heat rather than light (about 70% heat and 30% light). If this

heat is not removed, the LEDs run at high temperatures, which not only lower their efficiency, but

also make the LED less reliable. Thus, thermal management of high power LEDs is a crucial area

of research and development. It is highly necessary to keep the junction temperature below 120°C

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to run the LED's for maximum lifetime. In order to maintain a low junction temperature to keep

good performance of an LED, every method of removing heat from LEDs should be

considered. Conduction, convection, and radiation are the three means of heat transfer. Typically,

LEDs are encapsulated in a transparent resin, which is a poor thermal conductor. Nearly all heat

produced is conducted through the back side of the chip. Heat is generated from the PN junction by

electrical energy that was not converted to useful light, and conducted to outside ambience through

a long path, from junction to solder point, solder point to board, and board to the heat sink and

then to the atmosphere. A typical LED side view and its thermal model are shown in the figures.

The junction temperature will be lower if the thermal impedance is smaller and likewise, with a

lower ambient temperature. To maximize the useful ambient temperature range for a

given power dissipation, the total thermal resistance from junction to ambient must be minimized.

This is the basis for our project which is to maximize the thermal efficiency of LED and optimize

the output of light from the LED.

1.4 BLOCK DIAGRAM

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CHAPTER 2

LED THERMAL MODEL

2.1 IMPORTANCE OF THERMAL MANAGEMENT

A main cause of LED failures is improper thermal management. Many performance

characteristics of LED components are influenced by the operating temperature, so LED system

designers need a basic understanding of thermal design and performance.

All LED product data sheets detail the performance characteristics that are

impacted by temperature some performance characteristics experience a recoverable change, such

as light output, color and voltage, while others, such as lifetime, and can experience a non-

recoverable degradation due to high operating temperatures. However, exceeding the maximum

operating temperature specification, which is typically a 150 °C junction temperature, can cause

permanent and/or catastrophic damage to LEDs, so care must be taken to operate LEDs below this

limit.

2.1.1 Light Output

Elevated junction temperatures cause recoverable light output reduction, which is plotted on each

LED data sheets. As the junction temperature increases, the light output of the LED decreases, but

recovers when the LED cools.

Below in Figure 2.1 is a plot showing the relative flux versus junction temperature

from the Cree Lamp XB-D LED data sheet.2 The XB-D LED, among many other new Cree Lamp

LEDs, is binned at 85°C, so the relative flux data is based on 100% light output at an 85°C junction

temperature.

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Figure 2.1 X Lamp Xb-D Relative Flux Vs. Steady-State Junction Temperature

2.1.2 Voltage

Forward voltage decreases as the junction temperature of an LED increases. This is shown on

each of Cree’s Lamp data sheets as the temperature coefficient of voltage, and varies slightly

depending on the color and package type. This value varies from approximately -1 to -4 mV/°C

per LED. It is important to understand the full operating conditions for an LED system so the

driver can accommodate the potential range of drive voltages over the operating temperature of

the system. An example of this is shown later in this document.

2.1.3 Reliability

The reliability of any LED is a direct function of junction temperature. The higher the junction

temperature, the shorter the lifetime of the LED. Data from an IES LM-80-08 report can be used

to predict the lumen maintenance of an LED under various temperature and drive current operating

environments. Cree has published an LM-80 summary for its Lamp LEDs and full LM-80 data can

be obtained by contacting a sales representative.

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2.2 FUNDAMENTALS OF HEAT TRANSFER

There are three basic modes of heat transfer: conduction, convection and radiation. Each plays a

role in LED performance and final system design and must be understood for proper thermal

management.

2.2.1 Conduction

Conduction is the transfer of heat through a solid material by direct contact. This is the first mode

of heat transfer to get thermal power from the LED junction to the heat sink. Metals are typically

the best conductors of heat. The heat conduction potential of all materials can be expressed as

thermal conductivity, typically abbreviated as ‘k’. Equation 2.1 below shows how to calculate the

quantity of heat transferred via conduction.

Qcond = -k A………………………………………… (2.1)

Where:

Qcond is the amount of heat transferred through conduction (W)

k is the thermal conductivity of the material (W/m-K)

A is the cross sectional area of the material through which the heat flows (m2)

ΔT is the temperature gradient across the material (°C)

Δx is the distance for the heat must travel (m)

2.2.2 Convection

Convection is the transfer of heat through the movement of fluids and gases. In LED systems,

this is typically the transfer of heat from the heat sink to the ambient air. There are two sub-

categories of convection: natural and forced. Natural convection occurs with no artificial source

of field movement and is due to the buoyancy forces induced by thermal gradients between the

fluid and solid. Forced convection occurs when an external instrument such as a fan, pump, or

other device is used to artificially move the fluid or gas. In LED cooling systems, convection is

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the main mode of heat transfer to remove the generated heat from the LED system and heat sink.

Equation 2.2 below shows how to calculate the quantity of heat transferred via convection.

Qconv = h A ΔT……………………………….... (2.2)

Where:

Qconv is the amount of heat transferred through convection (W)

h is the heat transfer coefficient (W/m2K)

A is the surface area (m2)

ΔT is the temperature gradient across the material (°C), typically the difference between the

surface Temperature and ambient air temperature

The fundamental challenge in calculating the heat transfer via conduction is

determining and computing the heat transfer coefficient (h). Typical values for h can vary

significantly depending on boundary conditions, geometry and many other factors. However, for

natural convection h will usually be in the range of 5-20 W /m2K, while for forced convection h

can be as high as 100 W /m2K for air and up to 10,000 W/m2K for water. Typically, for natural

convection in air, a value of 10 W/m2K is a good assumption for an initial rough calculation.

2.2.3 Radiation

The transfer of thermal energy through an electromagnetic field is the third component of heat

transfer, radiation. The magnitude of radiation heat transfer is based on the emissivity of the

material, which is the ratio of how closely the surface approximates a blackbody. In an LED

system, radiation typically has a very small effect on the net system heat transfer since the surface

areas are typically fairly small and surface temperatures are relatively low, to keep the LED

junction temperatures below the maximum rated temperature of 150 °C. Equation 2.3 below shows

how to calculate the quantity of heat transferred via radiation.

