Hi, I have been using the following setup to power and control 12V led strip through my Arduino Uno R3:
Everything was fine when I used a short strip, but now I wanted to try and control a bit longer (5m = 14.4W/m) 2 pin led strip.
When I connect everything up and power on (12V, 5A source), the led strip is noticeably dimmer than it would be powered straight from the source. It's got red and blue lights mixed and there is no clear pattern, but a few leds in a row would be brighter, then like 3-4 dim, then some brighter and so on.
The MOSFET was also very hot. Burned my finger instantly upon touch.
Please advise.
The picture doesn't show an UNO. How is the controller powered?
What dimension of the longer LED cables are You using? Maybe to thin cables.
Yes, the pictured scheme is the same except the version of Arduino, which is Arduino Uno 3 in my case, powered through laptop's USB port.
My first thought was thin cables too, but I use the same 10cm long thin plug cables used for breadboards to power them straight from the power source and they get way brighter.
Also hot? What else was hot?
What model of MOSFET? If it is not a logic-level mosfet, that could be the reason. Try something like IRLZ44 or STP16NF30L STP36NF06L.
That does not make any sense.
5m x 14.4W/m = 72W
72W / 12V = 6A
Your 5A PSU is not enough.
Add a 180 Ohm resistor in the yellow wire path.
USB powering is okey...
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Wrong FET! 
Also hot? What else was hot?
I'm sorry I worded that wrong. I meant that MOSFET was hot in addition to LED strip being dim. Thought maybe there would be a connection between those two.
And yes, the LED strip wattage is 72W and my power source is 60W, but powering it directly the brightness is sufficient.
What model of MOSFET? If it is not a logic-level mosfet, that could be the reason. Try something like IRLZ44 or STP16NF30L.
It's IRF520 that I'm using. I'll see if I can get my hands on any of your suggested ones and try that.
Add a 180 Ohm resistor in the yellow wire path.
I assume this is meant to alleviate the heat but not brightness of LEDs, right?
It's very bad choice.
That resistor won't help with heat or affect brightness. It's to protect the Arduino pin from excess current. In practice, unless the MOSFET had a very high gate capacitance and you use a very high frequency of PWM, I don't think there is much danger to to pin, but if you have a resistor, add it anyway.
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It's very bad choice.
What's a good one for this?
I can't seem to find IRLZ44 or STP16NF30L locally and I don't want to wait for one to get shipped. But I see electronics shops have a bunch of other MOSFETs. What do I look for in terms of specs?
Sorry! Should be STP36NF06L. I should double check before I hit reply. I will fix above.
You should find IRLZ44 easily.
A low on-resistance between source and drain (Rds) when there's 5V from an Arduino pin across gate and source (Vgs).
For example:
Here, you can see Rds is less than 0.03 ohms when Vgs is 5V.
The lower that Rds figure, the less power/heat will be dissipated in the MOSFET.
Remember, power P = I²R
If the current flowing through the mosfet and led strip is 6A,
P = 6² x 0.028 = 1W (slightly warm...)
Looking at your MOSFET:
The Rds is far higher, even with 10V on the gate. It will be higher still at 5V, maybe much higher.
P = 6² x 0.27 = 9.7W (ouch! hot hot hot!)
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You could put a heatsink on it (a large one). That is why it has that tab with a hole - to bolt it to a heatsink. It is not necessarily supposed to be used without a heatsink unless severely underrated - which you @eightvs have not managed to do.
But your other problem is that V = 6 x 0.27 = 1.6 V so your LED strip has dropped to 10.4 V with that 10 V on the gate. Now because LED strips have a threshold voltage, the voltage you supply is relatively critical so that would result in the dimming.
OK, 6 Amps, 5 V on the gate - no, you clearly will not get 6 Amps. You might get 2 Amps. The FET is now operating for you as a constant-current driver. 
There's another lesson there. LED strips need to be powered by a regulated power supply. Fortunately, most mains supplies now used, are but for example, batteries are not.
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The word "logic level gate" is a dead giveaway in the spec sheet.
Maybe, but your power supply is overloaded. It could fail suddenly or only have a short life. It could catch fire, I'm not joking. For safety and long life, you should not exceed around 80% of the PSU's capacity. Limit your strip's power by shortening it to around 3.3m or replace the PSU with a 7.5A, 90W model or better.
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Thanks everyone for good advices! I took them and got a 100W power supply and replaced IRF520 with IRLZ44N.
The LED strip got a bit brighter, but still not as bright as it is when powered directly through power supply 
So what is the voltage drop?
I didn't get the chance to measure, but I rewired everything onto a bigger breadboard and it seems I now get same brightness as I would directly from power source. I previously used the little 170 points board and everything got a bit cramped.
Maybe it's a bit less bright as it is when powered directly, but it might be just my brain playing tricks as it's hard to tell if the brightness is exactly same.
Anyways thanks for the suggestions everyone! I've learned a lot.
I'm surprised changing the breadboard helped with the brightness problem. Breadboards are not designed for high currents. 1A is really the most you should have flowing through them. Your strip pulls 6A! I'm kinda surprised your old and new breadboards have not melted!
Or should that be not surprised?
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