Download DPKC_Mod02_Part03_v08

Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Dynamic Presentation of Key
Concepts
Module 2 – Part 3
Meters
Filename: DPKC_Mod02_Part03.ppt
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Overview of this Part
Meters
In this part of Module 2, we will cover the
following topics:
• Voltmeters
• Ammeters
• Ohmmeters
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Textbook Coverage
This material is introduced in different ways in different
textbooks. Approximately this same material is covered in
your textbook in the following sections:
• Circuits by Carlson: Section 3.5
• Electric Circuits 6th Ed. by Nilsson and Riedel: Section 3.5
• Basic Engineering Circuit Analysis 6th Ed. by Irwin and
Wu: Section 2.8
• Fundamentals of Electric Circuits by Alexander and
Sadiku: Section 2.8
• Introduction to Electric Circuits 2nd Ed. by Dorf: Section
1.6
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Meters –
Making Measurements
The subject of this part of Module 2 is meters. We will
consider devices to measure voltage, current, and
resistance. We have two primary goals in this study:
1. Learning how to
connect and use these
devices.
2. Understanding the
limitations of the
measurements.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeters –
Fundamental Concepts
A voltmeter is a device that measures voltage. There are a few
things we should know about voltmeters:
1. Voltmeters must be placed in parallel with the voltage they are
to measure. Generally, this means that the two terminals, or
probes, of the voltmeter are connected or touched to the two points
between which the voltage is to be measured.
2. Voltmeters can be modeled as resistances. That is to say, from
the standpoint of circuit analysis, a voltmeter behaves the same
way as a resistor. The value of this resistance may, or may not, be
very important.
3. The addition of a voltmeter to a circuit adds a resistance to the
circuit, and thus can change the circuit behavior. This change may,
or may not, be significant.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeters –
Fundamental Concept #1
Voltmeters must be placed in parallel with the voltage
they are to measure. Generally, this means that the two
terminals, or probes, of the voltmeter are connected or
touched to the two points between which the voltage is to
be measured.
We usually say that we don’t
have to break any connections to
connect a voltmeter to a circuit.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeters –
Fundamental Concept #2
Voltmeters can be modeled as resistances. That is to say, from the
standpoint of circuit analysis, a voltmeter behaves in the same way as a resistor.
The value of this resistance may, or may not, be very important.
Generally, we will know the resistance of the voltmeter. For most digital
voltmeters, this value is 1[MW] or higher, and constant for each range of
measurement. For most analog voltmeters, this value is lower, and depends on
the voltage range being measured. The larger the resistance, the better, since this
will cause a smaller change in the circuit it is connected to.
For analog voltmeters, the sensitivity of the
meter is the resistance of the voltmeter per [Volt]
on the full-scale range being used. A meter with
a sensitivity of 20[kW/V], will have a resistance
of 40[kW] if used on a 2[V] scale.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeters –
Fundamental Concept #3
The addition of a voltmeter to a circuit adds a
resistance to the circuit, and thus can change the circuit
behavior. This change may, or may not, be significant.
Of course, we would like to know if it is going to be
significant.
There are ways to determine whether it will be
significant, such as by comparing the resistance to the
Thevenin resistance of the circuit being measured.
However, we have not yet covered Thevenin’s Theorem.
Therefore, for now, we will solve the circuit, with and
without the resistance of the meter included, and look at
the difference.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeter Errors
Two kinds of errors are possible with voltmeter measurements.
1. One error is that the meter does not measure the voltage across it accurately.
This is a function of how the meter is made, and perhaps the user’s reading of the
scale.
2. The other error is that from the addition of a resistance to the circuit. This
added resistance is the resistance of the meter. This can change the circuit
behavior.
In a circuits course, the primary concern is with the second kind of error,
since it relates to circuit concepts. Generally, we assume for circuits problems
that the first type of error is zero.
