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Lab 3.2 Watch: (https://drive.google.com/file/d/1-yerZiPFE61uIrc9MSijEg65uWFkpulR/view)
3.2.1 Purpose of this lab:
The purpose of this lab is to explore the limitation of a simple DMM in measuring resistance using what is referred to as the 2-wire method and then see how low a resistance you can measure using the 4-wire method. The 4-wire method, also called the Kelvin method, can measure resistances lower than 1 uohm. In this lab you should be able to measure resistances as low as 1 mohm.
3.2.2 What you will need:
A DMM
Resistors
A SBB
Arduino
3.2.3 What you will do:
1. Watch the video for the lab and learn about the 4-wire or Kelvin method for measuring low resistances.
2. Finding a real device that has a small resistance and for which you know the value, is sometimes difficult. Here are some examples you can measure in this lab:
a. Short the ends of the DMM leads
b. Use a 10 ohm resistor
c. Use a small resistor, either 1.5 ohms or 1 ohm
d. Use a resistor labeled as 0 ohms.
e. Cut a 12 inch length of 22 AWG wire
f. Use one of the long power rail buses in your SBB
3. Measure the resistances of the above DUTs (“devices” under test) with the DMM. Use the traditional method which we call the 2-wire method. For now, assume the DMM is an ideal ohmmeter.
4. Set up to use the 4-wire method, as described in the video. Use the 10 ohm resistor as the sense resistor.
5. Set up the Arduino as the current source. Write a sketch that will drive 6 digital pins as a HIGH for 7 seconds and a LOW for 7 seconds on a loop. You can use this code:
void setup() {
pinMode(13, OUTPUT);
pinMode(12, OUTPUT);
pinMode(11, OUTPUT);
pinMode(10, OUTPUT);
pinMode(9, OUTPUT);
pinMode(8, OUTPUT); }
void loop() {
digitalWrite(13, HIGH);
digitalWrite(12, HIGH);
digitalWrite(11, HIGH);
digitalWrite(10, HIGH);
digitalWrite(9, HIGH);
digitalWrite(8, HIGH);
delay(7000);
digitalWrite(13, LOW);
digitalWrite(12, LOW);
digitalWrite(11, LOW);
digitalWrite(10, LOW);
digitalWrite(9, LOW);
digitalWrite(8, LOW);
delay(7000);
}
6. Using the DMM, measure the voltage from an Arduino pin into an open circuit. Then measure the voltage on the output when this single pin drives a 10 ohm resistor load.
Why does the voltage drop with the 10 ohm resistor load? Since the sense resistor is 10 ohms, and you measure the voltage across it, what is the current through the 10 ohm resistor?
7. One output pin will only deliver a limited current into a 10 ohm load. Measure the output current of the pin by the voltage you get across the 10 ohm resistor. Connect all 6 output Arduino pins in parallel to drive the same 10 ohm resistor. Measure the current you get from the voltage across the 10 ohm resistor.
8. What is the difference in current with 1 I/O and 6 I/O switching in parallel?
9. To gain confidence with a new instrument or method, always start measuring something for which you know the answer. As the first DUT, you will perform a 4-point resistance measurement of a 10 ohm resistor as the DUT and a 10 ohm resistor as the current sense
10. Calculate the resistance of the 10 ohm resistor using the 4-wire method. How does it compare to the resistance you measured with the 2-wire method?
11. Estimate the lowest resistance you think you can measure with the 4-wire method with the equipment you have available. For example, if the current is 200 mA and the smallest voltage you can measure is 1 mV, the smallest resistance you can measure is 1 mV/200 mA ＝ 0.005 ohms. If you can measure as low as 0.2 mV as the uncertainty in the voltage, then the lowest resistance you can measure is 1 mohm.
12. Use the 4-wire method to measure the resistance of a 22 AWG, 12-inch length wire, look up the resistance you expect to see from a table such as this one, and compare this to what you measure. For this measurement, measure the voltage across the wire by clipping directly to the wire and not through the SBB connection. This eliminates the contact resistance in the SBB.
13. Measure the resistance of the 12-inch long 22 AWG wire, with the influence of contact resistance, by using leads from the DMM connected directly to the 22 AWG wire. What do you estimate as the contact resistance for the wires plugged into the SBB?
14. Measure the series resistance of the long SBB power bus and the contact resistance in a connection.
3.2.4 What you will turn in
Section 1: answer the following questions: (2 points)
1. What is the limitation of the 2-wire method and how does the 4-wire method get around this problem?
2. What did you measure as the 2-wire resistance of the 12 inch long 22 AWG wire compared to the 4-wire method?
3. What is current you are able to get through the 10 ohm sense resistor with only 1 I/O driving or with 6 I/O driving?
4. What is the series resistance of a long column in the SBB? And how did you measure it?
Section 2: (3 points)
Describe how you set up your circuit to measure the 4-wire resistance of the 12 inch length of wire. How did your measurement compare with the specified value? Explain why you are not measuring the contact resistance in this measurement. Show a picture of your system.
Describe how you measured the resistance of the long column in the SBB and why this measurement does not include contact resistance.
What did you expect to measure as the resistance of the 12 inch length of wire? How did it compare to what you actually measured? What do you estimate to be the uncertainty of your measurement? For example, if the smallest voltage you could measure is 1 mV, what is the lowest resistance you could measure? What is the smallest voltage you can measure?
Using this method, what did you measure to be the resistance of the column interconnects in the SBB. So what? How will you use this information for future circuits in the SBB?
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