685 Series
ComboPak
User’s Manua
l · Page 31
the resistance sensitivity of the thermistor at your application temperature, which
varies greatly by temperature and thermistor types.
The
ComboPak
measures sensor resistance using a 100
μ
A constant current
source, with an upper measurement range of 45k
Ω
. This will limit low
temperature range operation depending on the thermistor type. For Arroyo
Instruments’ standard thermistors, this means a lower operating range of about
–
7°C.
The Steinhart-Hart Equation
As can be seen from the temperature versus resistance graph above, resistance
varies inversely with temperature in a non-linear fashion. This relationship can be
accurately modeled by polynomial equations, and one such being the Steinhart-
Hart equation:
3
)
ln(
*
)
ln(
*
1
R
C
R
B
A
T
+
+
=
The coefficients A, B, and C can usually be obtained from the thermistor
manufacturer. The
ComboPak
defaults to the coefficients for the BetaTHERM
10K3A1 thermistor (A = 1.12924x10
-3
, B = 2.34108x10
-4
, C = 0.87755x10
-7
). You
can change the coefficients using the TEC:CONST command.
External Fan Control
The
ComboPak
has a built-in user adjustable 4-12V DC power supply designed
to provide up to 250mA to an external fan, such as those built into the
200
and
240 Series
Series
LaserMounts
. When using the
ComboPak
with mounts that
require a fan, no additional external power supply is needed, however, the cable
must include the wiring needed for the fan. The
1260B
,
1261B
,
1262B
, and
1263B
cables
include the necessary wiring.
Fan control is set using the TEC:FAN command. The fan can be
set to
Off
,
Slow
,
Medium
,
Fast
, or to a specific fan voltage. When set to
Off
, the fan power is
never turned on.
Slow
,
Medium
, and
Fast
correspond to 9V, 10.5V, and 12V.
The fan on/off mode can be controlled using five different modes:
Auto
Fan is turned on whenever the TEC output is on, and turns off when the
TEC output is turned off.
On
Fan is always on.
Delay
Similar to the
Auto
mode, the fan is turned on whenever the TEC output
is on, but when the TEC is turned off, the fan will remaining running for
an addition number of minutes.