Project 2 Feasibility Report
ENG1002- Engineering Design: Cleaner, Safer, Smarter
Team no: 03
Written by: Liew Ze Ching,
Clement Chong Kha Hian,
Aksha Mehjabin,
Giridhar Gopal Sharma
Executive Summary
A solid-state device such as a portable desk lamp which only needs a small scale of energy source is needed to improve the lives of people from the underdeveloped nations who are still using kerosene wick lamp and candle. The kerosene wick lamp and the candles are expensive, not clean in terms of energy conversion and produce harmful gases such as carbon monoxide and sulfur dioxide.
The analysis and evaluation of an inexpensive, clean, reliable and rechargeable portable desk lamp are discussed and provided in this report. Three different types of electronic circuit configuration were investigated and compared in terms of the feasibility, the power consumption of the electrical components, costs as well as the robustness. Besides, the resistance of the components and the circuit current consumption are also calculated for different types of circuit. The configuration C was selected because it is feasible, high robustness and lesser power comsumption.
In conclusion, our mother earth has limited natural resources and these resources are depleting day by day. Therefore, it is recommended that people start to practise using these resources wisely and create more efficient as well as power saving device such as this portable desk lamp in order to save our mother earth.
Table of Contents
Executive Summary 1
Table of Contents 2
1.0 Background/ Introduction 3
2.0 LED array configuration evaluation 4
Evaluation of Given Configurations 4
Proposed Design 10
3.0 Conclusion 12
4.0 References 14
5.0 Appendices 14
1.0 Background/ Introduction
One cannot imagine their life on earth without light. The sun was the only natural source of light until people used friction(rubbing stones) to create firesticks(torches) which acted as a source of light. Then came candles which were made of wax but they didn't last long and had to be replaced every time one exhausted. Other lighting methods used were very simple and primitive but indigenous solutions used by humans to fight darkness, for instance gas lamps which became very popular in mid 1700s. With the development of technology and advancement of science, a lot of scientists tried to produce light artificially until in 1878, the first light bulb was discovered by Thomas Edison and filed his first patent for “Improvement in light history”. The principle was very simple which included the heating of a carbon filament upon passing electricity inside a vacuum which produced light.
Further modification and simplification of light bulbs gave rise to the discovery of LEDs or Light Emitting Diodes. They are basically electronic components that emit light when passed through a DC current. They work on a principle called electroluminescent and have the ability to emit light in visible,infrared and ultraviolet region. It consists of a chip that is made up of a semiconductor doped with different impurities that makes it a p-n junction. Current inside an LED flows from p to n and cannot be reversed. They have high efficacy when subjected to low temperature and currents and hence are used in freezing devices. Sudden failure of LEDs are very rare. Overall they are the present and most popular lighting technology used, however no one has seen the future.
LED with visible light became the replacement for the neon and incandescent lamps. The infrared LEDs were also used in TVs, Video games and also in remotes providing wireless control. They are also used in aviation navigation, car headlights, traffic signals and camera flashes as well.
In the end, this report provides an analysis of different LED array circuits provided by the management and recommends an LED array circuit that best meets the requirement of being energy-efficient, safe to use, low in cost, and capable of producing 250 lux of light intensity on a 15 ×15 cm square which is located 20 cm away.
2.0 LED array configuration evaluation
Evaluation of Given Configurations
For each configuration shown below, a 5V voltage source is supplied to the circuit. Each LED has a forward voltage of 3.2V and forward current of 20mA as given in the data sheet. Each LED gives approximately 40 added lux at distance of 20cm when turned on.
Configuration A
As this is a series circuit, the total voltage drop from the LEDs will be the sum of their forward voltages.
VTOTAL = VLED + VLED + VLED + VLED
= 3.2V + 3.2V + 3.2V + 3.2V
= 12.8V
Since the total voltage drop is greater than the voltage supplied, the circuit will not light up.
This configuration is not feasible.
Figure 1: Configuration A
Configuration B
Taking the pairs of LEDs and resistors in each parallel branch as one component, the voltages across every component of the parallel circuits would be the same.
