Basic Circuitry and
Measurements
Lab 3
1
Equipment List
•
•
•
•
•
•
Multimeter
DC power supply
Breadboard
Decade resistance box (2)
R-C combination box
Resistance: 1kΩ, 2.2kΩ
2
Circuit Testing
• To see if a circuit works, voltages and currents
have to be measured
• Multimeters are used to measure DC voltages and
currents
3
Using the Multimeter
• Switch the multimeter on. We will first use it to measure
resistance
• Push the third soft key in from the left on the top row. The
display should say that it is measuring ohms
• There are five terminals on the right in two columns. Never
use the terminals on the left column
• Place one wire in the ‘HI’ and and the other in the ‘LO’
terminal.
4
Using a Decade Box
• Find one of the decade resistance boxes.
The four terminals on it form six possible
pairs e.g., top left and bottom right. List
these six combinations
• Set the box to 1 kilo-ohm and then use the
multimeter to measure the resistance for
each of the six possible pairs
• Record the values on your data sheet
5
Valid Terminal Pairs
• Internal circuitry of a decade box:
1
3
2
4
• Any valid terminal pair consists of one red
and one black terminal
6
“Measuring” Resistance
• The multimeter generates a small test
current which it runs through the resistance
being measured
• The multimeter then measures the voltage
drop across the resistance
• Ohm’s Law is used to compute the
resistance value which is then displayed
7
Measurement Limitations
• Can the multimeter be used to measure the
resistor’s value without first removing the
resistor from the network?
• Record your answer clearly and legibly on
your data sheet
8
Connecting a Circuit
• Can we connect the following circuit using
one decade box? (Assume R1 and R2 have
the same values)
9
Can These Connections Be Used?
1
3
2
4
• Why or why not? Record your answers on
the data sheet
10
Connecting a Voltage Divider
• Connect the following circuit using two individual
decade boxes and a DC power supply
• Values for R2 are given on your grading sheet
11
Taking Measurements
• Use the multimeter to measure the voltages
across the resistances and the current
flowing in through the circuit
– Voltage Measurement: Connect the multimeter
in parallel. Use the Rightmost HI and the
rightmost LO terminals. Push the DC V button
– Current Measurement: Use Ohm’s Law
– It is usually easier to measure voltages rather
than current
12
Limitations of Measuring Devices
• For the voltage divider circuit, set both
decade resistance boxes up to their
maximum value (999,999 ohms)
• Experimentally check KVL by measuring
the voltage drop across each resistance
individually
– Do the voltage drops add up to the source
voltage? Explain your findings
13
Reason For The Limitation
• Connect two decade resistors in parallel. Set
one decade box to 100 kilo-ohms, There is
NO source in this circuit
• Use the multimeter to measure the resistance
of the parallel combination for different
resistance settings of the second decade
resistor given in the grading sheet
14
Reason For The Limitation
• When two resistances are connected in
parallel, the resistance of the overall
combination decreases
– Less voltage is dropped by the same current
across a lower valued resistance
• Therefore, if the resistance across which the
voltage is being measured is comparable to
the internal resistance of the multimeter,
errors in measurements are introduced
15
Effect of Lower Internal Resistance
• Connect the following circuit in PSPICE
16
Circuit Significance
• This circuit is to simulate the case when a
multimeter with low internal resistance
100k (depicted by R3) is used to make
measurements
– This circuit is similar to the case when an actual
multimeter is connected across R2 in the
original voltage divider
17
Measurements
• Measure the voltage across R2 for each of
the 3 values of R2
• Record the data on the grading sheet
18
Understanding the Difference
• When we use the multimeter to make DC
voltage measurements, it has an input
resistance of 10 MW
• In the second circuit, R3 depicts a
multimeter with relatively low input
resistance. Use the measurements you made
to explain why low input resistance is not
ideal
19
Building Circuits Differently
• Circuits can be built with components used
in front end circuitry of equipment such as
oscilloscopes, wave form generators
• Components commonly used are
– Resistances, inductances and capacitances
– Chips, integrated circuits
20
Component Specifications
• Information supplied about each component
–
–
–
–
–
Nominal value
Range of variability about the nominal value
Power ratings
Temperature ratings
Noise issues
• We will need only nominal values and the
variability for our purposes
21
Reading Resistance Values
• Resistances come as 3 band or 4 band
resistances. We deal only with 3 band
resistors
• The colour of the bands indicate the value
of the resistance
0 - Black
1 - Brown
2 - Red
3 - Orange
4 - Yellow
5 - Green
6 - Blue
7 - Violet
8 - Grey
9 - White
1st band = 1st digit of resistance value
2nd band = 2nd digit of resistance value
3rd band = Multiplier
Example: Brown = 1, Black = 0, Red = 2
Resistance = 10 (Brown, Black) X 10^2(Red)
= 1000
22
Variation About Nominal Value
• There is usually an extra band that denotes
the variability of the value of the resistance
about the stated value
– Gold - 5% variability
– Silver - 10% variability
– No band - 20% variability
23
Reading and Measuring Resistance
• Using the colour codes, figure out the value
of the two resistances
• Use the multimeter to measure the actual
resistance of each of the two resistors (Clip
a wire onto each end of the resistance)
• Record your values on your data sheet
24
Breadboard
• The breadboard has two halves separated by an
indentation. The holes in the breadboard are electrically
connected on the bottom of the breadboard
25
Breadboard
• The holes between the indentations are electrically connected column
wise
• The holes at along the red and blue lines are electrically connected
along the rows
• The indentations denote electrical separation
• The top and bottom halves are symmetrical
…
Indentations
…
Electrical connections
26
Placing and Removing Components
• Make sure that the pins on chips or wires
are straight and that they line up well with
the holes you intend to use
• Extract things slowly and carefully. Take
your time. For chips, slide something (a
wire or jeweler’s screw driver blade that
you have handy) under the item you are
extracting and gently raise it
27
Build a Voltage Divider
+
Series
Connection
Power Supply
R2 (2.2k)
R1 (1k)
28
Measurements
• Use the multimeter to measure voltage
across each resistance individually
• The multimeter just clips on to either side of
the resistance across which you are
measuring voltage
29