Note: as shown in the
schematic, the 4-wire PRT can
be directly measured without
the bridge resistors. The blue
and yellow wires are electrically
connected to one side of the
PRT while the green and black
wires are electrically connected
to the other side of the PRT.
NOTE: The wiring diagram
below is based off the new
wiring colors for serial numbers
1076 and above.
Measurement of PRT resistance is very similar to measurement of thermistor resistance, where a half-bridge
measurement is used. An excitation voltage is input across the bridge resistor and an output voltage is measured
across the PRT.
An excitation voltage of 2.1 V DC is recommended to minimize current drain, while still maintaining an adequate
voltage signal. However, other excitation voltages can be used. Decreasing the excitation voltage will decrease
current drain, but will also decrease output voltage. Increasing the excitation voltage will increase output voltage,
but will also increase current drain.
Conversion of PRT Resistance to Temperature
The PRT is a resistive element, where resistance changes with temperature. PRT resistance (R
PRT
, in Ω) is measured
with a half-bridge measurement, requiring a known excitation voltage input (V
EX
) and a measurement of output
voltage (V
OUT
):
=
100
P RT
P RT
V
V
100
R
(1)
where 100 Ω is resistance of the bridge resistor, V
100Ω
is the voltage measured across the 100 Ω bridge resistor, and
V
PRT
is the voltage measured across the PRT. From resistance, temperature (T
C
, in Celsius) is calculated with:
for R
PRT
/ 100 < 1 (T
C
< 0)
T
C
= gK
4
+ hK
3
+ iK
2
+ jK
(2)
for R
PRT
/ 100 ≥ 1 (T
C
≥ 0)
T
C
=
√d(
RPRT
100
)+e −a
f
(3)
where K = (R
PRT
/ 100)
–
1, a = 3.9083 x 10
-3
, d = -2.3100000 x 10
-6
, e = 1.7584810 x 10
-5
, f = -1.1550000 x 10
-6
, g =
1.7909000, h = -2.9236300, i = 9.1455000, and j = 2.55819 x 10
2
.
