Exp 2 Resistors, Ohm's Law,Power Rating and I-V Caharacteristic (2012)

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EEE 125 LAB 2

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Universiti Sains MalaysiaElectrical & Electronic Engineering DepartmentBasic Circuits Laboratory EEE125

Resistors, Ohms Law, Power Rating andI-V Characteristic

OBJECTIVE

To learn about basic voltage and current measurements.

To learn about resistors, color codes, I-V characteristics and its power rating.

To learn about resistor networks using Ohms Law.

To learn about series and parallel resistors.

INTRODUCTION

Resistor

A resistor is a passive two-terminal electrical component that implements electrical resistance asa circuit element. The current through a resistor is in direct proportion to the voltage across theresistor's terminals. Thus, the ratio of the voltage applied across a resistor's terminals to theintensity of current through the circuit is called resistance. Resistance is measured in ohm ().Resistors are common elements ofelectrical networks and electronic circuits and are ubiquitousin electronic equipment. Practical resistors can be made of various compounds and films, as wellas resistance wire (wire made of a high-resistivity alloy, such as nickel-chrome). Resistors arealso implemented within integrated circuits, particularly analog devices, and can also beintegrated into hybrid and printed circuits.

Resistor Color Codes

Resistors are labelled with an approximate, or nominal, resistance value and a tolerancespecification. For example, a resistor might be labelled as 1k +/- 5%, meaning that themanufacturer guarantees that the actual resistance will be between 950 (1k - 5%) and 1,050(1k + 5%). The nominal resistance value and its tolerance for 5, 10, and 20% resistors areeither printed numerically on the resistor body (if the resistor is large enough), or indicated by fourcolor bands (ABCD) on the resistor body as shown in Figure 1.

Figure 1: Resistor color bands
http://en.wikipedia.org/wiki/Passivity_(engineering)http://en.wikipedia.org/wiki/Terminal_(electronics)http://en.wikipedia.org/wiki/Electronic_componenthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Direct_proportionhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Electronic_circuithttp://en.wikipedia.org/wiki/Resistance_wirehttp://en.wikipedia.org/wiki/Integrated_circuitshttp://en.wikipedia.org/wiki/Hybrid_circuithttp://en.wikipedia.org/wiki/Printed_circuit_boardhttp://en.wikipedia.org/wiki/Printed_circuit_boardhttp://en.wikipedia.org/wiki/Hybrid_circuithttp://en.wikipedia.org/wiki/Integrated_circuitshttp://en.wikipedia.org/wiki/Resistance_wirehttp://en.wikipedia.org/wiki/Electronic_circuithttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Direct_proportionhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electronic_componenthttp://en.wikipedia.org/wiki/Terminal_(electronics)http://en.wikipedia.org/wiki/Passivity_(engineering)


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Table 1

Resistor Color Codes

Value Tolerance

Color Numerical Value Color Tolerance

Black 0

Brown 1

Red 2 Gold 5%

Orange 3 Silver 10%

Yellow 4 No Band 20%

Green 5

Blue 6

Violet 7

Gray 8

White 9

The procedure for reading the resistor values using Table 1 is:a) First find the tolerance band (gold 5%, silver 10%, no band 20%)b) Starting from the other end, identify the first band - write down the number associated with

that color. If for example the color is Brown the corresponding number is 1.c) Now identify the next color. If the color is Black the number is 0. So far we have the

number 10 in our example.d) The color of the third band gives the multiplier. If the color is orange the multiplier is 1000

(103

).e) So the resistor value is 10,000 or 10 k.

The notation to state a resistor's value in a circuit diagram varies. For example, 8k2 in a circuitdiagram indicates a resistor value of 8.2 k, 1R2 indicates 1.2 and 18R indicates 18 .

Ohms Law

Ohm's law states that the current through a conductor between two points is directly proportionalto the potential difference across the two points. Introducing the constant of proportionality, theresistance, one arrives at the usual mathematical equation that describes this relationship.

Where I is the current through the conductor in units of amperes, V is the potential differencemeasured across the conductor in units ofvolts, and Ris the resistance of the conductor in unitsofohms. More specifically, Ohm's law states that the Rin this relation is constant, independent ofthe current.

This formulation states that the current (I) is proportional to the voltage (V) and inverselyproportional to the resistance (R). This is directly used in practical computations. For example, if a300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 =0.04 amperes (or 40 milliamperes) flows through that resistor.
http://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Potential_differencehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Amperehttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Ampereshttp://en.wikipedia.org/wiki/Ampereshttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Amperehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Potential_differencehttp://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Electric_current


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Series and parallel resistors

In a series configuration, the current through all of the resistors is the same, but the voltageacross each resistor will be in proportion to its resistance. The potential difference (voltage) seenacross the network is the sum of those voltages, thus the total resistance can be found as thesum of those resistances.

