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.
A windshield washer pump is turned on by the mosfet, connected like in this circuit (the part to the right of r2):
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?
Best Answer
So, based on your comment, and your as-yet-unknown circuit that isn't the one you posted a picture of, even if it's similar on the right side, you discovered the joys of gate charge on MOSFETs; where not connecting the gate can leave you with an active, conducting MOSFET since there happens to be voltage on the gate, and being a MOSFET, that voltage isn't going anywhere in a hurry (leakage might pull it down over a few weeks, or sooner if you put your finger on it.)
I strongly suggest some active playing with a simple MOSFET - resistor - LED circuit if you don't have this one well-ingrained in your head.
simulate this circuit – Schematic created using CircuitLab
Connect the gate, briefly, to + or - and then disconnect. Observe behavior. Turn power supply off, go to lunch, come back, turn power supply on, observe behavior. Learn that you need to actively shut a MOSFET off, as well as turn it on. The above circuit is assuming the more-common enhancement mode MOSFET which is off when the Gate-Source voltage is 0V.
The good news would be that you evidently haven't killed your MOSFET, and perhaps you learned something, too.