Qrad = ε σ A (Ts4- Tf

4)…………………………….. (2.3)

Where:

Qrad is the amount of heat transferred through radiation (W)

ε is the emissivity of the surface (dimensionless)

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σ is the Stefan-Boltzmann constant (5.67 x 10-8 W/m2K4)

A is the surface area (m2)

Ts is the surface temperature of the material (°C)

Tf is the fluid temperature of the medium (°C), typically referenced to the ambient air

temperature

2.3 THERMAL PATH

Thermal path of an LED system can be illustrated by a simple resistor network similar to an

electrical circuit. Thermal resistances are represented by the resistors, the heat flow is

approximated by the electrical current, and the corresponding temperatures within the system

correspond to the electrical voltages. Below in Figure 2.2 is a resistor network representation of a

multiple-LED system on a printed circuit board (PCB) mounted to a heat sink in ambient air.

Fig 2.2 Thermal Circuit Of An Led Array

Where:

T is the temperature at each corresponding location (°C)

Θa-b is the thermal resistance from point a to point b (°C/W)

n is the number of LED components on a single PCB

To summarize the heat path illustrated in the Figure 2.2 above, heat is conducted

from the LED junctions through the LED components to the PCB, through the thermal interface

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material (TIM) to the heat sink and then convected and radiated to the ambient air. The nodes in

the circuit represent the individual sections within the system and the locations where temperatures

may be measured. For example, the solder point temperature (solder point) represents the location

on the board, as specified in the corresponding data sheet for each Cree X Lamp LED, where the

temperature on the top of the PCB can be measured. This can be used to calculate the junction

temperature, which is detailed in a later section. The resistors represent the thermal resistances of

the individual contributors.

For example, ΘJ-SP represents the thermal resistance of the LED component from

junction to solder point. The system is divided into a network of parallel connections for the

multiple LEDs and series connections for the singular components. If the system includes only one

LED, or n =1 in Figure 2.2, the entire thermal path is simply in series. The individual thermal

resistances described above can be calculated from Equation 2.4 below.

Θa-b = (Ta-Tb)*Pt ……………………………….… (2.4)

Where:

Θa-b is the thermal resistance from point “a” to point “b” (°C/W)

Ta is the temperature at point “a” (°C)

Tb is the temperature at point “b” (°C)

Pt is the thermal power.

The thermal resistance of the entire system can also be compared to an electrical

circuit in series, where the system thermal resistance can be calculated as shown below in Equation

2.5.

Θsys,a-z= Θa-b + Θb-c + … + Θy-z ……………………..…. (2.5)

Where:

Θsys,a-z is the system thermal resistance from point “a” to point “z” (°C/W)

Θa-b is the thermal resistance from point “a” to point “b” (°C/W)

Θb-c is the thermal resistance from point “b” to point “c” (°C/W)

Θy-z is the thermal resistance from point “y” to point “z” (°C/W)

In an LED system, the total system-level thermal resistance is typically defined as

“junction to ambient”, or ΘJ-A. This quantifies how well each component transfers thermal power.

Each Cree X Lamp LED has a current de-rating curve in its data sheet that gives the maximum

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drive current versus ambient temperature for a few system thermal resistances, or ΘJ-A. Shown

below in Figure 2.3 is the current de-rating curve for an X Lamp XB-D LED from the XB-D data

sheet 6.

Fig 2.3 Current De-Rating Curve

To use the current de-rating curve in Figure 2.3, the system-level thermal resistance

must be calculated, as detailed above in Equation 2.5. The ambient temperature must also be

known, and from this, the maximum drive current for this thermal design can be determined from

the graph in Figure 2.3.

2.4 THERMAL STACK

For purposes of thermal analysis, an LED system typically consists of a multi-component

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assembly, called a thermal stack, in which all components contribute in varying degrees to the total

system thermal performance. In a typical system, the LED is soldered to a PCB, either metal core

or FR4, which is then usually attached to a heat sink. It is critical to maximize heat transfer between

the heat sink and PCB, so a good TIM is needed to fill any air voids. The best method to enhance

the thermal path is to minimize the number of materials in the thermal stack and use the most

thermally conductive materials available. Below is a description of this typical LED system with

some suggestions and comments to consider for each element.

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CHAPTER 3

LED COMPONENTS

Heat is generated at the junction of the LED chip within the X Lamp component. The amount of

heat can be calculated from Equation 3.1 below based on the measured TSP and the thermal

resistance of the LED, as stated on its data sheet.

TJ = TSP + Θth*Ptotal …………………………………. (3.1)

Where:

TJ is the junction temperature (°C)

TSP is the measured solder point temperature (°C)

Θth is the thermal resistance of the component (°C/W)

Ptotal is the total power (W) input to the LED.

All Cree X Lamp LEDs must not exceed their maximum junction temperature of

150 °C, as specified on each data sheet.

3.1 PRINTED CIRCUIT BOARD

Most Cree’s X Lamp LEDs are required to be mounted on a PCB to provide electrical and

mechanical connections to additional components such as the driver and heat sink. The thermal

effect of the PCB can be significant, so care must be used when choosing or designing a PCB.

Cree has published a technical article, “Optimizing PCB Thermal Performance” that details PCB

design recommendations and offers tips.

3.2 THERMAL INTERFACE MATERIAL

The thermal interface material can play a large role in the system thermal performance, depending

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on the design choices made. TIMs are critical to minimize the air gaps between the heat sink and

the PCB. TIMs not only provide a thermal interface between the PCB and the heat sink, but

depending on the application these can have other functions as well, such as electrical insulation

or making a mechanical connection. Many types of TIMs are used in LED systems including

greases, tapes, pads, and epoxies. Each has its advantages and disadvantages depending on the

application. Table 3.1 below shows some of the benefits and drawbacks of various materials.

Table 3.1 Properties of Various Thermal Interface Materials

Many characteristics must be considered when selecting a TIM, not just the thermal conductivity.

Often overlooked is the bond line thickness of the material, and as shown below in Equation 3.2,

the thermal resistance of the material is highly dependent on this thickness. The TIM manufacturer

will provide these basic characteristics on their own data sheet, and it is important to understand

how all the characteristics work together and to decide which is the most important for each

specific application.

Sometimes a thinner TIM with poor thermal conductivity has a lower thermal

resistance than a thicker TIM with better thermal conductivity. Both these attributes must be

considered when selecting a TIM and their relative effects can be quantified with Equation 3.2

below. However, though a TIM may have better thermal conductivity than air, its conductivity will

not be nearly as good as metal’s, so the approach is not to add material between metal components

but rather to fill the voids that are typically occupied by air.

