Simple Circuits and the Electronic Breadboard Experiment #30(Alward/Harlow web file: “circuit2.doc” 3-23-04) (2 hours)

Name: _____________________ Partners: _______________________Section No. _______

Purpose: To become familiar with the CADET breadboard and to explore further Ohm's Law. The temperature dependence of resistance; power dissipation, I2R.

Equipment: cadet breadboardRadio Shack multimeterred and black multimeter cables w/ alligator clips3/4 watt light bulb (with one inch leads)wire cutters8 cm red wire8 cm black wire½ watt resistors: 100 (brown-black-brown)330 (orange-orange-brown)430 (yellow-orange-brown)1000 (brown-black-red)

Reserve Equipment: red and black wirewire cuttersmultimeter fuses100 ohm resistorscadet fuses

Part I. The CADET Breadboard

There is a large white area full of holes called "tie points". We call this area the "breadboard". The separate white strip at the top has four horizontal sets of tie points which are connected to three power supplies and ground. See picture of breadboard above.

Three sets of short wires (three red, three yellow, three blue, and three black) lead to twelve vertical strips of tie points on the main breadboard; these strips provide convenient places from which voltage may be accessed. The remaining tie points are grouped in sets of five; all five tie points in a horizontal set are in common; they are connected to the same piece of metal underneath the breadboard.

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The "Ground"

In the upper right corner of the cadet there are four colored terminal posts; the black post is physically connected to "ground", which may be thought of as a neutral --or zero voltage--terminal. Notice that there is a symbol to the right of the black terminal; it has three horizontal lines of diminishing width. This the symbol for "ground".

To the left of the ground terminal there is a line drawn to the end of the fourth of four horizontal rows of fifty tie points. Each of these fifty tie points are physically touching an unseen conducting metal strip beneath the tie points. Thus, all fifty points are directly connect to ground by a conducting wire; these fifty points all do the exact same thing: they provide a path from ground along which electrons can travel. The bottom strip has three short black wires leading to the lower portion of the breadboard.

The Voltage Terminals

5 Volts

The 5-volt terminal is the red one; it's connected to the fifty tie-points at the very top of the cadet.

A voltmeter will measure the voltage of a terminal relative to the ground terminal, or relative to some other terminal's voltage. Measure the voltage of the red terminal now with respect to ground. Place the red multimeter cable in the "+" terminal of the meter, and the black cable in the "-" terminal. The "-" terminal will always have a cable in it whenever any kind of measurement is made of any quantity (current, resistance, or voltage), and that cable will always be black. Red is for "positive", and black is for "negative" or "ground".

Turn on the multimeter and switch the selector to DC-V (direct current, voltage). There are settings ranging from 200 m (that's 200 millivolts, or 200 thousandths of a volt, or 0.200 volt), to 1000 volts. Select 20; this will allow your meter to measure voltages as low as zero and as high as 20 volts; of course, if your voltage is less than 2 volts, you will select the 2 volt or 200 m scale.

With your meter set to DC-V (20), attach a short length of black extension wire to the alligator clip of the black multimeter cable and insert the other end of the wire into any one of the fifty ground tie points. Next, attach a short length of red extension wire to the alligator clip of the red multimeter cable and insert the other end of the wire into any one of the fifty tie points connected to the red terminal, or to the terminal itself. What does your meter read? Record the value in volts in Table 1.

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Note that the 5 V supply is fixed; it cannot be varied; it is always about 5 volts.

Positive Variable Voltage (1.3 - 15V)

Repeat the procedure for the variable power supply connected to the yellow terminal. Note: the thin black twist knob labeled V+ near the top center of the box controls the variable positive voltage. With the meter's probes in place, observe the change in the voltage as you twist the voltage knob. Observe and record in Table 1 the smallest and largest voltages (they may not be 1.3 and 15 V, as shown on the label; you may have to switch to a voltage scale on the multimeter higher than 20 V to read the maximum voltage).