That is, we will assume that the
voltmeter accurately measures the
voltage across it; the error occurs
from the change in the circuit caused
by the resistance added to the circuit
by the voltmeter. The next slide
shows an example of what we mean
by this.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeter Error Example
Here is an example on voltmeter
errors. We will assume that the
voltmeter accurately measures the
voltage across it; the error occurs from
the change in the circuit caused by the
resistance added to the circuit by the
voltmeter.
Let’s add a voltmeter with a
resistance of 50[kW] to terminals A
and B in the circuit shown here. The
goal would be to measure the voltage
across R2, labeled here as vX. We will
calculate the voltage it is intended to
measure, and then the voltage it
actually measures. The difference
between these values is the error.
R1=
83[kW]
A
+
vX
+
vS=
4[V]
-
R2=
33[kW]
B
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeter Error Example –
Intended Measurement
The voltage without the voltmeter
in place is the voltage that we intend
to measure. Stated another way, this
is the voltage that would be measured
with an ideal voltmeter, with a
resistance that is infinite. Performing
the circuit analysis, we can say that
without the voltmeter in place, the
voltage vX can be found from the
Voltage Divider Rule,
R1=
83[kW]
A
+
vX
+
vS=
4[V]
-
R2=
33[kW]
B
R2
33[kW]
v X  vS
 4[V]
 1.14[V].
R2  R1
33[kW]  83[kW]
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeter Error Example –
Actual Measurement
Next, we want to find the voltage vX again, this time with the voltmeter in
place. We have shown the voltmeter in its place to measure the voltage across
R2. Notice that the circuit does not have to be broken to make the measurement.
The next step is to convert this to a circuit that we can solve; this means that we
will replace the voltmeter with its equivalent resistance.
The voltmeter schematic
symbol is shown here. It
has an arrow at an angle
to the connection wires,
implying a
measurement. The same
symbol is often used
with ammeters.
R1=
83[kW]
A
+
vX
+
vS=
4[V]
-
R 2=
33[kW]
Voltmeter
B
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeter Error Example –
Solving the Circuit
We have replaced the voltmeter with its equivalent resistance, RM, and now
we can solve the circuit. We may be tempted to use the voltage divider rule
using R1 and R2 again, but this will not work since R1 and R2 are no longer in
series.
However, if we combine RM and R2 to an equivalent resistance in parallel,
this parallel combination will indeed be in series with R1. We can do this, and
still solve for vX, since vX can be identified outside the equivalent parallel
combination. This is shown by identifying vX in the diagram at right, showing
the voltage between two other points on the same nodes.
R 1=
83[kW]
R 1=
83[kW]
A
+
+
vX
+
vS=
4[V]
-
R 2=
33[kW]
A
+
RM =
50[kW]
vS=
4[V]
vX
R2=
33[kW]
-
-
-
B
B
RM =
50[kW]
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Voltmeter Error Example –
The Resulting Error
We have replaced the parallel combination of RM and R2 with an
equivalent resistance, called RP. Now, RP is in series with R1, and
we can use the voltage divider rule to find vX. We get
20[kW]
v X  4[V]

20[kW]  83[kW]
v X  0.78[V].
As we can see, in this case, the
resistance of the voltmeter was too low
to make a very accurate measurement.
Repeat this problem, with RM equal to
1[MW], and you will see that the
measured voltage will then be 1.11[V],
which is much closer to the voltage we
intend to measure (1.14[V]) for this
circuit.
R 1=
83[kW]
A
+
+
vS=
4[V]
vX
RP=
20[kW]
B
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Extended Range and
Multirange Voltmeters
A voltmeter with a certain full scale reading,
can be made to measure even larger voltages by
placing a resistor in series with it. The resistor
and the voltmeter combination can then be
viewed as a new voltmeter, with a larger range.
