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First off, in electronics we look at current flowing from ground up to source. I'm looking at the diagram as if +12VDC is at the Source, and Ground (not -12VDC) is at the bottom of the diagram.
The pot and thermistor form a voltage divider. You set the pot so that the voltage at the Gate is correct to drive the transistor while the thermistor's resistance is low (hot). The current path is from ground, thermistor, pot, +12VDC; does that look like it's in series now? You're just tapping the voltage between the thermistor and the pot and appying it to the Gate.
Build it like circuit #2; I like that one since the load current is running through a relay; your FET will stay cool.
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Henri
Owned for 21 years: '87 Carrera coupe Venetian blue
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