Friday 28 August 2015

power supply - Do N-channel MOSFETs require a pull-down resistor?


I'm using an N-channel mosfet to supply power to a servo. The problem is that even with the gate set at low, I'm still getting some amount of voltage (~0.9V in a 5V system) coming through. Do I need a pull-down in case the pin is floating?


Here's the current schematic:


enter image description here



Answer



Yes, if you just tristate after pulling it high, then the gate will stay floating high. You either need a resistor to pull it down to ground or you need the input signal to drive it low.



The resistor can/should be relatively high valued compared to your input resistor to prevent excessive voltage drop when you have it set as a high input. You only have to drain the inherent capacitance on the MOSFET gate when you're pulling it low so even at a high resistance to ground the RC time constant is usually relatively short.


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