user7624: I'm using three PWM outputs from an Arduino Mini Pro to drive the R/G/B parts of some strips of LEDs. The circuit is basically this:

I've got a single 12V power supply, rated at 200W, to light the LEDs and to power the Arduino via the RAW pin, and there are 5 metres of 60LEDs/m RGB LEDs, i.e. a total of 300 LEDs, which should be drawing ~72W according to the spec sheet (14.4W/m).
The problem is that even when the sketch says digitalWrite(9, 255); digitalWrite(10, 255); digitalWrite(11,255), the LEDs are considerably dimmer than when connected directly to the 12V supply. My cheap, digital volt meter measures ~8.1V between GND and the PWM pins across the LEDs, not the 12V that the transformer is supplying.
Am I doing something wrong? Does the MOSFET really cause such a large drop in voltage? Is there anything I can do to mitigate it? (It's important to get as much brightness as possible from the LEDs for this project).

user10794: By my calculations you are sinking 6 amps:
72 W / 12 V = 6 A

I have some doubt that the Arduino board is designed to handle that amount of current through its ground plane. Try running a separate ground wire to the MOSFET sources.
I would also be concerned about the gauge of wire you are using, and whether the prototyping board can handle that amount of current.

7624: <p>I'm using three PWM outputs from an Arduino Mini Pro to drive the R/G/B parts of some strips of LEDs. The circuit is basically this:
<img src="https://i.stack.imgur.com/GWrfB.gif" alt="circuit diagram"></p>

<p>I've got a single 12V power supply, rated at 200W, to light the LEDs and to power the Arduino via the RAW pin, and there are 5 metres of 60LEDs/m RGB LEDs, i.e. a total of 300 LEDs, which should be drawing ~72W according to the spec sheet (14.4W/m).</p>

<p>The problem is that even when the sketch says <strong><code>digitalWrite(9, 255); digitalWrite(10, 255); digitalWrite(11,255)</code></strong>, the LEDs are considerably dimmer than when connected directly to the 12V supply. My cheap, digital volt meter measures ~8.1V <del>between GND and the PWM pins</del> across the LEDs, not the 12V that the transformer is supplying.</p>

<p>Am I doing something wrong? Does the MOSFET really cause such a large drop in voltage? Is there anything I can do to mitigate it? (It's important to get as much brightness as possible from the LEDs for this project).</p>
 10794: <p>By my calculations you are sinking 6 amps:</p>

<pre><code>72 W / 12 V = 6 A
</code></pre>

<p>I have some doubt that the Arduino board is designed to handle that amount of current through its ground plane. Try running a separate ground wire to the MOSFET sources.</p>

<p>I would also be concerned about the gauge of wire you are using, and whether the prototyping board can handle that amount of current.</p>