The measurement requires that the meter
resistance be known. This can be used to
calculate a multiplying factor for what the
voltmeter reads. Once done, this can be repeated
for other resistance values, to get a voltmeter with
multiple ranges. This allows for simple and
inexpensive analog multiple range voltmeters.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Extended Range Voltmeters
A voltmeter with a certain full scale reading, can be made to measure even
larger voltages by placing a resistor, RV, in series with it. The resistor and the
voltmeter can then be viewed as a new voltmeter, with a larger range. This is
shown here.
By using the Voltage
Divider Rule, we can find
the multiplying factor to
+
+
use to find the reading for
vT
vT
the new extended range
voltmeter. We replace the
RV
RV
voltmeter with its
equivalent resistance, RM,
and then write the
+
+
expression relating vT and
vM,
Existing
vM
RM
vM  vT
.
RM  RV
-
vM
Voltmeter
-
Extended Range Voltmeter
-
RM
-
Extended Range Voltmeter
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Multiplying Factor for Extended
Range Voltmeters
We solve the VDR
equation we wrote on the
last slide for vT and we get
the multiplying factor,
which is the sum of the
resistances over the meter
resistance.
A voltmeter with a certain full scale reading, can
be made to measure even larger voltages by placing a
resistor, RV, in series with it. The resistor and the
voltmeter can then be viewed as a new voltmeter,
with a larger range.
+
vT
RV
RM
vM  vT

RM  RV
RM  RV
vT  vM
.
RM
+
vT
RV
+
vM
-
+
Existing
Voltmeter
-
Extended Range Voltmeter
vM
-
RM
-
Extended Range Voltmeter
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Extended Range Voltmeters -Notes
The new Extended Range Voltmeter can now be used to read larger voltages. The
reading of the Existing Voltmeter is multiplied by the sum of the resistances divided by the
meter resistance. Thus, the Extended Range Voltmeter can read larger voltages, and in
addition has a larger effective meter resistance, which is the sum of the resistances.
By choosing different values of RV, we can also obtain a multirange voltmeter.
Inexpensive multirange analog voltmeters are built by using a switch, or a series of
connection points, to connect different series resistances to a single analog meter.
+
vT
+
vT
RV
RM  RV
vT  vM
.
RM
RV
+
vM
-
+
Existing
Voltmeter
-
Extended Range Voltmeter
vM
-
RM
-
Extended Range Voltmeter
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Go back to
Overview
slide.
Extended Range Voltmeters –
Proportional Scales
The new Extended Range Voltmeter can now be used to read larger voltages. The reading of the
Existing Voltmeter is multiplied by the sum of the resistances divided by the meter resistance. Thus, the
Extended Range Voltmeter can read larger voltages, and in addition has a larger effective meter
resistance, which is the sum of the resistances.
By choosing different values of RV, we can also obtain a multirange voltmeter. Inexpensive
multirange analog voltmeters are built by using a switch, or a series of connection points, to connect
different series resistances to a single analog meter.
RM  RV
vT  vM
.
RM
Since the scale on an
analog voltmeter is
linear, several scales
can be easily labeled
on the same meter,
each proportional to
the other.
+
vT
+
vT
RV
RV
+
vM
-
+
Existing
Voltmeter
-
Extended Range Voltmeter
vM
-
RM
-
Extended Range Voltmeter
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeters –
Fundamental Concepts
An ammeter is a device that measures current. There are a few
things we should know about ammeters:
1. Ammeters must be placed in series with the current they are to
measure. Generally, this means that the circuit is broken, and then
the two terminals, or probes, of the ammeter are connected or
touched to the two points where the break was made.
2. Ammeters can be modeled as resistances. That is to say, from
the standpoint of circuit analysis, an ammeter behaves the same
way as a resistor. The value of this resistance may, or may not, be
very important.
3. The addition of an ammeter to a circuit adds a resistance to the
circuit, and thus can change the circuit behavior. This change may,
or may not, be significant.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeters –
Fundamental Concept #1
Ammeters must be placed in series with the current
they are to measure. Generally, this means that the circuit
is broken, and then the two terminals, or probes, of the
ammeter are connected or touched to the two points
where the break was made.
