Download ELEC 103 LABORATORY EXERCISE 4 RESISTANCE

ELEC 103 LABORATORY EXERCISE 4 RESISTANCE MEASUREMENTS
PURPOSE OF EXPERIMENT
To familiarize the user with the DC Ohmmeter scales and ranges of a general purpose multimeter such
as the Simpson, Model 260 multimeter. And, to acquaint the user with safety precautions, equipment
precautions, and measurement methods.
EQUIPMENT LIST
1 − Simpson, Model 260 multimeter
1 − 1000Ω, 10,000Ω, and 1,000,000Ω 1 Watt Resistor
1 − 10kΩ potentiometer.
PROCEDURE
1. In addition to being used as a voltmeter, and a
milliammeter, the Simpson, Model 260, multimeter
can be used as an Ohmmeter. Observe the top scale
line on you meter. Notice the following significant
differences between the Ohms scale and the DC
scale used previously:
a.) Zero is at the right end of the scale and the scale
markings increase in value going to the left.
b.) The divisions on the Ohms scale are not linear
(equal in size). In fact, note that the physical
length of the segment between 2 and 5 on the
Ohms scale is roughly the same as the segment
between 5 and 10. Yet 5 minus 2 = 3 units,
whereas 10 minus 5 = 5 units, different values
for roughly equal length scale segments. This
type of scale is generally referred to as a
nonlinear scale. Nonlinear scales are used for
Ohmmeters while linear scales are used for
voltage and current meters.
Now turn your attention to the RANGE/FUNCTION SWITCH. There are three resistance or
Ohmmeter ranges, R X 1, R X 100, and R X 10000. Depending upon the size of the resistance
to be measured, one of these ranges is selected and used with the Ohms scale.
2. On the R X 1 range, any scale reading is interpreted using the numbers printed on the scale.
That is, all meter readings are multiplied by 1. On the R X 100 range or the R X 10,000 range,
the meter readings are multiplied by 100 or by 10,000 respectively. Consequently, if the needle
is pointing at 5 on the Ohms scale, it means the reading is 5 Ω if you are using the R X 1 range.
It would mean 500 Ω if using the R X 100 range or 50,000 Ω if using the R X 10,000 range.
3. Before proceeding with the experiment, the following Ohmmeter precautions must be reviewed
since serious damage to the multimeter can result if they are not carefully followed.
OHMMETER PRECAUTIONS
A. Move the status switch to the +DC position.
B. Move the range switch to one of the three resistor ranges. Unlike the voltmeter and the
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ELEC 103 LABORATORY EXERCISE 4 RESISTANCE MEASUREMENTS
milliammeter, the Ohmmeter does not need to be adjusted to the highest range. The
Ohmmeter can be adjusted to any range if it is connected to the circuit properly.
C. Remove all power from the circuit.
D. Remove at least one end of the component being measured from the circuit.
E. Connect the ohmmeter leads together and use the zero ohms adjust knob to make the pointer
read zero ohms before each use.
4. Adjust the STATUS/POLARITY switch to the +DC position and make certain the multimeter
test leads are in the proper receptacles. Turn the RANGE switch to the R X 1 range and short
(connect together) the two meter leads. Observe that the meter pointer moves up the scale and
comes to rest somewhere around zero ohms. With your test leads shorted, you want your meter
to "think" it is reading ZERO OHMS of resistance since there is no resistance between the
meter leads.
To accomplish this strategy leave the meter test leads shorted. Turn the ZERO OHMS adjust
knob slowly to the extreme counterclockwise position and then to the extreme clockwise
position. Observe the pointer movement. You should see the pointer move from below full
scale to greater than full scale. If this does not occur, contact you lab instructor immediately.
5.
Adjust the pointer, using the ZERO OHMS ADJUST knob, until the pointer is over the zero on
the Ohms scale. Once this has been accomplished, you have calibrated your Ohmmeter for this
measurement and you can UNSHORT the meter test leads.
NOTE: It is mandatory that you ZERO OHMS ADJUST the Ohmmeter before each
initial measurement on a specific range. Therefore, each time you change ranges you must
zero ohms adjust the meter.
6.
Using appropriate ranges, measure the resistance of the 1000Ω resistor, the 10,000Ω resistor,
and the 1,000,000Ω resistor. Record the measured values on the datasheet. MAKE CERTAIN
YOU RE−ZERO THE METER WHENEVER CHANGING RESISTANCE RANGES.
7.
Re−measure the 1,000,000Ω resistor using the appropriate range. However, this time place one
hand on each end of the resistor so your hands are in contact with the resistor and the meter test
leads. Record the results of the measurement on the datasheet.
8.
Repeat Step 7, only wet your fingers before touching the resistor and the meter leads. Record
the results of the measurement on the datasheet.
9.
The potentiometer is a variable resistor that has three terminals. One pair
of terminals is fixed and you will measure the total potentiometer
resistance between these terminals. The other lead connected to a wiper.
When you connect the ohmmeter between the wiper and one of the fixed
terminals, you will measure a resistance that may be any value between 0
Ω and the maximum resistance value of the potentiometer. The resistance
value that you measure will depend on the position of the wiper.
Connect the ohmmeter across the fixed leads of a potentiometer. Adjust
the meter to make an appropriate resistance reading. Slowly adjust the
FIGURE 4−1
wiper and observe the reading on the meter but be careful not to touch the
leads with your fingers. Record your observation on the Data Sheet as either remains constant
or varies.
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ELEC 103 LABORATORY EXERCISE 4 RESISTANCE MEASUREMENTS
10.
Move one of the leads to the wiper terminal. Do not change the range setting of the
ohmmeter. Slowly adjust the wiper and observe the reading on the meter but be careful not to
touch the leads with your fingers. Record your observation on the Data Sheet as either remains
constant or varies.
Answer the questions on the Data Sheet in the space provided
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ELEC 103 LABORATORY EXERCISE 4 RESISTANCE MEASUREMENTS
DATASHEET FOR EXPERIMENT 4
STEP 6:
Color Code Value Measured Value
Meter Range
Used
1,000Ω
10,000Ω
1,000,000Ω
STEP 7:
Measured resistance = _____________ Range = ___________
STEP 8:
Measured resistance = _____________ Range = ___________
STEP 9:
As you move the wiper, the resistance between the two fixed terminals ___________________.
As you move the wiper, the resistance between the wiper and the fixed terminal
____________.
QUESTIONS:
1.
When you read the Ohmmeter in STEP 6, you probably noticed a difference between the meter
reading and the resistance value predicted by the color code. Give a reason for the difference.
2.
What was the purpose of STEP 7 ?
3.
Using the measured values of the resistors in STEP 6, calculate the value you would measure if
you placed the three resistors in parallel. Show the formula and the calculations used to arrive
at the answer.
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