MOSFET Power Control Kit

I'm wondering if I could get some help connecting my MOSFET Power Control Kit (http://www.sparkfun.com/products/10256) to my Arduino. Basically, for my application I'm trying to send a current through 12" of a Shape Memory Alloy wire (0.006" dia.) The data sheet on the SMA wire recommends that you should send 400mA of current through the wire to heat it up to its transition temperature to get it to contract. Since, a pin on the Arduino can only source 40mA, I figured I'm going to need to use an external power source to supply more power to the SMA's. I've got a 12V 1000mA regulated DC power source that I want to use and I figured I could use one of the digital pins on the Arduino to trigger when the MOSFET should send the power from the external power source over to the SMA wire. Hopefully, my logic is sound. Since this is my one of the first times I've used a little bit more power in one of my projects, I just want to make sure I set everything up properly so I don't burn anything out. Looking at the Power Control Kit break out board, it seems like what I need to do is to connect a wire from a digital pin on the Arduino to the C pin on the break out board. Then connect the + and - side of the power source to the other two pins on that side. Obviously the SMA wire ends will go on the device side. My question is, do I need a resistor in circuit with the power source to limit the amount of current? Using Ohm's law, it seems like if I really want 400mA current to flow through and I'm supplying 12V that I'll need a 30 Ohm resistor. Would this go inline on the high side of the external power supply? Also, is there any reason to be concerned that the Digital Pin wont be able to supply enough current to trigger the MOSFET? Sorry, if a lot of these are beginner level questions, but I would really appreciate any feedback before I test this out.

My question is, do I need a resistor in circuit with the power source to limit the amount of current? Using Ohm's law, it seems like if I really want 400mA current to flow through and I'm supplying 12V that I'll need a 30 Ohm resistor. Would this go inline on the high side of the external power supply? Also, is there any reason to be concerned that the Digital Pin wont be able to supply enough current to trigger the MOSFET? Sorry, if a lot of these are beginner level questions, but I would really appreciate any feedback before I test this out.

Well you gave us a spec of 400ma for the wire, but not what resistance the wire has at that current. The memory wire will 'drop' a specific amount of voltage and 400ma, and once you know what that voltage is you then can size the series resistor between the +12vdc and the wire. Resistor size will be calculated as (12vdc - wire drop voltage) /.400A

The mosfet will then just switch the other end of the wire to ground and current can then flow. Also by applying a pwm output to the mosfet gate you can control the amount of current going through the wire from 0ma up to the max of 400ma.

The Mosfet gate draw zero continous current, it only draws current charging and discharging the gate capacitance during transition from on to off and off to on. Generally a 300-500 ohm resistor between the output pin and the gate is all that is required to protect the output pin. Also be sure to wire between a arduino ground pin and the negitive terminal of your +12vdc supply.

Lefty Lefty

Well you gave us a spec of 400ma for the wire, but not what resistance the wire has at that current.

Thanks for the detailed reply. The technical info on the SMA wire says that the resistance is 1.4 Ohms/inch and I'm using a 12" piece so that equates to 16.8 (or roughly 17 ohms) resistance. I could be wrong, but if my calculation is correct... then the voltage drop at 400mA and 16.8 ohms resistance would be 6.72Volts (does that seem correct?). If it is, then using your equation (12vdc - wire drop voltage)/0.400A = 13.2 ohms. So, should I use a 13 ohm resistor between the +12vdc and the + pin on the MOSFET? (sorry if this is obvious)

Also, you mentioned using a PWM for the MOSFET control signal. I thought I was supposed to use a digital signal (HIGH/LOW) which would trigger the MOSFET to allow the 12vdc to flow through the wire... Is this not correct? If I use a PWM... does that mean I could send a PWM value of 0 to turn it off and 255 would be full bore... but I could get gradients in between? That would actually be great if so.... I just didn't know that was possible.

Also, I was a little confused by your last paragraph. The power supply I'm using is a version of a wall wart (http://www.adafruit.com/index.php?main_page=product_info&cPath=44&products_id=329). I also bought the barrel jack connector (http://www.adafruit.com/index.php?main_page=product_info&cPath=44&products_id=327) and thought that I one side of this connector would be power and the other side ground... I had thought I could hook up the side with the white stripe running down the side to the positive pin on the MOSFET and the other side to the negative pin on the MOSFET. You suggested that I run a wire from my ground pin on my arduino to the negative pin on the MOSFET instead. Does it matter which ground I use?

Thanks again