LAB 2
Circuit Tools
OBJECTIVES
1. Become familiar with a dc power supply and setting the output voltage.
2. Learn how to measure voltages & currents using a Digital Multimeter.
3. Apply and Plot Ohm’s Law to find the resistance of a Resistor.
EQUIPMENT
Power Supply, Digital Multimeter (DMM), Leads, 1k Resistor, Breadboard
THEORY
The purpose of this laboratory exercise is to acquaint you with the equipment, so do not
rush. Don’t let one individual make all the measurements. You must become comfortable
with the instruments if you expect to perform your future job function in a professional
manner.
Circuit Symbols:
Most power supplies have three terminals, labeled as shown in the figure.
The three terminals permit the establishment of a positive (A or red) or negative (B or
black) voltage, which can be grounded (green). The variable voltage is available only
between terminals A and B. Both A and B must be part of any connection scheme.
When measuring voltage levels, make sure the voltmeter (the voltage setting on the
DMM) is connected in “parallel” (or across) to the element being measured, as shown in
the figure.
In addition, recognize that if the leads are connected as shown in the figure, the DMM
reading will be positive. If the meter were hooked up in the reverse manner, a negative
reading would result. The voltmeter is therefore an excellent instrument not only for
measuring the voltage level but also for determining the polarity. Since the meter is
always placed in parallel with the element, there is no need to disturb the network when
the measurement is made.
Ammeters (the current setting on the DMM) are always connected in “series” with the
branch in which the current is being measured. This requires that the branch to be
opened and the ammeter inserted into the circuit.
2-1
Ammeters also have polarity markings to indicate the manner in which they should be
connected to obtain a positive reading.
PROCEDURE
Part 1A: DC Power Supply
a. Find the positive (red) and negative (black) output terminals and check that the
negative (black) is connected to the ground (green) terminal in the center. Record
the min/max output voltage and current specs written below the negative terminal.
Identify the coarse and fine voltage adjustment knobs – these will be the controls you
use most often. The current adjust knobs and the “hi/lo” amps switch are used only
to set an upper limit on the current to protect the circuit that will be tested in our
experiment.
b. Turn on and test the power supply. Note that whenever power is on, either a green or
a red indicator light will light up: green indicates normal (constant voltage) operation
and red indicates the current limit has been reached.
 Use the coarse and fine adjust knobs to vary the output voltage. What is the
maximum voltage? How big is the adjustment range of the fine adjusting knob?
 Test and measure the current limit by “shorting” across the output terminals with
a wire (NOTE: never “short” any unprotected power supply!). Adjust the current
limit to 0.5A and then remove the short from the terminal points. You have now
set the upper current limit.
Part 1B: Setting the Output Voltage of a Power Supply with a Digital Multimeter
a. Connect a Digital Multimeter (DMM) to the dc power supply.
b. Using the analog scale on the power supply and the digital scale of a DMM, compare
the following voltage levels VAB using a percent difference; 1 V, 4 V, 5.5 V, 8.25 V,
and 9.6 V. For each setting choose that scale of the DMM that will result in the
highest degree of accuracy. How do they compare? (Remember to record your data
in a table.)
c. Is the magnitude of the percent difference for each level sufficiently small to verify
the fact that the reading of one meter will be very close to the other even though one
is analog and the other digital? Explain.
d. We will now investigate the effect of reversing the leads of the meter when
measuring a voltage. Reset the source voltage VAB to 5 V using the DMM. Then
disconnect the DMM and connect the red, or V-, lead to the B terminal and the
black, or COMM, lead to the A terminal. What is the effect on the reading of the
magnitude and sign?
Part 2: Plotting Ohm’s Law to find the Resistance of a Resistor
In this section we will determine the resistance of a resistor by determining the slope of
an “I-V curve,” given by Ohm’s law. The current and voltage of the resistive circuit will be
determined by direct measurements.
a. Construct the following circuit.
2-2
b.
c.
d.
e.
f.
Remember to use a DMM to set the voltage of the power supply.
Adjust the power supply until the voltage reading across the resistor (not the power
supply) is 2V.
Measure the current IR through the 1k resistor using the DMM. Record in a table
the voltage VR and the current IR using mA as the unit of measurement.
Repeat this procedure for the following additional levels of VR: 4V, 6V, 8V, and 10V.
Using the data from your table, plot the current IR versus the voltage VR on graph
paper (or Excel). Clearly indicate each data point on the graph.
The resistance level can be determined from the slope of this curve using Ohm’s law:
I
1
slope  R 
VR Rthy
g. Measured the resistance level Rexpt of the 1k-resistor using a DMM as an
ohmmeter.
h. Compare the predicted and measured resistance levels using a percent difference
from the following equation:
% Difference 
i.
Rexpt  R thy
Rexpt
 100%
Comment on the level of percent difference for the current values. Are the percent
differences sufficiently small to establish firmly the fact that the current determined by
Ohm’s law will be very close (to that measured directly? Answer the question in
sentence form.
2-3