We usually say that we
have to break a connection to
connect a ammeter to a circuit.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeters –
Fundamental Concept #2
Ammeters can be modeled as resistances. That is to say, from
the standpoint of circuit analysis, an ammeter behaves in the same
way as a resistor. The value of this resistance may, or may not, be
very important.
Generally, we will know the resistance of the ammeter. The
smaller the resistance, the better, since this will cause a smaller
change in the circuit it is connected to.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeters –
Fundamental Concept #3
The addition of an ammeter to a circuit adds a
resistance to the circuit, and thus can change the circuit
behavior. This change may, or may not, be significant.
Of course, we would like to know if it is going to be
significant.
There are ways to determine whether it will be
significant, such as by comparing the resistance to the
Thevenin resistance of the circuit being measured.
However, we have not yet covered Thevenin’s Theorem.
Therefore, for now, we will solve the circuit, with and
without the resistance of the meter included, and look at
the difference.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeter Errors
Two kinds of errors are possible with ammeter measurements.
1. One error is that the meter does not measure the voltage across it accurately.
This is a function of how the meter is made, and perhaps the user’s reading of the
scale.
2. The other error is that from the addition of a resistance to the circuit. This
added resistance is the resistance of the meter. This can change the circuit
behavior.
In a circuits course, the primary concern is with the second kind of error,
since it relates to circuit concepts. Generally, we assume for circuits problems
that the first type of error is zero.
That is, we will assume that the
ammeter accurately measures the
current through it; the error occurs
from the change in the circuit caused
by the resistance added to the circuit
by the ammeter. The next slide
shows an example of what we mean
by this.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeter Error Example
Here is an example on ammeter
errors. We will assume that the
ammeter accurately measures the
current through it; the error occurs
from the change in the circuit caused
by the resistance added to the circuit
by the ammeter.
Let’s add an ammeter with a
resistance of 50[W] to terminals A and
B in the circuit shown here. The goal
would be to measure the current
through R2, labeled here as iX. We
will calculate the current it is intended
to measure, and then the current it
actually measures. The difference
between these values is the error.
A
B
iX
iS=
2[A]
R1=
150[W]
R 2=
330[W]
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeter Error Example –
Intended Measurement
The current without the ammeter
in place is the current that we intend
to measure. Stated another way, this
is the current that would be measured
with an ideal ammeter, with a
resistance that is zero. Performing the
circuit analysis, we can say that
without the ammeter in place, the
current iX can be found from the
Current Divider Rule,
A
B
iX
iS=
2[A]
R1=
150[W]
R 2=
330[W]
R1
150[W]
iX  iS
 2[A]
 0.63[A].
R2  R1
150[W]  330[W]
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeter Error Example –
Actual Measurement
Next, we want to find the current iX again, this time with the ammeter in
place. We have shown the ammeter in its place to measure the current through
R2. Notice that the circuit had to be broken to make the measurement. The next
step is to convert this to a circuit that we can solve; this means that we will
replace the ammeter with its equivalent resistance.
The ammeter schematic
symbol is shown here. It
has an arrow at an angle
to the connection wires,
implying a
measurement. The same
symbol is often used
with voltmeters.
Ammeter
A
B
iX
iS=
2[A]
R 1=
150[W]
R2=
330[W]
Ammeter Error Example –
Solving the Circuit
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
We have replaced the ammeter with its equivalent resistance, RM, and now
we can solve the circuit. We may be tempted to use the current divider rule using
R1 and R2 again, but this will not work since R1 and R2 are no longer in parallel.
However, if we combine RM and R2 to an equivalent resistance in series, this
series combination will indeed be in parallel with R1. We can do this, and still
solve for iX, since iX can be identified outside the equivalent series combination.
This is shown by identifying iX in the diagram at right, showing the current
entering the same combination.
