Mosfet Type and Sizing Help

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i discovered mosfets better switching led 25w lights rather noisy relay bought irf40n mosfets off ebay.
done research, hooked em up, 1 mosfet per light resistor on gate limit current flow , pull-down resistor earth.
has worked flawlessly not switching them on & off dimming, problem had remove power circuit off breadboard when caused problems , started melting it, put mosfets on heatsink.

thought same dc fan invention , control lights, mosfets rated 33a, fan 80w / 12v = 6.66a. fan being 10inch radiator fan mounted on adjustable tv bracket arm.
anyway, wanted test first mosfet set high/low (on/off), fan turns on, won't turn off, touching mosfet , heatsink located directly behind fan take advantage of air flow, , hot after 1 minute run-time.
i took mosfet test bench, , conclusion is burn't out (shorted), current flows drain source regardless applied gate.

so long story short, recommendations on mosfet should using?
also further info me looking stupid in future great...









you need logic level fet, 1 have needs 10v turn on , have 5v not turned on. means fet running in linear mode , hot.

solution use logic level fet turn on 5v. @ data sheet fet voltage give when quoting drain source resistance. ignor gate threshold value in data sheet, not think is.


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