Voltage is a fundamental
quantity that is important in every phase of electrical engineering from
power systems to voltages inside VLSI chips.
If you are an Mechanical
You will want to measure
things like temperature. If you do that, you will use some sort of
temperature sensor, and the odds are high that it will produce a voltage
that you have to measure.
If you are a Chemical
You will want to measure
things like pH. If you do that, you will use some sort of pHsensor,
and the odds are high that it will produce a voltage that you have to measure.
If you are a Civil Engineering
You will want to measure
things like strain. If you do that, you will use a strain gage in
an electrical circuit, and you will need to know how to measure voltage,
and quite possibly you will need to know how to set up the circuit.
If you are a Bioengineering
You may want to measure
voltages produced by nerve cells.
Whatever your engineering persuasion, you will need to make measurements
that will invariably require you to deal with a voltage from a sensor.
You might not need to be the world's greatest expert on how to measure
voltage, but you will need to be knowledgable even if you just want to
talk to the person who designs the measurement system. (And, click
here if you need to review basic ideas about voltage.)
That leads us to the question of what you should know at the end of this
lesson. Consider the following:
Given a need for a physical
Be able to select and
use basic sensors to measure temperature, strain, etc.
Given a voltage output
from a sensor:
To be able to connect
a voltmeter - or other voltage measurement instrument - to the circuit
at proper points,
Be able to use a voltmeter,
oscilloscope or A/D card to measure the voltage
Eventually, you will also
want to do the following - even though it is not explicitly covered in
Given a voltage measurement
Be able to record voltage
measurements in a computer file, and,
Be able to use that file
in an analysis program, including Mathcad, Matlab or Excel.
conclusion that you have to come to is that everyone who makes measurements
- of almost any physical variable - is going to deal with voltages, voltage
measurements and digital representations of voltages, whether they are
a biologist, a mechanical engineer, an automobile mechanic or any number
of other occupations. Voltage is ubiquitous, and you have to deal
with it - whether you want to or not. You may not want to be an electrical
enginer, but you will probably need to understand enough about basic electrical
measurements to be able to use modern sensors, instruments and analysis
programs in your work.
In this section we'll look at how you use a voltmeter. Here's a representation
of a voltmeter.
For our introduction to the voltmeter, we
need to be aware of three items on the voltmeter.
The display. This
is where the result of the measurement is displayed. You meter might
be either analog or digital. If it's analog you need to read a reading
off a scale. If it's digital, it will usually have an LED or LCD
display panel where you can see what the voltage measurement is.
The positive input terminal,
and it's almost always red.
The negative input terminal,
and it's almost always black.
you need to be aware of what the voltmeter measures. Here it is in
A voltmeter measures the
voltage difference between the positive input terminal of the voltmeter
and the negative input terminal.
it. That's what it measures. Nothing more, nothing less - just
that voltage difference. That means you can measure voltage differences
in a circuit by connecting the positive input terminal and the negative
input terminal to locations in a circuit.
We'll show a voltmeter connected to the circuit diagram - a mixed metaphor
approach. Forgive us for that, but let's look at it.
This figure shows where you would place the
leads if you wanted to measure the voltage across element #4.
Notice that the voltmeter
measures the voltage across element #4, +V4.
Notice the polarity definitions
for V4, and notice how the red terminal is connected
to the "+" end of element #4. If you reversed the leads, by connecting
the red lead to the "-" terminal on element #4 and the black lead to the
"+" end of element #4, you would be measuring -V4.
are some important things to note about taking a voltage measurement.
The most important point is this.
Voltage is an across
That means that when you
measure voltage you measure a difference between two points in space.
There are other variables
of this type. For example, if you use a pressure sensor, you measure
the pressure difference between two points, much like you measure a voltage
There are other kinds
of variables. For example, there are numerous variables that are
variables. Current and fluid flow variables are example of flow variables.
They usually have units of something per second. (Current is couloumbs/sec,
while water flow might be in gallons/sec. - for example.)
