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2
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EXPERIMENT dc
Resistors and the
Color Code
OBJECTIVES
1. Become familiar with the digital and analog ohmmeter.
2. Learn to read and use the resistor color code.
3. Become aware of the magnitude and impact of the internal
resistance of a voltmeter and ammeter.
EQUIPMENT REQUIRED
Resistors
1-1-MA, 1-W film resistor
1-1-M/2, 2-W film resistor
1-1-M/2, 1/2-W film resistor
1-6.8-12, 91-12, 220-22, 3.3-k. 2, 10-K02, 470-k1, 1-MA, 1/4-W film resistors
Instruments
1-VOM (Volt-Ohm-Milliammeter)
1-DMM (Digital Multimeter)
EXPERIMENT de 2
RESISTORS AND THE COLOR CODE
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EQUIPMENT ISSUED
Laboratory Serial No.
TABLE 2.0
Manufacturer and Model No.
Item
VOM
DMM
DESCRIPTION OF EQUIPMENT
For increasing wattage ratings, the size of the film resistor will increase to provide the
"body" required to dissipate the resulting heating effects.
Resistance is never measured by an ohmmeter in a live network, due to the possibility
of damaging the meter with excessively high currents and obtaining readings that have no meaning.
It is usually best to remove the resistor from the circuit before measuring its resistance to ensure
that the measured value does not include the effect of other resistors in the system. However, if this
maneuver is undesirable or impossible, make sure that one end of the resistor is not connected to
any other element.
The use of the VOM and DMM will be described early in the laboratory session. For
the VOM, always reset the zero-adjust whenever you change scales. In addition, always choose the
dial setting (RX1,R X 10, and so on) that will place the pointer in the region of the scale that will
give the best reading. Finally, do not forget to multiply the reading by the proper multiplying fac-
tor. For the DMM, remember that any scale marked "K" will be reading in kilohms, and any
"MO" scale in megohms. For instance, a 91-2 resistor may read 90.7 N on a 200-12 scale, 0.091
on a 2-k12 scale, and 0.000 on a 2-MI scale. There is no zero-adjust on a DMM meter, but make
sure that R=0 when the leads are touching, or else an adjustment internal to the meter may have
to be made. Any resistance above the maximum for a chosen scale will result in an O.L. indication.
It is important to remember that there is no polarity to resistance measurements.
Either lead of the meter can be placed on either end of the resistor the resistance will be the
same. Polarities will become important, however, when we measure voltages and currents in the
experiments to follow.
Both the VOM and the DMM will be used in this experiment to measure the resistance of the pro-
vided resistors. The VOM employs an analog scale to read resistance, voltage, and current,
whereas the DMM has a digital display. An analog scale is a continuous scale, requiring that the
user be able to interpret the location of the pointer using the scale divisions provided. For resise
tance measurements, the analog scale is also nonlinear, resulting in smaller distances between in
creasing values of resistance. Note on the VOM scale the relatively large distance between 10
and 10 N and the smaller distance between 100 N and 1000 N. The digital display provides a nu.
merical value with the accuracy determined by the chosen scale. For many years the analog me
ter was the instrument employed throughout the industry. In recent years, the digital meter has
grown in popularity, making it important that the graduate of any technical program be adept at
using both types of meters.
PROCEDURE
RÉSUMÉ OF THEORY
The purpose of this experiment is to acquaint you with the laboratory equipment, so do not rush.
Learn how to read the meter scales accurately, and take your data carefully. If you are uncertain
about anything, do not hesitate to ask your instructor
Part 1
Body Size
In this experiment, the resistance of a series of 1/2-W film resistors first will be determined from
the color code and then compared with the measured value using both the VOM and the DMM
The two meters were applied to ensure familiarity with reading both an analog and a digital scale.
The procedure for determining the resistance of a color-coded resistor is described in
Appendix I with a listing of the numerical value associated with each color. The numerical value as
sociated with each color has been repeated in Fig. 2.1. The first two bands (those closest to the end
of the resistor) determine the first two digits of the resistor value, while the third band determines
the power of the power-of-10 multiplier (actually the number of zeros to follow the first two digits).