VTOTAL = V1 = V2 = V3 = V4
Figure 2: Configuration B = 5V
The LEDs and resistors are in series, so the voltage across each resistor would be found by subtracting the forward voltage of the LEDs.
VACROSS = VTOTAL – VLED
= 5V – 3.2V
= 1.8V
The total current drawn would be the sum of the currents through each bulb.
ITOTAL = I1 + I2 + I3 + I4
= 20mA + 20mA + 20mA + 20mA
= 80 mA
To calculate the resistance of the resistors RB, we will use Ohm’s law.
RB = V/I
= 1.8V/20mA
= 90Ω
To get this equivalent resistance, we can choose the following resistors from the E12 series of standard resistor values and arrange them in series
68Ω+10Ω+12Ω = 90Ω
The power consumption of each LED would be its forward voltage multiplied by the current through it.
PLED = 3.2V x 20mA
= 64mW
Applying the same formula to find the power consumption of the resistors:
PR = 1.8 x 20mA
= 36mW
Thus, the total power consumed by the circuit is
Ptotal = 64mW x 4 + 36mW x 4
= 400mW
Configuration C
In this configuration, the LEDs which are parallel to one another, are connected in series with the resistor RC.
The total voltage of the parallel LEDs is the same as the voltages across each of them, which is 3.2V.
Figure 3: Configuration C
So, the voltage across RC would be
VACROSS = VTOTAL – VLED
= 5V – 3.2V
= 1.8V
The total current drawn would be the sum of the currents through each bulb.
ITOTAL = I1 + I2 + I3 + I4
= 20mA + 20mA + 20mA + 20mA
= 80 mA
Since the RC is in series with the parallel LEDs, and the total current passing through the bulbs is 80mA, applying Kirchhoff's Current Law we know that the current through RC has to be 80mA. To calculate the value of RC, we will use Ohm's law.
RC = V/I
= 1.8V/80mA
= 22.5Ω
If the power absorbed by a resistor exceeds 100 mW, the resistor has a 20% chance of failure. Checking the power absorbed by RC :
PR = V2/ RC
= 1.82 V / 22.5 Ω
= 144mW
which is greater than 100mW.
In order to prevent this, we calculate the new value of RC by using 1.8V and the 100mW limit:
RC = V2/ PR
= 1. 82 V / 100Mw
= 32.4 Ω
Based on the E12 series, the closest value would be 33 Ω.
Using this new resistor value, the current would then be:
IR = V/RC
= 1. 82 V / 33 Ω
= 54.54mA
This means that the current flowing through each of the identical bulbs are:
ILED = IR/4
= 13.64mA
Consequentially, the power consumed by the resistor and each LED would be:
PR = IR x V
= 54.54mA x 1.8V
= 98.17mW
PLED = ILED x V
= 13.64mA x 3.2V
= 43.65mW
Total power consumed by circuit:
PTOTAL = PLED + PR
= 43.65mW x 4 + 98.17mW
= 272.76mW
The table below shows the results of our evaluations comparing the three configurations
with regard to the criterias listed in the project brief.
Table 1: Comparison of Configurations A, B and C
Configurations
Criteria
A
B
C
Feasibility
Not feasible (insufficient voltage)
Feasible
Feasible
Power Consumption of each LED (mW)
N/A
64
43.65
Power Consumption of resistors (mW)
N/A
36 x 4 = 144
98.17
Total power consumed (mW)
N/A
400
272.76
Current drawn (mA)
N/A
80
54.54
Cost
N/A
Relatively high (three more resistors used)
Relatively low
Robustness
N/A
Robust (failure of one LED would not cause the maximum current rating to be exceeded)
Quite robust (maximum current rating would only be exceeded if 3 of the LEDs failed simultaneously)
Proposed Design
According to the project guide, our LED array should meet the following requirements:
give a 250 lux difference from a 5V DC power supply at 20cm distance from the light meter over a 15x15cm square
draw less than 300mA maximum current draw from the DC power supply
Assuming that each LED gives about 40 lux when turned on, as stated in the project specifications, we decided to use 7 LEDs in our proposed design.
The figure below shows our circuit diagram.
