5. 100 Ohm PRT in 3-wire half
bridge
The advantages of the 3-wire half bridge over other measurements that correct for wire resistance
such as a 4-wire half bridge, are that it only requires 3 wires going to the sensor and takes 2
single-ended terminals, whereas the 4-wire half bridge requires 4 wires and 2 differential
channels.
The result of the 3-wire half bridge instruction is equivalent to the ratio of the PRT resistance, R
s
to the resistance of the 10 k fixed resistor, R
f
.
The
PRTCalc()
instruction computes the temperature (°C) for a DIN 43760 standard PRT from
the ratio of the PRT resistance at the temperature being measured (R
s
) to its resistance at 0 °C
(R
0
). Thus, a multiplier of R
f
/R
0
is used with the 3-wire half bridge instruction to obtain the
desired intermediate, R
s
/R
0
= (R
s
/R
f
x R
f
/R
0
). If R
f
and R
0
are equal, the multiplier is 1.
The fixed resistor must be thermally stable. The 0.8 ppm/°C temperature coefficient would result
in a maximum error of 0.035 °C at 125 °C. This measurement is ratiometric (R
s
/R
f
) and does not
rely on the absolute values of either R
s
or R
f
.
The properties of the 10 kΩ resistor do not affect the result. The purpose of this resistor in the
circuit is to limit current.
5.1 Excitation voltage
When determining the excitation voltage, it is important to consider the maximum excitation
current the sensor can experience without self-heating. This is typically less than 35 mA. Refer to
the manufacturer's data sheet for the sensor for the specific value.
Once the maximum excitation current is known, the excitation voltage is then calculated.
V
x
= I
x
(R
f
+ R
Smax
)
Where:
R
f
= PRT completion resistor value
3WHB10K 3-Wire Half Bridge Terminal Input Module
8
