Temperature Sensors TD Series
Honeywell1Sensing and Control11-800-537-6945 USA1F1-815-235-6847 International11-800-737-3360 Canada
133
REFERENCE AND APPLICATION DATA
INTRODUCTION
Linear temperature sensors have a major
advantage. The output can be easily con-
ditioned to achieve a desired voltage out-
put span over a particular temperature
range. A linear output voltage allows ease
of interface to data acquisition systems
and programmable controllers. By ad-
justing the circuit gain, the sensitivity of
the output can be adjusted over the total
range such as 10° to +40°C.
INTERFACING WITH 1-5V CIRCUIT
If more than 1 mA of current flows through
the TD, self-heating will occur. The self-
heating effect is typically 0.2°C/milliwatt.
The circuits in Figure 1 and Figure 2 pro-
vide a maximum current flow of 1 mA.
SETTING DESIRED SPAN
The circuit gain depends on the temper-
ature range you want to sense. The offset
adjustment is, in turn, dependent on the
chosen gain. The transfer function for
both circuits (Figures 1 and 2) is as fol-
lows:
(R
5
/R
4
+ 1) 1 V[R
TD
/(R
TD
+ R
7
)] -
(R
5
/R
4
)(1 + R
3
/R
2
)V
1
J v
O
Only two elements are unknown: the off-
set (v
1
), and the circuit gain (R
5
/R
4
+ 1). To
set the desired span, two equations for
the two unknowns must be created and
solved. To simplify these calculations, the
following assumption is made:
R
5
/R
4
J R
2
/R
3
The second assumption is that no self-
heating of the TD element will occur: the
values of V and R
7
are constant at the
values indicated.
V[R
TD
/(R
TD
+ R
7
)] J 5[R
TD
/(R
TD
+ 5110)]
These assumptions reduce the transfer
function to:
(R
5
/R
4
+ 1) 1 5[R
TD
/(R
TD
+ 5110)] -
(R
5
/R
4
+ 1)v
1
J v
O
To create the first of the two simultaneous
equations, the value of R
TD
for the desired
minimum temperature is taken from
Table 1. (R
TD
at 20°C equals 2000 Ohms,
and v
O
J 1V.) For the second equation,
the value of R
TD
for the desired maximum
temperature is taken from the table, and
v
O
J 5 V.
The two equations are then solved for the
gain (R
5
/R
4
+ 1) and the offset (v
1
). The
following example shows how this is ac-
complished.
Desired temperature range: 0° to 60°C.
Voltage output over range: 1 to 5 V.
Equation 1: R
TD
at 0°C is 1854 Ohms.
(R
5
/R
4
+ 1) 1 5[1854/(1854 + 5110)] - (R
5
/R
4
+ 1)v
1
J 1V
Equation 2: R
TD
at 60°C is 2314 Ohms.
(R
5
/R
4
+ 1) 1 5[2314/(2314 + 5110)] - (R
5
/R
4
+ 1)v
1
J 5 V
Step 1: subtract equation 1 from equation
2.
(R
5
/R
4
+ 1)(1.558) - (R
5
/R
4
+ 1)V
1
J 5
(R
5
/R
4
+ 1)(1.331) - (R
5
/R
4
+ 1)v
1
J 1
(R
5
/R
4
+ 1)(.227) - 0 J 4
(R
5
/R
4
+ 1) J 4(1/.227)
(R
5
/R
4
+ 1) J 17.62 J GAIN
Step 2: substitute (R
5
/R
4
+ 1) J 17.62 into
equation 1 and solve for V
1
.
(17.62)(1.331) - (17.62)v
1
J 1
23.454 - 17.62v
1
J 1
22.452 J 17.62v
1
1.274 J v
1
J OFFSET
In order to transfer this information into
the circuit in Figure 1, choose appropriate
values for R
4
and R
5
such that:
(R
5
/R
4
+ 1) J GAIN
For this example, R
4
J 1 K Ohm and R
5
J
16.62 K Ohm would be appropriate.
Choose R
2
and R
3
based on R
2
/R
3
J R
5
/R
4
.
For this example, choose R
2
J R
5
J 16.62
K Ohm, and R
3
J R
4
J 1 K Ohm.
To set the offset v
1
using potentiometer R
1
,
temporarily insert an equivalent discrete
resistor in place of the TD element. It
should be equal to the TD resistance at
the minimum desired temperature (1854
Ohms from the example). Adjust R
1
until
the output voltage is 1 V. Replace the
discrete resistor with the TD element. The
circuit is now set and ready to give 1 V to 5
V output over the chosen temperature
range.
Reference/Index
Temperature Sensors TD Series
134
Honeywell1 Sensing and Control11-800-537-6945 USA1F1-815-235-6847 International11-800-737-3360 Canada
REFERENCE AND APPLICATION DATA
Figure 1
5.0 V Regulated Circuit
1. LM358 is a general purpose oper-
ational amplifier.
2. 2N2222 is a general purpose NPN
transistor.
3. Resistor accuracy should be within
±1%.
4. v
O
is measured with respect to ground.
Figure 2
6.5-30 V Supply Voltage
Note: Any error on the 5.0 V regulator will
be seen directly on v
O
. This error can be
reduced when setting the span by as-
suming that V equals the actual output of
the regulator.
Figure 3
TD Series Resistance vs Temperature
ABSOLUTE MAXIMUM RATINGS
Operating temperature range −40 to +150°C (−40 to +302°F)
Storage temperature range −55 to +170°C (−67 to +338°F)
Voltage 10 VDC Continuous (24 hours)
Linearity
±2% (−25 to 85°C)
±3% (−40 to 150°C)
TD sensors can be linearized to within
±0.2%.
Repeatability
±1 V