V I R Lab 1

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1 Bob York

ECE 2A Lab #1

Lab 1Vol t age, Current ,

and Res is t anc e

Overv iew

In this lab you will learn to about two basic lab tools - the Digital Multimeter (DMM) and the

DC power supply and to use these tools to investigate simple resistive circuits and devices.

In particular you will examine the non-idealities of current (ammeter) and voltage (voltmeter)

measurements, and then use the DMMs to characterize series and parallel resistor

combinations, voltage dividers, and the current-voltage relationship of some simple nonlinear

devices like light bulbs and motors. You will also examine a simple variable-resistance

device called a potentiometer, and (for extra credit) a photocell.

Table o f Content sBackground Information 2

Resistors Solderless Breadboards 4

Digital Multimeters Further Reading 6

Pre-lab Preparation 7 Before Coming to the Lab 7 Required Equipment Parts List 7

In-Lab Procedure 81.1 Power Supply and Voltmeter 81.2 Resistance Measurements 9

Ammeter and Voltmeter Internal Resistance 10Continuity Testing 10

1.3 Series and Parallel Resistor Combinations 111.4 Voltage Dividers and Potentiometers 121.5 Current-Voltage Characteristics 14

Extra Credit: Photocell

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2 Voltage, Current, and Resistance

Bob York

Background In fo rmat ion

Resistors

There are a number of different types of resistors depending on the application. All obey

Ohms law with a specified resistance, of course, but the electrical behavior can also vary in

terms of power handling or current handling, temperature dependence of the resistance,

size/shape, and manufacturing tolerances. All factors must be considered in circuit design.

In ECE 2 we use so-called through-hole components which have long metal leads.

Figure 1-1 shows four common families of through-hole resistors. The carbon-film resistors

are the most ubiquitous by far, and dirt cheap: in large quantities, these resistors are priced in

fractions of a US cent (e.g. $0.004 per resistor in quantities of 1,000). They are so cheap that

the primary cost of using them in a product nowadays is associated with the insertion time in

the robotic assembly and not the cost of the part itself (the robotic labor cost is usually

about 1-2 cents per part).

Carbon Film 5%, 10% Cheap

General purpose

Metal Film Precision 1%

High Performance

Metal Oxide Higher Power

Wirewound High Power,

High Current

Figure 1-1 Here we show a few of the most common types of through-hole resistors. In

ECE 2 we will use the cheap carbon-film type almost exclusively.

Carbon-film resistors are made by

depositing a thin carbon film on a small

ceramic cylinder, and then etching the

film into a narrow helical pattern. It is

difficult to precisely control the

thickness and conductivity of the carbon

film so there are always some random

manufacturing errors. The statistical

distribution of resistor values is

characterized by the specified

tolerance. Although manufacturers do

not usually share the details of their processes it is generally assumed that

the resistor values can be described by a

Gaussian (normal) distribution with a

standard deviation such that the tolerance is around 3 . This effectively means that

99.7% of the resistors will fall within the specified tolerance range, clustered near the

nominal value. Figure 1-2 shows the probability-density functions computed for two

commonly-encountered tolerances, 5% and 10%.

0.00

0.05

0.10

0.15

0.20

0.25

-10 -5 0 5 10

% Deviation from Nominal

ProbabilityDensity

5%

10%

Figure 1-2 Probability density function describingresistor statistics, assume a 3 tolerance.

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Background Information 3

When companies make electrical components they try to avoid testing each part

individually; that would take too much time and would add significantly to the cost of each

component. Instead the engineers work to characterize the process statistics very well, and

then can test a few sample parts periodically to insure that components remain within the

acceptable range of values.That brings us to component values. There are a HUGE range of values to cover:

resistances can be anywhere from tiny fractions of an ohm to tens of mega-ohms

(mega=million). The resistor values that are selected for production are based on a system

that is tied closely to the expected manufacturing tolerances. The basic idea is to select

discrete values that cover as much of the full range of resistance as possible without having

the statistical distributions overlap too much. Consider two successive values of resistance,