Resistors in a parallel configuration are each subject to the same potential difference (voltage),however the currents through them add. The conductances of the resistors then add to determinethe conductance of the network. Thus the equivalent resistance (Req) of the network can becomputed.

or

A resistor network that is a combination of parallel and series connections can be broken up into

smaller parts that are either one or the other.

EQUIPMENTS

Resistors 100 X 1, 220 X 1, 390 X 1, 470 X 1, 820 X 1, 1k X3

Breadboard

Digital multimeter

Analog multimeter

Analog DC Miliammeter (Black case)

GW GPR3030 DC power supply

GW Function Generator GAG-809 X 2

DSO-X 2002A Oscilloscopes

Crocodile clips, BNC Connectors, Connection wires (single core)
http://en.wikipedia.org/wiki/Series_and_parallel_circuitshttp://en.wikipedia.org/wiki/Series_and_parallel_circuitshttp://en.wikipedia.org/wiki/Electrical_conductancehttp://en.wikipedia.org/wiki/File:Resistors_in_parallel.svghttp://en.wikipedia.org/wiki/File:Resistors_in_series.svghttp://en.wikipedia.org/wiki/Electrical_conductancehttp://en.wikipedia.org/wiki/Series_and_parallel_circuitshttp://en.wikipedia.org/wiki/Series_and_parallel_circuits


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Experiment: Part IResistor Color Codes and Resistance Value

1. Find the resistor value for the following color schemes:

First band: Red

Second band: BlackThird band: OrangeFourth band: SilverValue=_______________

First band: BrownSecond band: BlackThird band: BrownFourth band: GoldValue=_______________

First band: RedSecond band: BlackThird band: GreenFourth band: NoneValue_______________

2. Unfold the color code for a resistor with value of the following:

4.7k 5%First band: _______________Second band: _______________Third band: _______________Fourth band: _______________

10R 20%First band: _______________

Second band: _______________Third band: _______________Fourth band: _______________

2k2 10%First band: _______________Second band: _______________Third band: _______________Fourth band: _______________

3. Compute and note the error (in percent) between the measured resistance values andthe values indicated by the color code. Are your resistors within spec?

470Value=_______________ Error =_______________ %

820Value=_______________ Error =_______________ %

1kValue=_______________ Error =_______________ %


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Experiment: Part IIBasic voltage and current measurement circuit

1. Construct the circuit on a breadboard such as in Figure 2a. Use R1=100, R2=220 and

R3=470. The input voltage is 10 V.

Figure 2a: Basic circuit and its connection on breadboard

2. Use a digital multimeter to measure voltage. To measure voltage without loading thecircuit being examined, the voltmeter must be placed in parallel with the circuit andmust present very large input impedance so that negligible current flows through theprobes.

3. Use an analogue ammeter to measure current. However, in order to measure current,the ammeter must be placed in series with the circuit and must present very smallimpedance so that the flow of current through that branch is not disturbed.

4. Note that the ammeter is properly connected in series with the current s path youare measuring. Scale the ammeter to the highest at first.Any group who fails to dothis will lead to the damage/broken of the ammeter, causing the group to get 75%marks deduction (or RM1 penalty). This is considered as a highly mandatorymistake. You should be alert with this caution all the time especially whenmeasuring current.

5. Power up the circuit. Record the obtained voltage and current.V= __________ V (digital multimeter)IT = __________ mA (analog miliammeter)

6. Now change your measurement tools. Record the obtained voltage and current.V= __________ V (analog multimeter)IT = __________ mA (digital multimeter)

7. Can you compare the differences between step 5 and 6. Which one is the best tool tomeasure voltage and current. Explain.


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8. Figure 2b shows how you arrange the ammeter to measure currents through R2. Nowyou can see that R2 and R3 are no longer in parallel and separated by I 2. Power up thecircuit. Record I2.

Figure 2b: Measuring I2 on breadboard

I2 = __________ mA

9. In Figure 2c, draw the circuit and show how you arrange the ammeter to measure

currents through R3. Make sure you do this very well and know how to separate thedifferent nodes.

Figure 2c: Measuring I3 on breadboard

10. Note that the ammeter is properly connected in series with the currents path youare measuring. Scale the ammeter to the highest range at first. Any group who failsto do this will lead to the damage/broken of the ammeter, causing the group to get

75% marks deduction (or RM1 penalty). This is considered as a highly mandatorymistake. You should be alert with this caution all the time especially whenmeasuring current.

