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Phy132Experiment
Introduction to Instrumentation and Ohmic Circuit Elements
Name: ___________________
Date: _____________
Lab Partners: _______________________________________
Introduction: In general, an electric circuit consists of a group of components such as batteries, lamps,
switches, and motors that are connected together by conducting wires. Circuits containing semiconductor
devices, such as transistors, integrated circuits, or thermionic devices, such as vacuum tubes, among their
components are called electronic circuits. In a DC circuit, the direction of the electric currents in all of the
components is constant in time.
Any points where a given circuit connects to devices that are not considered to be part of the circuit itself
are called terminals. A single-loop circuit is one that consists of two or more components that are
connected together in series, one after the other, to form a single closed path.
In this lab you will study single-loop DC circuits. This will help you develop an intuitive feel for the basic
concepts and terminology used in electronics and gain hands-on experience with equipment that you will
use all semester!
Some Definitions:
CURRENT (I) is the rate at which charge flows past a given point in a circuit. It is usually measured in
amperes. One ampere (amp for short) is one coulomb of charge passing a given point in one second. The
current, at any instant of time in a single loop circuit, must be the same everywhere along the circuit.
Charge cannot accumulate at any point along the circuit path. This is the law of conservation of charge. In
a direct-current (DC) circuit the current always flows in one direction and is usually constant. In an
alternating current (AC) circuit the current oscillates, flowing in one direction and then reversing and
flowing in the other direction, and then reversing again, etc.
POTENTIAL DIFFERENCE, or voltage, (ΔV) is the work (energy) required per unit charge for a charge
to move from one point to another in an electric field. It is usually measured in volts. A potential
difference of one volt between two points means that it takes one joule of work to move one coulomb of
charge between the two points. In other words, when a charged particle is moved from point A to point B
through a region of space containing an electric field, the electric field does work on the particle either
giving energy to (positive work), or taking energy from (negative work), the particle. Potential
difference is the energy gained or lost by that charged particle divided by the charge of the particle.
POWER (P) is the rate at which energy is transferred to or from a portion of the circuit. It is measured in
Watts (W). The power delivered to, or taken from, a circuit component is equal to the product of the
current through the component and the potential difference across the component. There are two possible
situations that you might encounter in a circuit. The first is where the potential drops, or decreases, across
the component. In this case, the point where current enters the component has a higher potential (higher
voltage) than the point where the current leaves the component. This component draws, or uses, power
from the circuit. Examples of this include resistors, motors, diodes, capacitors and inductors. The second
case is where the potential rises, or increases, across the component. In this case the potential is lower
(lower voltage) where current enters the component and higher where it leaves. This component gives
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power to the circuit like a battery, generator, capacitor or inductor. Why are capacitors and inductors
listed as components that both give and take power? You will find out in a future lab.
RESISTANCE (R) is a property of electrical devices. All devices have a resistance to current flow. This
resistance is evident in the heat given off by a light bulb and the need for a fan in a computer. When you
use a laptop computer for an extended period of time you may have noticed that it is quite hot on the
bottom before you turn it off. The resistance is measured in ohms [Ω]. In this lab we will be looking at
how resistance is related to current and voltage in a circuit. It is important to keep in mind that resistors
always take power from the circuit.
Instruments:
NOTE: It is very important that you understand what these instruments do and how to use them
properly.
The POWER SUPPLY that you will use is shown in figure 1. This supply is voltage regulated, meaning
that when you set an output voltage and then attach it to a circuit that draws current from the supply, it
will maintain the set output voltage. This is different from a simple battery where the output voltage of
a battery goes down when you draw current from it. The power supply also has an adjustable currentlimiting feature. Current limited operation of the power supply will be explored in detail in a later lab. For
now, you should set the current limit control, the bottom knob, at its maximum value by turning it
clockwise as far as it will go and leaving it there for the duration of this lab. The top control sets the
regulated output voltage of the power supply.
The power supply has two digital meters, one to display the output voltage and the other to display the
output current. You will find that these readings are not as accurate as what you can measure with the
digital multimeter discussed below.