1r and 2r . If the component tolerance is defined as t, then the largestpossible value of 1r

would be 1(1 )r t . Similarly the smallestpossible value of 2r would be 2 (1 )r t . Equating

those limiting values give a ratio of successive resistor values as:

1

0

1

1

r t

r t

(1.1)

For example, if the tolerance is 20% ( 0.2t ), then (1.1) gives a ratio of consecutiveresistances of 1.5. Figure 1-3 illustrates a set of six values in the range of 1-10 that are

commonly used to describe components with this tolerance, and you can verify that the ratio

of successive values is indeed close to 1.5 in each case.

1.0 1.5 2.2 3.3 4.7 6.8 10

Figure 1-3 Illustration showing values appropriate to a 20% tolerance (E6),with expected normal distributions shown around each nominal value.

You will see that the ratio isnt exactly 1.5 for every pair of adjacent values. One reason is

that we want the pattern of numbers to repeat for each successive decade of values (e.g.

10,15,22,33..100,150,220,330.etc.). In order for the pattern to repeat afterN discrete

values, the ratio x must satisfy

1/10 10N Nx x (1.2)

So for 6N the ideal ratio would be 1.468x . But there are also some rounding errors thatcome into play, so the set of numbers in Figure 1-3 represent a reasonable compromise.

Its not hard to figure how many values are needed to cover the range 1-10 for a different

tolerance. For 10% we need 12N ; for 5% we need 24N , and so on. This has led theEIA (Electronics Industry Association) to standardize a set of values designated as E6, E12,

E24, etc. The first three are shown in Table 1-1.

Table 1-1 Standard EIA Decade Values for E6, E12, E24

E6 1.0 1.5 2.2 3.3 4.7 6.8

E12 1.0 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2

E24 1.0 1.1 1.2 1.3 1.5 1.6 1.8 2.0 2.2 2.4 2.7 3.0 3.3 3.6 3.9 4.3 4.7 5.1 5.6 6.2 6.8 7.5 8.2 9.1

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Bob York

A tighter tolerance typically costs more so there is a tradeoff between precision and cost.

As a future circuit designer it will be up to you to determine the sensitivity of your circuit in

order to choose the appropriate tolerance. In certain types of frequency-selective filter

circuits, for example, precision 1% metal-film components are often required. In the

circuits we will use in ECE 2, 10% tolerances will be fine so you should familiarize yourselfwith the values available in this range (E12).

Another practical issue is how to READ component values. Ideally the value would just

be printed on the side of the device, but most through-hole resistors use a multi-band color

code, described in Table 1-2. It is simple but takes a little getting used to, and you will only

figure it out with practice. Some common examples are shown on the right.

Table 1-2 Resistor color-code and examples

Color1

stStripe

(1st

digit)

2nd

Stripe

(2nd

digit)

3rd

Stripe

(Multiplier)

Black 0 0 1

Brown 1 1 10

Red 2 2 102

Orange 3 3 103

Yellow 4 4 104

Green 5 5 105

Blue 6 6 106

Violet 7 7 107

Gray 8 8 108

White 9 9 109

Examples of Common Values:

Brown-Black-Red

10102 = 1k

Yellow-Purple-Red47102 = 4.7k

Brown-Black-Orange

10103 = 10k

Orange-Orange-Orange

33103 = 33k

Brown-Black-Yellow

10104 = 100k

Red-Red-Yellow

22104 = 220k

A final practical issue: the carbon-film resistors come indifferent SIZES depending on power-handling. An

illustration of the relative sizes is shown in Figure 1-4. When

a current flows in a resistor some electrical energy is

dissipated, and this manifests itself as a rise in temperature

(heat). Ordinarily the resistors are mounted in a PC board or

breadboard where the only mechanism for getting rid of that

heat is convection to the surrounding air. Convection heat

transfer is enhanced by increasing the surface area, so resistors

that must handle higher powers have a larger surface area. For

very high power the resistors may have an integral metal

heatsink with fins for extra surface area. In ECE 2, the

resistors are almost always Watt resistors.

