jars121 wrote:On further research, it appears that the maximum Gate voltage for the above listed N-channel MOSFET is 2.5V...Seeing as it's receiving a 3.3V Gate signal when GPIO4 is set to High, I'm guessing it won't work?
Strange, seeing as this MOSFET is designed for use with the Arduino, which operates at similiar voltages I thought.
The maximum gate voltage of 2.5V just means that all NTD5867NL MOSFETs will turn on with Vgs >= 2.5V at 25C. Typically, they'll turn on at 1.8V. Some will turn on as low as 1.5V. The actual voltage Vgs you can apply can be much greater than 2.5V -- it can go as high as 20V continuous.
In fact, if you look at the NTD5867NL data sheet
many of the curves are for Vgs = 10. In a MOSFET, the Vgs threshold is when you first get conductivity between drain and source, and near Vgs it can be very small, i.e., the on-resistance may be quite high. Take a look at Figure 3: "On−Resistance vs. Gate−to−Source Voltage". Below 4.0V, the on-resistance from drain to source rises exponentially and Vgs = 3.3V isn't even on the chart. This MOSFET will work OK when driven by a 5V GPIO like Arduino, but RasPi's 3.3V is not going to turn it on very far. You're probably better off with an NPN Darlington pair.
From the way you describe your hook-up, it sounds like your transistor may have shorted out. I'd try playing with the module separate from RasPi and drive the gate through a 10K resistor pulled up to your 12V supply. Since the module has a 10K pull-down, that will give you about 6V on the gate which should turn it on nicely.
If the transistor is burned out, one possibility that you need a flyback diode
across the relay. The NTD5867NL appears to have a built-in zener so you may not need a flyback diode, but with most relay and motor circuits you do.