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ENGR201 Circuits I
Fall 2001
Basic DC Measurements
BACKGROUND
OBJECTIVES:
The goals of this laboratory exercise are to learn how to:
A. use some basic pieces of laboratory equipment, including
1. a circuit breadboard,
2. a dc power supply, and
3. a digital multimeter; (DMM)
B. use a breadboard to construct circuits from a circuit schematic and measure electrical
quantities of resistance, voltage, and current.
I. THE BREADBOARD
A breadboard is a device used to facilitate the building and testing (called prototyping) of
circuits. The breadboard accepts pre-stripped wires, making it easy to make connections
between various electrical components, power supplies, and meters. Breadboards allow
circuits to be tested and modified easily, since no permanent connections are required. Figure
1 shows a picture of typical breadboard that also has terminals for connecting to a power supply
(V1, V2, V3, and ground). One way to think of a breadboard is a collection of nodes with
multiple points (tie points) where components may be connected. A node (also called a bus) is
an area of a circuit where the voltage anywhere in the area is the same. Several nodes have
been highlighted on Figure I.
Figure 2 shows several resistors connected to two power sources. Resistor R1 and R2 are in
series, while R3 and R4 are connected in parallel. Note that one end of each of the resistors
R2, R3, R4, and R5 are all tied to the same node (one row of the breadboard). The schematic
corresponding to the connection is shown in Figure 3.
Use the most sensitive range of a DMM to measure the resistance between any two points on
the same node. What reading is displayed?
________________________________________
Use the least sensitive range of a DMM to measure the resistance between any two points on
different nodes. What reading is displayed?
________________________________________
ENGR201 Basic DC Measurements
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Figure 1
A node with 40 tie points
Nodes with 5 tie points
Nodes with 10 tie points
ENGR201 Basic DC Measurements
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Figure 2
C
A
R2
R1
R5
B
R3
D
R4
E
Figure 3
R1
A
R2
B
V1
R5
C
D
R3
R4
E
V3
ENGR201 Basic DC Measurements
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II. RESISTANCE COLOR CODE
Many, but not all, resistors have their nominal value specified by means of four (4) color-coded
bands positioned towards one end of the resistor. Figure 4 shows how to interpret the color
code of a resistor.
Figure 4 - Resistor Color Code
Example: Yellow/Violet/Red/Gold
47x102 = 4.7 K +5%
Multiplier x 10N
Color
Black
Brown
Red
Orange
Yellow
Value
0
1
2
3
4
N
Color
Green
Blue
Violet
Gray
White
Tolerance
Value
5
6
7
8
9
Tolerance
Gold
+ 5%
Silver
+ 10%
III. DC POWER SUPPLY
A dc power supply is used, to supply dc power at a “constant” voltage to electrical and
electronic circuits. The dc power supply used for this lab is an ac-to-dc converter that has three
nominal voltage levels: 5V @1A, 12V @300mA.
The three outputs share a common
reference, and the supply is connected to the breadboard so that +5V is available as V1, +12V
as V2, and –12V as V3.
IV. THE BEL MERIT DX405 DIGITAL MULTIMETER (DMM)
The DX405 DMM is a digital (computer-like) device capable of making the following
measurements:
1.
2.
3.
4.
5.
dc voltage from 200mV to 1000v
ac voltage from 200mV to 750V, RMS
dc current from 200A to 10A
ac current from 200A to 10A , RMS
resistance from 200 to 20M
The DMM can also measure capacitance and frequency and be used to perform continuity and
diode tests. Figure 5 shows a picture of the DX405. Figure 6 illustrates the proper connections
for measuring resistance, voltage, and current measurements.
ENGR201 Basic DC Measurements
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 Voltage measurements are performed by placing the DMM probes in parallel with the
element under test
 current measurements are performed by placing the DMM probes in series with the
element under test
 resistance measurements are performed by de-energizing the circuit and placing DMM
probes in parallel with the element under test and
Figure 5 – DX405 Digital Multimeter
Figure 6 - Connections for Measuring Resistance, Voltage and
Current
Resistance
V//Hz
RX
DMM
COMMON
Voltage
V//Hz
DMM
COMMON
- VX +
+
(Note different
connection.)
Current
mA
DMM
Note, circuit must be
“broken” to measure IX
COMMON
+
-
IX
ENGR201 Basic DC Measurements
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Series and Parallel Connections
series – two components and only two components share a common node; the two elements,
therefore, have the same current.
I2 = I1
I1
parallel – two (or more) components are connected between the same two nodes; the two
elements, therefore, have the same voltage.
A
+
+
V1
V2 = V1
-
-
B
Connect two resistors in series as shown:
How do the actual values of R1 and R2 compare with the nominal values:
R1 = 1k
What is the total resistance measured across the series connection?
Add a third resistor, R3 = 3.3k in series and measure the total resistance.
What conclusion can you make about series connected resistors?
R2 = 2.2k
ENGR201 Basic DC Measurements
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Connect the circuit shown and measure:
VAB _______________
VBC _______________
VCD _______________
VAC _______________
VBD _______________
What can you conclude about the way voltage is distributed across series-connected resistors?
How is the voltage across series connected resistors related to the amount of resistance?
Connect two, the three, then four 100k resistors in parallel, each time measuring the total
resistance:
100k || 100k = __________
100k || 100k || 100k = __________
100k || 100k || 100k || 100k = __________
What conclusion can you make about the relationship between R total, the value of R, and the
number of resistors connected in parallel?
ENGR201 Basic DC Measurements
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Connect the following size resistors in parallel and measure Rtotal :
1k || 10k = __________
1k || 47k = __________
1k || 100k = __________
What conclusion can you make about connecting a relatively small resistor in parallel with a
large resistor?