DEPARTMENT OF ELECTRICAL AND SYSTEMS ENGINEERING
ESE UNDERGRADUATE LABORATORY
ESE 205: Electrical Circuits and Systems I Laboratory
Lab: 2
Design project I (LED’s)
Independent sources, KCL, KVL, resistors in series and
parallel
Objective
The objective of this lab experiment is to design a display panel using light-emitting diodes (LED). You will get familiar with the process of circuit design (as opposed to circuit analysis) and the trade-offs that need to be made in a design which often goes beyond just technical considerations. You will also use a wire resistance to transducer a mechanical load.
Background
You are one of the founders of a high-tech start-up company that is developing a futuristic instrument for electronic games. You just received funding from a venture capitalist company that believes in your product. You and your partner will be designing and building the display panel.
The goal is to design and build this system at the lowest possible cost in order to keep your new invention competitive.
Cost, however, is not the only criteria. The product will be portable and needs to consume as little power as possible. . Your business associate, who is also a Penn student (Wharton), did a market study and has come up with a clever (although through some obscure reasoning) formula which he calls the Cost Function (CF). This function should be as low as possible and depends on both the component cost and the power dissipation. It takes into account the relative importance of price versus power dissipation, as valued by customers. The cost function is equal to:
CF = a . Price (in $) + b . Total_Power_dissipation (in Watt)
in which a=1.25 (per $) and b=2.0 (per Watt).
Project Definition
You have decided to use LED’s for the display elements instead of light bulbs since LEDs consume much less power, are smaller and much less fragile (LEDs are solid-state devices). The characteristic of an LED is highly non-linear as is shown in Figure 1. The optimal operating conditions are when the current through the diode is between 5 and 10 mA, corresponding to a voltage of 1.8 and 1.9 V. The maximum current in the LED should not exceed 60 mA.
Figure 1: A typical I-V characteristics of a red LED. (http://media.digikey.com/pdf/Data%20Sheets/Lite-On%20PDFs/LTL-10223W.pdf)
The instrument already has a power supply of 10V built in. As power supplies are very expensive and bulky, you have already ruled out adding another supply. After extensive discussions with your partner you have nailed the solutions down to two alternatives.
However, the choice between the two depends on the Cost Function (CF). The two solutions are shown in Figure 2.
The task at hand is first to decide which is the best solution (giving the lowest cost function).
- The cost
of the components are given in Table I. You have a choice of using two vendors - radioshack.com or jameco.com
-http://www.radioshack.com/family/index.jsp?categoryId=2032267&s=null
-www.jameco.com and browse for
Home > Category: Passive Components > Passive Components / Resistors
Figure 2: Solution # 1 and 2 for the LED display
|
Component |
Unit Price ($) |
Your Price (optional) |
|
1/8 Watt resistor |
0.02 |
. |
|
1/4 Watt resistor |
0.02 |
. |
|
1/2 Watt resistor |
0.06 |
. |
|
1 Watt resistor |
0.60 |
. |
|
10 Watt resistor |
4.00 |
. |
|
Bar or array display of 10 LEDs |
2.00 |
. |
|
Array of 8 DIP Switches |
2.50 |
. |
TABLE I: Price of the components
Only a limited number of resistance values are
available, as shown in Table II
|
R (Ohm) |
100; 150; 180; 270; 330; 510; 560; 680; 820; 1K; 2K, 5.1K; 6.8K; 10K; 47K; 180K; 200K; 470K; 560 K; 1M |
TABLE II: Available resistor values
Pre-Lab Assignment
The goal of the pre-lab is to decide which solution to use and to find the component values.
At first glance solution #2 (Fig. 2-2) is cheaper since it uses fewer components. However, it is not clear yet which will consume the least power, so you need to first calculate the power. In order to do this, you will need to know the values of each resistor:
1. Design circuit of solution #1 (Fig. 2-1)
a. Find the value of the resistor R in the circuit of Figure 2 (solution 1).
Hints: Find the resistor that gives a current of Id = 5mA through the diode and a corresponding voltage Vd of 1.8V over the diode (see diode characteristic of Fig. 1): use Ohm's law as illustrated in Fig. 3. Do the same for a value of 10 mA and a voltage of 1.9V. This gives you two limiting values for the resistance within which the actual resistance must lie.
b. Select a resistance value that lies in this range from Table 2. Calculate then the actual current that will flow in the diode based on the new resistor value (you can assume that the voltage Vd over the diode is constant and equal to 1.8V to calculate the current through the resistor and thus through the diode).
