SuperOxide
National Hazard
Posts: 539
Registered: 24-7-2019
Location: Devils Anus
Member Is Offline
|
|
What type of thermistor is this in my freezer?
Recently I purchased a used (listing), and when I got it, I noticed there was an after-market thermistor or thermocouple that had been affixed to the back wall on the inside
with its leads being through the wall out the back of the freezer, epoxied in place to create a good seal.
You can see it on the inside here:
 (more photos here)
I wasn't sure if it was a thermocouple or a thermistor, but the first thing I noticed was that the leads seemed to be the same metal which is
indicative of a thermistor. So I hooked up my multimeter and took some voltage and resistance readings as I would warm up the end of the sensor on the
inside and it looked like I got no voltage change but a slight resistance change. So I'm pretty confident its a thermistor. But I'm not entirely sure
what type of thermistor or even how I go about getting a temperature display in Celsius. I would prefer not to have to rip this thermistor out to
replace it just because I don't know what type it is. I'm really hoping I can just buy something like this Refrigerator Thermometer Digital Freezer Thermometer Fridge Thermometer and just splice the wires from the thermistor into the wire that comes
with it. But I first need to know its the correct type of thermistor.
According to the Google Gods:
| Quote: | - NTC behavior: If the resistance decreases as you apply heat, it's an NTC thermistor (most common type).
- PTC behavior: If the resistance increases as you apply heat, it's a PTC thermistor |
I used my K-type thermocouple, taped it right to the end of the thermistor sensor and turned the freezer on. Taking the temperature and resistance
readings as the freezer got down to temperature from about 20 °C to -11.3 °C. T, I created a graph:
So I guess that means its a PTC thermistor since the resistance decreases as the temperature goes down. But whats the "nominal resistance" (I guess
that's the resistance at 25°C). It was 0.995 R @ 21.7 °C, so I think its safe to assume that it would be approximately 1K R @ 25 °C.
Question
What do I buy to display the temperature using this 1K PTC thermistor? Most of the things I found use 10K NTC thermistors (example). I'm sure I could whip something up using Arduino or ESP32, but I would prefer to not have to create a whole project out of this. There
must have been a display that the previous owner was using that I could buy (the seller wasn't the previous owner, he doesn't know what was used or
what type of thermistor it is).
-------
Edit 1: I realize the beginning of the graph looks a bit uncharacteristic of what one would expect from a typical thermistor graph.
I'm not sure what to attribute this to other than maybe I didn't let the thermistor or thermocouple both stabilize before I started taking readings.
But the rest of the graph follows what one would expect from a typical thermistor output (ie: non-linear).
Edit 2: My last edit may be wrong, Maybe that graph is totally accurate, and its a "switching type" thermistor?: https://eepower.com/resistor-guide/resistor-types/ptc-thermi... The graph does look pretty similar..
[Edited on 1-2-2025 by SuperOxide]
|
|
|
Twospoons
International Hazard
Posts: 1349
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline
Mood: A trace of hope...
|
|
Resistance readings like that would be indicative of a thermocouple. Try using your meter in mV mode and see what you get.
Helicopter: "helico" -> spiral, "pter" -> with wings
|
|
|
SuperOxide
National Hazard
Posts: 539
Registered: 24-7-2019
Location: Devils Anus
Member Is Offline
|
|
Quote: Originally posted by Twospoons  | | Resistance readings like that would be indicative of a thermocouple. Try using your meter in mV mode and see what you get. |
That was one of the first things I tried, no luck.
I did try reversing the leads too, just in case, and still no reading.
|
|
|
Twospoons
International Hazard
Posts: 1349
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline
Mood: A trace of hope...
|
|
Your measurements show a resistance of just under 1 ohm - or is that typo? If not then your measurements are likely to be more than 10% out with
that meter.
Also note that PTCs are almost never used as temperature sensors - pretty much everyone uses 10k or 100k NTC thermistors, since they're cheap and
reasonably accurate.
The other possibility is that its a diode, or some other two wire electronic sensor (AD590 comes to mind).
Helicopter: "helico" -> spiral, "pter" -> with wings
|
|
|
Rainwater
National Hazard
Posts: 987
Registered: 22-12-2021
Member Is Offline
Mood: Break'n glass & kick'n a's
|
|
Cool it down and watch it go open circuit.
What does this device connect to? I.e. an ic chip, or a relay
[Edited on 2-2-2025 by Rainwater]
"You can't do that" - challenge accepted
|
|
|
SuperOxide
National Hazard
Posts: 539
Registered: 24-7-2019
Location: Devils Anus
Member Is Offline
|
|
Literally nothing. It just came as this sensor fixed inside the freezer and two leads on the outside.
I just ordered some wireless sensors, so I'm not sure I'll even be using this sensor on the back, but I'm still curious what it is.
The black stuff holding it in isn't as solid as I thought it was, its some really thick goop, not rock hard epoxy. So I may be able to just peel it
off and take the sensor out (then either replace it or plug the hole).
|
|
|
RedDwarf
Hazard to Others
Posts: 169
Registered: 16-2-2019
Location: UK (North West)
Member Is Offline
Mood: Variable
|
|
Did you disconnect the device from the circuit before measuring it's resistance? If not then you could well be measuring parallel resistances.
|
|
|
Twospoons
International Hazard
Posts: 1349
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline
Mood: A trace of hope...
|
|
A really close-up pic of the sensor would be more useful - can barely make it out in the first pic.
Helicopter: "helico" -> spiral, "pter" -> with wings
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