ΘTIM =L/KA …………………..……………………… (3.2)

Where:

ΘTIM is the thermal resistance of the TIM

L is the thickness of the TIM (m)

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k is the thermal conductivity of the TIM (W/m-K)

A is the area in (m2)

3.3 HEAT SINK

The heat sink is the last and most influential part of the thermal stack, and is needed to first

conduct heat away from the LEDs and then to convect and radiate heat to the ambient air. Thus,

the first task of the heat sink necessitates that the heat sink be fabricated from a high thermal

conductivity material to conduct heat away. The second task requires that the heat sink have a

large surface area to convect heat to ambient and also have good emissivity so it can radiate heat

away. In some cases, heat sinks are coupled to other heat dissipating devices such as housings,

enclosures, etc. For this document, we group these devices under the general term “heat sink”, but

this should not be overlooked in system design as it can contribute significantly to the performance

of the entire LED system. Table 3.2 below shows the thermal conductivity of some common

materials as well as a rough range of their emissivity, which can vary significantly depending on

the material finish. Choosing the highest thermal conductivity and/or emissivity is obviously not

always possible because of other factors that must be considered such as weight, cost, and

manufacturability. Each of these must be evaluated for each application to determine the best

material and manufacturing process. Cast or forged aluminum is typically used for heat sinks.

Anodizing aluminum gives a heat sink a much higher (to about 0.8) emissivity than standard

aluminum and helps with radiative heat transfer to the environment.

Heat sink design can be very complicated and limited by many restrictions such as

space constraints, cost, weight, manufacturability and countless other requirements. There is no

one right answer to heat sink design and each application must be approached on a case-by-case

basis, but the following general guidelines can help in the design process.

1. Maximize the surface area of the heat sink to maximize its ability to convect heat away

from the LED source.

2. A rough estimate of approximately 5-10 in2 of heat sink surface area per watt of heat can

be used for a first-order estimate of heat sink size.

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3. Do not restrict the airflow between the fins. Understand the orientation of the heat sink in

the application in which it will be used and maximize the airflow through the heat sink and

around as much surface area as possible.

4. Choose a material that has good thermal conductivity to spread heat away from the LEDs.

5. Use high surface emissivity heat sinks to maximize thermal radiation heat transfer.

Anodizing dramatically increases the emissivity of an aluminum heat sink.

6. Passive (natural convection) heat sinks are always preferred for many reasons, but if

appropriate, actively cooled heat sinks can significantly improve performance.

7. Use of thermal modeling can alleviate repetitive prototyping and indicate design

deficiencies and potential areas of improvement early in the design process.

A key aspect of heat sink design to account for is the manufacturing method that

will be used. These methods vary significantly and can produce extremely different heat sinks,

which can serve different applications and their specific needs. Some aspects of the more common

processes are compared below in Table 3.2.

Table 3.2 Thermal Conductivity of Various Material

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3.4 FANS / ACTIVE COOLING

When the thermal load of an LED system is too high to be properly dissipated by passive means,

active cooling may be the only solution. There are many types of actively cooled systems, from

fans to liquid cooling to heat pipes to other exotic methods. The effectiveness, reliability, noise,

cost, added power (and thus lower system efficiency) and maintenance of these devices need to be

weighed against the benefits of an actively cooled system. Very few active cooling devices can

equal the long LED lifetimes of many thousands to hundreds of thousands of hours, so care must

be taken to not compromise system lifetime with inept active cooling solutions. An LED system

is only as good as its weakest link, and active cooling can be this link without careful selection.

3.5 THERMAL MEASUREMENT

Accurate temperature measurements are required to appropriately design a thermal system and

to evaluate and assess an existing design. Whether for a final design or a prototype, the

measurement process is the same and requires due diligence to make sure realistic and accurate

measurements are made. LED reliability is a major advantage compared to traditional light

sources, so proper and realistic measurement procedures should be used so this benefit is not

jeopardized. When performing thermal measurements, it is critical to set up the test subject as close

as possible to the real-life, worst-case scenario to which the system may be subjected. Ensure that

the measurement setup accounts for similar airflow, material properties, orientation, ambient

conditions and any additional heat sources such as power supplies or contributory heat loads. This

ensures that the temperatures measured correspond to real-world, worst-case scenarios and could

identify potential problems that best-case scenarios may miss.

Another factor to note is the time required for the system to thermally stabilize.

Depending on the size of the heat load and the mass and effectiveness of the heat sink, some

systems take only a few minutes to stabilize while others take hours. It is best to monitor the

thermal stability and wait one hour at the very least for each thermal measurement. It is also

recommended to monitor the ambient temperature and look exclusively at the difference between

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the measurement point and ambient temperature, as any change in ambient will be reflected in the

measured data. Various methods to measure temperature exist, and for LED systems the most

common methods are simple thermocouples, infrared (IR) microscopes, and pulsed

voltage/transient response monitoring. The latter two methods require expensive, accurate and

specific tools that are beyond the scope of this document. Simple thermocouples are the most

common and simplest method to obtain accurate data and are recommended for precise absolute

LED system measurements.

3.5.1 Thermocouples

The Soldering and Handling application note Cree publishes for each X Lamp LED details the

location and process for attaching a thermocouple. Below in Figure 8 is an image from the X Lamp

XB-D Soldering and Handling document showing the proper location for a thermocouple 10. The

general guideline for thermocouple attachment is to place the thermocouple as close to the LED

as possible, mounted directly on a metal pad connected to the neutral thermal trace, if possible.

Thermally conductive epoxy or solder is recommended to ensure good heat transfer from the board

to the thermocouple. All thermocouples must be out of the optical light path or photons will

interfere with the readings, giving extremely inaccurate results.

Figure 3.1 Thermocouple

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3.5.2 Infrared camera

IR camera measurements can be useful to get a quick visual representation of the heat spreading

through an LED system to see any potential hot spots and for other relative comparisons. However,

using an IR camera for absolute temperature measurements can be very complex and lead to

inaccurate results. Knowing the exact emissivity of the material is crucial for accurate results, and

this often is not precisely known. One way to get this is to take a measurement with a thermocouple

and then adjust the emissivity setting on the IR camera to match these results.

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CHAPTER 4

OPTIMIZING PCB THERMAL PERFORMANCE

4.1 INTRODUCTION

One of the most critical design parameters for an LED illumination system is the system’s ability

to draw heat away from the LED junction. High operating temperatures at the LED junction

adversely affect the performance of LEDs, resulting in decreased light output and lifetime. To

properly manage this heat, specific practices should be followed in the design, assembly and

operation of LEDs in lighting applications. This application note outlines a technique for designing

a low-cost printed circuit board (PCB) layout that optimizes the transfer of heat from the LED.

