PC/CP200 Electronics Lab I

Resistors and Ohmmeter Lab

Objectives

1. To identify fixed resistors and determine their nominal value, tolerance, and expected resistance range.
2. To measure the actual resistance of a resistor using an ohmmeter.
3. To evaluate the resistance accuracy specifications for the digital multimeter.
4. To measure continuity using a continuity tester and an ohmmeter.
5. To determine the underlying wiring of a prototyping board and understand how this is used in circuit construction.
6. To determine the total resistance of a set of resistors connected in series.
7. To determine the total resistance of a set of resistors connected in parallel.

Preparation

Read the section on lab notebooks on this web site. Record all data for this lab in your lab notebook.

Read the section on fixed resistors on this web site. The following charts may also be of use; they all contain the same information but in slightly different formats.

• Resistor colour coding chart [PDF; NIC Components, May 2004]
• Resistor colour codes: 4- band, 5- band [GIF; © 1996-2003 Electronix Express, colour chart]
• Resistor colour code: 4, 5, 6 band [JPG; Tim May, University of Colorado, colour chart]

Equipment

• digital multimeter - Wavetek 33XR assumed
• 4 4-band colour coded resistors (beige/brown body) selected by lab instructor
• 2 5-band colour coded resistors (blue body) selected by lab instructor
• one of the above resistors should be less than 75 Ω
• boards with surface mount resistors, at least one per group
• mini magnifiers, one per group
• Central Scientific copper wire samples
• pencils [borrow survey pencils from department office]
• 2 breadboards, must be different types

Procedure

For each of the following sections, record resistor markings and required information in your lab notebook.

1. 
   

   The resistors to the right are higher power resistors. Since the resistors are physically large, the nominal value, tolerance, and wattage are written on the resistor. Complete the table.
   	nominal value	tolerance	wattage
   top resistor	8 Ω	±10%	6 Watts
   middle resistor
   bottom resistor			8348 = part number TRW = manufacturer PW= wattage prefix

   
   
2. 
   

   [click image to get a larger image]	The resistors to the left are carbon composition (3) and carbon film (1). Which one is the carbon film?  How did you determine that? [Note: carbon composition resistors are obsolete.] These resistors use the 4-band colour code to code the resistor's nominal value and tolerance. For the 4-band code, the resistor is oriented so that the right band is always gold, silver, or missing. Complete the table. Of the three carbon composition resistors, which resistor has the highest Wattage? How can you tell?
   		nominal value	tolerance	expected range
   	top resistor
   	middle top
   	middle bottom
   	bottom resistor	47 kΩ	± 10%	42.3 kΩ - 51.7 kΩ

   
   

3. On the bottom resistor, it is difficult to tell if the second band is blue or violet. I have assumed that the band is violet because yellow-blue-orange is not a valid combination for a resistor with a ± 10% tolerance. Why are some colour combinations not valid?

4. Circuit boards use surface mount resistors, which are too small to use colour bands. However, they are labeled in a way that should be understandable once you understand colour codes.
   
   

   For the image to the right, list as many resistor values as you can. Write down: the resistor identification that is silk-screened on the circuit board, the number on the resistor, the nominal value of the resistor. R72 doesn't follow the normal 3 or 4 digit numbering convention. What is R72 and why is it used?

   [click image to get a larger image] [Photograph of the circuit board inside a DMM similar to that used in this lab.]

   
   
5. If you need to know the actual value of the resistor (not the nominal value) or if the colour coding is too difficult to read, it may be necessary to measure the resistance using an ohmmeter, a DMM (Digital MultiMeter) or a VOM (Voltage/Ohm/Milliammeter). The DMM and VOM (analog version of DMM) are multifunction meters that have an ohmmeter function. Read Measuring Resistance for instructions on using the DMM as an ohmmeter before continuing.
   
   Before using an ohmmeter for the first time, determine the meter's symbol for overload. NOTE: depending on the function of the meter, "overload" may or may not be dangerous to the meter. For the ohmmeter function, overload is considered a normal output that indicates an open circuit. An open circuit means that two points are not connected.
   
   To determine how the meter will indicate overload:
   • You can look up the symbol in the Operator's Manual. Look it up.
   • Alternatively, if you don't have a copy of the manual, use an open circuit to show an overload. Set the meter to ohmmeter (any range) making sure the probes are not touching anything (including each other) and observe the display.
   • For your meter, what is the overload symbol(s)?
     
