I need some more help. Driving a simple LED light strip, would a pull down resistor on a mosfet gate be beneficial? I ordered a few FQP30N06L mosfets and didnt think I needed a pull down resistor, but I see sparkfun using one on their power control kit:
When Arduinos are powered up, the pins default to INPUT, which is essentially open circuit. An N-MOSFET gate connected to an input pin will be floating, and could turn on randomly, so it needs a pulldown resistor.
This is the usual circuit (don't forget R2, which protects the port pin from excessive current draw):
The resistor is there for Arduino 'Power Up' condition where the controller pins default to Inputs.
At power up, the MOSFET might turn on prematurely as the gate would be floating, hence the need
for the resistor resistor to ensure the gate is at GND level (MOSFET OFF) until the Arduino sketch turns it ON.
So once the pin is set to output, if I do analogWrite of 0 to that pin, would it reliably stay off, if controlling an LED strip?
Yes, the Arduino pin takes full control over the gate after bootup.
Leo..
jremington:
When Arduinos are powered up, the pins default to INPUT, which is essentially open circuit. An N-MOSFET gate connected to an input pin will be floating, and could turn on randomly, so it needs a pulldown resistor.This is the usual circuit (don't forget R2, which protects the port pin from excessive current draw):
I have seen a few values thrown around for the resistor at R2. It doesnt seem to matter much, but how do you determine what is best for a particular scenario?
For Arduino UNOs, R2 180 ohms is a good value so is 220
ex:
5v / 220R ~= 22mA which is a safe value for an output pin to provide.
The reason for the gate resistor is that the gate has very high resistance (it is voltage controlled), but it does have some capacitance. The gate capacitance must be charged to turn the MOSFET on. This capacitance looks, briefly, like a read short. The resistor limits the output pin current while the capacitance is charging. Use the lowest possible resistor so as to turn the gate on as fast as possible, but limit the output pin current to a reasonable value (around 20 mA).
Thanks. I had plan to use a RFP30N06LE mosfet to drive an led light strip. Only pulls about 1a at 12v Is that a good mosfet for my application?
RFP30N06LE this is an old part, it will work fine for you project.