Table 1

Voltages

Low Voltage (V)

High Voltage (V)

5 V red -------

+ (1.3 - 15 V) yellow

Test your breadboard with the multimeter to make sure that each of the three vertical 5 V strips show about 5 V, and the three vertical variable positive voltage strips are all functioning; likewise, check the three ground strips. It's not necessary to check the variable negative voltage strips, since we will not use them in this course.

Illustrating Voltage Differences with a Light Bulb Circuit

In the following investigation, students will note that the greater the voltage difference between the ends of the light bulb's leads, the brighter will the filament glow.

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Place one of the bulb's terminals into any one of the tie points in one of the three vertical (yellow) 1.3-15 V strips, and place the other terminal into any one of the (black) ground tie points; in order to reach the ground strip, it may be necessary to place the bulb's second lead into one of the set of 5-wide horizontal tie points, and then jump an extender wire from that set to the ground strip.

Measure Voltage

Next, put the multimeter in parallel with the light bulb's terminals to measure the voltage difference across the bulb while the bulb is illuminated. Figure 1 illustrates this circuit. Adjust the voltage to approximately 5 V using the slender black twist knob at the top of the cadet; record the voltage V in Table 2.

Measure Current

Remove the meter from the circuit, switch the meter to DC-A (direct current, amperes) to read current, and put the meter in series with the bulb as shown in Figure 2. Note that your meter will report current in milli-amperes, but you need to divide these numbers by 1000 to get the current in amperes. Record the current I in Table 2.

Calculate Resistance

Next, use Ohm's Law (R = V/I) to calculate the resistance of the filament for the current voltage and current values; report the resistance in Table 2.

Repeat voltage and current measurements and resistance calculation for the other approximate voltages in Table 2.

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Figure 1

Figure 2

Table 2

ApproximateVoltage (V)

Actual MeasuredVoltage V (volts)

Measured Current I (A)

Calculated Resistance R = V/I ()

5

6

7

8

10

12

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How Does Resistance of the Filament Depend on Its Temperature?

As the current I through the filament increases, the temperature of the filament increases. On the graph below, plot R along the vertical axis, versus I along the horizontal. How does the resistance change, if at all, as the temperature of the filament increases (i.e., as the current increases)? In other words, does the resistance increase, decrease, remain the same, appear to approach a limiting value?

R

I

Energy Dissipated as Heat in a Resistor

In the following investigation we will use resistors whose resistance is--unlike the light bulb filament--fairly constant over a wide range of currents. We wish to learn about the heat generated when electrons are pushed through a medium which resist their motion. Just as a sliding block warms up when it's bottom is pushed across a table which resists its motion, then so does a wire warm up when electrons are forced through it. The exact mechanism whereby this happens is discussed in more advanced courses, such as solid state physics or electromagnetism. All that the student needs to know now is that the rate at which thermal (heat) energy is being created in the resistor, i.e., the power dissipated, i.e., the number of joules of heat energy created each second, is given by the following equation

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P = I2 R

In the equation above, I is the current in amperes and R is the resistance (assumed constant in our investigation) in ohms. P is the power dissipated (energy per second lost), measured in units of Joules per second, or, watts.

If anything heats up too fast, it won't have enough time to release its heat to its environment, so it may expand and crack, or melt. In our case, the resistor will could all three things: expand, crack, and melt. We will stop short of the melting stage.

The larger a given resistor is, the more thermal energy it can hold without melting. There are 100 ohm resistors as small a the eraser on a pencil, or as large as a brick. The eraser-size 100 ohm resistor will melt if the power dissipated in it exceeds 1/4 watt, while the brick-sized 100 ohm resistor will withstand hundreds of watts of power dissipation. The resistors commonly used in our laboratory are 1/4 watt, ½ watt, and 1 watt.

