JimboZA:
Last visit to my preferred supplier I picked up a couple of IRL520N logic level mosfets. I've never used a mosfet before.If I want to use one connected to an Arduino digital output does it need a current limiting resistor on the
baseoops I mean gate?
Its not an easy question to answer definitively. The Arduino outputs are rated at an
absoute max of 40mA, yet will be designed to handle a certain amount of capacitive load
(although how much isn't given in the datasheet). Capacitive loads will mean brief
spikes of current as the outputs switch which could exceed 40mA, but last a few
nanoseconds.
So a small MOSFET with a few hundred pF of gate capacitance might be quite OK
to drive directly. But what the maximum capacitive load is before a resistor should be
used is unclear. Most modern MOSFETs have more gate capacitance that you would
expect to see on a logic signal (measured in nanofarads) so a resistor is generally
advised. 150 ohms is a good value, since the lower the better for switching speed.
If you have a beefy MOSFET and want to switch it fast, you should always consider
a MOSFET driver chip which boosts the current output to much higher levels safely,
reducing switching losses and appearing as a standard logic load to the microcontroller.