A
RM =
50[W]
A
B
R1=
150[W]
B
iX
iX
iS=
2[A]
RM =
50[W]
R2=
330[W]
iS=
2[A]
R1=
150[W]
R2=
330[W]
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ammeter Error Example –
The Resulting Error
We have replaced the series combination of RM and R2 with an
equivalent resistance, called RS. Now, RS is in parallel with R1, and
we can use the current divider rule to find iX. We get
150[W]
iX  2[A]

150[W]  380[W]
iX  0.57[A].
As we can see, in this case, the
resistance of the ammeter was too large
to make a very accurate measurement.
Repeat this problem, with RM equal to
0.5[W], and you will see that the
measured current will then be 0.62[A],
which is much closer to the current we
intend to measure (0.63[A]) for this
circuit.
iX
iS=
2[A]
R 1=
150[W]
RS=
380[W]
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Extended Range and
Multirange Ammeters
An ammeter with a certain full scale reading,
can be made to measure even larger currents by
placing a resistor in parallel with it. The resistor
and the ammeter combination can then be viewed
as a new ammeter, with a larger range. The
measurement requires that the meter resistance be
known. This can be used to calculate a
multiplying factor for what the ammeter reads.
Once done, this can be repeated for other
resistance values, to get an ammeter with multiple
ranges. This allows for simple and inexpensive
analog multiple range ammeters.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Extended Range Ammeters
An ammeter with a certain full scale reading, can be made to measure even
larger currents by placing a resistor, RA, in parallel with it. The resistor and the
ammeter can then be viewed as a new ammeter, with a larger range. This is shown
here.
By using the Current
iT
iM
Divider Rule, we can find
Existing
the multiplying factor to
RA
Ammeter
use to find the reading for
the new extended range
ammeter. We replace the
ammeter with its
Extended Range Ammeter
equivalent resistance, RM,
and then write the
iT
iM
expression relating iT and
iM,
RA
RM
RA
iM  iT
.
RM  RA
Extended Range Ammeter
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Multiplying Factor for Extended
Range Ammeters
An ammeter with a certain full scale reading, can be made to measure even
larger currents by placing a resistor, RA, in parallel with it. The resistor and the
ammeter can then be viewed as a new
ammeter, with a larger range.
We solve the CDR
equation we wrote on the
last slide for iT and we get
the multiplying factor,
which is the sum of the
resistances over the
parallel resistance.
RA
iM  iT

RA  RM
RM  RA
iT  iM
.
RA
iT
iM
Existing
Ammeter
RA
Extended Range Ammeter
iT
iM
RA
RM
Extended Range Ammeter
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Extended Range Ammeters -Notes
The new Extended Range Ammeter can
now be used to read larger currents. The
reading of the Existing Ammeter is multiplied
by the sum of the resistances divided by the
parallel resistance. Thus, the Extended Range
Ammeter can read larger currents, and in
addition has a smaller effective meter resistance,
which is the parallel combination of the
resistances.
By choosing different values of RA, we can
also obtain a multirange ammeter. Inexpensive
multirange analog ammeters are built by using a
switch, or a series of connection points, to
connect different parallel resistances to a single
analog meter.
RM  RA
iT  iM
.
RA
iT
iM
Existing
Ammeter
RA
Extended Range Ammeter
iT
iM
RA
RM
Extended Range Ammeter
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Extended Range Ammeters –
Proportional Scales
Go back to
Overview
slide.
The new Extended Range Ammeter can now be used to read larger currents. The reading of the
Existing Ammeter is multiplied by the sum of the resistances divided by the parallel resistance. Thus,
the Extended Range Ammeter can read larger currents, and in addition has a smaller effective meter
resistance, which is the parallel combination of the
resistances.
By choosing different values of RA, we can also
iT
iM
obtain a multirange ammeter. Inexpensive multirange
analog ammeters are built by using a switch, or a series
Existing
RA
of connection points, to connect different parallel
Ammeter
resistances to a single analog meter.
