[jyl]

Thank you! Your hand-drawn diagram is actually readable unlike my diagram.

Here's my math -

Suppose "turn on" temp is 41F.
Thermistor should be 27K ohm at 5C/41F. I got the specs (B value) and the formula that is supposed to calculate the thermistor curve (hope its right)
V(GS) threshold is 4.0v for this MOSFET per specs
To get V(GS) 4.0v I need resistance of potentiometer plus resistor 13.5K ohm, because

4.0 v = 13500 ohm * 12 v / ( 27000 ohm + 13500 ohm)

So if use the 10K resistor, a 10K potentiometer is set at 3.5K, nicely in the middle of its range.

Setting the potentiometer at 0K gives V(GS)=4.0v at 50F where thermistor is 20.7K ohm
Setting the potentiometer at 8K gives V(GS)=4.0v at 32F where thermistor is 35.2K ohm

So potentiometer seems nicely sized for desired range of "turn on" temps.

Well, I'll get the parts whenever they arrive (the average Radio Shack doesn't stock much anymore, had to order) and we'll see if this works.

Oh, I see V(GS) threshold is actually spec'd at "min 2.0v max 4.0v". If 2.0v I guess I'll need a smaller resistor. Maybe I'll just use a trimmer.