7 potentiometer

MAE 3005 Measurements Lab Lab 7

Lab 7 – Potentiometer Measurements

Objective

The objective of this lab is to construct a basic potentiometer, use it to make voltage measurements and then

determine the resistance of a resistor.

Equipment

Electrical project board 100 Ω resistor DC power supply (Agilent E3630 or E3620)

Connecting wires 10 kΩ potentiometer Multimeter (Agilent 34401A)

Theory

A simple voltage measurement circuit (include a complete circuit diagram in your report):

Potentiometer circuit (left) is connected to the circuit to be measured (right) by the ammeter.

When operated in the null mode, the potentiometer is varied (Rx changes) until the current IG is zero.

This requires that VA = Vm.

We can therefore balance the two circuits, then disconnect the potentiometer, and simply measure VA in any

way we choose.

A potentiometer is necessary when the circuit to be measured has very large impedance, so that drawing any

extra current through the circuit will create a large voltage drop across that impedance, therefore affecting the

measurement being made.


Procedure

Part 1 – Construct the potentiometer

1. Determine the uncertainty on the 100 Ω using the chart below.

Record this value in your lab book and state the resistance and uncertainty in your report.

2. Construct the circuit shown in the theory section and in the photo on the following page using:

VS = DC power supply. This will be set to 10 V later.

R = 10 kΩ potentiometer.

Ammeter = multimeter (do not fix permanently into the circuit, just use the probes).

Vm = supplied byDC power supply. This will be varied between 1 – 5 V

Rm = 100 Ω resistor. We will measure the voltage drop Vm across this resistor.

All of the lower sections of the circuits can be connected to the common (ground) terminals.

This will make the circuit look much neater.


Part 2 - Measurements

3. Create a table for recording measurements as follow

Voltage setting in resistive circuit (V)

VA (V) Potentiometer (Points A to B)

Vm (V) True value across

the resistor

ΔV (V) Uncertainty in voltage meas.

% error

1.00

…

4. Set the source voltage VS on the potentiometer to be 10.00 V.

Record this value and the uncertainty in your lab book.

5. Set the voltage source in the resistive circuit to be 1.00 V.

6. Connect the potentiometer circuit to the resistive circuit by inserting the multimeter as the ammeter.

7. With the two circuits connected by the ammeter, adjust the variable resistor until the current IG is as

close to zero as you can manage. Record in your lab book how close to zero you could achieve.

If the ammeter is blown, use the voltmeter setting and adjust until the voltage difference is zero.

8. Disconnect the ammeter from the circuit.

9. Measure the voltage difference VA between points A and B directly using the multimeter.

10. Measure the voltage difference Vm across the resistor directly using the multimeter.

11. Record the uncertainty in the voltage measurements.

12. Repeat steps 5 to 11 using voltages of 2, 3, 4 and 5 V across Rm.

13. Leave the circuit set up after the 5 V measurement for Part 3


Part 3 – “Unknown” resistor

14. With the voltage in the resistive circuit set to 5.00 V, record the current Im in the resistive circuit and the

uncertainty in the current.

Calculations for Part 2

1. Calculate the percent error between the measured VA and Vm using Vm as the true value.

Calculations for Part 3

2. Using Ohm’s law (using Vm and Im), calculate the resistance of the resistor in the resistive circuit.

3. Calculate the uncertainty in this value using propagation of error.

Include an equation showing the uncertainty in Rm in the theory section of your report.

4. Calculate percent error in Rm by comparing your result with the value marked on the resistor.

Questions

1. Part 2 - Why should you expect the measured value of Vm to be lower than the voltage setting?

2. Part 3 - Was your calculated Rm within the uncertainty range designated on the resistor?

7 potentiometer

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