The technique involves the use of FR-4-based PCBs, which cost less than metal core printed circuit

boards (MCPCB), but have greater thermal resistance. The use of metal-lined holes or vias

underneath LED thermal pads is a method to dissipate heat through an FR-4 PCB and into an

appropriate heat sink.

4.2 THERMAL CHARACTERSTICS

With increasing installed lumen package, heat dissipation from LEDs starts to be challenge.

Therefore, the thermal design of luminaires is essential for proper design of LED-based luminaires.

Thermal simulation based on finite element method is a widely used software tool in early design

stages. At present, LED packages have an electrically isolated thermal pad. The pad provides an

effective channel for heat transfer and optimizes thermal resistance from the LED chip junction to

the thermal pad. The pad is electrically isolated from the anode and cathode of the LED and can

be soldered or attached directly to grounded elements on the board or heat sink system.

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Fig 4.1 LED Package

4.2.1 PCB Thermal Characteristics

FR-4

FR-4 is one of the most commonly used PCB materials and is the National Electrical

Manufacturers Association (NEMA) designation for a flame retardant, fiberglass-reinforced epoxy

laminate. A by-product of this construction is that FR-4 has very low thermal conductivity. Figure

4.2 below shows a typical cross-sectional geometry for a two-layer FR-4 board.

Fig 4.2 FR-4 Cross Sectional Geometry

Using the thermal conductivity values in Table 2 below, the total thermal resistance for an FR-4

board can be calculated by adding the thermal resistances of the layers.

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θPCB = θlayer1 + θlayer2 + θlayer3 ... + θlayerN ………………………… (4.1)

For a given layer the thermal resistance is given by the formula:

θ = l / (k x A) ……………………………………… (4.2)

Where

l is the layer thickness

k is the thermal conductivity

A is the area normal to the heat source.

For a 1.6-mm thick star board approximately 270 mm2, the calculated through-

plane thermal resistance is approximately 30 ºC/W. Bear in mind that this calculation is one-

dimensional and does not account for the size of the heat source, spreading, convection thermal

resistances or boundary conditions. If a smaller heat source size is considered.

For example 3.3 mm x 3.3 mm, the resulting one-dimensional thermal resistance

increases to over 700 ºC/W.

Table 4.1 Typical Thermal Conductivities of FR4 Thermal Layer

4.3 DESIGNING THERMAL VIAS

An inexpensive way to improve thermal transfer for FR-4 PCBs is to add thermal vias - plated

27

through-holes (PTH) between conductive layers. Vias are created by drilling holes and copper

plating them, in the same way that a PTH is used for electrical inter connection between layers.

Fig 4.3 FR4 Cross Sectional Geometry with Thermal Vias

Fig 4.4 Cross-Sectional Geometry of Small And Large Thermal Vias In Fr-4 Substrate

Adding vias in an appropriate way will improve the thermal resistance of an FR-4

board. The thermal resistance of a single via can be calculated by the same formula, θ = l / (k x A).

28

Using the values in Table 4.2, a single solder-filled via with a diameter of 0.6 mm results in (1.588

x 10-3) / (58 x (� x (0.5 x 0.6 x 10-3)2)) = 96.8 ºC/W.

However, when N vias are used, the area increases by a factor of Nvias, resulting

in:

θvias = l / (Nvias x k x A) ……………….…………………. (4.4)

Note that this is applicable only if the heat source is directly normal to the thermal via otherwise,

the resistance increases due to thermal spreading effects. To calculate the total thermal resistance

for the region underneath (or normal to) the LED thermal pad, the equivalent thermal resistance

for the dielectric layer and vias must be determined. For simplicity, the two resistances are treated

as parallel applying this formula.

Using the values in Table 4.2, for a 270 mm2 board with five 0.6-mm-diameter

solder-filled vias results in an approximate thermal resistance of 12 ºC/W, a 60% improvement

over the initial 30ºC/W derived from the data in Table 4.2

Table 4.2 Typical Thermal Conductivities of FR-4 Board Layers Including Thermal Vias

4.3.1 Open vias vs. Filled vias

Open vias result in a higher thermal resistance than filled vias because the area normal to the heat

29

source is reduced as per the formula:

A = (D x t – t2) …………………………………….…. (4.5)

Where

D is the via diameter

t is the plating thickness.

For a 0.6-mm diameter via with 35μm copper plating, the area normal to the

thermal pad is only 0.06 mm2 compared to 0.28 mm2 for a solder-filled via resulting in a thermal

resistance of 64ºC/W per via compared to 42 ºC/W if filled with solder or compared to 96.8 ºC/W

for the same size.

In general, increasing plating thickness during PCB production improves the

thermal resistance of vias Non-filled vias may become filled with solder during reflow.

4.3.2 Solder voiding in open PTH vias

Figure 4.5.1 shows an example of unfilled vias after reflow. Figure 4.5.2 shows an example of

solder voids underneath the device (shown in red). The voids increase the thermal resistance of the

thermal interface. Also, the solder may overfill the hole leading to bumps on the bottom of the

board that can reduce the contact area between the board and the heat sink. Steps can be taken to

limit the amount of solder wicking. One way is to maintain a vias diameter smaller than 0.3 mm.

With smaller vias, the surface tension of the liquid solder inside the vias is better

able to counter the force of gravity on the solder. If the via structure is constructed following the

guideline above, holding the inside via diameter to around 0.25 mm – 0.3 mm, minimal solder

wicking is achieved. The drawback to this approach is that smaller open vias result in a higher

overall thermal resistance

30

Fig 4.5.1 Unfilled Vias

Fig 4.5.2 Solder Voiding

Another technique for limiting solder wicking involves using solder mask to restrict

the flow of solder from the top side of the PCB to the bottom side. One process, called tenting,

uses solder mask to prevent solder from either entering or exiting the thermal vias, depending on

the side of the board on which the solder mask is placed. Tenting the bottom side with solder mask

to cover and plug the thermal vias can prevent solder from flowing down into the vias and onto

31

the bottom of the board. In top side via tenting, small areas of solder masks are placed over the

thermal vias on the top side of the PCB to prevent solder from flowing into the vias from the top

side

Fig 4.6 Tented Vias

4.4 THERMAL PERFORMANCE SIMULATIONS

This section presents results obtained from computational thermal analysis for a series of PCB

configurations.