Solderless Breadboards

The most common method of experimenting with through-hole or leaded components like

resistors is using a solderless breadboard to simplify electrical connections. These get their

name from the early days of radio when it was common to build vacuum tube circuit

prototypes on a wooden surface. Today's breadboards are a grid of insulating plastic atop a

Aluminum

fins

Aluminum

fins

Figure 1-4 Size for power!

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Background Information 5

pattern of conducting metal strips. Figure 1-5 shows the top view of a typical breadboard

used in ECE2, and a close-up of the pattern of connection points.

Groove

isolates

holes on

either side

Rails

(lines denote internal connection)

Power/Ground terminals

Each group of 5 horizontal

holes are electrically

connected internally

by a metal clip

Figure 1-5 Solderless breadboard used in ECE 2

Component leads and wires are inserted into the holes to make contact with conducting

metal strips inside. On either side of the long channels or grooves, each perpendicular group

of five holes are electrically connected. There are also long groups of connected holes called

rails running the length of the breadboard, marked by colored lines. These are normally

used for power and ground connections. Breadboards come in a variety of sizes and shapesbut most have the same basic pattern of holes and rails.

Digital Multimeters

The digital multimeter (DMM) is a versatile measurement tool for electrical circuits. It is

capable of multiple types of measurements, including at least the three in Table 1-3:

Table 1-3 Basic Multimeter Measurements

Function Symbol Description

Voltmeter VV

Measures voltage between terminals, ideally with zero

current draw (infinite resistance)

Ammeter AA

Measured current flowing through the meter, ideally withzero voltage drop (zero resistance)

Ohmeter

Calculates resistance between terminals by applying a

known current and measuring the terminal voltage

At first glance the use of the DMM seems trivial: just hook it up and read the data, right?

But like all measurement instruments a real DMM is not perfect, and a clear understanding of

those non-idealities is essentially in order to use it properly.

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Bob York

Consider the voltmeter function: we want to measure the voltage between two points in

some circuit, but we dont want to the act of measuring to change the circuit in any way.

That means the voltmeter should not draw any current away from the circuit under test; it

should appear to the circuit as an infinite resistance. But a real voltmeter must draw some

current in order to make the measurement. So real voltmeters will always affect the circuit toa certain extent; they always have a finite resistance. This could be an issue if we are probing

circuits with tiny currents or extremely large resistances.

Similarly the ammeter is designed to measure the current flowing in some part of a circuit.

To do that we break the circuit at some point and insert the so that the current will passes

THROUGH the ammeter. To minimize the impact on the circuit the ammeter should not

introduce any additional voltage drops, which means it should appear like a perfect short-

circuit (zero resistance) to the circuit. But real ammeters necessarily have a non-zero

resistance, because the resulting voltage drop across

the internal resistance is used to calculate the

current. When measuring small currents a rather

large internal resistance may be necessary to create a

measureable voltage drop.In most DMMs there are three terminals: one

common terminal, one for voltage or resistance

measurements, and one for current measurements.

The user then manually attaches the leads to the

appropriate terminal, and manually selects the

appropriate measurement function. Some meters

also require you to manually select the measurement

range, whereas others are autoranging and choose

the range for you. The circuit behavior of the DMM

(how much current it draws, or equivalently its

internal resistance) usually depends on the

measurement range.A common and often destructive mistake by beginners is to configure the DMM for use as

an ammeter (zero internal resistance) and then attach the test leads across a carrying

component or voltage source. The ammeter then shorts the circuit and a large current may

flow through the DMM, causing an internal fuse to blow. The fuse will then have to be

replaced before any further measurements can be made. In the little hand-held DMMs like

that in Figure 1-6, the enclosure must be taken apart to replace the fuse.

Lastly, note that most multimeters can be used for both AC and DC measurements. In the

AC measurement mode where the currents and voltages are presumed to vary sinusoidally

with time, the instrument compute an RMS (root-mean-square) value for the waveform, NOT

the peak voltage or current.

