Newbie Here: having a hard time with a 10k NTC Thermistor

ideal2545
Posts: 4
Joined: Sat Jun 13, 2020 9:10 pm

Newbie Here: having a hard time with a 10k NTC Thermistor

Postby ideal2545 » Sat Jun 13, 2020 9:23 pm

I'm using this NTC thermistor: https://www.adafruit.com/product/372 with a B value of 3950.

I'm using this diagram:

Image
which looks like this:
Image

Because we're feeding it 3.3V and have a resistor in the middle I know that my expected voltages would be above 1.65v so I set attenuation to 6db to be able to read those voltages.

With this however I'm having a hard time getting the proper calibrated reading with a B constant of 3950. I think I may need to set my reference voltage but I'm not really sure what to use for that, i tried 1.65v but that does not work whatsoever.

Any help is appreciated. I am using ESPHOME for this project and this is currently my code:

Code: Select all

sensor:
  - platform: ntc
    sensor: resistance_sensor
    calibration:
      b_constant: 3950
      reference_temperature: 25°C
      reference_resistance: 10kOhm
    name: NTC Temperature B Constant

  - platform: resistance
    id: resistance_sensor
    sensor: source_sensor
    configuration: DOWNSTREAM
    resistor: 10kOhm
    reference_voltage: 3.3v
    name: Resistance Sensor

  - platform: adc
    id: source_sensor
    pin: 36
    attenuation: 6db
    update_interval: 5s


Beowulff
Posts: 17
Joined: Fri Mar 01, 2019 2:59 am

Re: Newbie Here: having a hard time with a 10k NTC Thermistor

Postby Beowulff » Sun Jun 14, 2020 11:20 pm

That’s not a 150KΩ resistor - it’s 120Ω (or maybe 10KΩ - hard to tell).

ideal2545
Posts: 4
Joined: Sat Jun 13, 2020 9:10 pm

Re: Newbie Here: having a hard time with a 10k NTC Thermistor

Postby ideal2545 » Mon Jun 15, 2020 4:30 am

Ah yes my apologies, I did use a 10k resistor instead of a 150k resistor

Beowulff
Posts: 17
Joined: Fri Mar 01, 2019 2:59 am

Re: Newbie Here: having a hard time with a 10k NTC Thermistor

Postby Beowulff » Mon Jun 15, 2020 3:08 pm

What kind of readings are you getting?
Use an ice cube and a hair dryer to expose the thermistor to a range of temperatures, and take readings, noting the actual temperature using a known-good thermometer.

ideal2545
Posts: 4
Joined: Sat Jun 13, 2020 9:10 pm

Re: Newbie Here: having a hard time with a 10k NTC Thermistor

Postby ideal2545 » Mon Jun 15, 2020 8:24 pm

I can create a curve using my own thermometer but I was hoping to use the B Constant value that i have on the spec sheets.

If I set different resister reference voltages and even a different attenuation, to say 11db then i get different voltages and resistance readings. I guess what I can't seem to do is figure out the best resistor reference voltage and the best attenuation to try and get the best temp readings based on the B constant value that I have. I sense I have a fundamental misunderstanding somewhere. For example if i set 3.9V for the reference resistance voltage and 11db, the ohms readings are substantially different from 3.6V and 11db, which is actually very close to matching up with the spec sheet at anything under 35C.

Beowulff
Posts: 17
Joined: Fri Mar 01, 2019 2:59 am

Re: Newbie Here: having a hard time with a 10k NTC Thermistor

Postby Beowulff » Tue Jun 16, 2020 3:18 pm

I’m not familiar at all with ESPHOME, but I looked on the web at the NTC, ADC and Resistance functions, and it’s a bit vague.
Obviously, the attenuator in the ADC directly affects the temperature readings, but I don’t see how the NTC function takes the attenuator setting into account.
You need to use at least 6db, since at room temperature, the thermistor is going to be around 10kΩ. 11db would allow for much colder temperature readings.

OK, I just did some searching, and came up with this:
https://community.home-assistant.io/t/1 ... y/203358/3
May help you.

ideal2545
Posts: 4
Joined: Sat Jun 13, 2020 9:10 pm

Re: Newbie Here: having a hard time with a 10k NTC Thermistor

Postby ideal2545 » Tue Jun 16, 2020 4:32 pm

Thank you! I appreciate your help.

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