Converter In-Rush Thermistors etc, UPDATE -4

Salvo wrote:
As I see it, the thermistor blew because it had too high resistance. You turned on the converter, waited a while and then connected converter leads to battery. And then, POW! During the delay, the thermistor cooled and its resistance increased.

So it sounds like you're damned if you connect converter to battery from the get-go, and damned if you don't.

There's a possibility, what you're looking for is a fast thermistor. You want it to heat up fast so that its resistance can quickly drop. That means that a 15A part might be better than a 20A part. The 15A part has less mass and can react faster to a surge in current.

You may say that the 20A part has a 300J to destruction spec, while the 15A part has only 200J rating. If it is designed correctly, it will never get near 200J. There are millions and millions of converters in use, they don't fail like that.

I like the 2.5 ohm, 15A thermistor. Is this the part that failed the very first time?

I can understand why it can use a heatsink. The thermistor temperature at 50% current is:

T = 25C + 7.5A^2 * 0.07 ohm / 32.4 mW/C = 147 C

That's only at 7.5A and it's cooking! The converter can probably pull 13A.

Sal

BFL13 wrote:

It was when the thermistor was already warmed up so its R was low that it got hit with the big current from a "too soon" restart and blew, so how could it help to have a lower R thermistor for the cold start too?