Campbell Scientific TC Psychrometer PROM and Cooling Current Interface (S3497X, A3497) - Manual (Page 7)

25 PELTIER THERMOCOUPLE PSYCHROMETER
3
Figure 1. Connection to CR7 using the A3497 Psychrometer Cooling Current Interface
5. THE A3497 INTERFACE AND
CURRENT CALCULATION
Figure 2 is a schematic of the A3497 Interface.
The A3497 performs several functions. The
switched analog output is held at ground
through a 10K resistor preventing leakage of
current through the TCP junction when the
analog output is disabled. The leakage current
is typically a negligible 2nA but can be as high
as 30nA. In addition, the low leakage diodes
isolate the TCPs from each other when excited
through a common analog output. The use of
parallel diodes with opposing polarity permits
both heating and cooling current through the
TCP. Finally, the 249 ohm resistors determine
the current values for a given excitation voltage.
NOTE: To obtain the proper direction for
the cooling current (Figure 1), a negative
excitation voltage must be applied.
The sign of the entry for Parameter 9 is for the
cooling current. The instruction uses the same
voltage but reverses the polarity when applying
the heating voltage. The voltage (mV) required
to produce a desired current I, (mA) is given by
V = I(249+Rs) +700
where Rs is the combined resistance of the
TCP junction and constantan lead length,
typically around 15 ohms for one meter TCPs.
Longer TCP lead lengths can have substantial
resistance since 24 awg constantan is around
2.4 ohms per meter.
The total current required is (I)(Np) where Np is
the number of TCPs connected to the A3497.
This value is limited by the available excitation
current; i.e., 25 mA at ±5 V, 50 mA at ±2 V. For
example, a current of 8 mA for five TCPs is
possible because 40 mA are delivered at an
excitation voltage of 2.81 volts, but 10 mA per
TCP requires a total of 50 mA at 3.34 V. The
latter pushes the limitation of the excitation.
When in doubt, measure the current supplied by
inserting a milliamp meter between the analog
port and the Ex terminal of the A3497. Use a
duration time sufficient to ensure proper meter
response.