Electric Circuit Engineering Lab assignment

Experiment 1

Introduction to DC Circuits

Lab #1 ENG 250-001

Name: Date:

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.

Experiment 1

Introduction to DC Circuits

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

Experiment 1

Introduction to DC Circuits

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: Black 5: Green

1: Brown 6: Blue

2: Red 7: Violet

3: Orange 8: Gray

4: Yellow 9: 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 1 %

Gold 5 %

Silver 10 %

None 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.)

Experiment 1

Introduction to DC Circuits

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.)

I1 meas = I2 meas =

Experiment 1

Introduction to DC Circuits

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?

I1 meas = I2 meas = I3 meas =

7. Using Multisim, 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. I1 meas =_______ ________________________

I2 meas =________ ________________________

V Meas =________ ________________________

Step 6. I1 meas =_______ ________________________

I2 meas =________ ________________________

I3 meas =________ ________________________

Did you find your measured and simulated value to be different?

Explain why.