FUNDAMENTAL DC CIRCUIT MEASUREMENTS

Phys/Geog 182

INTRODUCTION

Electrical circuits play a fundamental role in the practical application of electricity and magnetism. If you spend a few minutes contemplating the ways in which electrical circuits affect your life, you will soon realize that modern civilization could not exist without them.

We can group the applications of electrical circuits into several major categories:

1) Energy Transfer -- The potential energy stored in coal, oil, natural gas, flowing water, sunlight, etc., is transformed into electrical energy which is transported over power lines to a “user” where the energy is transformed into more usable forms such as light, sound, heat, and motion.

2) Information Transfer -- Information is transformed into electrical signals which are transmitted (over wires, fiber optic cables, or even through empty space) to a receiver where the signals are processed and the original information is recovered).

3) Control -- Electrical circuits are used to control the operation of a variety of devices -- e.g., door openers, water valves, airplane flaps, etc.

4) Information Processing -- Computers (from mainframes to micros) use electrical circuits to gather, analyze, store, and display information of all types.

5) Scientific Analysis -- Physical variables (such as velocity, acceleration, force, pressure, temperature) are often transformed into electrical signals which can then be analyzed by a variety of electrical instruments.

ELECTRICAL CIRCUITS

A basic electrical circuit is shown below. All electrical circuits, no matter how complicated, contain the same fundamental elements as this one.

EMF

Electrical Device

Connecting Wires

The EMF (Electromotive Force) is the source of electrical potential energy that drives the charge carriers through the circuit. It is connected by metallic wires to an electrical device (a light bulb, a carbon resistor, a motor, a loudspeaker, etc.) or several such devices connected together in some way. The strength of the EMF is called its potential difference or voltage and is measured in volts.

The EMF creates an electric field in the circuit that starts at the positive (high energy) side of the EMF and ends at the negative (low energy) side. This field exerts an electrical force on the charge carriers in the circuit and causes them to move through the circuit. This movement of charge is called electrical current and is measured in amperes.The electrical devices oppose (or resist) the motion of electrical charge through them. The amount of this opposition is called the resistance of the device and is measured in ohms.

An electrical circuit can be thought of as an energy transfer system. The EMF puts energy into the current flowing through it and the current gives up this energy in the process of moving through the electrical devices. Energy is thus transferred from the EMF to the devices. The energy given up by the current as it passes through an electrical device is determined by measuring the potential difference or voltage between the ends of the device.

ELECTRICAL CIRCUIT MEASUREMENTS

The fundamental quantities used in describing and analyzing DC circuits are voltage, current, and resistance. Measurements of these quantities are made using various types of electrical instruments. Voltage is measured by a voltmeter, current by an ammeter, and resistance by an ohmmeter. Some meters are devoted entirely to making measurements of one of the above quantities. Other meters can make measurements of two or more of the quantities and are called multimeters.

Meter readouts may be digital or analog. Digital scales are read directly. Analog scales are generally estimated to 1/10 of the smallest scale division. We will use digital scales for today’s lab.

When recording a meter reading, give the magnitude of the measurement as well as the unit of measure.

When making a measurement with any electrical meter, one must give careful consideration to the following factors:

1) Function -- Is the meter to be used to measure voltage, current, or resistance?

2) Range -- The term “range” refers to the range of values that can be measured by the instrument. For example, a voltmeter set on a 10 volt range can measure voltages between zero and 10 volts. The range setting of a meter thus indicates the maximum value that can be measured using that setting. Most meters have several ranges of operation. If you are not sure which range to use, start with the highest one. Then reduce the instrument setting to the lowest possible range which still contains the value you wish to measure -- this is the optimum or best range setting for that particular measurement and will give the most precise measurement possible. (The term “precision” refers to the number of significant figures in the measurement.)

3) Polarity -- Current and voltage measurements are directional -- i.e., both positive and negative values are possible. Voltmeters must be connected so that the + terminal of the meter is connected to the high potential side of the element being measured. Ammeters must be connected so that current flows into the + terminal of the meter.

4) Placement -- Meters must be properly placed in a circuit to make each type of measurement. Proper meter polarity and placement will be illustrated after a brief discussion of circuit diagrams.

Circuit diagrams are figures or drawings that depict particular electrical circuits. You will need to become familiar with circuit diagrams and be able to construct a circuit using the diagram as a guide. The following symbols are used to represent electrical quantities:

the resistance of a device

a voltage source (EMF)(long side indicates + side)

+

current

The electrical devices that we will examine in this exercise can be characterized entirely by their resistance. Thus, we often speak of an electrical device as “a resistance”. Lines joining voltage sources and resistances represent connecting wires. The circuit diagram below shows a resistance of R ohms connected to a voltage source of strength E volts with a current of I amps flowing in the circuit.

E+

R

I

The following diagrams show the proper polarity and placement of each type of electrical meter (note the symbols used to represent each type of meter):

Ammeter measuring the current through a resistor

+

V +

A+

Ohmmeter measuring the resistance of a resistor

Voltmeter measuring the voltage across a resistance

+

+

+

A

V+

The voltmeter must measure across the circuit element.The ohmmeter must measure across the resistor.The ammeter must be in the flow of current.

The following diagram shows an electrical circuit with meters properly positioned to make several different measurements:

A+

+

+ A

V+

+V

Before you start the lab, answer the following questions:

1. Read over the Operator’s Instruction Manual (at the end of this lab) for the DT830B Digital Multimeter.2. Below is a copy of the front of your multimeter. Answer the following questions.

