Temperature Sensors


In this section it will be presented different types of temperatures sensors (characteristics and different other useful details). At the final we have to choose one type which is considered optimum to be used in our system or other small projects. First of all we will take into consideration the energy consumption of the sensor, the accuracy of measuring the real temperature and the price.

TMP35/TMP36/TMP37

Features:

  • low voltage operation (2.7V to 5.5V)
  • calibrated directly in °C
  • 10mV/°C scale factor (20mV/°C on TMP37)
  • ±2°C accuracy over temperature
  • ±0.5°C linearity
  • stable with large capacitive loads
  • specified -40°C to + 125°C
  • less than 50μA supply current
  • shutdown current 0.5μA max
  • low self-heating

Applications:

  • environmental control systems
  • thermal protection
  • industrial process control
  • fire alarms
  • power system monitors
  • CPU thermal management

They provide a voltage output that is linearly proportional to the Celsius temperature. The TMP35/ TMP36/TMP37 do not require any external calibration to provide typical accuracies of ±1°C at +25°C and ±2°C over the −40°C to +125°C temperature range.

The low output impedance of the TMP35/TMP36/TMP37 and its linear output and precise calibration simplify interfacing to temperature control circuitry and ADCs. All three devices are intended for single-supply operation from 2.7 V to 5.5 V maxi-mum. The supply current runs well below 50 μA, providing very low self-heating—less than 0.1°C in still air. In addition, a shutdown function is provided to cut the supply current to less than 0.5 μA.

The TMP35 is functionally compatible with the LM35/LM45 and provides a 250 mV output at 25°C. The TMP35 reads temperatures from 10°C to 125°C. The TMP36 is specified from −40°C to +125°C, provides a 750 mV output at 25°C, and operates to 125°C from a single 2.7 V supply. The TMP36 is functionally compatible with the LM50. Both the TMP35 and TMP36 have an output scale factor of 10 mV/°C.

The TMP37 is intended for applications over the range of 5°C to 100°C and provides an output scale factor of 20 mV/°C. The TMP37 provides a 500 mV output at 25°C. Operation extends to 150°C with reduced accuracy for all devices when operating from a 5 V supply.

The output voltage vs temperature:

Supply current vs temperature:

Functional block diagram:

Pin configurations:

If the sensor is not used for reading the temperature value, for minimizing the energy consumption, it could be put into a stand by mode by setting the shutdown input (by connecting it to the voltage source pin).

Basic temperature sensor circuit configuration:

Note the 0.1 μF bypass capacitor on the input. This capacitor should be a ceramic type, have very short leads (surface-mount is preferable), and be located as close as possible in physical proximity to the temperature sensor supply pin. Because these temperature sensors operate on very little supply current and may be exposed to very hostile electrical environments, it is important to minimize the effects of radio frequency interference (RFI) on these devices. The effect of RFI on these temperature sensors specifically and on analog ICs in general is manifested as abnormal dc shifts in the output voltage due to the rectification of the high frequency ambient noise by the IC. When the devices are operated in the presence of high frequency radiated or conducted noise, a large value tantalum capacitor (±2.2 μF) placed across the 0.1 μF ceramic capacitor may offer additional noise immunity.

Bibliography:

  • Low Voltage Temperature Sensors – TMP35/TMP36/TMP37, Analog Devices DataSheet, 2008

TC1046

Features:

  • supply voltage range: 2.7V – 5.5V
  • wide temperature measurement range: -40°C to + 125°C
  • high temperature converter accuracy: ±2°C, max, at 25°C
  • linear temperature slope: 6.25mV/°C
  • very low supply current: 35μA
  • small 3-pin SOT-23B package

Applications:

  • cellular phones
  • power supply thermal shutdown
  • temperature controlled fans
  • temperature measurement/instrumentation
  • temperature regulators
  • consumer electronics
  • portable battery powered equipment

Package type:

The TC1046 is a linear output temperature sensor whose output voltage is directly proportional to measured temperature. The TC1046 can accurately measure temperature from -40°C to +125°C.

The output voltage range for these devices is typically 174mV at -40°C, 424mV at 0°C, 580 mV at +25°C, and 1205mV at +125°C. A 6.25mV/°C voltage slope allows for the wide temperature range.

The TC1046 is packaged in a 3-Pin SOT-23B package, making them ideal for space critical applications.

Functional block diagram:

Output voltage vs temperature:


Supply current vs temperature:


Bibliography:

  • TC1046 – High Precision Temperature-to-Voltage Converter, Microchip DataSheet, 2002

LM135/LM235/LM335, LM135A/LM235A/LM335A

Features:

  • directly calibrated in °Kelvin
  • 1°C initial accuracy available
  • operates from 400 μA to 5mA
  • less than 1 dynamic impedance
  • easily calibrated
  • wide operating temperature range
  • 200°C overrange
  • low cost

The LM135 series are precise, easily-calibrated, integrated circuit temperature sensors. Operating as a 2-terminal zener, the LM135 has a breakdown voltage directly proportional to absolute temperature at +10mV/°K. With less than 1Ω dynamic impedance the device operates over a current range of 400 μA to 5mA with virtually no change in performance. When calibrated at 25°C the LM135 has tipically less than 1°C error over a 100°C temperature range. Unlike other sensors the LM135 has a linear output.

Applications for the LM135 include almost any type of temperature sensing over a -55°C  to 150°C temperature range. The low impedance and linear output make interfacing to readout or control circuitry especially easy.

The LM135 operates over a -55°C  to 150°C temperature range while the LM235 operates over -40°C  to 125°C temperature range. The LM335 operates from -40°C  to 100°C.

Bibliography:

  • LM135/LM235/LM335, LM135A/LM235A/LM335A Precision Temperature Sensor, National Semiconductor, 2008

… in the next step … just search for low power temperature sensor

Since there are a lot of temperature sensors with different characteristics it was impossible just to read all of them description and characteristics. The next step was just to found the low power one, and after that filtering them after the precision and price. After a short search on Google I just found some interesting temperature sensors from Analog Devices, which for my luck could be orders as samples to be used. It was possible to order maximum 8 sample devices. So, I decided for some temperature sensors (analog and digital; one of the, was TMP20 – with a necessary current of 2.6μA; the precision is not very good but for testing purposes it could be considered the best) and some other devices (radio, operational amplifiers, etc).

In about a week the order just arrived. I was impressed by the small size of the TMP20; to create the sensor node of the network as small of possible, like the size of a coin, by example, seems to be possible. One of my following points in the development of the Wireless Sensor Network, used for monitoring, is to have  minimum impact to the medium and to nod disturb the design of the building.

I didn’t tried yet the TMP20 sensor, but I will do it soon and I will come with more details about it and my whole research activity.

Conclusions

…..

[1] High Precision Temperature-to-Voltage Converter, Microchip Datasheet, 2002

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  1. Augustina
    August 5, 2011 at 8:46 pm

    good. What of the thermistor?

    • August 6, 2011 at 2:28 pm

      I’m not sure I have understand your question …

    • xaiby
      December 6, 2011 at 8:32 pm

      i think thermistors are difficult to troubleshoot…..

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