Notes for the Ohm’s Law Lab
January 20, 2011
Dr. Buckley
This document relays some information regarding the use of power supplies and digital
multimeters that will be used in today’s lab as well as throughout the semester for other
experiments. Some of these tips are intended to help get good results and some are intended to
protect the equipment from misuse.
Power Supply
The face of the power supply looks like Figure 1. Notice this is a DC power supply.
Figure 1 DC Power Supply
To use the power supply:
1. Make sure the Power button in the lower left is out – the power supply is off.
2. Turn the Voltage knob entirely counterclockwise.
3. Connect your circuit that you wish to use. The connection to the power supply
comes through the red + port and the black – port with the red port being the
higher voltage side.
4. For today, have me check your circuit before you power it up.
5. Once cleared, push in the Power button in the lower left.
6. Increase the voltage by turning the voltage knob clockwise.
For measuring voltages, currents, and resistances we will typically use devices called
multimeters. A single multimeter provides the ability to measure these electrical properties over
a wide range of values.
The measurement of voltage and of resistance is done by placing the leads of the multimeter at
the two points to be measured. The measurement of current requires the multimeter to become
an actual part of the circuit – the circuit must be run through the meter. Following are brief
instructions for connecting the multimeter in these scenarios and some suggestions for protecting
the device from overload.
Types of Multimeters on Hand
Figure 2 Various Multimeters you May See
Type A (whitish)
Type B (orange)
Type C (blue)
You may work with any of these during the semester. Important points:
1. Each meter is capable of measuring either DC or AC signals. For meter Type A that
selection is made with the switch in the upper left corner. For meter Types B and C the
selection is made by the dial. The wavy line above a unit indicates AC and the straight
line indicates DC (for example, for amperes à vs. Ā). Make you select the correct setting
for your measurement.
2. The COM port is used in all measurements. For current, the other lead will go into the A
port and for voltage or resistance into the V-Ω port. On a Type C meter, the input for the
current is selected based on the range to be considered – up to 200 mA or up to 10 A.
3. The units are important to notice. For example, on a Type A meter the ampere ranges run
from 2 A to 200 μA. Be sure to properly record the units with your measurement.
Changing scales in any of the measurements allows you to better take advantage of the
accuracy of the meters. When taking readings, always start out with the range set to the
highest value and drop it down until you see your reading to the number of figures you
require.
4. Connections:
a. For voltage and resistance: In these cases you are measuring the difference
between two points. Taking the measurement is simply a matter of placing the
probes from the multimeter at the appropriate places in the circuit – the meter is
not a part of the circuit but is simply sort of an observer from the outside.
b. For current: The circuit must run through the meter. This is done by having the
circuit run into the A port and back out the COM port. This arrangement also
unfortunately provides the opportunity to blow fuses. A couple of tips may be
helpful to avoid blowing fuses.
i. Make sure the voltage on the power supply is turned low before powering
up the supply.
ii. When disconnecting the circuit, lower the voltage on the power supply to
zero and then power the supply off. Then disconnect the circuit.