An excitation voltage of 2.5 V DC is recommended to minimize self-heating and current drain, while still
maintaining adequate measurement sensitivity (mV output from thermistor per C). However, other excitation
voltages can be used. Decreasing the excitation voltage will decrease self-heating and current drain but will also
decrease thermistor measurement sensitivity. Increasing the excitation voltage will increase thermistor
measurement sensitivity but will also increase self-heating and current drain.
Conversion of Thermistor Resistance to Temperature
The thermistor in the simulated leaf has a resistive element where resistance changes with temperature.
Thermistor resistance (R
T
, in
Ω
) is measured with a half-bridge measurement, requiring an excitation voltage input
(V
EX
) and a measurement of output voltage (V
OUT
):
−
=
1
V
V
24900
R
OUT
EX
T
(1)
where 24900 is the resistance of the bridge resistor in
Ω
. From resistance, temperature (T
K
, in Kelvin) is calculated
with the Steinhart-Hart equation and thermistor specific coefficients:
3
T
T
K
))
R
(ln(
C
)
R
ln(
B
A
1
T
+
+
=
(2)
where A = 1.129241 x 10
-3
, B = 2.341077 x 10
-4
, and C = 8.775468 x 10
-8
(Steinhart-Hart coefficients). If desired,
measured temperature in Kelvin can be converted to Celsius (T
C
):
15
.
273
T
T
K
C
−
=
(3)
