Author Topic: Proper MOSFET Bench Test  (Read 1227 times)

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Offline OpenCircuit

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Offline w2aew

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Re: Proper MOSFET Bench Test
« Reply #1 on: March 21, 2017, 08:36:53 pm »
Not quite.  You should connect the fan between the positive power supply pin and the Drain.  Connect the Source to the Ground.  Also, include a resistor (value not critical, say 10K or so) between the Gate and Source so that the gate capacitance will discharge when you remove the 3.3V.  Finally, add a reverse biased diode (1N4001 etc.) across the fan to prevent inductive kickback from stressing the FET when you turn the fan off.  Note that 3.3V won't be enough to fully turn on this device according the datasheet.  See the curves on page 10 and later.  If it is a very low power fan, it might be sufficient...
« Last Edit: March 21, 2017, 08:41:18 pm by w2aew »
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Offline danadak

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Re: Proper MOSFET Bench Test
« Reply #2 on: March 21, 2017, 09:27:05 pm »
Without knowing the fan electrical specs or the switch used
to turn on the MOSFET (thinking bounce issues) or the frequency
of swicth cycling I would recommend you bolt the MOSFET to a
heat sink just in case it spends a extended period of time in the
linear area dissipating power and heating up.


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Offline OpenCircuit

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Re: Proper MOSFET Bench Test
« Reply #3 on: March 21, 2017, 09:44:34 pm »
Excellent, thanks for the help!


Dana, thanks for posting. Both FETS are mounted to an aluminum plate. removed this from an ATX switching power supply I am trying to fix: https://www.eevblog.com/forum/repair/atx-power-supply-dead/ 
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Offline OpenCircuit

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Re: Proper MOSFET Bench Test
« Reply #4 on: March 21, 2017, 10:21:56 pm »
You were correct; 3.3v was not enough to power the fan on completely. I did try 5v at the gate and it did turn the fan on completely. Was sure hoping they were bad so I could say problem likely solved. Oh well, at least I reached a conclusion. :)

« Last Edit: March 21, 2017, 10:30:38 pm by OpenCircuit »
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