Wavelength Electronics LFI-3751, User Manual

Page: 39 / 132

Chapter 2 - Front Panel Operation
Controlling Temperature with RTDs
Controlling Temperature with RTDs
1. Wiring an RTD
Wire the RTD to the Output Connector on the rear panel. Shielded
cables are available (Wavelength part number CAT-220). Do not exceed
1 meter cable length. Refer to
Sensor Error Condition on page 55 if
you can’t clear a Sensor Error.
1
9
4 5
2 wire RTD
1
9
4 5
3 wire RTD
Kelvin
Lead
2. Automatic Bias Current
A fi xed bias current, I
BIAS
, fl ows from pin 5, through the RTD and back into
pin 4 to produce a voltage, V
RTD
. As the RTD resistance (R
RTD
) changes,
the voltage changes. The voltage must be between 0 and +5 V for proper
operation. The LFI-3751 automatically ranges between four bias currents
(10
µ
A, 100
µ
A, 1 mA, and 10 mA) to optimize the feedback while allowing
a wider temperature range of operation. Hysteresis is designed in to
produce “bumpless” operation when switching from one bias current
to another.
P ACT T
LIM I
AUX T
TE V
SET T
TE I
B
C
I
A
D
V
RTD
= I
BIAS
* R
RTD
39
2
Default RTD Confi guration
[A1]
[A2]
[B1]
[B2]
[C1]
[C2]
= 1.000
= 1.000
= 0.00°C
= 0.100 k
Ω
= 100.00 °C
= 0.139 k
Ω
A
B
C
T
1
R
1
T
2
R
2
Bias Current and Sensor Resistance
25 k
Ω
2.5 k
Ω
0.25 k
Ω
0 k
Ω
10
µ
A
100
µ
A
1 mA
10 mA
Resistance Range Sensor Current
500 k
Ω
45 k
Ω
4.5 k
Ω
0.45 k
Ω
to
to
to
to
V
RTD
= Voltage across pins 4 & 5
I
BIAS
= LFI-3751 Bias Current
R
RTD
= Resistance of RTD
RTD Voltage (pins 5 & 4)
Key to accurately measuring and controlling temperature with an RTD
is properly entering the configuration data. An RTD’s temperature
response can be assumed to be linear or more precisely modeled by
the Callendar - Van Dusen Curve fi t. To indicate that the sensor is an
RTD,
A = (1.000, 1.000). To calculate the linear or Callendar - Van Dusen
coeffcients, the LFI-3751 requires two data pairs B = ( T
2
, R
2
), C = ( T
3
, R
3
).
With the appropriate equation and coeffi cients, the LFI-3751 can calculate
temperature from RTD resistance and properly calculate the setpoint
resistance from the setpoint temperature.