Lab #1 ENG 220-001
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Introduction to DC Circuits
Basic Circuit Definitions and Concepts
The concepts and definitions outlined here are extremely important for understanding how to construct and
analyze electric and electronic circuits.
In general, an electric circuit consists of a group of components such as batteries, lamps, switches and motors
connected together, in some pattern, by conducting wires. Circuits that contain semiconductor devices such as
transistors and diodes, or thermionic devices such as vacuum tubes, are called electronic circuits. A DC circuit
is an electric or electronic circuit in which the electric current through every component is constant in time. All
of the points where a given circuit connects to devices that are not considered to be part of the circuit itself are
called terminals of the circuit.
Current (we use the symbol I ) is the rate as which charge flows past a given point in a circuit. It is measured in
units of Amperes. One Ampere (or Amp for short) is one Coulomb of charge passing a given point along the
circuit in one second. I = Q / T
A single loop circuit is one that consists of two or more components connected together in series (one after the
other) to form a single closed path. The current in a single loop circuit will always have the same magnitude at
all points around the loop, otherwise charge would build up or disappear somewhere which, by the law of
conservation of charge, cannot happen.
Multi-loop circuits contain nodes, points where the current can divide up and take alternate paths, or where
currents can merge. Any circuit path between two nodes is called a branch. The sum of all branch currents
arriving at a node must always equal the sum of all branch currents leaving the same node. This is called
Kirchoff’s Node Law. Two or more circuit branches that connect together through the same pair of nodes are
said to be connected in parallel.
Potential difference, also known as voltage, (using the symbol V ) is the work required per unit of charge to
move charge from one point in a circuit to another. It is measured in Volts. One Volt is one Joule of work per
Coulomb of charge moved. When a potential difference is negative, it is often referred to as a potential drop.
The sum of potential differences along any closed path within a DC circuit is always zero. This principle is
known as Kirchoff’s Loop Law.
Power (using the symbol P) is the rate at which energy is transferred to or from a portion of the circuit. It is
usually measured in Watts or Joules per second. The amount of power delivered to or from a circuit component
is equal to the product of the current through the component and the potential difference across it.
Some components in a DC circuit will absorb energy and transform it to mechanical energy or to heat, or store
it as chemical potential energy, while other components will supply energy to the circuit. Rechargeable batteries
can perform either of these functions, depending upon whether they are being recharged or discharged at the
time. When the potential is lower at the point where current exits from a component than at the point where it
entered, power is being removed from the circuit by the component. But if the potential is higher at the point
where the current exits, then that component is acting as a power source, delivering power to the circuit.
Resistance (symbol = R) is a property of energy absorbing electrical devices that convert electrical energy into
heat. Often the current, I, through the device is directly proportional to the potential drop, V, across it. The
relation V = IR is known as Ohm's Law, and a device satisfying this linear relationship is called a resistor.
Resistance is measured in Ohms. A one-Ohm resistor carries a current of one Amp if there is a potential drop of
one Volt across its terminals.
The Digital Multimeter
The digital multimeter (DMM for short) is a multifunction instrument that can be used to measure Voltage,
Current or Resistance, depending upon which function is selected and how it is connected to the circuit. A large
rotary switch on the face of the DMM sets the range of the instrument as well as selecting the quantity to be
measured. The decimal point in the DMM’s digital display will move automatically, as you change ranges, but
you need to be aware of the units implied on each particular range of the instrument. Please Note: One of the
test leads must be plugged into a different jack on the meter when it is used to measure Current, than the jack
used when measuring Voltage or Resistance. One lead is always connected to the COM jack.
To measure the potential difference between any two points in a circuit, first select an appropriate DC Voltage
range, plug the test leads into the COM and V/ jacks, and then connect the test leads to the appropriate circuit
points. There is also a switch near the top to select DC versus AC.
To measure the current in a single loop circuit or in a particular branch of a multi-loop circuit, select an
appropriate DC Current range, plug the test leads into the COM and A terminals, break the loop or circuit
branch, and connect the DMM leads across the break, to complete the circuit. Note: To measure the current in a
circuit branch, the DMM must always be connected in series with the components that make up that branch.
To measure the resistance of a device, you must remove it from the circuit and connect it directly across the
DMM test leads, plugged into the COM and V/ jacks. Switch the DMM to one of the resistance ranges.
Attempting to use the DMM as an Ohmmeter (to measure the resistance of a component) while the component
is connected in a circuit will generally lead to erroneous results, and it may permanently damage the meter as
well.
The Power Supplies
There are several power supplies in your trainer kit. On the left side of the breadboards are jacks for the
following power supplies: A variable DC supply from +1.25 V to 20 V; A variable DC supply from –1.25 V to
–20 V; A fixed AC supply at 15 V and 30 V; A ground jack; and fixed +5 V, +12 V and –12 V jacks. In most
labs you will be using the +5 V DC fixed supply.
On the top of the kit are a series of knobs that control what is called a function generator. With these knobs you
can create a varying voltage supply with variable frequency and shape. The function generator will be discussed
in a later lab.
