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VeryNoob
Posts: 49
Joined: Thu Feb 14, 2013 1:07 pm
Location: Australia

MOSFET & Current Limiting Resistor

Tue Sep 05, 2017 6:25 am

Hi all, if I want to use a GPIO to drive a N-Channel MOSFET (https://www.adafruit.com/product/355 - Can be driven from 3.3v). I am going to use it to drive a couple of 12v fans (5amp max, probably less). I plan on having 12v go in to a buck to drop it to 5v to power the Pi. Run the 12v to the +ve of the fan, then use the MOSFET to switch it to ground.

Do I need to put in a current limiting reistor, as I would have to with a BJT transistor?
Do I need a pull down resistor (10k) to stop the pin floating? Or can I just set the pin High or Low? I assume I don't need a pull down while the Pi is running, but I don't want the fans to start if the Pi crashes or is shut down.

Thanks.

drgeoff
Posts: 10765
Joined: Wed Jan 25, 2012 6:39 pm

Re: MOSFET & Current Limiting Resistor

Tue Sep 05, 2017 8:44 am

You do not need a series resistor. A pull down resistor, say 22k, will ensure the FET stays off if the GPIO pin would otherwise float.

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