Using a 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, an analog input can be connected to the middle of the thermistor and resistor, measuring 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_{therm}R_{res}/V_{res}.  If the supply voltage is indeed 5 volts, then R_{therm} = V_{therm}R_{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 \beta and R_{0} value (usually the resistance at 25 C), the equation for temperature as a function of resistance can be written as:

T = \frac{\beta T_{0}}{\beta+T_{0}ln\frac{R}{R_{0}}}

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

T = \frac{1}{0.003354+0.0002502ln\frac{R}{10,000 \Omega}}

You may find yourself asking what value of resistor to use.  Well, nearly anything will work, but you can make some smart decisions based on the thermistor and the temperature range you expect to be operating at.

I’ll get back to you on how to be smart about it at a later date; be sure to check back…

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