Further Reading

Read the relevant sections of the Equipment Tutorial posted on the class website to

familiarize yourself with the details of the specific DMM and power supply in the ECE 2 lab.

Some useful/relevant Wikipedia Links:

http://en.wikipedia.org/wiki/Resistor and also http://en.wikibooks.org/wiki/Electronics/Resistors

http://en.wikipedia.org/wiki/Breadboard

http://en.wikipedia.org/wiki/Multimeter

Figure 1-6 Handheld DMM

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Bob York

In-Lab Proc edureRead the instructions carefully. If you skim through the text too quickly you may miss

something important.

Each critical step begins with a check box like the one at the left. When you complete astep, check the associated box.

Be sure to document all steps and results in your notebook for inclusion in your lab report.

1.1 Pow er Supply and Vol tm eter

Well start by using the bench DMM to probe the output voltage of the bench power supply.

This step is easy and should not take much time:

First set the bench DMM to measure DC Volts on the 20V full-scale range. Using one ofyour cables, connect the V/ terminal of the DMM to the +18V output of the power

supply, and with a second cable connect the COM terminals together. These connectionsare shown schematically in Figure 1-7. A common convention in microelectronics is to

use black wires for the ground or common lead,

but of course the color of the cable doesnt really

matter much here.1

Turn on the DMM and the power supply. Be surethe METER switch on the power supply is set

to correctly display the output of the +18V

terminal, and be sure to note which scale is

appropriate for this setting. Then adjust the power

supply to get a reading of +7.0V on the DMM.

To which full-scale range must the DMM be set ifthe display only shows one digit past the decimal? (i.e. 7.0 and not 7.00 or 7.000) What

is the lowest range that can be used to measure 7V? How does the DMM display an out-

of-range measurement?

When properly calibrated, DMMs are usually more accurate than the cheap indicators on the

power supplies, so in case there is a disagreement between what the power-supply says and

what the DMM says we will usually trust the DMM!

Repeat the steps above to set the +20V output on the power-supply to +10.0V. Be surethe METER switch on the power supply is set to correctly display the output of the

+20V terminal. Repeat again to set the -20V output to -5.0V, and then double check that

the three output terminals on the power supply are still at their desired values:

o +18V terminal: should be set to +7.0V,o +20V terminal: should be set to +10.0V

o -20V terminal: should be set to -5.0V.

In each case above we have defined the voltages at the various output terminals with respect

to the common terminal on the power supply.

1 Its a different story with the AC electrical wiring you find in buildings like your home. In that

case, white is always the neutral lead, black is the hot lead, and green is usually used for ground.

Vout

Power

SupplyVV

DMM

COM

Figure 1-7 Voltmeter test.

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Resistance Measurements 9

Lets now use the DMM to make measurements with respect to other reference points:

Using the DMM, record the following measurements between power-supply terminals:

o +18V terminal with respect to the +20V terminal

o

+18V terminal with respect to the -20V terminalo +20V terminal with respect to the -20V terminal

o COM terminal with respect to the +18V terminal

Clearly the voltage recorded by the DMM is critically dependent on the reference point.

You may have noticed that in addition to the COM terminal on the power supply there is

also a ground terminal. The ground terminal is electrically connected to the physical Earth

through the AC distribution system in the building, usually via a large copper pipe driven into

the ground outside the building somewhere. In some electrical instruments the ground

terminal is electrically connected to a metal case or enclosure to avoid hazardous shocks if

there is a wiring fault in the instrument. But it is important to note that COM and ground are

not the same. This is because power supplies are designed so that they can supply power to a

variety of loads, some of which may not be referenced to ground (so-called floating loads).The COM terminal is the point of reference for the output terminals in ALL situations, so that

any current flowing from one of the output terminals should return to the power supply via

the COM terminal.

There are times when it makes sense to electrically connect COM and ground, in which

case we will add a little jumper wire between the terminals. In fact there may already be a

jumper installed on your bench supply; it is usually considered good practice to keep that

jumper wire there unless you have a specific reason not to use it.

