-
KNE222
University of Tasmania
School Of
Engineering
KNE222 Electronic
Engineering
Operational
Amplifier Applications
The Resistive
Temperature Detector (RTD)
In
addition
to
thermocouples
for
measuring
temperature,
instrumentation
engineers
frequently
use
Resistive Temperature
Detectors
or RTDs. These are devices
whose DC resistance varies (almost)
linearly as a function of temperature.
Perhaps the most common of these is the PT100, a
platinum
based
sensor
whose
resistance
at
0?
C
is
exactly
100
Ohms,
(see
Table
1)
.
As
the
sensor’s
temperature
increases
so
does
its
resistance,
in
a
reasonably
linear
manner.
Table
1
shows
the
variation in resistance of a PT100
sensor with temperature. While the
temperature coefficient
varies
slightly
over
a
wide
range
of
temperatures,
(typically
0.0036
to
0.0042
Ohms/?
C),
it
can
be
considered
reasonably
constant
over
a
50
or
100
?
C
range.
The
commonly
accepted
average
temperature coefficient is 0.00385 Ohms
per ?
C. Accordingly the PT100 can often
be used without
linearization over such
a range provided the appropriate coefficient is
evaluated. This device is also
capable
of withstanding a wide range of temperatures, from
-200 to 800?
C, and for some
applications
the
variations
in
temperature
coefficient
can
be
tolerated.
Further,
the
PT100
provides
stable
and
reproducible temperature
characteristics.
For a
given base resistance R
o
,
the resistance of an RTD at T ?
C is
given by:
R
(
T
)
?
R
o
(
1
?
< br>?
(
T
?
T
0
))
Or
… (1)
R
< br>(
T
)
?
R
o
T
?
T
0
?
?
Where R
o
is the
base resistance corresponding to
T
o
, (100Ohms at 0
?
C) and
?
is the
temperature
coefficient,
(0.00385Ohms
per
?
C).
Thus
R(100
?
C
)
=
138.5
Ohms
.
This
approximation
provides
quite
a
good
estimate
of
temperature
up
to
about
300
?
C,
as
shown
in
Figure
1,
thereafter
the
nonlinearity becomes evident.
Figure 1.
Linear RTD model vs. the actual characteristic
Equation
(1)
assumes
that
the
nonlinearities
in
the
RTD
characteristic
are
negligible,
ie
that
the
device
is
entirely
linear,
and
while
for
many
applications
this
approximation
is
acceptable,
where
more precision is required a
nonlinear
model must be
used, as outlined in Equation (2).
R
(
T
)
?
R
o
(
1
?
AT
?
< br>BT
2
?
C
(
T
?
100
)
T
3
)
… (2)
Where: A
= 3.908E-3, B = -5.775E-7 and C = -4.183E-12 for
T<0 and C = 0 for T>0.
G
.
Vertigan
Page 1
2009
KNE222
University of Tasmania
Temperature
information
can
be
obtained
from
an
RTD
by
measuring
its
resistance;
either
by
applying
a
known
current
and
measuring
the
resulting
voltage
or
vice
versa.
Care
muse
be
taken
when
passing
a
current
through
an
RTD
as
internal
I
2
R
heating
will
also
affect
the
devi
ce’s
resistance. The degree to which this
occurs depends on the physical size of the RTD in
question, and
therefore
how
much
heat
it
can
dissipate
before
its
temperature
rises
significantly
above
ambient.
For small devices
sense currents must be kept quite low, typically
less than 3mA. A small (thick film)
PT100 device appears in figure 2.
Figure 2.
A Thick Film PT100 Temperature
Sensor Construction
Figure
3. Sample PT100 probes
RTDs generally have a small thermal
mass and therefore can exhibit a fast response to
rapid changes
in temperature. This can
be useful in process control applications.
Information Coding
Techniques.
Instrumentation
applications
frequently
use
Programmable
Logic
Controllers
(PLCs)
to
store
and
process
data,
and
therefore
the
analogue
output
signals
of
sensing
equipment
must
be
scaled
appropriately for the A-D converter
input card of the PLC concerned. This is generally
accomplished
by
the
sensor
driving
circuitry.
There
are
several
standard
voltage
ranges
used
by
manufacturers;
these include 0 to 1, 0 to 5 and 0 to
10 volts, each corresponding to the desired range
of temperatures
detected by the RTD.
In addition to the voltage
source based signals, it is also common to use a
current source
to carry
encoded analogue information. This
method offers significant noise immunity over
voltage carriers,
since both
common mode
and
normal mode
induced voltages
can be tolerated without significantly
corrupting the current flowing. Four to
twenty mA current loops are frequently used over
moderate
transmission
distances,
for
example
from
one
side
of
a
factory
to
the
other,
to
convey
analogue
information.
G
.
Vertigan
Page 2
2009
KNE222
University of Tasmania
The
loop
transmitter
is
generally
set
up
so
that
the
lower
end
of
the
required
temperature
range
corresponds to 4mA and the upper end to
20mA. Thus should the loop become broken,
resulting in a
total loss of current,
the fault can be readily detected. Effectively the
analogue signal is encoded as a
0-16mA,
current shifted from the origin by 4mA. The range
of temperatures that correspond to these
currents (usually known as the
span
) is determined by the
user, who must program the transmitter
accordingly.
Some
loop
transmitters
are
powered
by
the
4mA
current
component,
while
others
require an external
power supply.
An RTD Drive
Circuit.
The schematic shown in Figure
4 is designed to interface a PT100 to a PLC
analogue input card. It
offers two
output signals; a 0-5 volt
voltage
signal
and a 4-20mA
current
signal
. The circuit uses a
Wheatstone bridge
arrangement to derive a positive voltage,
proportional to the
increase
in sensor
resistance
beyond
the
base
resistance
R
o
,
which
corresponds
to
the
lower
end
of
the
desired
temperature range, (in this case 0
?
C).
Figure 4.
A Temperature Measuring Circuit for the PT100.
Thr RTD is
included in
a Wheatstone
bridge arrangement
(sometimes known as
a
quarter bridge
configuration),
which
operates
from
a
split
power
supply
.
However
in
this
circuit
the
voltage
supplies are not
quite equal. The negative rail is fixed at 0.265
volts while the positive rail is set so
that
the
voltage
on
the
top
side
of
the
RTD
is
zero,
i.e.
so
that
the
bridge
is
nulled.
The
voltage
required
to
null
the
bridge
will
vary,
depending
on
the
temperature
of
the
RTD.
Therefore
temperature
information is
encoded in the positive
supply potential
.
The left hand side of the bridge
consists of two identical resistors, which at
their union generate a
common
mode
voltage
containing
information
relating
only
to
the
temperature
of
the
RTD.
A
particularly good feature
of this technique is the fact that the output is
truly
linear with the
resistance
G
.
Vertigan
Page 3
2009