Sample-Formal-Lab Report

TENS 2146 Electric Devices and Measurements

Lab 3 Current and Voltage

This report was prepared by: L. Wall

Fall 2009
Prof. R. Alba-Flores
Team Members: J. White, L. Wall
Conducted on: September 17, 2009
Submitted on : September 24, 2009

Abstract:
In this lab students experimented with light emitting diodes. The student built a basic circuit with two LED's and resistors in parallel. The results showed that the voltage is the same in parallel. The items that were in series had the same current. The results also showed that the current from each branch could be summed up to equal the total current from the source. The experiment also helped the student to see the voltage drop across the diode was almost the same each time. This lab showed the effects of current and voltage in a parallel circuit. This experiment also showed how the brightness was effected by changing the voltage.
Equipment and materials:
• Multimeter
• Power Supply
• Connecting wires
• A bread board
• A 330 ohm resistor
• A l k ohm resistor
• 2 Red LED's
Theory:
A light-emitting diode (LED), is an electronic light source. The first LED was built in the 1925 by Oleg
Vladimirovich Losev, a radio technician who noticed that diodes used in radio receivers emitted light when current was passed through them. The LED was introduced as a practical electronic component in 1962. All early devices emitted low-intensity red light, but modern LEDs are available across the visible, ultraviolet and infra red wavelengths, with very high brightness.[1]

Figure 1. Basics Physics principle of an LED

LED's are semiconductors. They will only work if placed in the correct direction. Placing the LED in the improper direction could potentially damage it. The LED could also be damaged if it was not installed with the use of a resistor. They can not be connected directly to a power source. The anode is the positive end and the cathode is the negative end. LEDs are beneficial because they do not require much voltage to be illuminated.
The LEDs are great for conservation of energy .When we subtract the LED voltage from the supply voltage it gives you the voltage that must be dropped by the dropping resistor. A decrease in voltage will result in a decrease of the brightness of the bulb [1].

Figure 2 shows the electrical symbol and the actual shape of an LED.

Figure 2. Electrical symbol and the actual shape of an LED

Ohms law is used to be able to calculate the current and the resistance across each of the elements in the circuit. To analyze the circuit It must be known that the voltage is the same in a parallel circuit. The current is the same in a series. The current through each branch can be added up in order to determine the current from the source. From Kirchhoff's loop law it can be determined that the sum of all of the voltage drops around a closed loop must sum to equal zero.
The objective of this lab was for the student to use their knowledge of items such as LED's, series and parallel circuit configurations, Kirchhoff's laws, and Ohm's law in order to properly analyze and solve problems with given circuit.
[1] Light Emitting Diodes, http://en.wikipedia.org/wiki/Light-emitting_diode

Procedure:
First thing we did in the lab was to create the circuit. We created the circuit by using resistors with values of Rl
= 330 ohms and R2 = l k ohms and also by placing the LEDs after the Resistors so there would be no damage done to the LEDs. This s shown in figure 3.
The voltage supply was set at 8 Volts and then we tested the values for voltage and current. To measure voltage the meter has to be in Parallel with the circuit. Current is measured by placing the Multimeter in series with the circuit. In preparation for the experiment we built the circuit (shown below) using Multisim. We used simulations to get all the required measurements and used Ohm's Law (E=I*R) to solve for the rest. In the lab we set up the same circuit by connecting the resistors and the LED's in a parallel circuit to the power supply.
Most of the connections were done using the breadboard. We measured current by placing the Multimeter in series with the entire circuit. We set the Multimeter to measure amperes and turned the power on. We continued this using the 8, 6, 4, and 2 volts (adjusted on the power supply) while noting the brightness of the
LED's and writing down the value given by the Multimeter. We then connected the Multimeter in parallel with each resistor and LED's to measure voltage. We set the multimeter to volts then cycled through 8, 6, 4, and 2 volts on the power supply and noted the reading for each connection. We then used Ohm's Law to calculate the current through as well as the resistance for each LED. We also calculated the entire current to see if it matched what we measured.

Figure 3 Circuit built in the lab

Sample Calculations:
To calculate the current through each resistor-LED branch, Ohm's Law (V = IR) was used. In this Lab the equation used was I Rl = VRl / R 1
Example:
IRl = VRl / R1 = 5.8 V / 3300 Ω = 0.0176 A
To calculate the total resistance of each LED, Ohm's Law was used. In this lab the equation
RLED = V LED / ILED was used.
Example:
RLED = V LED / ILED = 2.18 v /0 .0175 mA = 124.57 Ω
To calculate the total current that the power supply was providing to the two branches, the equation
IE = IRl + IR2 was used. In this lab the equation that was given to use was ILEDl = IRl .
Example:
I LEDl + I LED2 = IE
.0175 + .0058 = .0233
Simulation Results:
Multisim was used to perform the simulations. Figures 4, 5, 6 and 7 shown the results obtained in the simulations. Table 1 summarizes these results.

Figure 4. Power supply= 8 volts: Voltages measured across R1 and R2 and currents through each LED

Figure 5 . Power supply= 6 volts: Voltages measured across R1 and R2 and currents through each LED

Figure 6. Power supply= 4 volts: Voltages measured across R1 and R2 and currents through each LED

Figure 7. Power supply= 2 volts: Voltages measured across R1 and R2 and currents through each LED

Table 1. Comparison of Pre-Lab simulations and actual Lab data

Looking at the comparison chart above we can see that the voltage and the total current was close in value when looking at the Pre-Lab and the Actual Lab. The actual Multisim simulation charts are printed and attached to this lab report. Looking at the comparison chart above we can see that the voltage and the total current was close in value when looking at the Pre-Lab and the Actual Lab. The actual Multisim simulation charts are printed and attached to this lab report.
Conclusion:
In conclusion when simulating the circuit in actuality or in Multisim; the LED voltage, current, and brightness are affected by the decreasing of the voltage supply. By decreasing the voltage supply the brightness of the LEDs also decrease in intensity.
When determining the factors that are involved in the brightness of the LED we must look at the circuit and see if the resistors and the LEDs are connected properly. We must also look at the value of the current passing through the current.
To determine the current through the LEDs Ohm's Law was applied. To find the current we must first measure the voltage and the resistance, and then after finding those two values we divide the voltage by the resistance. Which Ohm's Law is I (current) = V (voltage) / R (resistance). After finding the current in the LED it is seen that the current is almost equal to the resistor that is closes to that LED.
I am in agreement with the measurement that was taken for the voltage supply of 8 volts, 6 volts, and 4 volts; but I disagree with the values for the voltage supply of 2 volts. The measurement collected in Multisim fo and the actual measurement value more that the other voltage supply ranges.
When the LEDs were reversed the resistor and the LED current and their voltages changed to O or 'r' due to there was zero or no flow of current and voltage. The voltage is what supply energy to the components in the circuit. So decreasing the amount of voltage will decrease the amount of energy current, and the amount of current is what determines the intensity of the LED.
The pre-lab seemed to simulate more accurate values than the results of the values in Table 1. Due to the fact that there is more human value in the actual measurements than the simulated ones; plus the actual values have been round and round again. The simulated and actual values are very close in value; but do to human error the values are not and can not be exactly the same.