If there is a jumper wire connecting COM and ground, remove it and measure all of thethe voltages at the output terminals with respect to this ground terminal. The add the

jumper wire and repeat the measurement.

The key takeaway from this section is that voltage is always measured as a difference in

potential between two points, and that the choice of reference point is critical.

1.2 Res is tanc e Measurement s

Now lets explore the DMM as an Ohmeterfor resistance measurements. For convenience we

will use the bench decade box as an adjustable resistor in

this step. Again these measurements are quite simple,

but we want to pay close attention to the characteristics

of the meters themselves:

Configure the DMM for resistance measurement by pressing the button. Select the 20k range.

Connect the V/ terminal of the DMM to a redterminal on the decade resistor box, and the COM

terminal of the DMM to a black terminal. The

connections are shown schematically in Figure 1-8.

Set R to 5k. Record the actual value ofR asmeasured by the DMM. Adjust the range setting on the DMM to understand its effect on

the measurement resolution and record this in your notebook.

Bench

DMM

COM

R

Figure 1-8 Ohmmeter setup.

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Bob York

Ammeter and Voltmeter Internal Resistance

Now add your small handheld DMM to thecircuit as shown in Figure 1-9. Configure

the handheld DMM as a voltmeter by

selecting one of the DC Volt ranges withthe rotary switch. Does the resistance

measurement change when the voltmeter is

added? What can you conclude about the

internal resistance of the voltmeter?

Your hand-held DMM should indicate the

presence of a small voltage across the resistor.

In order to sense the value of the resistance the

ohmmeter must pass a small current through it!

Make a table in your lab notebook and record the measured resistance and the voltageacross the resistor for R=1k, 2k, 5k, and 10k, using the 20k measurement range on the

benchtop DMM. Using Ohms law with the measured resistance and voltage values,calculate current that must be flowing in the resistor each time. Repeat using the 200K

measurement range on the ohmmeter. What is the average current supplied by the

ohmmeter for each measurement ranges?

Remove the handheld DMM from the circuitand reconfigure it as an ammeter by selecting

the DCA 200A range. With the bench

ohmmeter on the 20k scale and R at 5k, add

the ammeter into the circuit as in Figure 1-10.

Is the measured current close to the current you

calculated in the previous step? What is the

measured resistance now? What must the seriesresistance of the ammeter be for this setting?

Set R to 0 so that only the resistance of theammeter is being measured. Change the

ammeter to the 2mA scale. Record the

ammeter resistance for each scale, and then move the lead from the ammeters V//mA

input to the 10A input and record its resistance.

Remove the handheld ammeter and reattach the test lead on the handheld DMM back tothe VmA input and put it in ohmmeter mode. This is a good precaution after making

current measurements!

A key takeaway here is that the meters will always perturb the electrical state of the system to

some extent, and we must always consider the possible influence of the meter itself on themeasurement.

Continuity Testing

A continuity test is done to confirm that things are electrically connecting in the manner we

want. We can use the ohmmeter to explore continuity in a circuit in the following way: if

two points are electrically connected (by a wire, for example) then the measured resistance

will be very low. If the two points are NOT connected, the measured resistance should be

very large (ideally infinite). It may sound trivial, but continuity testing is a very important

Bench

DMM

COM

R VV

Handheld

DMM

Figure 1-9 Voltmeter across the resistorunder test (parallel resistance).

Bench

DMM

COM

R

Handheld DMM

AA

Figure 1-10 Break circuit and addammeter (series resistance).

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Series and Parallel Resistor Combinations 11

tool in debugging malfunctioning circuits: in ECE 2, the most common reason for a

malfunctioning circuit is a simple wiring error!

Move the red test lead back to theUsing the ohmmeter and some jumper

wires, check for continuity betweenthe sockets in and around a sample row

in your breadboard to confirm the

connections described in the

background section. For example,

using the row and column designations

shown in Figure 1-11, draw lines

through the sockets which are

connected in row 6, for example. Is

their any continuity between rows 5

and 6, or 6 and 7, or across the gap

between columns e and f?

