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A blog by Son of Samurai in General

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Are Thermistors Interchangeable?

Son of Samurai

Entry posted by Son of Samurai in Tech Talk

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Thermistors are everywhere in appliances these days, and they're a relatively common-fail item, so wouldn't it be nice to stock a supply of them in your service vehicle to be used on any occasion?

Well, in order to determine how feasible that is, we need to answer a question: are all thermistors interchangeable?

The short answer is no. The long answer is that thermistors are not interchangeable brand-to-brand, but they can be interchangeable within the same brand, depending on the manufacturer.

Here's an example. This table is from a Samsung refrigerator manual:

Screenshot 2021-02-20 003557.png

Compare that to this spec table from a Whirlpool manual.

Screenshot 2021-02-20 004226.png

It only gives a single data point (typical Whirlpool), but we can at least compare that to the Samsung table. While 25 C in this Whirlpool fridge corresponds, to 2,700 ohms, in the Samsung it corresponds to 5,039 ohms. That's a big difference!

Speaking of Samsung, they're one of those brands where, as a general rule, their thermistors are all interchangeable. The only difference between them is the harness, which can be clipped and spliced in a pinch. So if you work on a lot of Samsungs, it may be worth keeping a stock of Samsung thermistors for general use.

If you're so inclined to keep Samsung thermistors on hand, then DA32-10109W is a good part number to replace any thermistor you find inside the box (thanks to Brother @Vance R for the recommendation).

Want to learn more about thermistors? Click below to watch a webinar recording about thermistors in refrigerators and how to troubleshoot them -- available only to premium members.

Screenshot 2021-02-20 005722.png

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Coley

Coley

Posted

Just for fun...Here's a chart someone sent me that apparently fleshes out the rest of the readings for that Whirlpool thermister. 25C=77F and it's 2700 Ohms. Why they don't include more datapoints..who knows. What the heck are you gonna do with a 77F datapoint in a fridge?? 

thermister chart.JPG

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Samurai Appliance Repair Man

Samurai Appliance Repair Man

Posted

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6 minutes ago, Coley said:

What the heck are you gonna do with a 77F datapoint in a fridge?? 

Yeah, the paucity of thermistor specs that Whirlpool includes on their tech sheets is obnoxious. The 77F data point would be helpful if the refrigerator was at room temperature. They'll sometimes include another data point at 32F which is useful for ice water testing the thermistor. But thermistor R-T values are not linear so you can't simply do a linear interpolation for temperatures in between the given values on the tech sheet. Most of the time, we're troubleshooting a problem where the FF section is too warm or warmer than programmed but not at room temperature. I hope someone from Whirlpool reads this and they start upping their game in the specs they provide on their tech sheets.  

And let's get away from defaulting to giving specs as ohms and start giving us voltage drop specs. If Samsung can do it, there's no reason all the manufacturers can't do it, too. 

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Chashb

Chashb

Posted

Good old varying R vs. T curves between different thermistor manufacturers thermistors... Software is configured to read a specific curve. Using a wrong thermistor could result in accurate readings in some portion(s) of the curve and inaccurate readings in others. In a past life as a software engineer changing a thermistor in one of my designs meant re-coding either the look-up tables or equations used to read the R vs. T curve. Come to think of it, the software actually referenced a resultant voltage drop via an A/D converter, never the resistance...  All any manufacturer needs to do is provide the software look-up table in a more presentable form...

Charlie

 

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ABA Tech

ABA Tech

Posted

This is great info, thanks!

Does anyone happen to know the part number for the Samsung thermistor with the longer wires? I remember reading a thread on here a while back where someone mentioned it, but I can't seem to find it now that I need it.

Rhubarb Tau

Rhubarb Tau

Posted (edited)

On 2/20/2021 at 6:12 AM, Samurai Appliance Repair Man said:

And let's get away from defaulting to giving specs as ohms and start giving us voltage drop specs

Measuring voltage drop on Whirlpool thermistors gets dicey on some /many units; the sense voltage isn't steady, but pulses. I've never looked at the waveform or measured it specifically, but I've seen the pulsing DC often enough to just disregard it. I'll try to remember to measure it next time I have the opportunity...

Edited by Rhubarb Tau
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Samurai Appliance Repair Man

Samurai Appliance Repair Man

Posted

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29 minutes ago, Rhubarb Tau said:

Measuring voltage drop on Whirlpool thermistors gets dicey


My comments were tongue in cheek directed at manufacturers. Whirlpool, among others, only gives resistance specs. If Samsung can give detailed specs with voltage drops, then all the manufactures can do it because all NTC thermistors function according to the same technology, just with different specs. And since we can’t derive the specs on the thermistors, we have to use the specs we’re given.

I’ve not seen Whirlpool pulsing the thermistors to periodically poll them. Not sure what functional benefit they would gain from that, either. But, alas, the point is moot since they don’t give a voltage drop spec anyway. Again, we have to use the specs we’re given. 

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