3. Below is another copy of the front of your multimeter. Label each of the arrows to indicate what the selected region on the dial measures.

Which lead gets plugged in here? Red or Black?

Which lead gets plugged in here? Red or Black?

What information is displayed here?

4. Within most of the regions there is a series of numbers. What do these numbers indicate?

5. Set the multimeter to 200 mV. Check the voltage on the multimeter by touching both leads together. Value: __________6. Check the voltage on the 9V battery by touching the leads to the terminals. Your multimeter should NOT be set to 200 mV!

What should it be set to?___________ Value: _______________7. Connect the battery connector to the 9V battery. Check the voltage using the multimeter by touching the multimeter leads

to the connector leads. What happens if you reverse the connection?

Now for the lab!8. Connect the 9V battery to the Breadboard and recheck the voltage.

a. The + (red) on the 9V should be connected to the + column on the Breadboard. The – (black) should be connected to the – column on the Breadboard. Push the lead wires in to the Breadboard.

b. Plug two short jump wires into the columns of the Breadboard that have voltage between them, one in the + column and one in the – column.

c. Use your multimeter to check the voltage. Be sure that you are using the correct range setting! (The entire + column is positive and the entire – column is negative.) Value: ___________

9. Use your jumper wires to determine which other rows and/or columns are connected. Place your jumper wires in the columns/rows and test the resistance by touching one lead wire to each jumper wire end. Your multimeter must be set to resistance. Use 200 Ohms to be sure you don’t miss anything. The resistance will be very small (~1 Ohm) if the column/row is connected.

a. Are a-e in each row connected? ______b. Are a-j in each row connected? ______c. Are f-j in each row connected? ______d. Are 1-30 in each column connected? _____e. Is the + column on the right connected to the + column on the left? ______f. Is the – column on the right connected to the – column on the left? ______g. On the Breadboard diagram below, very carefully “connect the dots” for columns/rows that are connected.

10. You have been given 2 resistors. Use your multimeter to determine the resistance for each resistor. Write down the color code next to the resistor value so you can keep track of Resistor 1 and Resistor 2.

a. Resistor 1 value: ________b. Resistor 2 value: ________

11. Using the values for the battery voltage (recheck this!) and resistors 1 and 2 that you got from the multimeter, calculate the following for the series circuit shown below:

a. the current passing through resistor 1, b. the current passing through resistor 2, c. the voltage across resistor 1,d. the voltage across resistor 2, e. the total circuit resistance, f. power through resistor 1, g. power through resistor 2, andh. the total circuit power usage.

Show your calculations on a SEPARATE SHEET OF PAPER. Your calculations should be done in a logical, neat format.

12. First, make sure that you are not going to blow any components! Each resistor is rated for a maximum of ¼ Watt of power. Power = Voltage * Current

What is the power reading across Resistor 1? ________What is the power reading across Resistor 2? ________Are you going to destroy any resistor if you activate this circuit? _______________

13. Build the series circuit above using your two resistors. Now you will determine the circuit information above using the multimeter.

a. First you will measure voltage. You are using a 9 V battery so the voltage range you should use is a maximum of 20 V. Turn the multimeter dial to 20 V setting and determine the voltage across resistor 1. In order to do this you will need to touch the multimeter leads to metal legs on either side of the resistor. Be careful not to touch any other metal wires.Voltage across resistor 1 is: ____________________________

b. Determine the voltage across resistor 2.Voltage across resistor 2 is: ____________________________

c. Now you will measure the total circuit resistance. Use the information from your calculations above to determine which range you should set the multimeter to for resistance. In order to measure total circuit resistance you need to put the multimeter leads on either side of the “total resistance”. The battery should not be connected during this measurement. Is the resistance the sum of the resistances of each resistor as measured previously?

14. Using the values for the battery voltage (recheck this!) and resistors 1 and 2 that you got from the multimeter, calculate the following for the parallel circuit shown below:

d. the current passing through resistor 1, e. the current passing through resistor 2, f. the voltage across resistor 1,g. the voltage across resistor 2, h. the total circuit resistance, i. power through resistor 1, j. power through resistor 2 andk. the total circuit power usage.

Show your calculations on a SEPARATE SHEET OF PAPER. Your calculations should be done in a logical, neat format.

15. First, make sure that you are not going to blow any components! Each resistor is rated for a maximum of ¼ Watt of power. Power = Voltage * Current

What is the power reading across Resistor 1? ________What is the power reading across Resistor 2? ________Are you going to destroy any resistor if you activate this circuit? _______________

16. Build the parallel circuit above using your two resistors. Now you will determine the circuit information above using the multimeter.

l. First you will measure voltage. You are using a 9 V battery so the voltage range you should use is a maximum of 20 V. Turn the multimeter dial to 20 V setting and determine the voltage across resistor 1. In order to do this you will need to touch the multimeter leads to metal legs on either side of the resistor. Be careful not to touch any other metal wires.Voltage across resistor 1 is: ____________________________

m. Determine the voltage across resistor 2.Voltage across resistor 2 is: ____________________________

n. Now you will measure the total circuit resistance. Use the information from your calculations above to determine which range you should set the multimeter to for resistance. In order to measure total circuit resistance you need to put the multimeter leads on either side of the “total resistance”. And remember that the battery must be disconnected from the circuit when you measure resistance.