When you measure a voltage
the two terminals of the voltmeter (in the figure, the red terminal and
the black terminal) are connected to the two points where the voltage appears
that you want to measure. One terminal - say it is the red terminal
- will then be at the same voltage as one of the points, and the other
terminal - the black terminal - will be at the same voltage as the other
point. The meter then responds to the difference between these two
look at an example. Here are three points. These points could
be anything and may be located in a circuit, for example. Wherever
they are, there is a voltage difference between any two of these points,
and you could theoretically measure the voltage difference between any
two of these points. There are actually three different choices for
voltage differences. (Red/Green, Green/Blue, Blue/Red) Then,
for each difference, there are two different ways you can connect the voltmeter
- switching red and black leads.
Let's check to see if you understand that.
Here are the same three points, but now they are points within a circuit.
In this particular circuit, the battery will produce a current that flows
through the two resistors in series.
This circuit has a schematic representation
And, here is the same circuit with the measurement
points (see above) marked.
Now, if you want to measure the voltage across
Rb, here is a connection that will do it.
And, the physical circuit would look like
Now, the reason for taking this so slowly is that students often have trouble
moving between circuit diagrams and the physical circuit and understanding
how to translate between them. What looks clear on a circuit diagram
is not always as clear in the physical situation. We'll get a little
closer to physical reality in this exercise.
Here's a portion of a circuit board. You want to measure the voltage
across R27. Click on both places where you should put the voltmeter
When you measure a voltage difference - whatever the instrument you use
- you will always have two leads coming from the instrument that will have
to be connected to the two points in your circuit across which the voltage
And, remember, the voltage might be any of the folowing.
The voltage might be across
an element embedded in a circuit.
The voltage might be the
output of a transducer measuring some physical variable like temperature,
pH, rotational velocity (a tachometer), etc.
Instruments for Measuring
In the material above, we assumed that you would measure voltage with a
voltmeter. Actually, there are often numerous options for the instruments
you use to measure voltage. Here are three common options.
An A/D card in a computer
We will examine each of these options separately
in the next section. Before we get there, however, note these common
points for each of these three instruments.
Each measures voltage.
To measure voltage, remember
that voltage is an "across" variable. Each instrument will therefore
have two leads to be connected to the circuit where you want to measure
voltage, and those leads should be placed across the two points defining
the voltage you want to measure.
Voltmeters (including oscilloscopes, etc. as voltmeters) will have an effect
on any circuit when they are used. Any time you take a measurement
- no matter what the measurement is - you disturb the thing you are measuring.
Attaching a voltmeter to a circuit will change the circuit - i.e. disturb
the circuit - and modify the voltage you are trying to measure. You
just have to ensure that the disturbance is negligible. That's what
we want to look at here.
Let's examine measuring the outut voltage of a voltage
divider circuit. Here is the circuit.
Now, the voltmeter is really equivalent to a resistor, so we can - for
purposes of analysis - replace the voltmeter by its equivalent resistance.
Here is the circuit with the voltmeter equivalent resistance. (Rm
is the resistance of the voltmeter.)
Now, you should be able to see that this isn't the same circuit that you
thought you were measuring. The addition of the voltmeter resistance
changes the circuit and the changed circuit will have a different output
voltage than the original circuit. The question is whether the output
voltage of the changed circuit is significantly different from the output
voltage of the original circuit.
To determine if the output voltage has changed, you need to consider that
the voltmeter and the resistance, Rb, are now in parallel.
That means that the output of the voltage divider is different. However,
you can compute the output without the meter and with the meter.
= Vin Rb/( Ra + Rb)
- without the meter
= Vin Re/( Ra + Re)
- with the meter, and
= Rm Rb/( Rm + Rb)
These two expressions are very similar, and
the how the close the two voltages will be depends upon how close the equivalent
resistance and the original resistance are. Note that the equivalent
parallel resistance is:
= Rm Rb/( Rm + Rb)
= Rb [Rm/( Rm + Rb)]
So, if the factor multiplying Rb
is close to one, there won't be much difference between the original voltage
and the voltage you have when you attach the voltmeter. In order
to be sure that is true, we need to have the factor multiplying Rb
as close to one as possible.
Rm + Rb)] = 1
or at least get as close to 1 as we can.
That's going to happen when the meter resistance is much larger than Rb.
The conclusion that you come to is that you want the resistance of a voltmeter
- any voltmeter, including osciloscopes, etc. - to be as large as possible.
