Thermistor calculation


Joined Apr 20, 2004
I think it's more like selecting a thermistor in a range you like, determining its resistance at the critical temperatures, and using that to set up your circuit.

Thermistors come with a rating at 25 C, and a stated non-linear response curve.


Joined Apr 24, 2007
I assume you're wanting an 'inrush-current limiter' type of NTC thermistor. They have a higher resistance when cold, and then fairly-quickly drop to a low resistance, as the current heats them.

About the best you can do, unless you can come up with spice models for them, is go to the manufacturer's website and look at the data for them. You could get there by going to or, for example, and searching for NTC inrush-limiter thermistors and then clicking on the manufacturer links and making your way to their websites.

For example, I found this:

If I was using an NTC thermistor on the mains side of a power transformer, I'd still want a relay circuit that shorted across the thermistor, after the inrush surge was finished, i.e. probably within 300 msec. But the same thing could be done with a properly-sized resistor, instead of a thermistor.

Inrush-current limiting can be a good thing to do. It also enables you to use a fuse that is rated closer to the actual operating requirements, which is safer for the equipment.