4.4.1 Surface Thermal Dissipation

The first configuration, shown in Figure 4.7, consists of a star FR-4 PCB with varying widths of

the thermal pad and two board thicknesses (0.8 mm and 1.6 mm); the bottom copper layer is solid

and there are no thermal vias

Fig 4.7 Variation in Thermal Pad Width On Top Side Of PCB

3.3 mm Wide 6.0 mm Wide 12.0 mm Wide 20 mm Wide

32

The analysis results in Chart 4.1 show that, for the 1.6-mm thick board, increasing the thermal pad

width beyond 12 mm provides little improvement and, for the 0.8-mm thick board, improvement

diminishes beyond a 16-mm width.

Chart 4.1 Thermal Resistance for Fr-4 PCB With No Vias With Varying Thermal Pad Size

4.4.2 Thermal Dissipation with Vias

Chart 4.2 shows the effects of various filling materials for 0.7-mm diameter vias with 1-mm

center-to-center spacing for both 1.6-mm and 0.8-mm board thicknesses, shown in Figure 10. The

analysis data indicate solid copper filled vias result in lower thermal resistance and unfilled vias

deliver higher thermal resistance. A vias filled with conductive epoxy performs only slightly better

than an unfilled via.

33

Fig 4.8.1 FR-4 Board with Five 0.7-mm Diameter Vias and 1-mm Pitch

Fig 4.8.2 FR-4 Board with Fifteen 0.7-mm Diameter Vias and 1-mm Pitch

Chart 4.2 Thermal Resistance for Fr-4 Vias Filled With Materials Of Different Conductivity

34

Chart 4.3 shows the effect of changing the diameter and number of vias. This chart assumes the

vias are filled with SnAgCu solder. As expected, the larger the diameter of the via, the lower the

thermal resistance becomes. Increasing the number of vias shows considerable improvement for

smaller via diameters.

Chart 4.3 Fr-4 PCB With Various Via Diameters And Numbers Of Vias

35

CHAPTER 5

ELECTRICAL OPTIMISATION

5.1 LED DRIVER

A LED driver is an electrical device that regulates the power to an LED or string(s) of LEDs.

What makes a driver different from conventional power supplies is that an LED driver responds

to the ever-changing needs of the LED, or circuit of LEDs, by supplying a constant amount of

power to the LED, as its electrical properties change with temperature.

Think of an LED driver as ‘Cruise Control’ (like in a car) for the LED, and the

temperature changes of the LED are the hills and valleys it is ‘driving’ over. The power level (or

‘Speed’) of the LED is maintained constant by the driver as the electrical properties change

(amount of ‘gas’ or power needed) throughout the temperature increases and decreases (or ‘hills

and valleys’) seen by the LED(s). Without the proper driver, the LED may become too hot (driving

too fast) and become unstable (out of control), causing poor performance (engine problems) or

complete failure (crash!)

Fig 5.1 Basic LED Driver

36

5.1.1 Type of LED driver

LED drivers may be constant voltage types (usually 10V, 12V and 24V) or constant current types

(350mA, 700mA and 1A). Some drivers are manufactured to operate specific LED devices or

arrays, while others can operate most commonly available LEDs. LED drivers are usually compact

enough to fit inside a junction box, include isolated Class 2 output for safe handling of the load,

operate at high system efficiency, and offer remote operation of the power supply.

5.1.2 PWM LED Driver

Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a modulation

technique that confirms the width of the pulse, formally the pulse duration, based on modulator

signal information. Although this modulation technique can be used to encode information for

transmission, its main use is to allow the control of the power supplied to electrical devices,

especially to inertial loads such as motors. In addition, PWM is one of the two principal algorithms

used in photovoltaic solar battery chargers, the other being MPPT. The average value of voltage

(and current) fed to the load is controlled by turning the switch between supply and load on and

off at a fast pace. The longer the switch is on compared to the off periods, the higher the power

supplied to the load is. The PWM switching frequency has to be much faster than what would

affect the load, which is to say the device that uses the power. Typically switching have to be done

several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to

tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and

computer power supplies. The term duty cycle describes the proportion of 'on' time to the regular

interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off

for most of the time. Duty cycle is expressed in percent, 100% being fully on.

The main advantage of PWM is that power loss in the switching devices is very

low. When a switch is off there is practically no current, and when it is on, there is almost no

voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both

37

cases close to zero. PWM also works well with digital controls, which, because of their on/off

nature, can easily set the needed duty cycle. PWM has also been used in certain communication

systems where its duty cycle has been used to convey information over a communications channel.

A key requirement for a “smart” LED driver is adjusting the LED brightness with dimming

controls, using one of two approaches: PWM and analog. PWM dimming controls the LED current

by adjusting the pulse duty cycle. If the frequency is above about 120 Hz, the human eye averages

these pulses to produce a perceived average luminosity. Analog dimming scales the LED current

at a constant (dc) value. PWM dimming could be implemented by opening and closing an NMOS

switch inserted in series with RSENSE. These current levels would require a power device, but

adding one of these would defeat the size and cost benefits obtained by using a buck regulator

containing its own power switches. Alternatively, PWM dimming can be performed by quickly

turning the regulator on and off. At low PWM frequencies (<1 kHz), this can still give great

accuracy. PWM dimming is very popular, but sometimes noiseless “analog” dimming is required.

Analog dimming simply scales the constant LED current, whereas PWM dimming chops it.

Analog dimming is required if two dimming inputs are used, since multiple PWM dimming signals

can create beat frequencies that cause flicker or audible noise. However, PWM might be used for

one dimming control and analog for another.

5.1.3 PWM v/s Analog dimming

With the phenomenal growth of the LED lighting market, there has been a natural growth in

demand for highly efficient and controlled LED drivers. Applications from ‘smart’ street lights,

flashlights, digital signage and many others require not only highly regulated currents, but in many

cases dimming capability in order to sustain the energy efficient scheme and end use flexibility

behind LED design. As there are several ways to achieve dimming of an LED, we describe here

the main methods that are used to provide dimming for LED’s from a switch mode LED driver.

38

5.1.3.1 PWM Dimming

PWM or Pulse Width Modulation dimming is actually turning on and off the LED current for

short periods of time. The on/off frequency has to be faster than what the eye can perceive so as

not to cause a flickering effect (typically over 100Hz). PWM dimming can be achieved a number

of ways:

1. Driving Vadj (adjust voltage) directly by a PWM signal.

2. Via open collector transistor.

3. By a microcontroller as detailed in our product datasheets.

The average current to the LED is the product of the total nominal current and the duty cycle of

the dimming function. A designer must also take into consideration the delays in shutdown and

start-up of the converter’s output which leads to limitations on the PWM dimming frequency and

range of duty cycles.

