WARNING: THIS IS AN EXAMPLE LAB ONLY. NUMBERS AND CONCLUSIONS
REACHED IN THIS EXAMPLE REPORT ARE NOT WHAT YOU WILL FIND
WHEN YOU PERFORM LAB 2!
Jan 1, 2009
Lab 2: DC Circuits
Heather Ray
Partner: Some Student
The purpose of this lab is to fully examine several fundamental properties of
circuits, using a DC power supply. In specific, we examine the current flowing
through a component as a function of applied voltage, experiment with voltage
dividers, and examine the effects of our measuring devices on the data we
collect. We find that there are two main categories of circuit components: linear
components follow Ohm’s V=IR law, and non-linear components such as the
diode that deviate drastically from this law. The instruments we use to perform
these measurements have a small, but non-negligible, effect on our readings.
This will need to be taken into account for all future measurements.
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2. Effects of Instruments on Measurements
Introduction
In this portion of the lab, we use the multimeter in the Voltmeter and in the
Ammeter setting. We determine the effect of the multimeter in these two settings
on our measurements.
Analysis and Results: Part A - Voltmeter
Our circuit to measure the effects of the voltmeter consists of a 1M resistor
connected in series with the multimeter, set to record voltage. We used the black
power supply box to provide +12V to our circuit. (See Figure 1)
1M
12V10V
VV
+
-
Vol tmeter
Figure 1: Left: Diagram of circuit used to measure the effects of the voltmeter on
our measurements. Right: Breadboard diagram of our circuit.
We first measured the resistance of the 1M resistor outside of the circuit, using
the multimeter on the ohmmeter setting. The measured resistance is 0.98 M.
The voltage supplied by the black box should be +12V. The true voltage output,
as measured by the voltmeter, is +11.58V. When the circuit was connected, the
voltmeter measured +2.4V.
The internal resistance of the voltmeter is given by:
Rm =
R * Vm
Vs – Vm
Using 0.98M for R, 11.58 for Vs, and 2.4 for Vm, we find the internal resistance of
the voltmeter is 0.26M. While this is a high value, the voltmeter will have a nonnegligible effect when connected in parallel to ~200kOhm and higher resistance
paths.
Analysis and Results: Part B - Ammeter
Our circuit to measure the effects of the ammeter consists of a 100 Ohm resistor
connected in series with the multimeter, set to record current. We used the black
power box to provide +12V to our circuit. (See Figure 2)
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100 ohms
+
12V10V VV
Ammeter
Figure 2: Left: Diagram of circuit used to measure the effects of the ammeter on
our measurements. Right: Breadboard diagram of our circuit.
We first measured the resistance of the 100 Ohm resistor outside of the circuit,
using the multimeter on the ohmmeter setting. The measured resistance is 96.4
Ohms. The voltage supplied by the box should be +10V. The true voltage
output, as measured by the scope, is +9.98V. When the circuit was connected,
the ammeter measured a current of 0.05 Amps.
Using these two equations:
Rtotal = R + Rmeter
and
I=
Vs
Rtotal
We find that
Rmeter = (Vs/I) - R
Using values of 9.98 for Vs, 0.05 for I, and 96.4 Ohms for R, we find the
measured resistance of the ammeter is 103.2 Ohms. This is a low resistance,
but the ammeter will impact measurements in circuits having a few hundred Ohm
resistance from other components.
Conclusions
We found the internal resistance of the multimeter, when used as a voltmeter, is
0.26M. The internal resistance of the multimeter, when used as an ammeter, is
103.2 Ohms. The voltmeter should have a very high resistance, so that it has
little impact on a circuit when connected in parallel. The ammeter should have a
low resistance, so that it has little impact on a circuit when connected in series.
The non-ideal values indicate that we cannot ignore the effect of the meter when
used in our circuits.
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