Datasheets : Temperature Sensor (Thermistor) - Input Sensor

What does it do?

The temperature sensor is used to detect changes in the temperature of its surroundings.

How does it operate?

Usually the temperature sensor produces a voltage signal that increases as the temperature increases. The inverted temperature sensor (cold sensor) produces a voltage signal that increases as the temperature decreases.

If the temperature sensor is being used with a digital process unit then it needs to be followed by a comparator or Schmitt inverter to give a sharp change of signal from low to high.

In the normal circuit, the thermistor is placed in the upper half of the potential divider.  In the inverted circuit, the thermistor is placed in the lower half of the potential divider.

Accurate temperature measurement

By using a precision thermistor e.g. the Rapid 10k thermistor 61-0515, it is possible to measure temperatures to an accuracy of „b2oC. The resistance if the thermistor varies with temperature as shown on the graph below:

If the thermistor is placed in a potential divider with a fixed value 10k resistor (not the variable resistor shown in the 'Normal circuit' above) then, with a 5V supply voltage, the Output signal voltage varies with temperature as shown below. For accurate temperature measurements, the 10k resistor needs to have a +/- 1% tolerance, rather than the usual +/- 5%.

If this output signal voltage is fed to a PIC with an analogue to digital converter (ADC) reading between 0 and 255, the reading on the ADC will look like:

Precise values of resistance, output signal voltage and ADC reading can be found by downloading an Excel spreadsheet from this web site. This makes use of what is called the 'B parameter equation' (explained in an article in Wikipedia).

Using these graphs or the spreadsheet, a monitoring or control system can be designed to respond at an accurately predicted temperature. By using a PIC with an ADC, the software can test if the ADC reading is close to the calculated ADC value for the temperatures of interest and activate an output device. By using a liquid crystal display this approach can be used to produce a digital thermometer.

Note - for precise measurements the thermistor should not be placed in water because water conducts electricity and the apparent resistance of the thermistor will be reduced.

Possible applications

• Fire alarm
• Frost warning
• Digital thermometer

Making

Before soldering it into the PCB, make sure (by testing with a multimeter on the resistance setting) that the resistance of the thermistor falls when it is warmed up.

In the case of the precision thermistor, note that a fixed value resistor needs to be used.

How part of the PCB might look

Testing

Turn the variable resistor to its mid-point.

Make sure that the voltage signal going out from the normal circuit (on the green PCB track) increases when the thermistor is warmed up.

In the case of the inverted circuit the signal voltage should fall as the temperature increases.

Fault finding

If there is a fault, check that:

• The voltage at the 'top' of the thermistor is high (the supply voltage)
• The resistance of the variable resistor changes when it is warmed

If there is a fault, check the tracks and solder joints.

Alternatives

• Thermal switch ¡V easy to use, acts like a temperature-activated switch at a fixed temperature.
• Linear temperature sensor ¡V more accurate but more expensive
• Thermocouple ¡V lower cost but needs a fixed reference temperature

Web links

• Inputs and outputs
• Manufacturer's data sheet for Rapid low-cost thermistors
• Manufacturer's data sheet for Rapid precision thermistors
• Wikipedia atical on thermistors

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