Discussion in 'General Electronics Chat' started by sonda, Dec 12, 2012.

1. ### sonda Thread Starter New Member

Oct 30, 2012
10
0
Hi,

i am using a N channel mosfet IRF 1405 to drive a 19A fan motor. the mosfet is controlled by a 10K potentiometer & 50K NTC. my question is what type of a heat sink do i need to use for this Mosfet. thanks In advance.

2. ### #12 Expert

Nov 30, 2010
16,664
7,311
Huge.

I think you just said you are using the mosfet in the linear region where P=IE and the voltage is unknown. Power = current times voltage. That's why mosfets are usually run as variable pulse width devices. Way complicated compared to just a potentiometer, but much easier to keep cool.

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3. ### nigelwright7557 Senior Member

May 10, 2008
488
71
A huge one !

Look up PWM controllers.

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4. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
You have to calculate the power dissipation, then select a heatsink based on allowable maximum temperature rise.

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5. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
However, if a FET is used as a "saturated" switch (fully cranked ON), power is calculated as:

P = (I)squared X (FET ON resistance)

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6. ### crutschow Expert

Mar 14, 2008
13,473
3,361
What is the fan supply voltage? From the maximum power theorem the maximum MOSFET dissipation will occur when the voltage across the MOSFET is 1/2 the supply voltage but that's for a resistive load. Since the fan is a variable load it's more difficult to accurately determine the maximum power point, but it's likely to be no more than 1/2 the supply voltage times 1/2 the maximum fan current.

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7. ### sonda Thread Starter New Member

Oct 30, 2012
10
0
its supply voltage is 12 volts. thanks a lot!