help Building a fan project

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The design of it that I had in mind was to turn on a small 40x10mm fan to blow on a heatsink when the LED reaches a temperature of around 48C the fans should turn on until it's below that temperature and the fans should turn back off and stay off without any voltage applied.

from what I heard so far I need a MOSFET for it and match a thermistor with it but I can't seem to find the flat kind that I need to put it between the underside of the LED and the backside of the heatsink.

 

[crutschow]

You can just attach or glue the thermistor to the side of the LED at the junction to the heatsink. That should give you a temperature close to the LED temperature. If you put it between the LED and heatsink it will add thermal resistance and cause the LED to run hotter.

 

[1-3-2-4]

One thing I was told if I run two fans on this that the thermistor would burn up? I'm not sure where to start or for sure do I need a heatsink with a fan for it.. it would be a Cree XRE running at 1A.

Edit*

Also I only wanted the fans to turn on when the temps reached around 100C Without the MOSFET some power would always be going to the fan (turning or not) So I don't want to drain my 12v battery over that...

Like one person said if I have a fan running at 0.5A and the thermistor goes down to 10 ohms from 2Kohms it has to dissipate 2.5W

 

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a schematic

 

[audioguru]

If the heatsink is at 100 degrees C then the chip inside the LED is probably way hotter than its max allowed temperature.

 

[1-3-2-4]

I don't foresee it getting that high since the LED will be for a car's reverse lights and wont be on for long.

 

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is using a thermistor for this type of job the right thing? It does not need to be complex at all something very simple.

 

[Boncuk]

You are using the thermistor in a voltage divider to control the transistor. No motor current flows across the thermistor and so it won't dissipate 2.5W.