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
Engineering student:
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
Engineering student:
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 student:
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
student:
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.
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
measurement:
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 this lesson.
Given a voltage measurement
problem:
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.
The 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.
Using a Voltmeter
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.
Next, you need to be aware of what the
voltmeter measures. Here it is in a nutshell.
A voltmeter measures the
voltage difference between the positive input terminal of the voltmeter and
the negative input terminal.
That's 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.
There are some important things to note about
taking a voltage measurement. The most important point is this.
Voltage is an
across variable.
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 difference.
There are other
kinds of variables. For example, there are numerous variables that are
flow
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 voltages.
Let's 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 shown
below.
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 this
one.
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.
Exercise 1
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 leads.
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 appears.
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.