So, the voltage across R would be
VACROSS = VTOTAL – VLED
= 5V – 3.2V
= 1.8V
Taking the optimum current flow through the LEDs as 20mA, total current drawn would be:
ITOTAL = I x 7
= 140mA
Since the RC is in series with the parallel LEDs, and the total current passing through the bulbs is 80mA, applying Kirchhoff's Current Law we know that the current through RC has to be 80mA. To calculate the value of RC, we will use Ohm's law.
RC = V/I
= 1.8V/140mA
= 12.76Ω
If the power absorbed by a resistor exceeds 100 mW, the resistor has a 20% chance of failure. Checking the power absorbed by R :
PR = V2/ R
= 1.82 V / 12.76 Ω
= 252mW
which is greater than 100mW.
In order to prevent this, we will use the minimum resistor value where the power absorbed would not exceed 100 mW which is 33 Ω. We will put 2 of these in parallel to get a closer value to the required resistance.
Req = (1/33 + 1/33)-1
= 16.5 Ω
Using this new resistor value, the current would then be:
IR = V/Req
= 1. 8 V / 16.5 Ω
= 109.09mA
This means that the current flowing through each of the identical bulbs are:
ILED = IR/7
= 15.58mA
Consequentially, the power consumed by the resistors and each LED would be:
PR = IR x V
= 109.09mA x 1.8V
= 196.36mW
PLED = ILED x V
= 15.58mA x 3.2V
= 49.87mW
Total power consumed by circuit:
PTOTAL = PLED + PR
= 49.87mW x 7 + 196.36mW
= 545.448mW
Criteria
Proposed Design
Feasibility
feasible
Power Consumption of LEDs (mW)
349.09
Power Consumption of resistors (mW)
196.36
Total power consumed (mW)
545.558
Current drawn (mA)
109.09
Cost
Relatively low
Robustness
Quite robust (maximum rating would only be exceeded if 4 of the LEDs failed simultaneously)
3.0 Conclusion
Given LED array circuits i.e. Configuration A,B and C were thoroughly analysed throughout this report. The most prioritised aspects when analysing these configurations were in terms of feasibility,cost, consumption and robustness.
Our recommended array design would be of Configuration C where four LEDs are arranged in parallel and connected to a single resistor in series. This is because we can see from the comparison table that Configuration A is not feasible, thus cannot be chosen which leaves Configuration B and C. Although Configuration B is more robust than C, it is still preferable to choose C due to factors like cost, power consumption and current drawn. We can see that C draws 54.54 mA current and 272.76 mW of power which is less compared to B. Other than that,C uses a single resistor while B uses four resistors thus costing less in manufacture of Configuration C.
Since Configuration C meets the requirements such as least resources, least power consumption and high efficiency of energy, our chosen LED array circuit would be configuration C.
4.0 References
[1] History of light bulb | History and development of light bulb
Available: https://www.energy.gov/articles/history-light-bulb [Accessed: 11-May-2018].
[2]"History of the Light Bulb | Lighting Basics | Bulbs.com", Bulbs.com, 2017. [Online]. Available: http://www.bulbs.com/learning/history.aspx. [Accessed: 11- May- 2018].
[3]"The History of the Light Bulb", Energy.gov, 2017. [Online]. Available: https://energy.gov/articles/history-light-bulb. [Accessed: 11- May- 2018].
5.0 Appendices
List of Equations :
1. Voltage across each LED, VACROSS (V)= VTOTAL – VDROP
2. Voltage, V (V) = I × R [I = Current (A); R = Resistance (]
3. Power Consumption, P (W) = V × I [V = Voltage (V); I = Current (A)]
4. For components arranged in series,
a. Total Voltage, VTOTAL = V1 + V2 + V3 + V4
b. Total current, ITOTAL = I1 = I2 = I3 = I4
c. Total Resistance, RTOTAL = R1 + R2 + R3 + R4
5. For components arranged in parallel,
a. Total Voltage, VTOTAL = V1 = V2 = V3 = V4
b. Total Current, ITOTAL = I1 + I2 + I3 + I4
c. Total Resistance, = (1/R1 + 1/R2 + 1/R3 + 1/R4)-1