If the third band is silver (0.01) or gold (0.1), it is a multiplying factor used to establish resistor val
ues less than 10 22. The fourth band is the percent tolerance for the chosen resistor (see Fig. 2.1).
(a) In the space below, draw the physical sizes of 1/4-W, 1/2-W, 1-W, and 2-W. 1-MA
film resistors (color bands = brown, black, green). Note in particular that the resistance of each is
the same but that the size increases with wattage rating. Identify each by its wattage rating.
Green
Blue
Purple
Gray
White
Black
Brown
Red
Yellow
Orange
6
7
8
5
9
0
2
1
3
4
Red Red B
Silver
2 2 1
10%
(b) How much larger (physically) is the 1-W resistor than the 1/2-W resistor? Is the
ratio about the same for the 2-W resistor as compared with the 1-W resistor? Answer the ques-
tions in sentence form.
FIG. 2.1
220 2 10% = 220 22 $ 222 = 1982 to 242 12
EXPERIMENT dc 2
RESISTORS AND THE COLOR CODE
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TABLE 2.2
Resistor
Minimum Resistance
Maximum Resistance
22 Ω
20,9 12
23.10
Part 2 Color Code
(a) Using the procedure described in the Resume of Theory, determine the color
bands for each resistor appearing in Table 2.1. Then find each resistor in your provided kit and
enter the colors for all four bands in Table 2.1, as shown by the example.
(b) Enter the numerical value of each color in the next column, as shown by the
example.
9112
220 Ω
3.3 k2
10 k2
TABLE 2.1
Color Bands-Numerical Value
3
2
4
470 ΚΩ
Color Bands-Color
1
4
Resistor
(Nominal
Value)
3
I MA2
2
0
1
5%
2.
2
2
Gold
Black
6.8 12
Red
2212
Red
9112
22012
% Difference
Nominal Measured
Nominal
X 100%
3.3 k12
(2.1)
10 kA
470 k 2
TABLE 2.3
1 ΜΩ
Meter
DMM
VOM
6.812
Normal
Resistor
Value
Measured
Value
Falls within
Specified
Tolerance
(Yes/No)
%
Difference
Measured
Value
Falls within
Specified
Tolerance
(Yes/No)
%
Difference
Sample:
22 Ω
22.9 22
Yes
4.09%
23 Ω
Yes
4.5%
(C) The percent tolerance is used to determine the range of resistance levels within
which the manufacturer guarantees the resistor will fall
. It is determined by first taking the percent
tolerance and multiplying by the nominal resistance level. For the example in Table 2.1, the
resulting resistance level
(5%)(22 12) = (0.05) (22 12) = 1.12
is added to and subtracted from the nominal value to determine the range as follows:
9112
220 Ω
3.3 kΩ
10 k 2
Maximum value = 22 2 + 1.1 12 = 23.1 12
Minimum value = 22 - 1.11 = 20.92
470 k 2
as shown in Table 2.2.
1 ΜΩ
6.8 22
Complete Table 2.2 for each resistor in Table 2.1.
(d) Read the resistance level using the DMM and insert the value in Table 2.3. For
each resistor, take the time to choose the scale that will result in the highest degree of accuracy
for the measurement. Ask for assistance if you need it.
Then determine the magnitude of the difference between the nominal and measured
values using the following equation. The vertical bars of the equation specify that the sign
resulting from the operation is not to be included in the solution.
(e) Repeat part (d) using the VOM and insert the data in Table 2.3. As with the DMM,
be sure to ask for assistance if you need it. It is very important to become familiar with the oper-
ation of each meter.
EXPERIMENT dc 2
RESISTORS AND THE COLOR CODE
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range? You can check by referencing Table 2.2 or comparing the percent tolerance to that listed
(1) For the DMM measurements, are all the resistors within the specified tolerance
in Table 2.1. If not, how many resistors did not fall within the tolerance range?
whereas the 250-V scale will have (250 V)(20,000 A/V) = 5 MI. No matter what voltage level
is measured on each scale, the internal resistance will remain the same.
Using the DMM as an ohmmeter, measure the resistance of each de voltage scale of
the VOM and record in Table 2.5. Complete the table only for those scales found on your VOM.
If the calculated resistance is greater than the range of your DMM, simply put a dash in the space
provided for the measured resistance.
