Thermistor Trouble

[coolmandan1]

Hi,

Hi again everyone. I want to connect a thermistor to the ADC port of a PICAXE 28X-1. But on looking at the 'Interfacing Circuits' data sheet i discovered that you have to add a resistor after the thermistor and connect it to 0V. I am fine with doing this but was just wondering what it was for and if the value of the resistor mattered. If you don't know what i mean then a diagram is located on the interfacing circuits datasheet and is on page 29.

Thanks,
Daniel

 

[Rickharris]

The resistor forms a potential divider with the thermister. see here http://electronics2000.co.uk/data/itemsmr/potdiv.php

The value depends much on your thermistor but for a start you can try 10K and see what results you get OR put a variable resistor in there so you can change the value easily.

 

[coolmandan1]

Thanks for that rickharris. Very helpfull but is the resistor necessary when connecting the thermistor to an analogue input??

Thanks,
Daniel

 

[Rickharris]

Short answer yes - Long answer look at the link I gave and try to figure out what is happening in a potential divider circuit.

 

[Jaguarjoe]

Be careful when picking the value of the fixed resistor in your potential divider. If it is too low it will cause a lot of current to flow through the circuit which inturn will cause the thermistor to heat itself up. This 'self heating' effect makes it harder to measure temperature.