5.1.3.2 Analog Dimming

Analog dimming of the LEDs is the adjustment of the LED current level. This can be by resistive

dimming or external DC control voltage. Since there is current level adjustment in analog

dimming, inherently there is a disadvantage where color temperature variation can occur. The use

of analog dimming is not recommended in applications where color of the LED is critical.

Table 5.1 Comparison between PWM Dimming and Analog Dimming

PWM DIMMING ANALOG DIMMING

Brightness adjusted by modulating the peak

current in the driver

Brightness adjusted by changing the DC

current going to the LED

No color shift Possible color shift as LED current changes

Possible current inrush problems No inrush current to device

Frequency limitation & possible frequency

concerns

No frequency concerns

39

Very linear change in brightness Brightness linearity not as good

Lower optical to electrical efficiency Higher optical to electrical efficiency

5.2 AUTO DIMMING OF LED

Auto dimming of LED's refer to automatic intensity adjustment of LED's according to the outside

light intensity which may be from natural light sources such as sun or due to the presence of other

lightning sources such as street lamp or focused lightning sources. This kind of adaptation would

definitely help to reduce the extra power consumption of led lights and it will also help to reduce

the power cost as well as the heat that is generated due to constant illumination of led lights.

According to an estimation of world energy forum their research have shown that by reducing the

unnecessary glowing of LED and other such lightning resources that use or employ LED

technology by adopting automatic dimming technology we can save at least 25% of power that

gets wasted due to unnecessary glowing of LED even when there is a good amount of natural light

available.

40

CHAPTER 6

WORKING OF THE PROJECT

6.1 FLOW DIAGRAM

NO

YES NO

YES

START

Press the switch to select

the mode

If auto

mode

LEDs will glow

LDR Works

If LED

Temp>35

PRESET works

LEDs will not

glow

If Manual

Mode

LEDs will glow

FAN will be on.

41

Fig 6.1 Model of the Project

6.2 WORKING OF THE CIRCUIT

1. Bridge rectifier converts the ac voltage to dc voltage.

2. A polarized capacitor is used to filter out the remaining ac components.

3. The regulator converts 12V to 5V which is operating voltage of the microcontroller.

4. LDR acts for the automatic adjusting of the light intensity by sensing the light of the

environment.

5. A preset or variable resistor is used to control the light intensity of the lamp.

6. L293D is a led driver which provides constant current to LED’S.

42

7. DS18B20 is a temperature sensor is used to monitor the ambient temperature of the LED, if

the temperature is greater than 35oC the cooling mechanism associated is turned ON.

43

CHAPTER 7

RESULT AND CONCLUSION

7.1 RESULT

Undergoing the process of making this project, we observe the following things and problems

which are discussed as follows:

As our project name is “Optimization of LED Lighting System”, thus we wanted to fabricate it

properly. But the research for this, we found that, it would be very costly to implement this and

also because of the resources available to us, we were only led to present our study in this report.

We have used 5W LED for greater illumination. Due to the complexity of driver circuit, instead

of deigning we have chosen to use the driver IC.

The following result has been observed:

This system may work in two modes: either automatic or manual mode depending on the

application.

In Manual mode:

The LEDs can illuminate the light intensity from minimum 229 lux to maximum 2023 lux.

In automatic mode:

LDR will work and decrease the light intensity from minimum 86 lux to maximum 2486 lux.

In both the modes, if LED temperature goes beyond 35oC; then FAN will be on and it

will decrease the temperature of LED die and hence improves the lifetime of LED system.

7.2 CONCLUSION

The design of LED lighting system has various advantages over existing incandescent lamp. It

does not produce toxic substance, has higher life span, consume less power, can operate manually

as well as automatically, and conserve energy.

44

A 5W LED produce approximate 695 lux. Whereas a 60W incandescent lamp produce 17

lumens/watt. It can be further improved by interfacing more driver IC’s for more wattage, and high

power LED’s to increase the illumination.

7.3 APPLICATIONS

There are various applications of LED lighting system and are listed below:

1. For residential for office and for industrial purpose

2. As ceiling lamp

3. TV backlight

4. Mobile backlight

5. In car for dimming and for headlamps

6. As a fog lamp

7. Traffic lights

8. Low voltage inspection light

45

APPENDIX A

ELECTRICAL COMPONENTS USED

ADAPTERS

The adapters are the device that has inbuilt circuitry for converting the 230V AC in to desired

DC like +5V adapter, +12V adapter, +9V adapter and many more. This consists of inbuilt circuit

for HIGH AC to low voltage DC conversion.

DC Adapter

DIP BASES

The case outlines of the plastic and ceramic Dual In-line Packages (DIPs) are nearly identical.

The lead configuration consists of two rows of leads, both with 100 mil pitch. The plastic DIP is

shown in Figure. If the DIP base is of 18 pin then 9 lines will be in one side and 9 on other side.

The IC bases of have round cut from the left of which the pin 1 of base is considered similar is the

case with integrated chips. Basically IC is sensitive to short circuit or voltage so in place of that

we first install the bases of the IC with same number of pins and before placing the IC’s we check

46

all voltage points of the IC then mount the IC once proper configuration is assured. The DIP base

depends on number of pins of the IC and ranges from 4pin configuration to 40 pin configuration.

They are available in different pin configuration and size depending on IC need.

Dip Bases

SWITCHES

In electrical engineering, a switch is an electrical component that can break an electrical circuit,

interrupting the current or diverting it from one conductor to another. The most familiar form of

switch is a manually operated electromechanical device with one or more sets of electrical

contacts, which are connected to external circuits. Each set of contacts can be in one of two states:

either "closed" meaning the contacts are touching and electricity can flow between them, or

"open", meaning the contacts are separated and the switch is non-conducting. The mechanism

actuating the transition between these two states (open or closed) can be either a "toggle" (flip

switch for continuous "on" or "off") or "momentary" (push-for "on" or push-for "off") type.

A switch may be directly manipulated by a human as a control signal to a system, such as

a computer keyboard button, or to control power flow in a circuit, such as a light switch.

Automatically operated switches can be used to control the motions of machines, for example, to

indicate that a garage door has reached its full open position or that a machine tool is in a position

to accept another work piece. Switches may be operated by process variables such as pressure,

temperature, flow, current, voltage, and force, acting as sensors in a process and used to

automatically control a system. For example, a thermostat is a temperature-operated switch used

47

to control a heating process. A switch that is operated by another electrical circuit is called a relay.