We'll look at typical values for instruments that are sold as we examine
Voltmeters are perhaps the commonest or most widely used instruments for
measuring voltage. While there are still many analog voltmeters,
most voltmeters today have digital displays, so that you get an LCD display
with several digits of resolution.
If an instrument has other capabilities (for example being able to measure
current and/or resistance) then it is a multimeter.
If it is a digital multimeter it is often referred to as a "DMM".
A digital voltmeter can be referred to as a DVM.
There are several things you will need to worry about when using a voltmeter
Voltmeters can often measure
either DC or AC voltages.
When measuring AC voltages,
a voltmeter will give you values for the RMS
value - not the peak value
of the sine wave. And, if the signal isn't sinusoidal, you may have
trouble getting the measured value(s) you want.
In many instances, it
is possible to connect the voltmeter to a computer. That allows you
to import your data into a computer and then use analysis programs like
Mathcad, Matlab, spreadsheets, etc. to extract information from your data.
You may need to learn how to use those kinds of connections.
Voltmeters have range
settings. Some common range settings are 0-0.3v, 0-3v, 0-30v, etc.
On lower ranges you will get more accuracy. On digital voltmeters,
for example these ranges are really:
As you go to higher ranges
you will get as many significant digits in the measured value.
If you want more significant
digits in a meter the cost will go up, and each additional digit is more
Voltmeters are not ideal.
The most common aspect of a voltmeter that you need to take into account
is the resistance of the voltmeter. Typically a DMM will have a resistance
of 10 MW.
When you connect the voltmeter to a circuit it would be like connecting
a 10 MW
resistance to the circuit. In many circuits that won't be a problem
because that will be a negligible disturbance to the circuit.
Voltmeters measure voltages
that are constant or at least do not change rapidly. A typical digital
voltmeter will measure voltage and display the results, then hold the results
long enough for you to see the number.
last point in the bullets above has a hidden question. That question
is "What if you have a voltage that changes rapidly and you want to see
details as it changes?". If you have that situation, a voltmeter
may not be your instrument of choice. You may need an oscilloscope
or an A/D card in a computer. That's what we will examine next.
can measure time-varying voltages and give you a graph of voltage vs. time.
When you think about how to connect them to a circuit, they are exactly
like voltmeters. You connect an oscilloscope across the two points
where you want to measure the voltage. However, what you get from
an oscilloscope is not what you get from a voltmeter. When you measure
a signal with an oscilioscope, you get a scaled picture of the voltage
time-function. That picture might look like this one if you were
measuring a sinusoidal voltage.
Currently oscilloscopes will also perform
some computations using data taken from the voltage waveform that is presented
on the oscilloscope face. These usually include things like the following.
Also, once those signal parameters are computed
and are in numerical form within the oscilloscope, they can be transmitted
- using a variety of ways - to a computer where you can use a program to
compute other properties you might be interested in. For example,
you might capture a transient temperature and measure the time it takes
your temperature control system to reach a steady state by computing a
time constant. You could use any number of analysis programs for
that including Mathcad, Matlab and spreadsheets.
If you want a more complete description of oscilloscopes, you can go to
the lesson on oscilloscopes by clicking
here. (That lesson has a number of interesting simulations you
can try, so that you can learn a little before you go into lab. It
also has links to laboratories that help you learn to use oscilloscopes.)
You can purchase numerous A/D (short for Analog-to-Digital Converter) (Click
here to go to the lesson on A/D converters.) converters that come on
boards that plug into computers. And, there are numerous ways to
interface with such boards including at least the following.
Pre-written programs you
Programming in C or C++
Programs that allow you
to build good-looking GUIs (That's Graphical User Interfaces) including:
Programming in Visual
Programming in LabView
Programming in Matlab
Programming in Visual
The ability to use these boards to get data
into a computer allows you to use analysis programs like Mathcad, Matlab
and spreadsheets to analyze your data, plot it, and to extract other information
from your data.
In many cases you may have
on the computer. Soft instruments are computer programs that simulate
voltmeters and oscilloscopes. In other words, they look and feel
like instruments (except that they are interactive images on a computer
screen). They are often designed to look and act like real instruments
as much as possible.