Using the STK525
2-16
STK525 Hardware User Guide
7608A–AVR–04/06
2.6.3
Temperature Sensor
The temperature sensor uses a thermistor (R18), or temperature-sensitive resistor. This
thermistor has a negative temperature coefficient (NTC), meaning the resistance goes
up as temperature goes down. Of all passive temperature measurement sensors,
thermistors have the highest sensitivity (resistance change per degree of temperature
change). Thermistors do not have a linear temperature/resistance curve.
The voltage over the NTC can be found using the A/D converter (connected to channel
0). See the AT90USBxxx datasheet for how to use the ADC. The thermistor value (R
T
)
is calculated with the following expression:
Where:
R
T
= Thermistor value (
Ω
) at T temperature (°Kelvin)
R
H
= Second resistor of the bridge -100 K
Ω
±
10% at 25°C
V
ADC0
= Voltage value on ADC-0 input (V)
VCC = Board power supply
The NTC thermistor used in STK525 has a resistance of 100 K
Ω
±
5%
at 25°C (T
0
) and a
beta-value of 4250 ±3%. By the use of the following equation, the temperature (T) can
be calculated:
Where:
R
T
= Thermistor value (
Ω
) at T temperature (°Kelvin)
ß
= 4250 ±3%
R
0
= 100 K
Ω
±
5% at 25°C
T
0
= 298 °K (273 °K + 25°K)
The following cross table also can be used. It is based on the above equation.
Table 2-4 . Thermistor Values versus Temperature
Temp.
(°C)
R
T
(K
Ω
)
Temp.
(°C)
R
T
(K
Ω
)
Temp.
(°C)
R
T
(K
Ω
)
Temp.
(°C)
R
T
(K
Ω
)
-20
1263,757
10
212,958
40
50,486
70
15,396
-19
1182,881
11
201,989
41
48,350
71
14,851
-18
1107,756
12
191,657
42
46,316
72
14,329
-17
1037,934
13
181,920
43
44,380
73
13,828
-16
973,006
14
172,740
44
42,537
74
13,347
-15
912,596
15
164,083
45
40,781
75
12,885
-14
856,361
16
155,914
46
39,107
76
12,442
-13
803,984
17
148,205
47
37,513
77
12,017
-12
755,175
18
140,926
48
35,992
78
11,608
-11
709,669
19
134,051
49
34,542
79
11,215
-10
667,221
20
127,555
50
33,159
80
10,838
R
T
R
H
V
ADC 0
⋅
(
)
V
C C
V
ADC 0
–
(
)
⁄
=
T
β
R
T
R
0
-------
ln
⎝
⎠
⎛
⎞
β
T
0
------
+
-------------------------------
=