     
6. 
   

   For the example shown to the left, the resistor is yellow-violet-orange-gold which corresponds to the nominal value 47,000 ± 5%. The expected resistance range is calculated to be 44.65 kΩ to 49.35 kΩ. A measurement range of 200K is selected because the nominal value of 47 kΩ is between 20KΩ and 200KΩ. Measuring the resistance shows 46.6 on the display with the measurement range at 200K which means the actual resistance is 46.6 kΩ.

   For each of the six resistors provided for this lab:

   • determine the nominal value,
   • determine the tolerance,
   • calculate the expected resistance range, and
   • measure the actual resistance.
   Leave space in your table for the true resistance range (next question).
   
   Until you are completely comfortable with the interpretation of the ranges, check the resistor across all of the ohmmeter range values, starting at the lowest range, and record the display at each range value.
   
   Demonstrate resistance measurement to the lab supervisor. Be prepared to explain the associated terminology [e.g. nominal resistance, tolerance, expected resistance range, actual resistance].
   
   Look over the lab so far: Is there a single point of useful information that would be good to include in a lab summary? If so, highlight it.
   
   
7. The accuracy of a digital ohmmeter is typically expressed as "±( x % of the reading + y digits)". For meters which measure over multiple ranges, each range may have a different accuracy specification. These specifications will be found in the Operator's Manual for the meter.
   
   If, for example, the specifications were ±(2% reading + 1 digit) and the reading was 13.75 kΩ, 2% of reading = 2% of 13.75 kΩ = 0.275 kΩ. The 1 digit equals 0.01 kΩ. Therefore the accuracy is ±0.285 kΩ. The true resistance should be between 13.465 kΩ and 14.035 kΩ.
   
   For each of the measurements made in the previous question, calculate the true resistance range given the resistance accuracy specifications in the meter's Operator's Manual (pdf, see Specifications section). Is the true resistance range within the expected resistance range?
   
   
8. Even simple wire has a resistance. Using the four copper (Cu) wire samples on the Central Scientific wire sample board, investigate the effects of wire length and wire radius on resistance. Note: The spool dimensions are given as length in metres and diameter in inches. [Hint: Resistance measured in ohms is equal to the electrical resistivity of the material measured in ohm metres, copper in this case, times the length of the specimen in metres divided by the cross-sectional area of the specimen measured in square meters.]
   
   
9. When measuring a very small resistance, it is important to account for the resistance of the meter's leads. Measure the resistance of the leads by touching the two probe tips together.
   
   
10. Many of the resistors under discussion are carbon composite or carbon film. You can create your own resistor by drawing a line on a piece of paper with pencil, preferably a softer pencil with a higher carbon content. The line should be at least 2mm thick and about 5cm long. Draw the line in your lab notebook and carefully record the points of measurement. With your multimeter, measure the resistance of this line. Make sure the probes are touching the carbon from the pencil and not puncturing the paper.
    
    The resistance measured will depend on the the length of the line, the thickness of the line and the type of pencil used but will probably be in the vicinity of 1 MOhm ±0.5 MΩ. What happens if you double the length of the line? What happens if you double the width of the line? [One should increase the resistance and the other should decrease the resistance.]
    
    Measure the resistance of the pencil itself. The survey pencils should have exposed carbon at both ends to facilitate this measure.
    
    Demonstrate and explain your results to the lab supervisor.
    
    Look over the lab since the previous demonstration point: Is there a single point of useful information that would be good to include in a lab summary? If so, highlight it.
    
    
11. In the instructions for using the digital multimeter, you were cautioned not to include yourself as part of the circuit. [Do not touch the component or the probe tip with your hands as your body's resistance will affect the measurement.] Measure your resistance and that of your partner.
    • Make sure you are not grounded (e.g. are not touching the case of any electronic device, are not using a static mat strap) and that you are not part of a circuit.
    • Place the meter on the lab bench.
    • Stand away from the bench and gently grasp one probe tip in each hand between your thumb and forefinger or gently press the probe tip into the pad of a finger or palm. How you hold the probe will affect the measurement.
    • Have your partner select the appropriate scale. It will be very difficult to get a single stable measurement; you will have to take a visual average
    
    How does the measurement change if the contact points on your hands are moist? Resistance will vary from person to person based on the percentage of body fat (fat has a higher resistance) and on their internal hydration (water is a better conductor). This is the principle behind body composition analysis scales and meters.
    