Similarly confirm continuity between the ends of one the red and blue bus-bar strips.

1.3 Series and Paral le l Res is tor Combina t ions

Now find the small Watt resistors in your parts kit. We will first measure the resistances of

these components and then explore the resistance of simple combinations of them

Create a table in your notebook like Table 1-4, and for each resistor value in the table,find the corresponding component in your parts kit and record the color-code in the

appropriate place. The use your ohmmeter to measure the actual resistance and determine

the % error in comparison to the nominal value (to save time you can do the calculations

later, outside of lab). It is convenient to mount the resistors on the breadboard and use

the pointed probes of your handheld meter for this measurement.

Table 1-4 Summary of Measurements on Individual Resistors

Nominal Color Code (e.g. red-orange-brown) Measured [] % Error

1k

1k

1k

2k

3k

4.7k

10k

Next, create a table like Table 1-5 in your notebook. For each row, interconnect theselected resistors on your breadboard in the desired configuration and measure the net

resistance between the terminals (open-circles in each figure) with the ohmeter. Then

compare your measured data.

Figure 1-11 Section of a solderless breadboard.

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Bob York

Table 1-5 Summary of Measurements on Resistors Combinations

Resistor

Configuration

Theoretical

Net

Resistance

Nominal

Values

Theory w/

Nominal

Values

Theory

w/Actual

Values

Measured

Resistance

R1=1k

R2=1kR1

R2

1 2R R

R1=2k

R2=3k

R1=1k

R2=1kR

1

R2

1 2

1

1 1

R R

R1=4.7k

R2=10k

R1

R2

R3

1 2 3R R R

R1=1k

R2=1kR3=1k

R1

R2

R3

1 2 3

1

1 1 1

R R R

R1=1k

R2=1k

R3=1k

R1

R3

R2

1 2 3

1

1 1

R R R

R1=1k

R2=1k

R3=2k

1.4 Vol tage D ividers and Potent iomet ers

In lecture we have discussed (or will soon) the fact that the voltage supplied to a circuit by a

battery or power supply will divide among the circuit components in proportion to the

relative resistances of the components.

Verify that the +20V output of thepower supply is still set at +10V and

make the connections shown in Figure

1-12, where the two resistors are

mounted on your solderless breadboard. Begin with

1 2 1kR R .

With the DMM set to measure DCvolts, measure the voltage across 2R .

Here the voltage is indicated by the

variable, outV , and the polarity of the variable is defined with + and signs. For DMM

measurements the corresponds to the reference node where the COM terminal is to be

Vout

R1

R2

1 k

+10V

Power

Supply

VV

DMM

Figure 1-12 Simple voltage divider.

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Voltage Dividers and Potentiometers 13

connected. Enter this result in your notebook using a table like that in Table 1-6, where

the theoretical expectation in this case is given by

2

1 2

10VoutR

VR R

(1.3)

Repeat for the remaining resistorvalues in Table 1-6, keeping 1R fixed

at 1k in each case.

The voltage divider circuit thus

provides a simple way reduce an applied

voltage by a desired amount, but it would

be nice if we could adjust the output

voltage more easily and continuously. The

potentiometer (pot for short) allows us

to do this. A potentiometer is a resistor

with a third terminal called the wiper.The wiper rubs along the length of the resistor material internally, creating a self-contained

adjustable voltage divider network. Figure 1-13a illustrates one particular type of

potentiometer (a thumbwheel type), and Figure 1-13b shows ones very simple voltage-

divider configuration using a potentiometer configuration. These kinds of circuits are often

used in audio volume controls to adjust the level of the audio signal before it is fed into a pre-

amp or power amp.

Wiper

Vout

R1

R2

1 k+10V

10 kVout

10 k+10V

(a) (b) (c)

Figure 1-13 Potentiometer and its use in simple voltage-divider circuits.

Potentiometers are specified in terms of the total resistance between the outer terminals.

