Looks like I killed a mosfet today, its an irf3205 http://www.irf.com/product-info/datasheets/data/irf3205.pdf
The dc motor always goes on as soon as +12v and GND for the motor are connected, no matter if the gate is connected or not.
This mosfet wasn't my first choice but the shop I was visiting was out of stock on the TTL mosfet I originally wanted. A quick google turned up the irf3205...but I missed that VGSon = 10v.
I was driving it via a bc547 from a avr a short test with a 35ma 5v motor worked even without the bc547 so I connected it directly.... there's a shottky diode in parrallel to the dc motor to kill spikes after shutdown.
The pump should take about 2a(found different sources for that) when running and of course a lot more when it starts.
I'm missing a RDSon graphic in the datasheet for different VGS voltages - shouldnt there be one?
The on time for the pump was at most for 30s...the avr code still got some hickups. Could this be enough with the not so good VGSon value to kill the mosfet?