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Ive just seen the graph on page 4, at the top left, this seems to explain the characteristics better than the rest of the datasheet.
I'm using a 14.4V DeWalt drill battery (about 15V fully charged) for the circuit. I have +5V going from a Voltage regulator to the gate and the full 15V going to the drain.
do you also have the GND from the voltage regulator connected to the source?
No wonder it doesn't work!You have to connect the source to ground and put the fan between drain and +15V, since the gate has to be positive with respect to source.
If you use P-channel for the high side devices and use eg. an NPN transistor for driving the gate (which has to be negative w. resp.to the source, which in this case should be connected to the positive supply), you can drive it like the N-channel devices.
I just looked at some random P-Channel MOSFET Datasheet and it said the Vgs(th) is -0.8V to -2V. Does this mean that the gate actually needs negative less than -0.8 to allow current from source to drain or is it that it has to be -0.8V or less relative to the source?
If its the relative to source way, I think I understand how the NPN transistor may fit in: pull-up resistor to the motor + and P-Channel Gate with the NPN between P-Channel Gate and Ground, maybe?