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[on:]

hi
for my first project im switching a relay, ive been researching about mosfets, am i right in thinking i can use the output (high) straight onto the gate of the mosfet without a resistor? below is the one i want to use.

thank you


http://www.rapidonline.com/Electronic-Components/Fdn359an-Mosfet-N-30v-2-7a-Sot-23-47-0208/?sid=9e663417-1e1c-4c70-b322-60a27b647168

[Reply #1 on:]

That mosfet is suitable for switching up to 2.4A current, which is the value for which Rds(on) is quoted at Vgs=4.5v.

For all but small-signal mosfets, it is recommended to connect a resistor between the Arduino pin and the mosfet gate. This is to limit the peak current drawn from the Arduino pin when the pin changes state and the mosfet input capacitance charges or discharges. A value between 100 and 220 ohms is typically used.

It is also recommended to connect a resistor between the Arduino output pin and ground. Without this, the mosfet gate voltage will be in an undefined state when the Arduino has powered up but before the pinMode call is made to define the pin as an output. In this condition, the mosfet might be on or off or somewhere in between, so that you can't be sure what state the relay will be in. A value between 10K to 100K is typically used.

For driving a relay, it is more usual to use a bipolar transistor, which just requires a resistor between its base and the Arduino pin. Bipolar transistors are essentially current operated, so you don't need the second resistor.

Whether you use a mosfet or a bipolar transistor, don't forget to connect a diode across the relay coil to catch the back emf when the transistor or mosfet turns off.

[Reply #2 on:]

thanks very much for the the help, do i need to use 10k rs for outputs that light led's?
(i already have a diode across the coil)

[Reply #3 on:]

No, you only need the 10K resistor when driving a load with very high input resistance, such as a mosfet gate.

[Reply #4 on:]

i thought as much, thanks very much for your help