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NTC Thermistors: What Are They, How Do They Work, and How Do They Fail?


Son of Samurai

2,249 views

Anytime you work on a refrigerator with a control board, you've got multiple NTC thermistors in play. With how ubiquitous these devices are, it's important to know them inside and out -- especially how to test them. That's why we've made a webinar all about NTC thermistor technology for your viewing pleasure!

In this short excerpt from one of our many technical training webinars, we cover topics like:

  • What is an NTC thermistor used for and where will you find them?
  • What does NTC mean?
  • What does an NTC thermistor graph look like and how do you use it?
  • How doe NTC thermistors fail?

Watch it now for free!

Want to watch the full webinar? Click below -- viewable only by premium members.

Screen Shot 2021-10-20 at 7.23.48 PM.png

10 Comments


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Charts and diagrams are interesting, but they don't answer your question.  NTC thermistors, are inrush protection resistors.  These resistors have a relative higher resistance when cold and a relatively lower resistance when warm/hot.  Their usual purpose is to protect motors and other electronic equipment from damage due to high inrush currents at startup.  Typically, these parts run around 20 ohms when cold, and only a few ohms when hot.  That behavior prevents excessive voltage drop when the inrush is over (a second or so). the part normally runs hot when the equipment is on.  So if the designer were otherwise distracted when specifying the correct part, it will tend to overheat, crack and self-destruct.  Since the part fails open, your equipment is usually saved, but doesn't work until a new part is installed.  Sometimes the part is out in the open and can be desoldered and replaced.  Other times the part is partially potted.  If you can leave pigtails exposed, a new part can be soldered to those pigtails.  Inrush protection is a big subject and needed almost universally.  It is a cost effective solution compared with more expensive relays and current detectors.  Hope that helps.

-Ghost

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  • Team Samurai
Samurai Appliance Repair Man

Posted

On 10/27/2021 at 5:40 PM, Ghost said:

NTC thermistors, are inrush protection resistors. 

You're way off the mark. NTC thermistors are semiconductor devices designed to decrease resistance with increasing temperature (hence the name NTC - negative temperature coefficient). They have nothing to do with inrush current or motor protection. They are used for temperature sensing (eg., refrigerator compartments, evaporator temperature, pass-through heater temperature, etc.,) because of the gentle curvature of their T-R response, which is what the graph is showing you. The webinar explains this in detail. Also, NTC thermistors are monitored by computer boards. They have nothing to do with motors. Whenever you see an NTC thermistor, you know there also has to be a computer monitoring that sensor. 

PTC thermistors (positive temperature coefficient-- also semiconductor devices), on the other hand, are designed so that their resistance increases rapidly with increasing temperature. The start devices on many modern compressors use a PTC thermistor (often incorrectly called a "relay" in the manufacturer's literature.) They are also used in wax motors and can also be used as motor protection devices, such as in the Whirlpool in-door ice maker that uses the DC motor. Maybe this is what you were thinking of? 

This post breaks down the difference between NTC and PTC thermistors in more detail.

 

  • Like 1
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Far field

Posted

There are both types I think

take a blower motor on a ecm electrical commentated motor. In the housing end where the blower has its power and control module there is a thermistor soldered onto the board. It is there to do ? I don’t know but overload at lower startup speeds makes sense. 
and when we encounter NTC on appliances the thermistor is used as a sensor. 
so I may not understand the way a ecm blower works in reguards to a thermistor. But it would make sense that that thermistor could protect da motor or the electronics in its board. Is this true? Or am I needing to understand more?

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  • Team Samurai
Samurai Appliance Repair Man

Posted

20 hours ago, Far field said:

But it would make sense that that thermistor could protect da motor or the electronics in its board. Is this true? Or am I needing to understand more?

As I mentioned above, thermistors come in two flavors: PTC and NTC. The function differently and are used in entirely different applications. In a nutshell: PTCs are used for temperature reaction and act independently; NTCs are used for temperature sensing and are always used with a control board. Watch the webinar that Sam posted in the blog post for a deep dive into this and morr. 

 

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Far field

Posted

I understand this from Scotts lessons and agree with it 

There is a part on the "brain end" of a electrically communicated motor used in higher end furnaces, that there is a part on that board that is referred to as a thermistor. It has 2 soldered connection points on the motor board. 

grayfurnaceman.com/the-ecm-motor.html

http://www.grayfurnaceman.com/the-ecm-motor.html

There is a thermistor used in this some ecm design mounted on board. Now I dont know if its a thermistor or not but I understand somewhat limited it is referred to as a thermistor. It does heatup. And from the definition given in wickapida the thermistor can act as a fuse with reseting ability  I E when the resistance is so great from heat it is blocking electrons. Once the load is allowed to cool it will be inrange of allowing electron flow. Thus resetting the fuse application. 

From Wikipedia:

"A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word is a combination of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors (negative temperature coefficient or NTC type typically), self-resetting overcurrent protectors, and self-regulating heating elements (positive temperature coefficient or PTC type typically). An operational temperature range of a thermistor is dependent on the probe type and is typically between −100 ⁰C and 300 ⁰C (−148 °F and 572 °F)".

a general definition +¹Qq q

I'm not missing or misslabeling the part? I'm only calling it what the people that I have dealt with call it.

 

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Far field

Posted

Oh I see now. The motor ecm type use a ptc so as they heatup the resistance increases. I'm sorry I'm so thick headed.  The thermistor general definition needs scotts charts to explaine the difference and also will show the major difference

Hit me in the head and call me golfball.

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Far field

Posted

So in a heating application using an ntc the resistance is decreasing and a board is seeing the lesser resistance and will decide to cut off power to the heater?  Is this deduction correct?

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  • Team Samurai
Samurai Appliance Repair Man

Posted

4 hours ago, Far field said:

So in a heating application using an ntc the resistance is decreasing and a board is seeing the lesser resistance and will decide to cut off power to the heater?  Is this deduction correct?

Yes. Works the other way, too. NTC thermistors are used in modern refrigerators. As temperature decreases, thermistor resistance increases. NTC stands for “negative temperature coefficient” meaning that temperature and thermistor resistance move in opposite directions.

You really should take the time to watch the webinar. 

 

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Far field

Posted

Ok the webinar I will try to find

on a related matter I’m wondering if I need a hz meter to check the water fill on washers that use this technology. What meter do you recommend? Approx cost? I was thinking about getting just a cheap hz meter but I have no idea on connecting or if I have a need of something better. If I followed you correctly in the washer advanced troubleshooting lesson we are looking for a difference in the cycles. If I start out with 60 hz (I’m assuming 60 is the starting point) then read the hz as it’s filling I should be loosing hz? Is hz connected to the meter just like volts? Black - red + ? If polarity is reversed does it have an effect?

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MarcusF

Posted

On 2/12/2022 at 5:23 PM, Far field said:

Ok the webinar I will try to find

on a related matter I’m wondering if I need a hz meter to check the water fill on washers that use this technology. What meter do you recommend? Approx cost? I was thinking about getting just a cheap hz meter but I have no idea on connecting or if I have a need of something better. If I followed you correctly in the washer advanced troubleshooting lesson we are looking for a difference in the cycles. If I start out with 60 hz (I’m assuming 60 is the starting point) then read the hz as it’s filling I should be loosing hz? Is hz connected to the meter just like volts? Black - red + ? If polarity is reversed does it have an effect?

The webinar is posted in this very blog post, no need to go searching for it. 😅

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