ECE 231 Laboratory Exercise 1Ohm’s Law

ECE 231 Laboratory Exercise 1 – Ohm’s Law

Laboratory Group (Names) ________________________ _______________________ ________________________

OBJECTIVES

Verify Ohm’s Law

Learn to read resistor color codes

Learn to use ohmmeter, voltmeter, and ammeter

Learn to calculate power loss in resistors

EQUIPMENT REQUIRED

ECE 231 Circuit Board (In Stock room) One banana cable One lot of colored clip leads (students must supply their own clip leads) DMM (digital multimeter) DC power supply

BACKGROUND

Resistors are used for many purposes such as electric heaters, voltage, and current dividing elements, and current-limiting devices. As such, their resistance values and tolerances vary widely. Resistance tolerances may range from +0.001 to +20%. The most common types of resistors are carbon composition, wire wound, metal film, carbon film, steel, and liquid. Their ratings can range from microwatts to megawatts. Variable resistors are called either potentiometers or rheostats. When used as a potentiometer their output is a variable voltage. When used as a rheostat they are used to control current. A good reference source is http://en.wikipedia.org/wiki/Electronic_color_code. Review this website before you come to the laboratory. Many types of resistors do not have a color code such as resistors made to military specifications and surface mount resistors. You might remember the following mnemonic to remember the color versus number code:

Bad (0) Boys (1) Race (2) Our (3) Young (4) Girls (5) But (6) Violet(7) Generally (8) Wins (9). Black Brown Red Orange Yellow Green Blue Violet Grey White

Most resistors use either 4 or 5 bands of colors. The 5 band color is usually used for 1% and 0.1 % resistors. This band represents 5% if gold, 1% if brown, and fire resistant if yellow.

When you observe a resistor it is not always possible to predict its wattage by just observing its size. There are many variables that affect a resistor’s wattage. Some such parameters are size, mounting, encapsulation, and cooling. There are three ways you can calculate the power being dissipated in a resistor in this laboratory. See Eq.1. In a thermodynamics' laboratory you could measure the rise in temperature of water in a calorimeter to determine the power being dissipated by a resistor. Consider the following design problem. What size (ohms

ECE 231 Laboratory Exercise 1Ohm’s Lawand wattage) resistor would you use for the heating element in a coffee maker or toaster? Assume 120 VAC and 300 watts.

P=V2

R=I 2R=V ∆R IR (1)

The resistance of a resistor can be approximated by equation (2):

Resistance (R)= ρLA (2)

Where ρ=¿resistivity of the material; L = length of material; and A is the area of the material. The material may be solid, liquid, or gaseous. Each of these parameters is often functions of temperature and stress. Liquid is often used for low resistances rated in the megawatts.

Part 1. There are 7 resistors and one potentiometer on the BOARD. Determine and record the values of the 7 resistors and the potentiometer and their associated color code if appropriate. See your text or the internet for the color code. Measure each resistor with an ohmmeter then see how that relates to the color code. We will assume the color code is the Theoretical Value. See Figure 1.

Figure 1. Experimental board for ECE 231 experiments.

ECE 231 Laboratory Exercise 1Ohm’s Law

Table 1. Resistor color codes

MeasuredValue

Color Code TheoreticalValue (Color Code)

% errorExperimental Discrepancy

Part 2. Connect a variable voltage supply to three different resistors and vary the voltage from 0 to 10 volts. See Figure 2. If the overload light is illuminated you may have tripped the overload protective device. Press the red reset button to reset the overload device.

Figure 2. Variable voltage supply. Use cable with banana plug. Notch side goes to black.

Plot the current versus the voltage in Figure 4 for each resistor. Label each curve with its resistance value. There is both a Fluke and Beckman multimeter that can be used to measure the current. See Figure 3. How does the plot verify Ohm’s Law? What can you say about the slope of the plots? Calculate the slopes and show that they are equal to 1/R.

Hint: All of the curves go through zero so only one additional point for each resistor is required to generate the Ohm’s Law curve. Simply set the voltage supply at one voltage (for example 10 volts) for all the resistors and then measure the current in each resistor. Verify the current using Ohm’s Law.

ECE 231 Laboratory Exercise 1Ohm’s Law

Figure 3. Laboratory bench equipment.

Figure 4. Plot for verifying Ohm's Law, ( 1R

= ∆ I∆V )

How does the plot verify Ohm’s Law? What can you say about the slope of the plots? Calculate the slopes and show that they are equal to 1/R.

What are the possible ways to measure the current through a resistor? There are several ways. How do you calculate the current through a resistor under test without using an ammeter? For a circuit board with surface

ECE 231 Laboratory Exercise 1Ohm’s Lawmounted resistors you would usually use the calculation method. Calculation of the measure of uncertainty for each method is different. A good reference source for error analysis is the Reference text or http://www.lhup.edu/~dsimanek/errors.htm.

Part 3. Connect a small resistor (less than 100 ohms) to the variable power supply. Gradually increase the voltage and feel, using your finger, the increase in the temperature of the resistor. Only increase the voltage so that the wattage lost in the resistor is less than 1/2 watt. What voltage created a ¼ watt loss? At what wattage does the resistor get too hot to touch? Comment on how hot the resistor gets when it is dissipating 1/4, 1/3, and 1/2 watt. Hint: Power = V2/R. Resistors are available on the 5th floor in the student work area and stock room.

CAUTION

Going beyond ¼ watt can cause the resistor to explode or ignite. A 100 ohm resistor will dissipate ¼ watt at 5 volts. You will usually see smoke or fire at ½ watt. Do NOT exceed 7 volts for a 100 ohm resistor.

Table 2. Wattage versus resistor temperature

Measured Test Value TemperatureCheck appropriate box

Comments

Resistance Voltage Wattage Ambient Warm Hot

1/4 1/3 1/2

Voltage

Part 4. Write a professional comprehensive laboratory report using a word processor. Show your results, calculations, error analysis, and include a comprehensive conclusion. There are lots of sample lab reports on the internet. (http://www.baylor.edu/content/services/document.php/118900.pdf ).

ECE 231 Laboratory Exercise 1Ohm’s LawEvery figure must be sequentially numbered and referenced in the preceding text. Your calculations may be handwritten and attached to the report if properly referenced in the text. Number all pages.

On the cover page of your laboratory report include the number and tile of the experiment, date performed, and laboratory partners.

Conclusion or comments.

___________________________________________________________________________________________