I'm gonna start to excuse me for my bad soldering/layout, but this is 
I am using a mosfets (IRF520) to which I have connected a 12V power source to the drain and source. They are connected up to a led strip. The gate I am controlling with my arduino MKR1000 (3.3v outputs). The circuit does exactly what I expect from it except for one thing: the leds dont burn as bright as I would expect. When I connect a 12v input to the gate they do burn as bright as I expect. My guess is that it is because the voltage I put on the gate is too low, and the mosfets are acting as a variable resistance.
The store I bought them though tells me that they should burn as bright with 3.3v as with 12v. Am I doing something wrong, or am I using the wrong mosfets?
Schematic I used:
Link to datasheet:
https://www.vishay.com/docs/91017/91017.pdf
Best Answer
I think your Vgs is too low. You could easily check this by connecting the gate to VUSB or even the 12V as you did. I used IRF520 with 3.3V logic and had to use a higher voltage transistor coupling to the gate to increase Vgs.