TABLE 2.5
range? Again, you can check using the range of Table 2.2 or the percent tolerance of Table 2.1. If
(g) For the VOM measurements, are all the resistors within the specified tolerance
not, how many resistors did not fall within the tolerance range?
VOM
ΩN Rating
Calculated
Resistance
% Difference -
Calc. Meas.
X 100%
Calc.
Measured
Resistance
2.5 V
10 V
50 V
Part 3
250 V
1000 V
Body Resistance
Guess the resistance of your body between your hands and record the value in Table 2.4. Measure
the resistance with the DMM by firmly holding one lead in each hand (wet your fingers and hold
the leads as tight as possible), and record in the same table. If 10 mA are “lethal," what voltage
(V = IR) would be required to produce the current through your body? Again, record in Table 2.4.
Do the resulting percent differences and verify that the internal resistance of a VOM
can be determined using the procedure just outlined? Answer the question in sentence form.
TABLE 2.4
Guessed body resistance
Measured body resistance
Lethal voltage
Calculation:
Most DMMs have the same internal resistance for all the de voltage scales. There are
a few meters, however, that have a lower internal resistance for some lower scales, such as less
than 1 V. An internal resistance of 10 MQ to 20 MQ is typical for a variety of commercially avail-
able DMMs.
Use your laboratory neighbor's DMM to measure the internal resistance of each do
voltage scale of your DMM and complete Table 2.6 for all the scales that appear on your meter.
TABLE 2.6
Specified Internal
Resistance
Measured Resistance
DMM
200 mV
Part 4 Meter Resistance
(a) Voltmeters Ideally, the internal resistance of the DMM and VOM should be
infinite (like an open circuit) when voltages in a network are being measured to ensure that the
meter does not alter the normal behavior of the network.
For the VOM, there is an ohm/volt (/V) rating written on the bottom of the scale that
permits determination of the internal resistance of each scale of the meter when used as a
voltmeter
. For the 260 Simpson VOM, the ohm/volt rating is 20,000 N/V. The internal resistance
of each setting then can be calculated by multiplying the maximum voltage reading of a scale by
the ohm/volt rating. For instance, the 10-V scale will have (10 V)(20,000 N/V) = 200 kN,
2 V
20 V
200 V
1500 V
EXPERIMENT de 2
RESISTORS AND THE COLOR CODE
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possible
network
EXERCISES
If the internal resistance is more than the maximum resistance the DMM can read, simply insert
the maximum possible reading for your DMM in the measured resistance column.
Based on the fact that the internal resistance of a meter should be as large as
for each dc voltage scale, which meter (the VOM or DMM) would appear to disturb a
1. What are the ohmic values and tolerances of the following commercially available carbon
resistors?
Color Bands-Color
1
2
3
4
Numerical Value
Tolerance
Brown
network
Black
Blue
Gold
Yellow
Violet
Orange
Gold
(b) Ammeters The ammeter is also an instrument that when inserted in a
possible,
should not adversely affect the normal current levels. However, since it is placed in series with
Brown
Gray
Gold
None
the branch in which the current is being measured, its resistance should be as small as
Red
Yellow
Silver
Gold
Using the DMM as an ohmmeter, measure the resistance of each dc current scale of
the VOM and record in Table 2.7. Change the maximum value for each current scale of Table 2.7
if different on your VOM.
Green
Brown
Green
Silver
Green
Blue
Black
None
TABLE 2.7
2. For the VOM, which region of the scale normally provides the best readings? Why?
VOM
Measured Resistance
1 mA
10 mA
100 mA
500 mA
3. In your own words, review the procedure for using a DMM to read the resistance of a resistor
Using the VOM as an ohmmeter, measure the resistance of each current scale of the
DMM and record in Table 2.8. Change the maximum value for each current scale of Table 2.8
if different for your DMM.
TABLE 2.8
DMM
Measured Resistance
2 mA
4. In your own words, review the procedure for using a VOM to read the resistance of a resistor
20 mA
200 mA
2 A
Based on the fact that the internal resistance of an ammeter should be as small as pos-
sible for all current ranges, which meter (the VOM or DMM) would appear to disturb the network
the least? Answer the question in sentence form.