••
~!
Activity: 3
DC Measurements:
Group Number:
Voltage and Current
Group Members with Reg #:
Date of Activity:
Scope:
At the end of the exercise the students will be able to
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Measure Voltage and Current in a circuit.
•
Verify Ohm's Law.
•
Explain the effect of Open Circuit.
Note:
•
Before using a DMM to measure any parameter (current, voltage or resistance), the range
switch hasto be properly selected first otherwise damages can happen to a multimeter or
circuit board.
•
Use the largest scale range before any measurement. This reduces damage to meters and also
the possible error from the readings.
Voltage Measurements
in a Series Circuit
1. Set DMM as Voltmeter by connecting positive meter lead (Red) to the Volts/Ohms terminal and
black meter lead to the Ground/Com, For dc voltage measurement locate and press ---V sign
button, you can select the range (mV, V) by pressing Hi or Low buttons.
aJ Change the range to Volts, without connecting the testleads to anything, what does the display
read? Why?
b) Connect the two test leads together, what happened? what is the reading now? Why?
..
2.
You are provided
with four different
from the color code table
resistances.
For each resistor,
determine
its nominal
value
(arrange Rl as lowest and R4 as highest), and connect the circuit as
in fig.
Value of Rl:
Value of R2:
Value of R3:
Value of R4:
Red Probe
Black Probe
R3
R4
Voltmeter
Power
Supply
+5V
3.
By using conventional
approach,
Measure
+5V side and the black probe between
COM
the voltage drop across Rl with the red probe on the
Rl and R2.
a)
What is the voltage
reading?
b)
Now exchange the red and black probe. What is the voltage reading? Comment.
4.
Measure the voltage drop across the iemaining
resistors R2, R3, and R4. What is the sum of the
voltage drops? Comment.
5.
Knowing the voltage drop across each resistor, use Ohm's Law to calculate the current
through
each resistor. Comment.
6.
Calculate the total resistance;
use Ohm's Law to find the current.
agree? Comment.
7.
Calculate power dissipation
of each resister (use any formula).
Do current values in 5 and 6
Current Measurements in a Parallel Circuit
For current measurements,
the probes must be in series with the load only and current
limited to less than 500 mAmps. If the circuit draws SOOmAmps or more, a fuse will blow and
disable the instrument.
•
Insert the black probe into the black terminal
red terminal
labeled A for current
labeled COM; then insert the red probe into the
measurements.
•
Press and locate the DC Ampare --- A button to measure DC current.
•
Range can be selected
•
Measure the current
by pressing Hi/La button.
with probes in series with the load; then view the current measurement
on
the display screen.
I
Red Probe
~
R1
A
R2
B
R3
C
R4
+5V
8.
Power
Supply
Set up the DMM to measure
displayed
on the DMM?
D
COM
dc current.
Without
connecting
the test leads to anything, what is
Why? Connect the two test leads together,
what is the reading? Why?
,
.
,
9.
Set up the parallel circuit,
Adjust the output of the power supply to 5 V, Place Rl, R2, R3, and R4
in parallel as shown in figure
second lead of resistor
a)
Place an ammeter
current reading?
Comment.
b)
(arrange Rl as lowest and R4 as highest), at this stage leave the
open (I.e. open connections
across A terminal
across A, B, C, and
to measure the current
OJ.
through
RI. What is the
Exchange the probes across point A. What is the current
Measure the current
the currents through
10. Knowing the current through
through the remaining
Rl to R4? Comment.
reading?
resistors R2, R3, and R4. What is the sum of
each resistor, use Ohm's Law to calculate the voltage drop across
each resistor. Comment.
11. Complete the connection
How does it compare
across points A, B, C, and D. Measure the total current of the circuit.
with the sum of the currents through
the resistors?