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CR510
PROGRAM
01:
3W Half Bridge (P7)
1:
1
2:
23
3:
1
4:
1
5: 2100
6:
1
7: 100.93
8:
0
02:
Temperature RTD (P16)
1:
1
2:
1
3:
2
4:
1
5:
0
7.8 100 OHM PRT IN 4 WIRE FULL
BRIDGE
This example describes obtaining the
temperature from a 100 ohm PRT in a 4 wire
full bridge (Instruction 6). The temperature
being measured is in a constant temperature
bath and is to be used as the input for a control
algorithm. The PRT in this case does not
adhere to the DIN standard (alpha = 0.00385)
used in the temperature calculating Instruction
16. Alpha is defined as ((R
where R
and R
100
0
PRT at 100 C and 0 C, respectively. In this
PRT alpha is equal to 0.00392.
The result given by Instruction 6 (X) is 1000
V
/V
(where V
is the measured bridge output
s
x
s
voltage, and V
is the excitation voltage) which
x
is:
X = 1000 (R
s
SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
E1
H1
L1
AG
FIGURE 7.8-1. Full Bridge Schematic for 100 ohm PRT
Reps
25 mV 60 Hz Rejection
Range
SE Channel
Excite all reps w/Exchan 1
mV Excitation
Loc [ Rs_Ro
]
Mult
Offset
Reps
R/Ro Loc [ Rs_Ro
Loc [ Temp_C
]
Mult
Offset
/R
)-1)/100,
100
0
are the resistances of the
/(R
+R
)-R
/(R
+R
))
s
1
3
2
3
The resistance of the PRT (R
with the Bridge Transform Instruction 59:
Where
Thus, to obtain the value R
0 C) for the temperature calculating Instruction
16, the multiplier and offset used in Instruction 6
are 0.001 and R
multiplier used in Instruction 59 to obtain R
is R
/R
(5000/100 = 50).
1
0
It is desired to control the temperature bath at
]
50 C with as little variation as possible. High
resolution is needed so that the control
algorithm will be able to respond to minute
changes in temperature. The highest resolution
is obtained when the temperature range results
in an output voltage (V
measurement range selected in Instruction 6.
The full bridge configuration allows the bridge to
be balanced (V
temperature. Thus, the output voltage can go
both positive and negative as the bath
temperature changes, allowing the full use of
the measurement range.
The resistance of the PRT is approximately
119.7 ohms at 50 C. The 120 ohm fixed
resistor balances the bridge at approximately
51 C. The output voltage is:
V
s
= Vx [R
The temperature range to be covered is 50
±10 C. At 40 C R
ohms, or:
) is calculated
s
R
= R
X'/(1-X')
s
1
X' = X/1000 + R
/(R
+R
3
2
/R
, (R
s
0
/(R
+R
), respectively. The
3
2
3
) range which fills the
s
= 0V) at or near the control
s
= V
[R
/(R
+R
) - R
/(R
x
s
s
1
3
/(R
+5000) - 0.023438]
s
s
is approximately 115.8
s
-4
V
= -8.0224x10
V
s
)
3
= R
@
0
s
/R
s
0
+R
)]
2
3
x
7-7
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