Figure 3
c. Calculate and record the power dissipated in the resistors. Decide which power rating for the resistors to use. Find the total power PT1 dissipated in the full circuit (4 diodes). Then calculate the total price of the solution 1 and the cost function. Summarize your calculations in table form, similar to table III shown below.
|
|
Min (Ohm) |
Max (Ohm) |
Actual value (Ohm) |
Current (mA) |
Power (mW) |
Cost ($) |
|
Resistor R |
. |
. |
. |
. |
. |
. |
|
|
||||||
|
LED |
- |
. |
. |
. |
||
|
Total Circuit |
- |
. |
. |
. |
||
|
Cost Factor |
. |
|||||
Table III
2. Do the same for the circuit of solution #2 (Fig. 2-2). This solution makes use of a voltage divider R1-R2.
a. Find the values of the resistors R1 and R2.
Hints: From Fig. 1 you know that the voltage Vd over the diode changes very little and lies between 1.8 - 1.9V as long as a current flows. For simplicity lets assume that Vd is constant and equal to 1.8V. This makes the calculation of R1 and R2 easy. Lets design for the worst case when all four diodes are on. Lets assume that they draw each a current of 5mA for a total of 20mA. This current has to be supplied from the resistor R2 according to KCL at node A in Fig. 4: I2= Id + I1=20mA + I1. Lets choose I1=10mA. You can now calculate the two resistors R1 and R2.
b. Select your resistors from Table II. For these new resistor values calculate the actual currents I1 and I2.
c. Calculate the power dissipation in each resistor and find the power rating. Find also the total power dissipated in the circuit. Now, calculate the total cost and the Cost Function of the circuit. Summarize the calculations in table form.
Figure 4
3. Compare the cost functions of each solution and decide which one you will
implement.
*Do you know what it costs to make iPhone 4 ? *
http://www.isuppli.com/Teardowns-Manufacturing-and-Pricing/News/Pages/iPhone-4-Carries-Bill-of-Materials-of-187-51-According-to-iSuppli.aspx
In-lab Assignment
A. Equipment:
A. Equipment:
- 1. Digital multimeter (HP34401A)
- 2. Triple output programmable power supply (HP E3631A): set at 10V
- 2. Protoboard
- 3. Box with cables and connectors
- 4. Resistors: to be chosen.
- 5. DIP
switche
- 6. LED bar
display, make sure you have the right polarity for connections
Jameco part number, 697477, http://www.jameco.com/Jameco/Products/ProdDS/697477.pdf
.
- 7. Multisim
B. Procedure
1. Get your components: resistors (value and
power rating), switches and diodes. Measure and record the actual resistance
values and compare with the nominal values.
|
Resistors |
Nominal Value (Ohm) |
Measured Value (Ohm) |
|
1 |
. |
. |
|
2 |
. |
. |
|
3 |
. |
. |
|
4 |
. |
. |
|
5 |
. |
. |
2. Build the circuit of your choice. Organize the components on your protoboard
so that it reflects the topography of your circuit. Use the long horizontal red
or blue lines A and B for power distribution; the short vertical lines (with 5
contacts each) are the nodes of your circuit. Remember that the division
between short vertical lines isolates these lines from each other and can be
used to put the DIP switches over. See protoboard figure in "Good Lab
Practice".
3. Set the power supply for 10V (set the current limit to 60 mA to prevent damage to the diodes) and test your display while changing the switch positions. Verify that you can switch on each diode separately as well as all of them together. Record your findings in your lab notebook. Measure and record the voltage over the diodes and current supplied by the power supply for each switch setting in table form (do this for the case that one; two, three, four and no diode is on). Have your circuit checked by the instructor and sign your notebook. What do you notice about the measured currents and voltages? Can you explain it?
4. Measure the total current that is being supplied. Calculate the total power dissipation when all diodes are on. Compare the results with the calculations.
5. Simulate the circuit on Multisim to confirm your results. For the LED, select the LED icon in the parts bin. Double click on the LED. You will see different models from which you can choose: ideal, red, and green. For this lab, you select the "Red LED". The model of the LED need to be modified so that it corresponds to the LEDs in the bar graph display. Clicking on the LED symbol will allow you to change the forward voltage, set it to 1.8 volt.
To implement the switch select the switch icon from the parts bin (looks like two horizontal lines, one shorter than the other). You can define a key (space is the default key) that will toggle the switch on and off. (Double click on the switch and define a different key for each switch)
Do the same measurements on Multisim as you did on your actual circuit: current and voltage measurements. Compare the results with the measured data and record the results in your lab notebook. See the screen captures for your reference, solution1.gif, solution2.gif
*EXTRA, SHOULD YOU CONNECT LEDs in parallal ? *, http://www.kpsec.freeuk.com/components/led.htm
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Created by Jan Van der Spiegel:
Updated by Sid Deliwala 9.22.10