Obtain a ½ watt, 100-ohm resistor (it's brown-black-brown); check its resistance with the multimeter to make sure it's approximately 100 . Using the same circuit as in Figure 2, raise the current to the approximate values shown in Table 3 by twisting the variable voltage supply knob, each time waiting about 30 seconds, then touching the resistor to note its warmth and calculating the power dissipated. It's not necessary to obtain the exact values shown in the table.

Calculate the power dissipated using the power equation above, P = I2R and the value of the current shown on the meter, assuming R = 100 ohms. Report that value, along with the current, in Table 3.

If your meter shows zero current, it's either because the red meter cable is not in the "A" socket, or you burned out the fuse. Touch the resistor and note whether it's warm.

The resistor's advertised power limit is 0.5 watt, but that doesn't mean it will burn up at exactly. 0.5 watt. At what power does the resistor become too warm to comfortably touch? Record this power in Table 3; this should be close to the power limit, 0.5 watt, but the resistor will actually withstand more power dissipation than its rated value.

Watch for a browning of the resistor or wisps of white smoke. At what power does this occur? Record this power in Table 3. Don't proceed with higher currents in Table 3 after smoking occurs.

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Table 3

I (ma) I (A) P = I2R (watts)

40

60

80

100

120

140

160

180

200

Resistor too warm to touch at P = watts

Resistor browns at P = watts

Resistor smokes at P = watts

Throw the burned-out or discolored 100-ohm resistor away.

Voltages Across Resistors Connected in Series

Take apart the circuit used to test power dissipation. Obtain three resistors of approximately 330, 430, and 1000 ohms, and use the multimeter to measure their exact resistances. Record these resistances as R1, R2, and R3 in Table 4.

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After measuring the exact resistances, adjust the variable (yellow) voltage supply to about 10 V (it's not necessary to be exact); record this value in Table 4.

Switch the multimeter to read DC-A (direct current, amperes), and make sure the black multimeter cable is in the negative jack, and the red cable is in the A jack.

Construct a chain of of the three resistors by using the five-wide tie points; these resistors are to be in series with the 10-volt power supply, a multimeter, and ground, as shown in Figure 3.

The circuit diagram is shown in Figure 4. The circle in the figure represents the 10 V voltage source. Attached to it are the three resistors connected in series with the power supply and multimeter and ground; ground is symbolized by the three horizontal lines. Conventional current passes from the power supply, through the three resistors and multimeter to ground.

Figure 3 Figure 4

Record in Table 4 the measured current Im shown on the multimeter.

According to Ohm's Law, the expected current in this series connection is V/(R1+R2+R3); record this expected current as Iexp in Table 4.

Calculate the percentage difference between the expected current Iexp and the measured current Im by subtracting the currents, dividing by the expected, and multiplying by 100.

Table 4

V (volts)

R1 ()

R2 ()

R3 ()

Iexp (A)

Im (A)

% Diff

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Calculate and report in Table 5 the expected value of the voltage differences V1, V2, and V3 across each resistor using Ohm's Law (V= IR) and the measured values of the current and resistance.

Remove the multimeter from the circuit an re-establish a connection to ground by inserting the lead of the last resistor into one of the ground strips.

Switch your meter to read voltage (DC-V) and measure the voltage difference across each of the three resistors and report your values in Table 5. Figure 5 illustrates the measurement of the voltage across R2. These voltage measurements are made by merely touching the meter's probes to the exposed wires on each side of each resistor.

Table 5. Voltage Differences Across the Resistors

V1 V2 V3

Calculated(V = IR)

Measured

The sum of all of the measured voltages across the three resistors should equal the voltage of the power supply, recorded earlier in Table 4. Report the power supply voltage again, this time in Table 6, and report in Table 6 the sum of all of the measured voltages reported in Table 4. Calculate the percentage by subtracting the values, dividing by the power supply voltage, and multiplying by 100.

Table 6

Power Supply Voltage (V)

Sum of All Voltages (V)

Percentage Difference

When you are finished, straighten all wires and resistor leads and place all components at the end of the table for inventory.

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Figure 5