Large switches may be remotely operated by a motor drive mechanism. Some switches are used

to isolate electric power from a system, providing a visible point of isolation that can be padlocked

if necessary to prevent accidental operation of a machine during maintenance, or to prevent electric

shock. An ideal switch would have no voltage drop when closed, and would have no limits on

voltage or current rating. It would have zero rise time and fall time during state changes, and would

change state without "bouncing" between on and off positions. Practical switches fall short of this

ideal; they have resistance, limits on the current and voltage they can handle, finite switching time,

etc. The ideal switch is often used in circuit analysis as it greatly simplifies the system of equations

to be solved, however this can lead to a less accurate solution. Theoretical treatment of the effects

of non-ideal properties is required in the design of large networks of switches, as for example used

in telephone exchanges.

There are varying types of switches:

Micro-switch: This is small switch for interconnection. It has 4 terminals with 2 in pair already

connected .when you press the switch all four get connected.

Power switch: This is 6 terminal switches for bidirectional connectivity on press.

Toggle switch: A toggle switch is a class of electrical switches that are manually actuated by a

mechanical lever, handle, or rocking mechanism. This is a two state switch that is not connected,

and connected. It remains in the state till not forced again to change the state.

DPDT switch: A DPDT switch is a class of electrical switches that are manually actuated by a

mechanical rocking mechanism. This is a three state switch that is not connected, connected to one

and connected to second. It remains in the state till forced in that particular state els4e goes to

normal state.

DIP switches: These are combination of multiple small switches in one package to put on/off

multiple channels in circuitry. A DIP switch is a manual electric switch that is packaged with

others in a group in a standard dual in-line package (DIP). The term may refer to each individual

switch, or to the unit as a whole. This type of switch is designed to be used on a printed circuit

board along with other electronic components and is commonly used to customize the behavior of

an electronic device for specific situations.DIP switches are an alternative to jumper blocks. Their

48

main advantages are that they are quicker to change and there are no parts to lose. These are

available in different configuration for example 8 pin configurations, 16 pin configuration and

many more.

DC CONNECTORS

A DC connector (or DC plug, for one common type of connector) is an electrical connector for

supplying direct current (DC) power. Compared to domestic AC power plugs and sockets, DC

connectors have many more standard types that are not interchangeable. The dimensions and

arrangement of DC connectors can be chosen to prevent accidental interconnection of

incompatible sources and loads. Types vary from small coaxial connectors used to power portable

electronic devices from AC adapters, to connectors used for automotive accessories and for battery

packs in portable equipment.

DS18B20 TEMPERATURE SENSOR

The DS18B20 digital thermometer provides 9-bit to 12-bit Celsius temperature measurements

and has an alarm function with nonvolatile user-programmable upper and lower trigger points. The

DS18B20 communicates over a 1-Wire bus that by definition requires only one data line (and

ground) for communication with a central microprocessor. It has an operating temperature range

of -55°C to +125°C and is accurate to ±0.5°C over the range of -10°C to +85°C. In addition, the

DS18B20 can derive power directly from the data line ("parasite power"), eliminating the need for

an external power supply. Each DS18B20 has a unique 64-bit serial code, which allows multiple

DS18B20s to function on the same 1-Wire bus. Thus, it is simple to use one microprocessor to

control many DS18B20s distributed over a large area. Applications that can benefit from this

feature include HVAC environmental controls, temperature monitoring systems inside buildings,

equipment, or machinery, and process monitoring and control systems.

49

Pin Assignment of Temperature Sensor

CRYSTAL OSCILLATORS

A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating

crystal of piezoelectric material to create an electrical signal with a very precise frequency. This

frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable

clock signal for. digital integrated circuits, and to stabilize frequencies for radio transmitters and

receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator

circuits designed around them were called "crystal oscillators". A crystal oscillator is an electronic

circuit that produces electrical oscillations at a particular designed frequency determined by the

physical characteristics of one or more crystals, generally of quartz, positioned in the circuit

feedback loop. A piezoelectric effect causes a crystal such as quartz to vibrate and resonate at a

particular frequency. The quartz crystal naturally oscillates at a particular frequency, its

fundamental frequency that can be hundreds of megahertz. The crystal oscillator is generally used

in various forms such as a frequency generator, a frequency modulator and a frequency converter.

The crystal oscillator utilizes crystal having excellent piezoelectric characteristics, in which crystal

functions as a stable mechanical vibrator. There are many types of crystal oscillators. One of them

is a crystal oscillator employing an inverting amplifier including a CMOS (complementary metal

50

oxide semiconductor) circuit, and used, for example, as a reference signal source of a PLL (phase-

pocked poop) circuit of a mobile phone. Crystal oscillator circuits using crystal have a number of

advantages in actual application since crystals show high frequency stability and stable

temperature characteristic as well as excellent processing ability. Temperature-compensated

crystal oscillators, in which variation in oscillation frequency that arises from the frequency-

temperature characteristic of the quartz-crystal unit is compensated, find particularly wide use in

devices such as wireless phones used in a mobile environment. A surface mounting crystal

oscillator is used mainly as a frequency reference source particularly for a variety of portable

electronic devices such as portable telephones because of its compact size and light weight.

L293D & H-BRIDGE (LED DRIVER)

The most common method to drive DC motors in two directions under control of a computer is

with an H-bridge motor driver. H-bridges can be built from scratch with bi-polar junction

transistors (BJT) or with field effect transistors (FET), or can be purchased as an integrated unit in

a single integrated circuit package such as the L293. The L293 is simplest and inexpensive for low

current motors, for high current motors, it is less expensive to build your own H-bridge from

scratch. The L293D is an integrated circuit motor driver that can be used for simultaneous, bi-

directional control of two small motors (small means small). The L293D is limited to 600 mA, but

in reality can only handle much small currents unless you have done some serious heat sinking to

keep the case temperature down. Unsure about whether the L293D will work with your motor?

Hook up the circuit and run your motor while keeping your finger on the chip. If it gets too hot to

touch, you can't use it with your motor. The L293D is a quadruple high-current half-H driver

designed to provide bidirectional drive currents of up to 600-mA at voltages from 4.5 V to36 V. It

is designed to drive inductive loads such as relays, solenoids, dc and bipolar stepping motors, as

well as other high-current/high-voltage loads in positive-supply applications .All inputs are TTL-

compatible. Each output is a complete totem-pole drive circuit with a Darlington transistor as sink

and a pseudo-Darlington as a source. Drivers are enabled in pairs with drivers 1 and 2 enabled by

51

1,2 EN and drivers 3 and 4 enabled b3, 4 EN. When enable input is high, the associated drivers

are enabled, and their outputs are active and in phase with their inputs. External high-speed output

clamp diodes should be used for inductive transient suppression. When the enable input is low,

those drivers are disabled, and their outputs are off and in a high-impedance state. With the proper

data inputs, each pair of drivers forms a full-H (or bridge) reversible drive suitable for solenoid or

motor applications.

