Thermistor

A thermistor, as the name indicates, is nothing more than a resistor, whose resistance value is sensitive to temperature. Like a normal resistor, it has no polarity, that is, you can't hook it up backwards. Depending on the type of thermistor, the resistance may increase or decrease with increasing temperature.

To use a thermistor to measure temperature in a project, you must monitor the resistance across the thermistor. Generally, this is done by connecting a resistor of a known value in series with the thermistor, bridging a higher voltage (say, + 5V) to ground. To eliminate confusion, let's just say that the thermistor is the component connected to ground. Then, between the thermistor and resistor can be connected to an ADC or "analog in" to measure the voltage across the thermistor.

Knowing that V_therm / R_therm = V_res / R_res, the resistance of the thermistor can be measured to be R_therm = V_thermR_res / V_res. If the supply voltage is indeed 5 volts, then R_therm = V_thermR_res / (5 − V_therm).

That just leaves converting resistance to temperature. This conversion will depend on the particular thermistor and the information supplied. If given a B or β and R₀ value (the resistance at T₀ which is usually taken to be 298 K), the equation for temperature as a function of resistance can be written as:

<math>T = \frac{\beta T_{0}}{\beta+T_{0}ln\frac{R_{therm}}{R_{0}}}</math>

For instance, if you have a 10kΩ, β = 3977K thermistor, the hard-coded equation for temperature (in Kelvin) would become:

<math>T = \frac{1}{0.003354+0.0002502ln\frac{R_{therm}}{10,000 \Omega}}</math>

This page is an Article on bildr. Articles are pages that define or explain a concept, method, or generic item.