Wavelength Electronics LFI-3751, Руководство пользователя, Страница: 35 / 132

35
2
Controlling Temperature with Thermistors
Key to accurately measuring and controlling temperature with a thermistor
is properly entering the confi guration data. A thermistor’s temperature
response can be modeled by the Steinhart-Hart equation. To calculate the
three Steinhart-Hart coeffcients, the LFI-3751 requires three data pairs
A = ( T
1
, R
1
), B = ( T
2
, R
2
), and C = ( T
3
, R
3
). Thermistor manufacturers
provide this information in a table of Resistance vs. Temperature data. With
the Steinhart-Hart equation and coeffi cients, the LFI-3751 can calculate
temperature from thermistor resistance and properly calculate the setpoint
resistance from the setpoint temperature.
1. Wiring a Thermistor
Wire the thermistor 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
Thermistor
2. Automatic Bias Current
A fi xed bias current, I
BIAS
, fl ows from pin 5, through the thermistor and
back into pin 4 to produce a voltage, V
T
. As the thermistor resistance (R
T
)
changes, the voltage changes. V
T
must be maintained 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 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.
Chapter 2 - Front Panel Operation
Controlling Temperature with Thermistors
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
P ACT T
LIM I
AUX T
TE V
SET T
TE I
B
C
I
A
D
Default Thermistor
Confi guration
[A1]
[A2]
[B1]
[B2]
[C1]
[C2]
= 10.00°C
= 19.90 k
Ω
= 25.00°C
= 10.00 k
Ω
= 40.00 °C
= 5.326 k
Ω
A
B
C
T
1
R
1
T
2
R
2
T
3
R
3
V
T
= Voltage across pins 4 & 5
I
BIAS
= LFI-3751 Bias Current
R
T
= Resistance of thermistor
Thermistor Voltage
(pins 5 & 4)
V
T
= I
BIAS
* R
T