Vaguely related to this question but with some different questions and a different component.

For an inrush-limiting NTC thermistor such as the Ametherm SL series:


It seems purposefully designed to run hot when at the steady-state current, and this heat is necessary to minimize the working resistance. But I wonder about this "body temperature". Will that 112C be exposed to the surface of the component? If so, I would be worried about the following:

  • risk to human skin (until it's enclosed in a box)
  • ambient heating through air or the PCB that could degrade other component performance, particularly in this application since the cooling will be passive
  • risk to some polymer materials whose maximum rated temperature is well below 112C, such as PVC - care would need to be taken to secure wires etc. away from this component

Is it assumed that these are the typical risks incurred when using inrush thermistors? It's not like a heatsink can be added, because that would fight the purpose of the NTC.

  • \$\begingroup\$ if it were insulated, it wouldn't help inrush much when the power dropped for 10 seconds due to a squirrel or whatever. \$\endgroup\$
    – dandavis
    Commented Apr 20, 2022 at 21:10

2 Answers 2


Yes, the body temperature is (or nearly is) the surface temperature of the device, and it can be a burn hazard.

Most regulations require the user accessible parts of a device be at a safe temperature so that would require a box or frame around the thermistor to keep a user from being burned.

Inrush thermistors are often through-hole mounted with the body above and away from other components that could be impacted. If surface mounted, the necessary thermal design has to be done to make sure it doesn't impact the PCB or other components.


"Inrush Thermistors" are negative temperature dependent devices. Looking at your chart the resistance doubles at 50% load because it is cooler. They are designed to be hot and there characters are dependent on their temperature. The chart tells you they are hotter then boiling water at full load. Adding a heatsink or any other method of cooling might make you feel better; but it will change the characteristics of the device and possibly cause premature failure of the device it is protecting. If you see no serviceable parts inside, that could indicate there are parts that are hot thermally/electrically or both.


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