LAB #02: Practical Voltage & Current Measurement

OVERVIEW:
The purpose of this lab was to explore how non-ideal meters affect our measurements of voltage and current in a physical circuit.

PROCEDURE:
Pre-Lab:
As illustrated in the ENGR 44 Lab book, analyze the circuit in Figure 1 to determine the value of voltage Vout for the cases in which:

1. Vout is determined using an ideal voltmeter.
2. Vout is determined using a non-ideal voltmeter of internal resistance Rm.

Lab:

1. Construct the circuit given in the lab book, measure Vout using DMM, and use pre-lab results to estimate the internal resistance of DMM.
2. Repeat step 1, but use the Analog Discovery  module to measure Vout instead; as well as determine the internal resistance of the scope instrument.

Physical Circuit: Two 10MΩ resistors in series

ANALYSIS:
Pre-Lab:
A) Vout = 2.5 V
B) Vout = (Rm / (10MΩ + 2Rm))*5V

Pre-Lab Calculations

Lab:
Part 1:
Vout (DMM) = 1.66V
Rm = 10MΩ

Part 2:
Vout (scope) = 2.5V
Rm = infinity Ω

Scope Reading

CONCLUSION:
In part 1, we measured Vout to be 1.66V using the DMM. This value makes sense because it implies that the internal resistance, Rm of the DMM is approximately 10MΩ, which is common for many digital meters today. For part 2, we measured Vout to be 2.5V using the Analog Discovery Module. This value is promising, yet misleading because it implies that the Analog Discovery Module has infinite resistance, thereby allowing Vout to be exactly half of the input voltage like our calculations predicted. This, however, is improbable because we should expect physical meters to have a high, yet measureable internal resistance.