RM  RA
iT  iM
.
RA
Since the scale on an
analog ammeter is
linear, several scales
can be easily labeled
on the same meter,
each proportional to
the other.
Extended Range Ammeter
iT
iM
RA
RM
Extended Range Ammeter
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ohmmeters –
Fundamental Concepts
An ohmmeter is a device that measures resistance.
There are a few things we should know about ohmmeters:
1. Ohmmeters must have a source in them.
2. An ohmmeter measures the ratio of the voltage at its
terminals, to the current through its terminals, and reports
the ratio as a resistance.
3. An analog ohmmeter is often characterized by its halfscale reading.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ohmmeters –
Fundamental Concept #1
Ohmmeters must have a source in
them.
The voltmeters and ammeters we
discussed earlier may or may not have a
source within them; they may use the
voltage or current that they are measuring
to power the measurement. However, a
resistor does not provide power, and a
source must be present to provide this.
Thus, while an analog voltmeter or
ammeter may work without a battery, it is
not possible for an ohmmeter to work
without a battery or other source of power.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ohmmeters –
Fundamental Concept #2
An ohmmeter measures the ratio of the voltage
at its terminals, to the current through its terminals,
and reports the ratio as a resistance.
This is a key idea about ohmmeters. We could
say that an ohmmeter assumes that everything is a
resistor. If we connect the ohmmeter to something
other than a resistor, such as a battery, it will report
the ratio of the voltage to the current at its
terminals, even though this may be a meaningless
number.
Electrical-Engineer General’s Warning: It
is important to remove a resistor from its
circuit before measuring it with an
ohmmeter. If we do not, the measurement
we obtain may not have any meaning.
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Ohmmeters –
Fundamental Concept #3
An analog ohmmeter is
often characterized by its halfscale reading.
An analog ohmmeter will have a
scale which has zero on one end, and
infinity on the other end. This is true
no matter what the “range” it is set
to. To understand this, it is useful to
look at the internal circuit of the
ohmmeter. A typical circuit for a
simple analog ohmmeter is shown
here.
Meter
vB
(Battery
voltage)
+
Unknown
Resistor RX
-
Adjustable
Resistor RO
Ohmmeter Circuit
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Simple Ohmmeter Circuit Notes
We may note several
things about this circuit.
1. If the resistor RX is infinity
(an open circuit), the current
through the meter will be zero.
The meter will be at one end
of its scale.
2. If the resistor RX is zero (a
short circuit), the resistor RO is
adjusted to make the meter
read full scale.
Meter
vB
(Battery
voltage)
+
Unknown
Resistor RX
-
Adjustable
Resistor RO
Ohmmeter Circuit
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
Simple Ohmmeter Circuit –
More Notes
Thus, the value of the
resistor RO is adjusted to make
the meter read full scale when
RX is zero. Thus, the full-scale
current must be equal to vB
divided by the series
combinations of the meter
resistance and RO. It follows
that half the full-scale current
will result when RX equals this
series combination.
A potentially useful bit of information is
this: the half-scale reading of an analog
ohmmeter is equal to the internal
resistance of the meter.
Go back to
Overview
slide.
Meter
vB
(Battery
voltage)
+
Unknown
Resistor RX
-
Adjustable
Resistor RO
Ohmmeter Circuit
Dave Shattuck
University of Houston
© Brooks/Cole Publishing Co.
What is the Point of
Considering Analog Meters?
• This is a good question, considering how accurate,
inexpensive, and easy to use digital meters have become.
• The answer is two fold: First, there are still several
applications for analog meters, and it is important to
understand them. The benefits are made more important
since the meters themselves are relatively simple and easy to
understand.
• Second, an understanding of these meter concepts allow
digital meters to be understood, from an applications
standpoint. For example, we can extend
the operating range of a digital voltmeter
by adding a series resistor, just as we did
with analog voltmeters.
Go back to
Overview
slide.