The Breadboard
Permanent circuits are usually constructed by soldering the components to a fiberglass circuit board that is
constructed with metal “traces” that interconnect all of the components, completing the circuit. These
permanent circuits are inexpensive to construct, easy to test and reliable, but difficult to modify or experiment
with. For experimentation with new or modified circuits, the best strategy is to build the circuit with a
“breadboard” and jumper wires. This is an ideal way to make temporary but fairly reliable electrical
connections and keep things spaced out to avoid accidental short circuits (connections where you don't want
them).
The layout of connection points on the breadboard is designed for dual-in-line package integrated circuits,
which you will be using in a few weeks, but it's useful in working with discrete component circuits as well. The
narrower strips on the breadboard are called bus strips; they have two long lines of connection points and all the
points in one line are connected together under the board. Typically you'll connect the +5 Volt output of the
power supply to one bus line and the power supply common (ground) output to another bus line; then wires
plugged into the bus lines at any point can conveniently connect power to your circuit. The wider breadboard
strips with a groove down the middle are wired quite differently: Groups of 5 connection points running
perpendicular to the length of the board on each side of the center groove are connected together. This allows an
integrated circuit to be plugged in straddling the groove with four available connection points to each pin of the
integrated circuit. ( See a copy of layout at the end of your lab.)
Resistors
Carbon-film resistors obey Ohm's law quite accurately and come in a broad range of resistance values: from 10
Ohms to 22 Mega ohms. (“Meg” means million). They carry a color-code to make it easy to identify their
resistance value.
The color of the first color band (closest to one end) represents the first decimal digit of the resistance value.
The next band is the second decimal digit. The third band represents the number of zeroes that must be added
behind the two digits to represent the resistance as an integer.
0:
1:
2:
3:
4:
Black
Brown
Red
Orange
Yellow
5:
6:
7:
8:
9:
Green
Blue
Violet
Gray
White
There is also a fourth color band that gives you the tolerance of the resistor. The tolerance tells you how close
the color code resistance is to the actual resistance. The following table shows the colors of the fourth band and
the corresponding tolerances:
Brown
Gold
Silver
None
1%
5%
10 %
20 %
Experimental Procedures
1. Using a DMM check the output voltages available from your power supply and record them.
(Notice that you may connect the DMM leads to the power supply either way and that the DMM gives you a
minus sign on its display when the potential is lower at the V/ terminal than at the COM terminal. Use the DC
Volt range that gives you the greatest number of significant figures.)
2. Select three different resistors from the front of the class, record their color codes from lowest to highest
using the above table, and record the nominal value. Measure the resistance of each resistor with the DMM's
ohmmeter function and compare the measured values with the nominal (approximate) color-code values. Adjust
the range to get the largest number of significant figures.
Color Code
Nominal Value
Measured Value
a.
________________________
________________________
________________________
b.
________________________
________________________
________________________
c.
________________________
________________________
________________________
3. Construct a series circuit using the 1.5 Volt power supply and tow of your lowest resistor. Calculate the
current in this circuit. Draw a schematic of your circuit with the polarity and insert an ammeter to show how
you connect your meter in the circuit.
I cal =
Measure the current using your DMM as an ammeter (start on the 200 milliamp DC Amp scale). Change the
range until you get the largest number of significant figures. Record your value.
I Meas =
4. Repeat part 3 using all three resistors in series. Draw a schematic with the polarity, calculate and measure
voltage drop across each resistor. Insert the ammeter at different points in the circuit loop to confirm that the
current is the same everywhere around a single-loop circuit.
V1 cal =
V2 cal =
V3 cal =
I meas =
5. Construct a circuit with two highest value resistors in parallel. Predict and then measure the current in each
branch of the circuit. Draw a schematic of your circuit and insert an ammeter to show how you connect your
meter in the circuit. Is Kirchoff’s node law obeyed? Can you think of a reason that it might not seem to be
obeyed exactly? (Hint: Use one DMM to measure the voltage drop across the terminals of the other DMM.)
6. Add the third resistor in series with that parallel pair. Draw a schematic of your circuit and insert an ammeter
to show how you connect your meter in the circuit. Predict and then measure the voltage drop across each
resistor in this circuit. Do the results make sense?
7. Using Multi SIM, build each of the circuits and verify your data for each of the steps (3-6.).
Measured Value
Simulated Value
Step 3.
I Meas
=______
____________________
Step 4.
V1 Meas =_______
V2 Meas =_______
V 3Meas =_______
I Meas =_______
________________________
________________________
________________________
________________________
Step 5. ________________________
________________________
_______________________
________________________
________________________
________________________
Step 6. ________________________
_______________________
_______________________
________________________
________________________
________________________
Did you find your measured and simulated value to be different?
Explain why.
8. Using Multi SIM, build Fig 2.22 circuits in page 52 of your book and verify your calculated data.
9. Using Multi SIM, build Fig 2.23 circuits in page 52 of your book and verify your calculated data.
10. Build the circuit in the figure below. The voltage drop across the 330 ohm resistor is 2.8V. Calculate the
current in (I1, I2, I3, I4 and I5), Voltage (VG, V1, V3, V4 and V5) and calculate Power (P1, P2, P3, P4, P5 and PT).
Using the DMM measure the current in (I1, I2, I3, I4 and IT), Voltage (VG, V1, V2, V3, V4 and V5) and compare
your result.
Are the measured and calculated values different?
Explain why.