11. Build and power up the circuit. Record I3.I3 = __________ mA


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Experiment: Part IIIOhms Law

1. Calculate and measure the total resistance of RAB in Figure 3a using digital multimeter.

R1= R2= R3= 1k.

Figure 3a: Series resistor

RAB = __________ (calculated)

RAB = __________ (measured)

2. Calculate and measure the total current, I if the resistors are connected to 10 V DCpower supply by using Ohms Law.

Current, IT = __________ mA (calculated)Current, IT = __________ mA (measured)

3. Repeat step 1 and 2 for resistors connected in parallel as in Figure 3b.

Figure 3b: Parallel resistor


RAB = __________ (measured)Current, IT = __________ mA (calculated)Current, IT = __________ mA (measured)

4. Repeat step 1 and 2 for combination of resistors as in Figure 3c.

Figure 3c: Combination of parallel and series


RAB = __________ (measured)Current, IT = __________ mA (calculated)Current, IT = __________ mA (measured)


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Experiment: Part IVPower Rating of Resistors

1. Power rating is a very important characteristic of electronic devices. These devices maybe complex electronic systems or simple electronic components such as resistors. Mostof the resistors used in our laboratory are rated for Watt. Some of them are rated at

1/2

Watt or 1 Watt.

2. Let us experiment with our resistors to see what actually happens when the stated powerrating is exceeded. Construct the circuit shown below in Figure 4a.

Figure 4a: Measuring power rating

3. Resistors R1 and R2 form a current divider network. We will experiment with resistor R2 todetermine the minimum resistance that it can have without violating the power rating.Violation of the power rating will simply result in the destruction of the component.

4. Consider Vin = 10 V and calculate the power in column 2 of Table 4a.

Table 4a: Calculated values

R2() Calculated Power in R2, W

Observation of increasein temperature (Y/N)

1k

820

470

390

5. Calculate the minimum resistance of R2 before the power dissipated from it exceed morethan Watt (0.25 Watt). Consider Vin = 10 V.

R2 (minimum) = __________ (calculated)

6. Set the voltage Vin to +10 Volts. Experiment and record your observation in column 3 ofTable 4a above. Start with R2 =1k and decrease it until you violate the power rating (untilthe resistor get hot) or approaching R2 (minimum).

7. It is expected that you do not destroy at least one resistor in this experiment. Becareful here. The resistor will get hot and it will eventually burn with a verycharacteristic odor if the power rating is exceeded.


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Experiment: Part VI-V Characteristic of a Resistor

1. In general the I-V characteristic of devices is one of the most important tools inunderstanding their behaviour. From Ohms law, V= IR, we see that in the I-V space theresistor is represented with a straight line of slope 1 / R.

2. We will do the experiment to determine the I-V characteristic of a resistor. Construct thecircuit such as in Figure 5a. In our case the Device Under Test (DUT) is a 1k resistor.

Figure 5a: I-V characteristic setup

3. Run the experiment. Find the voltage and current range of interest. Use digital multimeteras a voltmeter to measure voltage and analog miliammeter as an ammeter to measurecurrent.

4. Perform the same experiment for the following Vin as in Table 5a. Record your results.

Table 5a: Results

Vin, V Voltage, V Current, mA

5

10

15

20

25

5. Ideally, the system should be calibrated so that there is no offset (bias voltage or current)in the measurement. Plot I-V characteristic of a resistor on a graph as in Figure 5b. Onthe same graph, plot the line which is the least-squares best-fit linear curve of the form

y = mx + C. The Y-intercept should be zero Volts.

6. Determine the slope from the graph. What is the significant of the slope in this case?

Slope = __________


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Figure 5b: I-V characteristic

REVIEW QUESTIONS

1. How far the tolerance of a resistor affects the reading of voltage and current?

2. What will happen when we measure the current using multimeter in parallel with thecircuit?

3. How far the Ohms Law is verified for series and parallel circuit?

4. Explain briefly the meaning and importance of power rating.

5. Draw the I-V characteristic and indicate the region inside which you are able to safelyoperate your Watt resistors (according to Figure 5b).

6. Calculate the maximum current that may flow through a wire-wound resistor before thepower dissipated from it exceed more than 5 Watt. The resistance is 1k.

I (maximum) = __________ mA

TURN IN ONE REPORT PER GROUP AT THE END OF YOUR LAB SESSION.THERE IS NO TAKE HOME REPORT.

Exp 2 Resistors, Ohm's Law,Power Rating and I-V Caharacteristic (2012)

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Transcript of Exp 2 Resistors, Ohm's Law,Power Rating and I-V Caharacteristic (2012)