Thermistor
Created by lady ada
Last updated on 2018-08-22 03:30:26 PM UTC
Guide Contents
Guide Contents
Overview
Some Stats
2
3
4
Testing a Thermistor
Using a Thermistor
Connecting to a Thermistor
Analog Voltage Reading Method
Better Readings
Converting to Temperature
How Accurate is the Reading?
5
6
6
6
8
10
12
Self-Heating
CircuitPython
Convert to Temperature
Thermistor Module
Usage
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Python Docs
Buy a Thermistor
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20
© Adafruit Industries
https://learn.adafruit.com/thermistor
Page 2 of 20
Overview
A thermistor is a thermal resistor - a resistor that changes its resistance with temperature. Technically, all resistors are
thermistors - their resistance changes slightly with temperature - but the change is usually very very small and difficult
to measure. Thermistors are made so that the resistance changes drastically with temperature so that it can be 100
ohms or more of change per degree!
There are two kinds of of thermistors, NTC (negative temperature coefficient) and PTC (positive temperature
coefficient). In general, you will see NTC sensors used for temperature measurement. PTC's are often used as
resettable fuses - an increase in temperature increases the resistance which means that as more current passes
through them, they heat up and 'choke back' the current, quite handy for protecting circuits!
Thermistors have some benefits over other kinds of temperature sensors such as analog output chips
(LM35/TMP36
(https://adafru.it/aK2)
) or digital temperature sensor chips (DS18B20) or
thermocouples
(https://adafru.it/cl0).
First off, they are much much cheaper than all the above! A bare 5% thermistor is only 10 cents in bulk.
They are also much easier to waterproof since its just a resistor.
They work at any voltage (digital sensors require 3 or 5V logic).
Compared to a thermocouple, they don't require an amplifier to read the minute voltages - you can use any
microcontroller to read a thermistor.
They can also be incredibly accurate for the price. For example, the 10K 1% thermistor in the shop is good for
measuring with ±0.25°C accuracy! (Assuming you have an accurate enough analog converter)
They are difficult to break or damage - they are much simpler and more reliable
On the other hand, they require a little more work to interpret readings, and they dont work at very high temperatures
like thermocouples. Without a digital-to-analog converter on board, you might be better off with a digital temperature
sensor.
© Adafruit Industries
https://learn.adafruit.com/thermistor
Page 3 of 20
Their simplicity makes them incredibly popular for basic temperature feedback control. For example, lets say you
wanted to have a fan that turns on when the temperature gets high. You could use a microcontroller, a digital sensor,
and have that control the relay. Or you could use the thermistor to feed the base of a transistor, as the temperature
rises, the resistance goes down, feeding more current into the transistor until it turns on. (This is a rough idea, you
would need a few more components to make it work)
Even if you do use a microcontroller or complex system, for the price you can't beat 'em!
You can pick up a 10K 1% waterproof thermistor in the Adafruit shop
(http://adafru.it/372)
Some Stats
Here are technical details for the thermistor in our shop
(http://adafru.it/372)
Resistance at 25°C: 10K ±1%
B25/50: 3950 ±1%
Thermal time constant ? 15 seconds
Thermistor temperature range -55°C to 125°C
Wire temperature range -55°C to 105°C
28 AWG PVC Wire
Diameter: 3.5mm/0.13in
Length: 18in/45cm
Resistance/Temperature table
(https://adafru.it/aK4)
Note that even though the thermistor can go up to 125°C the cable itself maxes out at 105°C so this thermistor is not
good for measuring very very hot liquids
© Adafruit Industries
https://learn.adafruit.com/thermistor
Page 4 of 20
Testing a Thermistor
Because thermistors are simply resistors, its easy to test it out. Simply measure the resistance using a multimeter:
You should read about 10Kohm assuming its room temperature where you're sitting. The resistance of course may be
higher or lower depending on the room temperature. Also, your hands may end up touching the contacts so your
body-resistance will lower the value a bit too. But you should still get about 10 Kohm
For example, its warm here in an un-airconditioned room in the middle of the summer, so we read 8Kohm (30°C -
86°F!)
© Adafruit Industries
https://learn.adafruit.com/thermistor
Page 5 of 20
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