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OPERATION
Measurement/Control (J2)
8 LDT-5100
CHAPTER 2
Output Monitors
The LDT-5100 has two buffered output monitors present on the measurement /
control connector (J2). These monitor the actual thermistor resistance and I
TE
(TE drive current).
Thermistor Monitor
The thermistor monitor (pin 10) provides an output voltage which is proportional to
the thermistor sense resistor value. The transfer function with the 100µA
thermistor current range selected is 10kΩ/V and with the 10µA thermistor current
range selected,100kΩ/V. The accuracy is specified to 5V although higher output
voltages can be measured.
TE Drive Current Monitor
A TE drive current monitor, I
TE
, is also provided as a buffered output on pin 8 of
the measurement / control connector (J2). This voltage output is proportional to
I
TE
with a 400mA/V transfer function.
Note: This is a bipolar output.
Status Monitors
The LDT-5100 measurement / control connector (J2) includes three status lines
which provide information about the instrument operation. They are thermistor
open, TEC open and output on. All outputs are open collector, allowing for a wide
variety of logic compatibility, including wired “or” operation. A pull-up resistor from
the user’s power supply is required. This power supply must also be connected to
the LDT-5100 ground reference for proper operation. Sink current in excess of 25
mA should be avoided when choosing the value of the pull-up resistor.
Thermistor Open (pin 6)
This control line is grounded (low) when the thermistor input, pins 7 and 8 of J1,
have a high impedance. The threshold for determination of high impedance is
dependent on the thermistor current range selected. The thresholds are 90 kΩ
and 900 kΩ for the 100 µA and 10 µA thermistor current ranges respectively.
When the sensor resistance is within the ranges indicated, the control line will be
high impedance, indicating an open thermistor.
TEC Open (pin 4)
This control line is grounded (low) when the TE module (+) and (-) inputs have a
high impedance. If the LDT-5100 tries to drive current to the TE, but is unable to
do so, an open TEC is assumed. Because of variation in TE load impedance and