how to size a resistor to read a 10K ohm Thermistor

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how to size a resistor to read a 10K ohm Thermistor

By f4u5t
- Mon Apr 11, 2016 5:51 pm #45317 I am attempting to read a 10 K ohm Type 3 Thermistor.
I have it wired up like this:

[Ground] ---- [10k-Resistor] -------|------- [Thermistor] ---- [+5v]
|
Analog Pin 0

I realize I need to reduce the voltage to 1Vdc max for the ESP8266

I do not understand how to size the resistor, can someone please help?

Image
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Re: how to size a resistor to read a 10K ohm Thermistor

By trackerj
- Mon Apr 11, 2016 11:04 pm #45320
f4u5t wrote:I am attempting to read a 10 K ohm Type 3 Thermistor.
I have it wired up like this:

[Ground] ---- [10k-Resistor] -------|------- [Thermistor] ---- [+5v]
|
Analog Pin 0

I realize I need to reduce the voltage to 1Vdc max for the ESP8266

I do not understand how to size the resistor, can someone please help?

Image


It's a negative temperature coefficient one?

If you read my post about Internal ADC and Voltage Divider calculations you will find all the info that you need.
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Re: how to size a resistor to read a 10K ohm Thermistor

By trackerj
- Tue Apr 12, 2016 5:01 am #45340 I really think you want to use this configuration:

[Vcc] ---- [Resistor Rdiv] -------|------- [Thermistor] ---- [GND]
|
[Analog Pin 0]

For Vcc = 5V (why 5V and not 3.3V?)
If is a NTC then (just example values, check NTC table for precise values for your specific Thermistor):
Rntc/25C = 10kohm
Rntc/125C = 300 ohm

- At 25C
ADC Input Voltage = 0.98 V -> Rdiv = 41k / Rntc =10k

- At 125C
ADC Input Voltage = 0.036 V <- Rdiv = 41k / Rntc =300

This will be your scale, from 0.036V to 0.98V.

To calculate Temperature, you can use table lookup technique or the Steinhart–Hart equation.

For more details read also : http://en.wikipedia.org/wiki/Thermistor
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