Electronic IC Temperature Sensors

Electronic IC temperature sensors are specialized integrated circuits that are used for sensing temperatures in the -55° C to +150°C range.

Basic Principle of IC Temperature Sensors

The working principle of IC temperature sensors is based on semiconductor physics. When the temperature of a p-n semiconductor junction changes, the mobility of the charge carriers in the material changes and this affects the rate at which electrons and holes can diffuse across the junction. As a result, when a p-n junction has a constant potential difference across it, the current through the junction, is a function of the temperature or, if we have a constant current through the junction, then the voltage across it is a function of the temperature. Thus a semiconductor diode can be used as a temperature sensor. Such a sensor is compact like thermistor, but has the great advantage of giving response which is a linear function of temperature. Semiconductor diodes for use as temperature sensors, together with the necessary signal conditioning are supplied as integrated circuits for instance, the LM3911 is supplied complete with an internal operational amplifier and gives a linear output of 10 mV/°C.

In the same way to the thermo-diode, for a thermo-transistor, the voltage across the junction between the base and emitter depends on the temperature and can be used as a measure of temperature. A common technique is to use two transistors with different collector currents and use the difference in the base-emitter voltages between them as a measure of the temperature. Such transistors can be combined with other circuit components on a single chip to give a temperature sensor with its associated signal conditioning. For instance, the LM35 illustrated below can give an output voltage directly proportional to temperature of 10 mV/°C with a range of -55 °C to +150 °C. The LM35 can be used in electronic application with an analogue-to-digital converter such as ADC0804 to give a digital output suitable for a microprocessor.

IC Temperature Sensors – Packaging & Examples

Electronic IC temperature sensors can be packaged in the form of a small metal (TO-52) can, a flat pack (F-2A) or a plastic (TO-92) case. AD590 and AD592, two terminal, temperature-to-current sensors from Analog Devices are other good examples of IC temperature sensors. The AD sensors typically operate from a 4-30 V dc supply and provide an output current which is a linear function of the sensor’s kelvin temperature. That is to say, at 0° C, the output current is a nominal 273.15 μA, while at 25°, the output current is 298.15 μA, and so on. Over the entire operating temperature range, the AD590M has a maximum nonlinearity of ±0.3 °C, maximum repeatability and long term drift are ±1.0 °C and the absolute error with the sensor calibrated for zero at 25 °C is no more than ±1.0 °C. The AD temperature-to-current sensors are typically used with an op-amp current-to-voltage amplifier, which allows the generation of a voltage output proportional to the Celcius or Fahrenheit scales.

Additional examples of IC temperature sensors are the LM34, LM35, LM134, LM135, LM234, LM235, LM335 series from National semiconductors. These sensors are three terminal devices, which produce an output voltage proportional to the temperature. For instance, the AD34CA has a 10 mV/°F output voltage gain, a range from -50 °F to +300 °F, and a nonlinearity of ±0.3 °F. The LM35 series has a 10 mV/°C output voltage gain over a -55 °C to +150 °C range, and ±0.15 °C nonlinearity. IC temperature sensors are generally appropriate for environmental monitoring and control applications within their temperature ranges.

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