NAME
DATE
CLASS
LAB
2
Resistance
Measurements
Resistan ce Measu remen ts
OBJECTIVES
After completing this lab, you will be able to
measure resistance with analog and digital ohmmeters,
compare color band values with measured values of resistance,
compare the physical dimensions of resistors of different power
rating,
• measure the resistance of a diode,
• observe how temperature affects resistors and thermistors.
•
•
•
EQUIPMENT REQUIRED
Digital multimeter (DMM)
Volt-ohm-milliammeter (VOM)
Ruler or caliper scaled in centimeters
Note: Record this equipment in Table 2-1.
COMPONENTS:
resistors:
thermistor:
diode:
47-Ω, 220-Ω, 3.9-kΩ, 10-kΩ, 180-kΩ, 5.6-MΩ
(all resistors are 1/4-W, 5% tolerance)
470-Ω (1/8-W), 470-Ω (1/4-W), 470-Ω (1/2-W),
470-Ω (1-W), 470-Ω (2-W).
10-kΩ @ 25˚C (or equivalent)
1N4004 (or equivalent)
EQUIPMENT USED
Instrument
Manufacturer/Model No.
DMM
VOM
Other
Table 2-1
11
Serial No.
12
Resistance Measurements
TEXT REFERENCE
Section 3.6
Section 3.7
Section 3.8
Section 3.9
COLOR CODING OF RESISTORS
MEASURING RESISTANCE - THE OHMMETER
THERMISTORS
NON-LINEAR RESISTANCE
DISCUSSION
Fixed resistors are used extensively in electrical and electronic circuits to limit the amount of current in the circuits. Although the
value of a resistor (in ohms) is occasionally stamped on the resistor
(e.g., 330[330-Ω], 2k0[2.0-kΩ]), it is more common to designate the
resistance value with a standardized color code. Figure 2-1 shows a
typical color-coded resistor.
The procedure for determining the value of a color-coded resistor is
described in Section 3.6 of the textbook and is briefly outlined in Appendix A. Resistor color codes are read from left to right; left being defined as the end of the resistor with the band nearest to the end. The first
two bands of the resistor provide the first and second digits of the resistance value. The third band is the multiplier and represents the number
of zeros following the first two digits. The fourth band provides the tolerance (or uncertainty) of the resistance value. A fifth band is occasionally used to represent the reliability of the resistor. This last band is
used primarily in military applications and seldom used in commercial
electronics. The physical size of the resistor is an indication of how
much power the resistor can safely dissipate. Resistors with a large surface area can handle more power than small resistors.
When measuring resistance with an ohmmeter, several important steps must be followed.
1. All power supplies must be disconnected from the circuit. If
power is not removed, the ohmmeter may be damaged.
Figure 2-1 Resistor Color Codes.
Lab 2
13
2. If the component is part of a larger circuit, it is necessary to isolate
the component from the rest of the circuit. This is done by disconnecting at least one terminal of the component from the circuit.
3. Connect the two probes of the ohmmeter across the component
to be measured. Be careful not to touch both probes of the meter,
since your body’s resistance may introduce additional error.
When measuring resistance, the red and black leads of the ohmmeter can usually be interchanged without affecting the reading.
The exception is diodes and most active components such as transistors and integrated circuits (ICs).
4. Select a range which is appropriate for the measurement to be taken.
Prior to taking measurements a meter movement may need to be
zeroed. This is generally done by adjusting a ZERO OHMS potentiometer. When using a meter with a PMMC (permanent magnet
moving coil) movement (such as a VOM), the most accurate reading
is obtained when the needle is approximately midscale. You must
remember to multiply the reading by the corresponding scale multiplier. For example, a reading of 2.4 on the R×100-Ω range corresponds
to a resistance of 240 Ω. When using a digital ohmmeter, the ranges
represent the largest resistance which can be measured on the scale.
For example, a reading of 2.4 on the 200-Ω range corresponds to a
resistance of 2.4 Ω, while the same reading on the 20-kΩ range corresponds to a resistance of 2.4 kΩ.
5. When storing the ohmmeter, it is necessary to ensure that the ohmmeter is turned off, since it is possible to drain the internal battery if
the probes are accidently connected together for any length of time.
CALCULATIONS
1. Refer to the color coded resistors given by your lab instructor. Determine the resistor value and the corresponding tolerance for
each resistor and enter your results in Table 2-2.
Example
R1
R2
R3
R4
R5
R6
Table 2-2
Color Codes
Resistance and Tolerance
Brown Black Orange Gold
10 kΩ ± 5%
14
Resistance Measurements
2. The tolerance of the resistor is a specification indicating the range
of possible values in which the actual resistance of the component will occur. The resistor in the example of Table 2-2 has a
nominal value of 10 kΩ with a tolerance determined as follows:
Tolerance = (5%)(10 kΩ) = 0.5 kΩ = 500 Ω
By specifying a tolerance of 5%, the manufacturer of the resistor is providing assurance that the actual value of the resistor is
between 9.5 kΩ and 10.5 kΩ. Calculate the expected minimum
and maximum values for each of the resistors in Table 2-2. Enter
the data in Table 2-3.
