Sensors - Tranducers CircuitsTemperature compensated

Smartec Temperature Sensor Schematic Circuit Diagram

Temperature Sensor Overview: Smartec Type SMT160-30-18

The Smartec Type SMT160-30-18 is a temperature sensor equipped with a digital output, encased in a TO-18 housing. The supply input pins are designated as Pins 1 and 3, drawing a current not exceeding 200 μA with a nominal 5 V supply. Pin 2 provides a short-circuit-proof and TTL-compatible rectangular output voltage with a pulse repetition frequency (p.r.f.) of 3 kHz. The specific frequency value is less critical, as the actual temperature data is encoded in the duty factor, denoting the ratio of pulse width to pulse spacing. This relationship is expressed as:

duty factor = 0.32 + 0.0047x T.

The linear correlation between temperature (T) and duty factor allows for precise temperature measurement. For instance, at the sensor’s minimum operating temperature of -45 °C, the duty factor is 0.109. The sensor’s operational range extends to a maximum temperature of 130 °C. During production, the sensors undergo calibration, ensuring an accuracy of ±0.25 °C.

Smartec temperature sensor Schematic diagram

Integration of Rectangular Signal for Temperature Measurement

In theory, it is conceivable to employ a rectangular signal for driving a moving coil meter, providing a readout directly proportional to the average voltage level of the signal. This correlation extends to the duty factor and, consequently, the temperature. In practical terms, however, a more pragmatic approach involves linking the sensor to a digital input port on a peripheral interface or microcontroller. Sampling the rectangular signal allows the computer system to conduct temperature measurements with minimal external components, as illustrated in the accompanying diagram.

A referenced PC measurement card (Ref. 1) serves as a suitable interface. Connector K1, a 26-way box header, establishes a connection with connector K6 of the measurement card through a short length of flat cable. The 5 V supply is drawn directly from the computer. On the prototype, the inclusion of L1-C1-C2 was crucial to mitigate signal jitter. Which otherwise caused oscillation in the first digit after the decimal point.


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