Week 3

Blog sheet week 3

1.       Compare the calculated and measured equivalent resistance values between the nodes A and B for three circuit configurations given below. Choose your own resistors. (Table)

Image 1: Figure (a) shows three resistors in parallel. Figures (b) and (c) are a combination of series and parallel

Resistors used:

R1: 120 Ω

R2: 150 kΩ

R3:  41 Ω

R4: 360 Ω

Chart	Calculated Value	Measured Value
A	32.7 Ω	34.8 Ω
B	160.98 Ω	165.89 Ω
C	519.93 Ω	522 Ω

Our calculated values were fairly close to the measured values. We may have conducted minor mistakes in our calculations.

2.       Apply 5V on a 120 Ω resistor. Measure the current by putting the multimeter in series and parallel. Why are they different?

When measured in series, the current was measured at 39 mA and in parallel, the current was measured at 0.12 A. They're different because in parallel, the DMM will short out the circuit, thus not giving you an accurate reading.

3.       Apply 5 V to two resistors (47 Ω and 120 Ω) that are in series. Compare the measured and calculated values of voltage and current values on each resistor.

Our calculated & measured current values and voltage values were close to each other. The values are fairly close to each other because the two resistors are in series, meaning that the two would have similar current values. The only difference between the two resistors are the voltage values; in series, only the same current flows through the resistors, making the voltage values varying upon which resistor it flows through.

Resistors in Series:

Resistor Value	Measured Current	Calculated Current	Measured Voltage	Calculated Voltage
47 Ω	27 mA	29.94 mA	1.38 V	1.41 V
120 Ω	29.9 mA	29.93 mA	3.34 V	3.42 V

 4.       Apply 5 V to two resistors (47 Ω and 120 Ω) that are in parallel. Compare the measured and calculated values of voltage and current values on each resistor. 

Our measured and calculated current for the 47 Ω were off by 10.9 mA. We may have had a minor calculation error. The measured and calculated values for the current and voltage are different between each resistor because they're in parallel. When in parallel, the voltage is the same. However, the current through each resistor will be different, varying upon the resistor value. From the chart, the larger the resistor value is, the lower the current value will be.

Resistors in Parallel:

Resistor Value	Measured Current	Calculated Current	Measured Voltage	Calculated Voltage
47 Ω	99 mA	88.1 mA	5 V	5 V
120 Ω	38 mA	37.7 mA	5 V	4.8 V

 

5.       Compare the calculated and measured values of the following current and voltage for the circuit below: (breadboard photo)

a.       Current on 2 kΩ resistor,

b.      Voltage across both 1.2 kΩ resistors. 

Image 2: Our breadboard setup

In chart (a), Our calculated value was very close with the measured value. In chart (b), our calculated values and measured values were pretty close as well for the first 1.2 kΩ resistor, and slightly off for the second 1.2 kΩ resistor. The reason why the second resistor had a lower value than the first resistor is because they were in parallel; the voltage read across one resistor will not be the same as the next one. 

a.) 2 kΩ resistor

Calculated Value	Measured Value
1.80 mA	1.83 mA

       b.) 1.2 kΩ resistor 

Resistor	Calculated Value	Measured Value
1.2kΩ	1.21 V	1.27 V
1.2kΩ	0.5158 V	0.45 V

6.       What would be the equivalent resistance value of the circuit above (between the power supply nodes)?

The equivalent resistance value of the circuit is 2.176 k

7.       Measure the equivalent resistance with and without the 5 V power supply. Are they different? Why?

• The equivalent resistance with the 5V power supply was 25.26 MΩ. We weren't sure why we were getting such a high value, but something about it didn't seem right. We were expecting the value to be much lower.

• The equivalent resistance without the 5V power supply was 2.5 kΩ

They are different because of how the DMM measures the resistance; it supplies a current to the circuit and then measures a voltage. If an outside voltage source is added, it should throw off the measurement.

8.       Explain the operation of a potentiometer by measuring the resistance values between the terminals (there are 3 terminals, so there would be 3 combinations). (video)

A potentiometer operates by spinning its dial to either increase or decrease the resistance value. The video below provides a more detailed explanation.

 Video 1: Explanation of the potentiometer

Lead Combination	Value
1 and 2	5.3 k Ω
2 and 3	5.3 k Ω
1 and 3	1.3 k Ω

9.       What would be the minimum and maximum voltage that can be obtained at V1 by changing the knob position of the 5 KΩ pot? Explain.

The maximum would be 5V because if the knob on the pot is almost turned all the way up, you’ll get close to 5V.

The minimum would be 0V because the knob on the pot is almost turned all the way down, you’ll get close to 0 V.

10.   How are V1 and V2 related and how do they change with the position of the knob of the pot? (video)

Video 2: Showing how the knob affects the DMM readings

(Note: Our leads were switched, hence the negative reading)

V1 is constantly set at 5V because there’s no other component between that and the voltage source. V2 would be different because of the 1k resistor. They are related by:

 (R2/R1+R2) *V1 = V2

11. For the circuit shown in the video below, you should NOT turn down the potentiometer all the way down to reach 0 Ω. Why?

If you turn the potentiometer all the way down to reach 0 Ω, it will short the voltage source, thus creating high current and it will destroy the potentiometer. 

 12.   How are current values of 1 kΩ resistor and 5 kΩ pot related and how do they change with the position of the knob of the pot? (video)

Video 3: Showing how current values change according to the knob's position 

The current would be the same because it’s in parallel if it’s at 1k. If the pot increases resistance, current should decrease; as the pot decreases, the current should increase.

13.   Explain what a voltage divider is and how it works based on your experiments.

A voltage divider is a series of resistors or capacitors and an input that turns larger voltage into a smaller one. It can produce a specific output voltage, which is a fraction of the input voltage. From our experiment, we used the potentiometer to adjust the voltage. By turning the knob either left or right, the resistant value is adjusted; either increasing the voltage or decreasing the voltage of the output.

14.   Explain what a current divider is and how it works based on your experiments.

A current divider is a group of resistors connected in parallel and divides the current from the power source. This allows different amount of current to be allocated to different parts of a circuit. In our experiment, it's based on the resistor value. As the potentiometer's resistance decreased, more current goes through it, causing less to travel through the 1k resistor.