Example
Minimum Resistance
Maximum Resistance
9.5 kΩ
10.5 kΩ
R1
R2
R3
R4
R5
R6
Table 2-3
MEASUREMENTS
3. Connect the ohmmeters as shown in Figure 2-2 and measure each
of the resistors. Record the results in Table 2-4. Reverse the leads
of the ohmmeter and measure again. There should be no difference in the reading.
Example
R1
R2
R3
R4
R5
R6
Table 2-4
DMM
VOM
Other
9.83 kΩ
9.8 kΩ
9.90 kΩ
Lab 2
15
Figure 2-2 Measuring Resistance With an Ohmmeter.
4. Select five resistors with the same color bands but different power
ratings. Record the color codes and expected resistance values for
the resistors on the top portion of Table 2-5. Since the codes are
the same, the resistors will all have the same value. Use the DMM
to measure each of the five resistors. Enter the results in the appropriate column of Table 2-5.
Resistor Color Code
Resistance and Tolerance
Measured Resistance
Volume
R1 (1/8-W)
R2 (1/4-W)
R3 (1/2-W)
R4(1-W)
R5(1-w)
Table 2-5
5. Use a ruler or caliper to measure the physical dimensions (length
and diameter) of the resistors. Calculate and record the results in
Table 2-5.
16
Resistance Measurements
Diodes are electronic devices which permit current in one direction, while preventing current in the opposite direction. The principle is the same as a check value in a water line. Although the theory
of operation is outside the scope of this book, we will observe how
the resistance of a diode is affected by the way it is connected into a
circuit.
6. Connect the VOM to the diode as illustrated in Figure 2-3(a). The
diode is said to be operating in its forward region. Record the results in Table 2-6.
7. Connect the VOM to the diode as illustrated in Figure 2-3(b). The
diode is said to be operating in its reverse region. Record the results in Table 2-6.
8. Repeat the diode measurements using a DMM. It may be necessary to move the range switch to the “diode” position (if one is
available). Record the results in Table 2-6.
VOM
Forward
Reverse
DMM
Forward
Diode
Table 2-6
Figure 2-3 Measuring Diode Resistance With an Ohmmeter.
Reverse
Lab 2
17
A thermistor is a transducer in which the resistance changes with
changes in temperature. If the resistance decreases as the temperature increases, the component is said to have a negative temperature
coefficient (NTC). Conversely, if the resistance increases as the temperature increases, the component is said to have a positive temperature coefficient (PTC).
9. Connect a DMM to the terminals of a thermistor. Measure the
resistance of the thermistor at room temperature and record the
result in Table 2-7. With the DMM still connected to the thermistor, hold the body of the thermistor with two fingers. The additional temperature from your fingers will raise the temperature
of the component to approximately 30˚ C. Once the meter display has settled, record the approximate resistance of the thermistor in Table 2-7.
10. Repeat the measurement of Step 9 with a resistor having a similar resistance. You should observe that the resistance remains
relatively constant in comparison to the thermistor. Record the
results in Table 2-7.
Thermistor
Resistor
T = 20˚C
T = 30˚C
Table 2-7
CONCLUSIONS
11. Compare the resistance measurements of Table 2-4 to the expected tolerances in Table 2-3. Are the measured values of all resistors within the tolerances specified by the colored bands? If
not, indicate which resistors fall outside the accepted values and
calculate the actual percent deviation of the measured values
from the color code values.
12. Refer to the data of Table 2-5.
a. Are all the resistor values within the specified tolerance? If
not, indicate which resistors fall outside the accepted values.
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18
Resistance Measurements
b. What conclusion can you make about the power rating of a
resistor and its physical size?
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13. Compare the forward resistance of a diode to its reverse resistance. Do both meters provide the same readings?
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14. Compare the resistance of the thermistor at room temperature
to the reading when the temperature was increased by the heat
in your fingers. Does the thermistor have a negative temperature coefficient or a positive temperature coefficient?
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15. Compare the resistance of a resistor at room temperature to the
reading when the temperature was increased by the heat in your
fingers. Does the resistor have a negative temperature coefficient, positive temperature coefficient, or is resistance constant?
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PROBLEMS
16. Complete Table 2-8 by giving the resistance value of each resistor and the corresponding tolerance (in percent).
Color Codes
R1
Brown Red Yellow Gold
R2
Orange White Gold Gold
R3
Blue Gray Green Silver
R4
Yellow Orange Brown Silver
R5
Brown Green Orange
Table 2-8
Resistance and Tolerance
Lab 2
19
17. Explain how an ohmmeter could be used to determine whether
a light bulb is burned out.
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18. A VOM is used to measure the resistance of a diode. In one direction, the resistance of the diode is measured to be 250 Ω while in
the other direction, the resistance is measured to be 380 Ω. Is
this diode faulty? Explain your answer.
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