DussanR:
but what i mean is if my power supply is 12V at 2A. there's no chance of it damaging the circuit? it will only draw the 1.1 to work for the whole circuit?
No there isn't.
Look at a car battery, it is rated to supply over 100A to turn the starter motor, yet you can still just run the interior 1/2A light off it on its own.
The power supply delivers constant output volts until the current being drawn by the load, meets and exceeds its rated current rating.
Tom... 
Hey guys, so i found out the tip120 gets quite hot! i read is somewhat normal. that i could use 2 in parallel or a heatsink. even use a mosfet? i am going to read up about mosfets, but wanted to know what is your opinion.
thanks!
Yes, then you need a logic level n-channel MOSFET, e.g. the IRL504.
Drain to the coil; source to GND; 10k pull-down resistor between gate and GND, small resistor (100 Ohm or so) between gate and Arduino pin. No need for optocoupler.
Shouldn't be producing any heat that way.
Forget about transistor gain. It has nothing to do with saturating a transistor switch.
It is clearly stated in the TIP120 datasheet that for saturation a base:collector current ratio of 1:250 should be used.
For a collector current of 1100mA, you should use a base current of about 1100/250 = 4.4mA.
BE voltage at that current is about 1.5volt (saturation graph).
And yes, the TIP120 gets hot with 1.1A collector current.
Saturation voltage is about 0.7volt at 1.1A, so 0.7*1.1 = 0.77watt is dissipated.
It could easilly get >50C without a heatsink. That's why we all use mosfets.
Leo..
hey everyone! sorry to keep on posting, but where i live i couldn't find any logic level n channel mosfets. Although i came across a popular one, the IRFZ46N. Its not logic level, but from the datasheet i seem to understand that i can actually keep a similar circuit i was using with the TIP120 (using the optocoupler). I just replace it and use the 10k resistor between gate and source.
is this correct?
Thanks for everything!
The IRFZ46N is not logic level.
It will pe partially 'on' with 5volt logic, and could also get hot.
Leo..
I know, but in my circuit iām using the 12V of my PS on the transistor! I wanted to know if this would work just by replacing my TIP120 and adding the 10k resistor.
Voltage on the drain (supply for the load) is not relevant.
This fet needs a gate drive voltage of 10volt, which the Arduino (5volt) can't supply.
Leo..
please take a look on the first picture i posted. i'm using a circuit based on that picture, i'm using the 12V of the PS for the load and the same 12V goes to the optocoupler, and then to the gate for the 12Vgs. I'm wondering if this would work!
Ahhh, yes.
Thought you were using direct drive from an Arduino pin, but you're using an opto coupler and a 12volt supply.
Yes, that will work for this mosfet, since the gate now gets 12volt drive.
Leo..
great, thanks!!
last thing, there's no problem using 12V instead instead of 10 for the gate?
Not a problem, as long as you stay under the 20volt absolute max for the gate (see datasheet).
Leo..
perfect, thanks!