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


Son of Samurai

780 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

2 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

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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.

 

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