![]() ![]() ![]() PWM stands for Pulse Width Modulation and is a method of controlling the average voltage output of a microcontroller pin by turning the output on (5V) and off (0V) very quickly. This becomes particularly important if you are switching the MOSFET on and off quickly with a PWM output. ![]() The more resistance we put between the microcontroller output pin and the MOSFET gate, the more slowly the gate will charge and the hotter the MOSFET will run. Charging speedĪ MOSFET is an efficient switch when completely on or off, but it is inefficient when switching and dissipates those losses as heat. So with this in mind it’s important to protect the microcontroller with a current limiting resistor. The datasheet quotes a 47nS on/rise time with a 4.7 ohm gate resistor, so it took an average of 7.3nC/47nS = 155mA average current to charge the gate – that’s far more than the 40mA maximum current for an Atmel AVR output pin. Coulombs are a measure of capacity, current time in units of amp seconds. The datasheet for a STP16NF06L 16A 60V logic level MOSFET shows a 345nF input capacitance and a 7.3 nC total gate charge. However during switching a MOSFET behaves more like a capacitor that needs to be charged in order to open and discharged in order to close. So with this simplistic model of MOSFET operation a current limiting resistor is not required. In this model very little current is required at the gate. Voltage controlled deviceĪ MOSFET is normally thought of as a voltage controlled device, sufficient voltage at the gate opens the MOSFET and allows a large current to flow through the drain/source. But others insist that you need a current limiting resistor to protect the microcontroller output pin. In the tutorials referenced by Adafruit and Sparkfun they connect an Arduino output pin directly to the MOSFET gate. If you are driving a logic level MOSFET directly from an Arduino, or another Atmel AVR such as an ATTiny85, you may have wondered what value resistor should be placed between the output pin and the MOSFET Gate. ![]()
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