In your parts kit you should have a 10k potentiometer. Figure 1-13c shows another

common configuration where the wiper is intentionally shorted to one of the other terminals.This creates a two-terminal variable resistance.

Construct the circuit in Figure 1-13c on your breadboard. Monitor the output voltageusing the DMM as the thumbwheel is varied over its extremes.

Table 1-6 Voltage Divider Measurements

R2 Vout (Theory) Vout (Meas.)

1k

2k

3k

4.7k

10k

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There are many other types of potentiometers on the market, the one in our kit is just one

example. Some must be adjusted with a screwdriver and may allow for multiple turns for

better control over the resistance. Some have large rotating shafts that can be fitted with a

knob for instrument panels. Potentiometers are sometimes used to tweak a circuit after

production to compensate for other component errors; in this application they are sometimesreferred to as trimpots.

1.5 Current -Vol tage Charac t er is t ic s

Resistors are engineered to obey Ohms law over a wide range of voltages and currents. The

relationship between voltage and current is not always as simple in other devices, but

knowing this relationship is critical for circuit design. We will examine the I-V

characteristics of two simple devices: a

small incandescent bulb, and a small

DC toy motor.

To determine the current-voltage

(I-V) characteristics of a device, weapply a certain voltage and measure the

resulting current, or vice versa. Figure

1-14 illustrates the basic experimental

setup. An adjustable DC power supply

provides the stimulus, and we use two

DMMs to monitor the current through the device and the voltage across it.

To protect both the device and ammeter from accidental excessive currents it is advisable

to include a small current-limiting resistor in series with the device as shown.

Set up the circuit shown in Figure 1-14 using the decade-box for the 50 current-limitingresistor. Set the power-supply output to zero volts initially.

In your notebook, create a table for the voltage-current measurements. The first datapoint is of course (0 V, 0 mA).

With the handheld DMM, measure the cold DC resistance of the lamp prior to puttingit in the circuit.

Now add the lamp to the circuit. Slowly raise the power supply voltage until the voltageacross the lamp increases by about 0.1V and record the current in [mA]. Repeat this until

you reach +0.5V across the bulb (five data points). Note that the voltage across the bulb

is what we want to record, not the power-supply voltage. Also, your voltage increments

do not need to be exactly 0.1V, just something close (within 10%)

Continuing from where you left off, raise the voltage in larger increments of 0.5V andkeep recording the currents in this fashion until you reach +6V (the recommended

voltage for this particular bulb).You should now have enough data to make a nice plot in your lab report. Can you tell that

the data is not linear? Incandescent bulbs have a tiny resistive tungsten filament inside, but

when the filament gets really hot its resistance increases substantially, changing the slope of

the I-V curve.

Now replace the lamp with the toy DC motor and repeat the measurement to characterizeits I-V curve. The motor is a +3V device, so you will need to choose your voltage

increments differently. Note that at low voltages and currents the motor does nothing,

50AA

Power

Supply VV

DMM

D.U.T.

DMM

Figure 1-14 Measuring I-V curves

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Current-Voltage Characteristics 15

and there is a particular threshold at which the motor begins to turn. To get an accurate

representation of the I-V behavior near this threshold you may need to use finer

increments.

In your lab report, make a nice plot of the I-V curves for the lamp and the motor, and contrast

with Ohms law. There is a short tutorial on the course website for making plots in MicrosoftExcel, but you can also use MATLAB, Mathematica, or any other program of your choice.

NO HAND-DRAWN PLOTS WILL BE ACCEPTED.

Extra Credit: Photocell

For students who finish early or want an extra challenge, there is a cadmium-sulfide (CdS)

photocell included in your parts kit (the data sheet is on the course web site). Characterize

the I-V curve for the photocell under bright light and dark conditions. Can you think of a

way to pair the lamp and photocell to make an electronically-variable resistor?

Congratu la t ions !You have now c omple ted Lab 1

Note: keep all your electrical components! Over the course of the year you willaccumulate a number of parts and we will make use of parts from prior labs in orderto save you some money on lab kits.

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V I R Lab 1