PC/CP320 Physical Computing

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 fixed a resistor using an ohmmeter.
3. To understand the operation of a potentiometer or variable resistor.

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
• 1 4-band colour coded resistors (beige/brown body) of your choice
• 1 5-band colour coded resistors (blue body) of your choice
• one of the above resistors should be less than 75 Ω
• trimmer and small screwdriver, any value
• boards with surface mount resistors, at least one per group
• mini magnifiers, one per group

Procedure

Resistor Identification

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. Resistance Measurement

   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)?
     
     
5. 
   

   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 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.
   
   

6. Variable Resistors

   A potentiometer or variable resistor is like a pair of resistors in series with variable values but a fixed sum. The diagram on the left shows the symbols for a potentiometer. The diagram on the right (hopefully) makes clear the relationship between a potentiometer and the two resistances R₁ and R₂. (Note: Sometimes a potentiometer is also called a "trimmer".)
   
   The total resistance of the potentiometer, R₁ + R₂, is constant. The arrow represents a slider or knob which will vary the values of R₁ and R₂ subject to the constraint that the total resistance does not change.
   Note: It may be easier to do these measurements by plugging the potentiometer into a breadboard.
   
   
   Use an ohmmeter to study the resistance of the (unconnected) potentiometer. Correlate your observations with the diagram above. Determine the minimum and maximum values of R₁, R₂, and R₁+R₂.
   Do the resistances R₁ and R₂ vary linearly with the adjustment of the wiper?
   
   
7. Note the numbers on the potentiometer. (Note: There are usually two numbers; one of the numbers is a lot number for the manufacturer, and the other indicates the component value.) How do they indicate the resistance of the potentiometer? Is it similar to anything else you've seen?
   
   

8. Problems with Resistance Measurement

   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.
   
   
9. 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.
   
   

10. Continuity Testing

    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?
    
    Can you determine what threshhold resistance is required in order to not identify as a short circuit?
    
    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.

11. Surface Mount Resistors

    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.
    
    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. _________________________________________
       
       
       
12. Look at the image below.

    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.]

    
    
13. Before you leave the lab, have the lab supervisor sign your lab notebook immediately after your last entry.