L293D is a bipolar motor driver IC. This is a high voltage, high current push pull

four channel driver compatible to TTL logic levels and drive inductive loads. It has 600 mA output

current capabilities per channel and internal clamp diodes. The L293 is designed to provide

bidirectional drive currents of up to 1A at voltages from 4.5 V to 36 V. The L293D is designed to

provide bidirectional drive currents of up to 600-mA at voltages from 4.5 V to 36 V. Both devices

are designed to drive inductive loads such as relays, solenoids, dc and bipolar stepping motors, as

well as other high-current/high-voltage loads in positive supply applications. All inputs are TTL

compatible. Each output is a complete totem-pole drive circuit, with a Darlington transistor sink

and a pseudo-Darlington source. Drivers are enabled in pairs, with drivers 1 and 2 enabled by

1,2EN and drivers 3 and 4 enabled by 3,4EN. When enable input high is given then the associated

drivers are enabled, and their outputs are active and in phase with their inputs. When the enable

input is low, those drivers are disabled, and their outputs are off and in the high-impedance state.

With the proper data inputs, each pair of drivers forms a full-H (or bridge) reversible drive suitable

for solenoid or motor applications.

LDR

The light dependent resistor is an electronic component whose resistance decreases with

increasing light intensity. It is also called as “Photo Resistor” or “Photo conductor” The light

dependent resistor uses high resistance semiconductor material. (Cadmium Sulphide). When light

falls on such a semiconductor the bound electrons [i.e. Valence electrons] get the light energy from

52

the incident photos. Due to this additional energy, these electrons become free and jump in to the

conduction band. The electron –hole pairs are generated. Due to these charge carriers, the

conductivity of the device increases, decreasing its resistivity.

Generation of charge carriers due to light

Symbol of the LDR

As we define R= p*l/A

Where p= resistivity of the material,

l= length of the medium

And A= area of cross section of the medium.

Now resistivity depends upon the nature of the material. Now, Cadmium sulphide

is such a material which changes it resistivity on different illumination of the light. As the

resistivity changes the resistance of the LDR also changes. Now, according to OHM’s law for a

fixed value of current if the resistance changes the voltage drop across that also changes. So for

varying luminance of light on the LDR surface the voltage drop across it will also vary that we

will have an analog variation of voltage.

53

VOLTAGE REGULATOR LM 7805

It is a voltage regulator. The 78 indicates a positive regulator the 05 indicates the voltage output.

At 1 amp if adequate heats sink is provided. Never fear it has thermal protection to shut it down

only if the internal heating exceeds the safety zone. It will not destroy itself by removing or

reducing the load it will come- back alive after cooling

Pin diagram of Voltage Regulator

LED- 5 WATT WARM WHITE HIGH POWER SQUARE SHAPE

Products Model #: BY-HP5SWW

Specifications of Various Parameters

Parameter Min. Max. Unit

Luminous Intensity 200 250 lm

Viewing Angle 115 125 deg

Color Temperature 2700 3500 K

Wave Length / / nm

Forward Voltage 6.5 7.5 V

Forward Current 700 800 MA*

54

5 Watt LED

5 watt LED bulbs are a type of high intensity LED. LED bulbs produce much more light for each

watt of power they consume than a regular incandescent bulb would. 5 watt LED bulbs also

produce light with a narrower color spectrum than incandescents. Whereas a regular bulb produces

every color in the rainbow, a “white” LED only produces the most common colors. This means

that an LED will not produce ultraviolet or UV radiation that might attract bugs. Compared to

fluorescent bulbs, LEDs also do not require a ballast that could produce electromagnetic radiation

and interfere with other electronics. LEDs are extremely simple devices. Like an incandescent

bulb, they consist of a special material through which electricity is fed. Whereas an incandescent

bulb uses material that heats up, LEDs use a block of two separate materials layered on top of each

other. These materials have highly specialized electrical properties. The color of the light created

is determined by the materials: if the materials favor creating fewer higher energy particles of light,

the color is different than if the materials favor creating more lower energy particles.

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APPENDIX B

ATMEGA 168 MICROCONTROLLER

Pin Diagram of Atmega 168

PIN DESCRIPTION:

VCC: Digital supply voltage.

GND: Ground.

Port B (PB7:0) XTAL1/XTAL2/TOSC1/TOSC2

Port B is an 8-bit bi-directional I/O port with internal pull- up resistors (selected for each bit).

The Port B output buffers have symmetrical drive characteristics with both high sink and source

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Capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port B pins are tri-stated when a reset condition becomes active, even

if the clock is not running. Depending on the clock selection fuse settings, PB6 can be used as

input to the inverting Oscillator amplifier and input to the internal clock operating circuit.

Depending on the clock selection fuse settings, PB7 can be used as output from the inverting

Oscillator amplifier. If the Internal Calibrated RC Oscillator is used as chip clock source, PB7.6 is

used as TOSC2.1 input for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set.

Port C (PC5:0)

Port C is a 7-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

PC5.0 output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port C pins are tri-stated when a reset condition becomes active, even

if the clock is not running.

PC6/RESET:

If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical

characteristics of PC6 differ from those of the other pins of Port C. If the RSTDISBL Fuse is un-

programmed, PC6 is used as a Reset input. A low level on this pin for longer than the minimum

pulse length will generate a Reset, even if the clock is not running. Shorter pulses are not

guarantee to generate a reset.

Port D (PD7:0):

Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

Port D output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up

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resistors are activated. The Port D pins are tri-stated when a reset condition becomes active, even

if the clock is not running.

AVCC:

AVCC is the supply voltage pin for the A/D Converter PC3:0, and ADC7:6. It should be

externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be

connected to VCC through a low-pass filter. Note that PC6.4 use digital supply voltage, VCC.

AREF: AREF is the analog reference pin for the A/D Converter.

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REFERENCES

1. www.cree.com/Xlamp

2. www.stmicroelecronics.com

3. www.nationalsemiconductor.com

4. www.choicegroupofcompanies.com

5. www.datasheetcatalog.com

6. www.wikipedia.com

7. Grid logics Technology Insight Report-LEDs in Lighting

8. CooLam Lighting Technology Article

9. IEE_Ecova_LED