University of Southern Mississippi EET 110 Introductory Circuit Analysis Lab 2: Resistors and the Color Code Lab Instructor: Dan Garcia Date Performed: 2/2/2006 Date Submitted: 2/9/2006 Submitted by: Chris Mills Akio Lofton Latara Hudson Cortez Davis Matt Landry Elizabeth Buckley Bonnie Williams Domingo Megia Planet Abstract In this lab the group validated the nominal value of various ¼ watt resistors by direct measurement. In addition to these measurements, the participants in the lab measured their skin resistance and used Ohm’s Law to predict the value of a lethal voltage. Next the effect of a meter’s connection to an energized circuit was noted. Finally, the power rating of a resistor was related to its size as ability to release heat and survive the resultant rise in temperature due to its power dissipation. Introduction Common commercial resistors are marked with color-coded bands that represent digits in the nominal value of the resistor and specify accuracy with an additional band if applicable. In addition to variations in resistors there are two major types of meters, analog and digital, which must be investigated to uncover their specific relative strengths and weaknesses. Materials Archer Kit Volt-Ohm-Milliameter Model 14A Hewlett Packard Digital Multimeter Model 114z 220, 3.3k, 10k, 470k, and 1M ¼ watt resistors A resistor placard indicating sample values of carbon resistors at ¼, watt ½ watt, 1 watt, 2 watt, and higher power ratings Methods In parts 1 and 2 the color code table [Table 1] was used to predict te resistance in ohms of the ¼ watt carbon resistors supplied. The color bands are read from left to right, where the band closest to the end of the resistor’s body is the first significant digit; the next is the second significant digit and the third is the power of ten of the multiplier. A band succeeding these is an indication of accuracy where a gold band signifies 5% tolerance, a silver band indicates 10% tolerance and no band signifies 20% tolerance. Thus, based on the bands present on the body of a resistor its nominal specification can be calculated. Color Black Brown Red Orange Yellow Green Blue Violet Grey White Value 0 1 2 3 4 5 6 7 8 9 Table 1. The Resistor Color Code. In parts 3 the digital multimeter was used to measure the skin and body resistance from one hand to the other. In part 4 the digital multimeter was used to measure the resistances of the various scales of voltage and current of another meter. Results Part 1 of the lab was completed through the observation of a placard displaying resistors of various sizes and wattages. It was observed that as the size of the resistor increased it would be able to handle more power traveling through it. Instructor Garcia pointed out this observation at the close of the lab, and said that though we were not required to sketch the different wattages, the fundamental ideas of an increase in wattage for a resistor were the important things to note for this portion of the lab. Part 2 consisted of determining the nominal resistance of the resistors based on the color code and verifying this value through testing with the digital multimeter. The expected values based on the color code are dispayed in Table 2. Resistor Color Bands - Color Color Bands – Numerical Value (Nominal 1 2 3 4 1 2 3 4 Value) 220 Red Red Brown Gold 2 2 1 5% 3.3k Orange Orange Red Gold 3 3 2 5% 10k Brown Black Orange Gold 1 0 3 5% 470k Yellow Violet Yellow Gold 4 7 4 5% 1M Brown Black Green Gold 1 0 5 5% Table 2. Expected Values of Resistors Based on Color Code In order to verify the nominal values of the resistors, the accuracy band was considered and a range of acceptable resistances was calculated using Equation 1. Equation 1: (Value of Accuracy Band)(Nominal Value) = Acceptable Deviation The number achieved through this equation is added to the end of the nominal value in order to achieve the acceptable range, for example, in the 220 resistor the acceptable deviation is 11. Thus the acceptable range for the 220 resistor would be written as: 220 +/- 11. From this range a set of minimum and maximum acceptable values were calculated and these are shown in Table 3. As a side note, the lab did not have any 1M resistors and because of this even though they appear in the materials list and in the nominal value calculation table [Table 2], they were not tested in the lab. Resistor Minimum Value Maximum Value 220 209 231 3.3k 3.135k 3.46k 10k 9.5k 10.5k 470k 446.5 493.5 Table 3. Minimum and Maximum Acceptable Values for Resistors Using the above calculated acceptable range the resistors were tested, their percent difference was calculated, and their nominal values were either confirmed or rejected. The results of this part of the experiment are shown in Table 4. Meter VOM DMM Normal Measured Nominal % Difference Measured Nominal Value % Difference Resistor Value Value Value Confirmed? Value Confirmed? 220 215 Yes 2.27% 214 Yes 2.7% 3.3k 3.2k Yes 3.03% 3.27k Yes .9% 10k 9.9k Yes 1.00% 9.97k Yes .3% 470k 480k Yes 2.13% 466k Yes .85% Table 4. Results of the Digital Multimeter Analysis of the Nominal Values of the Resistors As can be seen through this graphical display of the data gathered in part 2, the nominal values of all the resistors used in the experiment were verified. This verification can be issued through the use of either the calculated % difference or the measured value because they are both measurements of the deviation in the nominal value from the actual measured value. In part 3 of the lab each member of the group measured their body/skin resistance from one hand to the other. By using this value and an assumed constant 10mA as a lethal current in Ohm’s Law the members of the group calculated the amount of voltage that would be lethal. The results of these calculations are shown in Table 5. Group Member Name Measured Resistance Calculated Voltage (in V) (in ) Akio Lofton 880000 880 Latara Hudson ? ? Cortez Davis 530000 530 Matt Landry 340000 340 Elizabeth Buckley ? ? Bonnie Williams 1701000 1701 Chris Mills 340000 340 Domingo Megia Planet 246700 246.7 Table 5. Measured Body Resistance and Calculated Lethal Voltages From these measurements and calculations it can be seen that Ohm’s Law can be used to calculate a lethal voltage and that any voltages exceeding those calculations shown in the second column of Table 5 for any specific member of the group would be lethal. In part 4 the group measured the internal resistance of the voltmeters when the meter was shorted out (by connecting the negative and positive leads). Only the VOMs (Volt-Ohm Meters, analog) were tested in this part because the equipment required for testing DMMs was not available in the lab. The results of the measurements are shown in Table 6. VOM /V Rating Calculated Measured % Difference Resistance Resistance 10V 20000 /V .200M .201M .5% 1000V 20000 /V 20M 20.08M .4% Table 6. Measurements of the Internal Resistance of Laboratory Owned VOMs Conclusions In part one it was acknowledged that the size of a resistor is directly proportional to both its ability to dissipate heat, and its power rating. The conclusions derived from part 2 are that the nominal values of resistors in the laboratory are well within the expected acceptable range, and that through the calculation of an expected range the validity of an accuracy band of any resistor can be tested. In part 3 Ohm’s Law was shown to be useful in calculating voltages given a resistance and current measurement, and it was concluded that a lethal voltage can be calculated through the measurement of skin resistance and the assumption that 10mA is a sufficiently deadly current. Through part 4 it was concluded that the internal resistance of a VOM could be calculated by shorting the leads and measuring the resultant ohms. This serves also as a means of compensating for errors introduced by the connection of a meter to an energized circuit.
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