    
12. Prototype boards or breadboards come in various configurations.

    The image to the right shows two single strip boards. The board at the top is our standard board, labeled Experimentor™ 300, but the department has various other single strip and multistrip boards (multiple single strips side by side). A single strip board has a trough down the centre of the board. Any dual inline pin (DIP) chip, a chip with legs on both sides of the chip, would be installed straddling the trough; the green rectangle indicates proper placement, the red rectangle indicates improper placement. Discrete components that are not in a chip format, such as a resistor, do not have to straddle the trough. On either side of the trough, there is an array of sockets. The sockets contain spring clips to make contact with any component lead inserted in the socket. The vertical groups of five sockets are electrically connected (shown by the blue vertical lines in the image). If you flip our standard board over, you can see the electrical connections through the paper. Do NOT remove the paper.

    [click for larger image]

    At the top and bottom of the boards in the image are an additional one or two rows of sockets. These rows are referred to as the bus lines. The sockets in the bus lines are partitioned into one or more electrically connected segments. For example on the lower board in the image, the blue horizontal lines indicate two possible electrical connection options.
    
    Verify the electrical connections of the two sample breadboards using the audible continuity tester mode on the digital multimeter. An audible continuity tester is used to identify electrical connections or shorts, where the electrical connection is interpreted as a 'wire'. Sketch the electrical connections for both boards.

13. Use the audible continuity tester mode on the resistors from earlier in the lab. In theory, a resistor should not appear as an electrical connection. What are your results? Why?
    
    
14. A continuity test is actually testing resistance. Therefore, if you do not have a continuity tester, you can use an ohmmeter to test continuity, by looking for a resistance close to zero. Recheck the electrical connections of the two sample breadboards using the ohmmeter mode on the digital multimeter. Note your readings. Did a resistance of zero Ohms indicate continuity? Did you expect it to be zero? [If you answered yes, review your results for questions 8 and 9.]
    
    
15. At the simplest level, analog components will be connected:
    • in series, placed end-to-end, or
    • in parallel, components in parallel and ends connected

    The diagram to the right shows both the schematic diagram and one wiring possibility for two resistors in series and parallel. Using two resistors with resistances of similar orders of magnitude, construct the series circuit. When fitting resistors into a circuit, do not bend the leads right at the edge of the resistor body as this is the weakest point of the component. If doing a tight fit to the board for a project, use one of the two bending techniques illustrated to the right. For the labs, tight board fit is not required. Measure the resistance across resistor 1, measurement marked as M1 on the diagram. Do you get the same resistance when measuring across the exposed leads of the resistor as when you measure hole-to-hole as shown on the diagram? How does this measure compare to measuring the component outside the circuit? Measure the resistance of resistor 2, shown as M2, and the resistance of circuit segment, shown as M3. Construct the parallel circuit and do the measures comparable to M1, M2, and M3. What did you learn?

    Rseries = R1 + R2 Rparallel = (R1 x R2) / (R1 + R2) Rparallel = 1 / (1/R1 + 1/R2)

    
    Demonstrate and explain your results to the lab supervisor/demonstrator.
    
    Look over the lab since the previous demonstration point: Is there a single point of useful information that would be good to include in a lab summary? If so, highlight it.
    
    
16. Take a circuit board with surface mount resistors from the sample box and a mini magnifier. This investigation will be easier if the SMT resistors are physically larger; easier to read the numbers and easier to do the measurements. Note the board number - written number in silver on the front or back of the board. Find 2 to 3 resistors of different orders of magnitude. For these resistors, record the resistor identification number that is silk-screened on the circuit board [should start with the letter R], the number as written on the resistor, the nominal value of the resistor as determined by you, and the actual value of the resistor as measured by the DMM.
    
    Note: the nominal value may not match the actual value. There are three possible reasons for a mismatch.
    1. You determined the nominal value incorrectly.
    2. You measured the actual value incorrectly.
    3. _________________________________________
       
       
       
17. Before you leave the lab, have the lab supervisor sign your lab notebook immediately after your last entry.