Figure 1: The Power Supply
The DIGITAL MULTIMETER (DMM) is a multifunction instrument that can be used as an ohmmeter to
measure resistance, an ammeter to measure current or a voltmeter to measure voltage depending upon
which of its functions is selected with the large rotary switch on its face.
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Figures 2, 3, and 4 show three slightly different DMMs. When using a DMM you will always have one
test lead, or wire, connected to the COM port and a second connected to one of the other three ports as
discussed below.
Figure 2: The Fluke 77
Figure 3: The Fluke 83
Figure 3: The Fluke 87
Measuring Resistance: To measure the resistance of a resistor you would use the DMM as an ohmmeter
by first setting the rotary switch to the  symbol and then connecting one wire to the port marked with
and a second wire to the port marked COM. (These DMMs are self-adjusting in the sense that you do
not need to make any adjustment to the meter if you intend to measure a very large resistance versus a
very small resistance.) You would then attach one test lead to each end of the resistor with the resistor
completely removed from the circuit. When the resistance measurement is displayed you will also see
units such as M, k, or . Be sure to make note of the units along with the measured resistance.
Always convert any measured values from M or k to before using data in calculations or plots.
Measuring Voltage: To measure the voltage (or potential difference) across a component you would use
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
~ 
the DMM as a voltmeter by first setting the rotary switch to either the V , V
, or mV symbol and then
connecting one wire to the port with the V symbol and the second to the port marked COM. Set the rotary
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
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 , or mV , symbol when measuring DC
switch to the V symbol when measuring AC voltages or to the V
voltages. For now we will be working with DC circuits.
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
The mV setting is used to make more precise measurements of small voltages, but the maximum voltage
that can be measured using this setting depends upon the specific meter. For both the FLUKE 83 and 87,
the maximum voltage is 400 mV, while for the FLUKE 77 the maximum is 300 mV. If you do not know
 setting. If you see
approximately what voltage to expect always start with the rotary switch set to the V
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
that your measured voltage is less than the maximum mentioned above, then switch to the mV setting to
obtain a more precise measurement.
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To make an actual measurement you would connect one wire to each side of a component in your circuit
without disconnecting the component from the circuit. We say that the voltmeter is connected in
parallel with the component. This is VERY important so if this is not clear ask your instructor to
demonstrate.
Measuring Current: The following discussion is very important. Be sure you understand this
otherwise you could cause serious damage to the DMM.

To measure the current passing through a component you first need to set the rotary switch to the A
symbol and connect one wire to the port with the A symbol and the second wire to the port marked COM.
This sets up the DMM as an ammeter. Next, disconnect one end of the component from the circuit.
Connect one wire of the ammeter to the disconnected end of the component and connect the other
ammeter wire to the circuit where the component had been connected. Connected in this way we say that
the ammeter is in series with the component. Remember, you must always break the circuit to
measure current. (A negative sign in the ammeter display simply means the current is traveling into the
COM port and out of the A port. Normally you would connect the ammeter such that current flows into
the A port and out of the COM port, but often you may not know the direction of the current. Don’t worry
though, this won’t hurt the ammeter.)
If you try to measure a current larger than 10 A, a fuse will blow in the DMM and it is possible that the
DMM will be damaged. If you see that your measured current is small, you may obtain a more precise
measurement by switching the second lead to the mA port, but be careful. The FLUKE 77 can handle
currents only up to 300 mA while the FLUKE 83 and 87 can handle currents up to 400 mA on this more
precise setting. So always measure current first with the second lead in the A port and then switch if the
current is small enough depending on the meter being used.
Even more precision can be obtained using the FLUKE 83 and 87 by moving the rotary switch to the A
symbol. The second lead must be in the mA A port and the current must be less than 400 mA.
Safety Check: When current is flowing in the circuit be careful not to touch the metal alligator clips
or leads on the resistor. It is possible that you could get a good shock. When you need to make
changes to the circuit like adding a resistor, or inserting an ammeter, turn the power supply off or
include a switch in your circuit so the power supply can be safely disconnected temporarily.
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