A resistance that falls as it warms

One number at 25 °C

An ordinary resistor is made to keep its value when it warms up. A thermistor is made to change. This one is an NTC thermistor, for negative temperature coefficient: warmer means lower resistance.

Slide it from cold to hot. The resistance falls all the way, but not evenly: between 0 and 25 °C it drops from about 34 kΩ to 10 kΩ, and between 75 and 100 °C only from about 1.5 kΩ to 0.7 kΩ. A straight line through any two points on that curve is wrong everywhere else, which is why the sketches in this book never use map() for it.

The curve is drawn on a scale where each gridline is ten times the one below. On an ordinary scale the cold end would fill the drawing and everything above 40 °C would be lying on the floor.

What the part number says

The part is a Sunlord SDNT2012X103F3950FTF. Its datasheet could not be fetched while this book was written, so what follows is the part number read the way this family is usually decoded:

• 2012: 2.0 × 1.2 mm, an 0805, which the board's layout agrees with.
• 103: 10 × 10³ Ω, so 10 kΩ at 25 °C.
• F: ±1 % on that 10 kΩ.
• 3950: the B value, in kelvin, which says how steeply the resistance falls. It is usually measured between 25 and 50 °C.
• F, the second one: most likely ±1 % on the B value.

The B model

Two of those numbers are all a sketch needs. The resistance at any temperature is, to a good approximation:

R = 10 kΩ × exp(3950 × (1/T − 1/298.15))

with T in kelvin, 273.15 plus the Celsius figure, and 298.15 K being 25 °C. At 25 °C the bracket is zero, exp(0) is 1, and R is 10 kΩ exactly.

It is called the B model because one number, B, sets the whole shape. It is closest to the real part between the two temperatures B was measured at and drifts away from it further out. How far to trust it comes back to that.