User Guide
The result of the 4-wire half bridge Instruction is:
V
V
2
1
the voltage drop is equal to the current (I), times the resistance thus:
V
V
I x R
I x R
R
R
2
1
s
f
s
f
==
The RTD Instruction (Instruction 16) 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 4-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 4 ppm/°C temperature
coefficient would result in a maximum error of 0.05°C at 60°C. The 8ppm/°C
temperature coefficient would result in a maximum error of 0.33°C at 125°C.
Because the 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 10kΩ resistor do not affect the
result.
3.1 Excitation Voltage
The best resolution is obtained when the excitation voltage is large enough to
cause the signal voltage to fill the measurement voltage range. The voltage drop
across the PRT is equal to the current, I, multiplied by the resistance of the PRT,
R
s
, and is greatest when R
s
is greatest. For example, if it is desired to measure a
temperature in the range of -10 to 40°C, the maximum voltage drop will be at
40°C when R
s
=115.54 ohms. To find the maximum excitation voltage that can
be used when the measurement range is ±25mV, we assume V
2
equal to 25mV
and use Ohm’s Law to solve for the resulting current, I.
I = 25mV/R
s
= 25mV/115.54 ohms
= 0.216mA
V
x
is equal to I multiplied by the total resistance:
V
x
= I(R
1
+R
s
+R
f
) = 2.21V
If the actual resistances were the nominal values, the 25mV range would not be
exceeded with V
x
= 2.2V. To allow for the tolerances in the actual resistances, it
is decided to set V
x
equal to 2.1V (e.g. if the 10kΩ resistor is 5% low, then
R
s
/(R
1
+R
s
+R
f
)=115.54/9715.54, and V
x
must be 2.102V to keep V
s
less than
25mV).
3.2 Calibrating a PRT
The greatest source of error in a PRT is likely to be that the resistance at 0°C
deviates from the nominal value. Calibrating the PRT in an ice bath can correct
this offset and any offset in the fixed resistor in the Terminal Input Module.
The result of the 4-wire half bridge is:
V
V
I x R
I x R
R
R
2
1
s
f
s
f
==
3
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