Just bought an arduino and my plan is to control a 6A, 12v, 300 LED (5 meter) strip (around 72 watts)
Being a beginner I have a few questions about the circuit to the arduino. I have researched and found out that you need to use transistors, ideally mosfets in between your led strip and arduino. Correct me if im wrong but I think its because the pins cannot power that many leds. So I bought 3 " 40A 120V STP40NF12 N-Channel Power MOSFET TO-220"(amazon title) MOSFETs
so before I start plugging thing in with little knowledge of the fragile nature of this microcontroller,
my questions are:
are these the correct or suitable type of MOSFETS(power wise) to link an arduino to the above specified LED strip?
are they too much or too little power?
I know to little isnt a big deal since it just wont power the strip but Is overpowering a danger to the strip or arduino?
thanks
That MOSFET needs 10V+ on the gate to fully turn on. You can use a 1K resistor from gate to 12V to turn it on, and an NPN with collector to gate, emitter to Gnd, driven by Arduino pin thru 270 ohm resistor to base to turn it off.
Add 1K from base to 5V to turn on NPN/keep MOSFET off while Arduino comes out of reset.
With just a quick glance at the specs, I see the threshold voltage is 4V. That means it just barely starts to turn on at 4V and it won't be fully on at the Arduino 5V level. You need a "logic level" MOSFET. One of the other members here will usually give you a DigiKey link but it may be bedtime in his part of the world right now.
Does the LED strip have its own current control? You can just hook it up to a 12V battery with no other components? Depending on what is inside it, it may not work well with PWM control from the Arduino.
You can't "overpower" the strip. Each group of 3 LEDs has a current limit resistor so you could connect to +12V/Gnd direcly and have it work correctly.
The MOSFET you selected has pretty high Rds,
http://www.st.com/web/en/resource/technical/document/datasheet/CD00003615.pdf
Power dissipated by the MOSFET is IIR, so 6A x 6A x .032ohm = 1.15W , it will be warm.
A better MOSFET, AOI510,
has Rds of <.004ohm at 4.5V gate voltage
6A x 6A x .004 = 0.144W, much cooler, and gate can be driven from Arduino thru 270 ohm resistor, with 10K pulldown to Gnd to keep it off during Reset.
Yeah, took me a bit to note that it was an "A" oh eye 510 (and not A oh 1510). Looks like a nice part - added to my list of next order from Digikey ($15 for 25 of them).
thanks for the guidance guys!
CrossRoads:
You can't "overpower" the strip. Each group of 3 LEDs has a current limit resistor so you could connect to +12V/Gnd direcly and have it work correctly.
The MOSFET you selected has pretty high Rds,
http://www.st.com/web/en/resource/technical/document/datasheet/CD00003615.pdf
Power dissipated by the MOSFET is IIR, so 6A x 6A x .032ohm = 1.15W , it will be warm.
A better MOSFET, AOI510,
http://aosmd.com/res/data_sheets/AOI510.pdf
has Rds of <.004ohm at 4.5V gate voltage
6A x 6A x .004 = 0.144W, much cooler, and gate can be driven from Arduino thru 270 ohm resistor, with 10K pulldown to Gnd to keep it off during Reset.
so i have ordered 3 of these mosfets and and went ahead an orded 3 (270 ohm) resistors. the resistor part math still confuses me a bit because most video tutorial using mostfets i have watched didnt bother using resistors. how did you arrive at a 270 ohm resistor being appropriate for this setup?
also, i am using a 12v, 30A power supply(plan on controlling 3 strips total) but i want to get the hang of using one strip flawlessly first before i connect the other 2.
Hi,
Cross posting, makes t very difficult to get solutions across.
http://forum.arduino.cc/index.php?topic=388931.0
Tom.. 
The 270 ohm resistor is there because the gate of a MOSFET can be viewed as a capacitor and when the Aruino turns on or off, it has to charge/discharge the capacitor. The 270 ohm resistor in series with the gate limits the current into the "capacitor" (gate) to a safe level for the Arduino. It will also slow down the turn on/off times of the MOSFET, but for your use, that is not an issue. If you take 5v (output of Arduino in a perfect world) and divide that by 20 ma (max current sink/source) you arrive at 250 ohms. He picked 270 which is a standard value.