November 6th 2009
The I.C. Temperature Transducer
This is an integrated circuit containing 16 transistors, 9 resistors and 2 capacitorscontained in a transistor type package. The device reference number is LM 335 and it provides an output of 10 mV/oK. A measurement of the output voltage therefore indicates the temperature directly inoK. For example, at a temperature of 20 oC (293 oK), the output voltage will be 2.93 V. The circuit arrangement provided with the DIGIAC 1750 unit is shown in Figure 2.
An LM 335 unit is mountedexternal to the heated enclosure and fitted in a heat sink with a type “K” thermocouple, its output being obtained from the “Ref” socket. The output from this can be used as an indication of theambient temperature outside the heated enclosure and that from the “Int” socket in Figure 2 indicates the temperature within the heated enclosure. The output from the “Ref” socket does not give an accuratevalue of the room ambient temperature when the heater is in use, due mainly to heat passing along the baseboard by conduction from the heater. An LM 335 remotely mounted or some other method isnecessary for accurate values of ambient temperature.
The Platinum R.T.D: (Resistance Temperature Dependent) Transducer
The construction of the platinum R.T:D. transducer is shown in Figure 3,consisting basically of a thin film of platinum deposited on a ceramic substrate and having gold contact plates at each end that make contact with the film.
The platinum film is trimmed with a laserbeam so that the resistance is 100 Ω at 0oC. The resistance of the film increases as the temperature increases, i.e. it has a positive temperature coefficient. The increase in resistance is linear, therelationship between resistance change and temperature rise being 0.385 Ω/oC for the unit.
Where R, is the resistance at temperature T(C) and Ro is the resistance at 0 C, which is equal to 100...