Lab Date
Lab 1: DC Circuits
Student 1, student1@ufl.edu
Partner : Student 2, student2@ufl.edu
I. Introduction
The purpose of this lab is to allow the students to become comfortable with the
use of lab equipment, through exercises involving simple DC circuits. This lab
introduces us to the use of a power supply, multimeter, and voltmeter. We examine the
properties of DC circuits containing resistors and diodes. We also become familiar with
the use of an oscilloscope and a function generator.
In this lab we will be connecting a DC power supply to circuits containing various
resistive sources (resistors, light bulbs, diodes). We plot voltage (x-axis) vs current (yaxis) and compare the plots for the various setups. This will allow us to understand the
following: how voltage and current are affected by devices that follow Ohm’s law,
devices that are not ideal resistors, and devices that do not follow Ohm’s law. We also
try to calculate and build a Thevenin equivalent circuit.
II. Results
1-1. Ohm’s Law
A. Experimental Setup
We connect a power supply, voltmeter, multimeter, and resistor to the breadboard.
Circuit Diagram:
Breadboard Diagram:
B. Results
B.1 Values of V and I for 20K and 10K resistor.
18 kΩ Resistor
V (V)
I (mA)
1.00
0.055
2.00
0.112
3.00
0.168
4.00
0.224
5.00
0.280
10.00
0.561
12 kΩ Resistor
V (V)
I (mA)
1.00
0.084
2.00
0.169
2.99
0.252
4.00
0.338
4.99
0.422
10.00
0.846
B.2 Plot the data points as I (y axis) vs V (x axis).
V vs. I
Current (mA)
1
0.8
0.6
18k
0.4
12k
0.2
0
0
2
4
6
8
10
12
Voltage (V)
B.3 Compare to calculated from Ohm’s Law
The curves are clearly linear, as expected by Ohm’s Law. The slopes are 17.82 ohm and
11.82 ohm, close to the resistance of the corresponding resistor used.
B.4 How to fix circuit to get voltmeter to measure what we want
B.5 Accuracy of current measurement
B.6 Ideal voltmeter should do
B.7 Ideal ammeter should do
In the circuit diagram above we can see that the voltmeter is measuring any voltage drop
across the resistor or ammeter. If the ammeter were ideal, then its internal resistance
would be zero and there would be no voltage drop across it. In reality, the ammeter will
have non-zero resistance. We can try to fix the problem by moving the terminals of the
voltmeter to cross only the resistor. However, this introduces a new problem. The
voltmeter is now directly in parallel to the resistor, so that the measured current will now
be the current passing through the resistor plus the current passing through the voltmeter.
If the voltmeter were ideal, it would have infinite resistance and there would be no
current passing through it. In that case, our measurements would not be affected.
B.8 Measure R of ammeter
B.9 Measure R of voltmeter
It is possible to measure the internal resistance of the ammeter and voltmeter (configured
for this experiment) by placing each device in series with a resistor comparable to the
internal resistance of the device. In the case of the voltmeter, we placed it in series with a
20M resistor, and set the input voltage to 5.00 V. The voltage measured by the voltmeter
was only 1.66 V. We can compare the internal resistance knowing the voltage divider
equation:
V_meas = [ R_int / (R_int + R_ext)] * V_in,
or
R_int = [Vmeas / (V_in – V_meas)] * R_ext.
R_int = [1.66 / 3.34] * 20 Mohms = 9.94 Mohms, ~10 Mohms
In the case of the ammeter, we placed it in series with a 1kohm resistor and set the input
voltage to be 0.20 V. We know
I_meas * (R_ext + R_int) = V_in,
or, after solving,
R_int = (V_in / I_meas) – R_ext,
R_int = (0.20V / 0.173 mA) – (1.0 kohm) = 150 ohms.
B.10 Quantitative view: how large is each error, given 20K resistor
Knowledge of the internal resistance of our measuring devices allows us to estimate the
error for the two possible positions our voltmeter may be in. If the voltmeter is across
only the resistor, then it draws about R_ext / (R_ext + R_int) = 20k/10M = 0.002 (or
0.2%) of the current away from the 20K resistor. If the voltmeter is across both resistor
and ammeter, then the ammeter draws about R_int / (R_ext + R_int) = 150/20k = 0.0075
(or 0.8%) of the voltage away from the resistor.
B.11 Which of two alternative hookups is preferable
B. 12 Estimate error for 20 M resistor
It is preferable by a small margin to place the voltmeter across the resistor. This
conclusion would be reversed if we were trying to measure a 20M resistor since then
placing the voltmeter directly across the resistor will have it draw out 1/3 of the current.