# NTC resistor max current and max power conflict

Hi!

I have a question regarding the NTC resistor. I plan to use it to limit the startup current of a machine. I am looking at this resistor right now.

I went over the datasheet it show me the max power is 3.6W and its max current is 4A. Since its resistance is 60 ohm at 25 cel degrees, the above two-parameter can be conflict at 25 degrees. Can someone explain how should I understand those two parameters?

Hello bbg8842,

Welcome to the DigiKey TechForum.
This is a Thermistor that will have 60 ohms of resistance at 25C temp.
As the temperature of the part heats up, the resistance will go down.
Maybe one of the engineers can comment on any possible conflict in the ratings.

As you pass current through an NTC thermistor it self heats lowering its resistance. This deficiency from the theoretical ideal thermistor is intentionally exaggerated in thermistors designed for startup current limiting, since it’s helpful for that application.

See the “Dependence of NTC resistance on current” section of the data sheet for an equation. It should make the seemingly impossible W vs. A ratings make sense.

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Hi bbg8842,

@PaulHutch is right on there. Note on page 2 of the datasheet, they give the following specs for the parameters “k” and “n”:

The formula on page 3 gives an approximate steady-state value for RNTC for any given value of current (I) at an ambient 25°C.

For example, for a current of 3A, one can calculate RNTC as:

RNTC = 1.77 x (3A)-1.44 = 1.77 x 0.2056 = 0.364Ω

And this equates to power dissipation across the NTC inrush current limiter of:

NTC power dissipation = (3A)2 x 0.364Ω = 3.27W

Using the same formula for the max specified current of 4A, the power dissipation calculates to 3.85W, which is slightly above the max power at 25°C of 3.6W, but they qualify that by stating that the formula is only an approximation, and even then, only for a specific set of circumstances. However, to that point, I would never use this particular part with a continuous 4A load. I personally would want to keep a margin of safety by not using it for anything above about 3A.