mrmu wrote:2) How does the MOSFET N-channel 'complete the circuit' that allows the pump to turn on?
Quite nicely, thanks. ;-)
The detailed answer exceeds the Recommended Daily Allowance of Quantum Physics for the average adult, but at a human level, the mosfet is basically a drawbridge for electrons.
Electronics deals with the movement of 'charge', which usually means 'electrons' but can technically be any particle that carries an electrical charge.. in electroplating tanks, the moving charges are actual metal atoms drifting through the liquid.
Voltage is the force that makes charged particles move. The amount of energy available to do work is a combination of how many partices you have and how fast they're moving, and is called 'current'. Obstacles that prevent current from flowing are called 'resistance'. Voltage pushes current through resistance, and when current meets resistance you get voltage. If you know any two, you can work out the third according to Ohm's Law.
Mosfets are made from silicon, which is.. complicated.. all "conductance and valence bands", "localization", "dopants" and the like. The upshot is that you can change the resistance in a piece of silicon by applying voltage to it.
A mosfet is arranged so the resistance is really high (several million ohms) in its natural state, but drops to only a few ohms (or thousandths of an ohm for power mosfets) when you apply voltage to it. Current flows between a mosfet's 'source' and 'drain', and the amount of current that flows between them depends on the voltage at the 'gate'.
So.. when a mosfet's drain-source resistance is high, it's much larger than the resistance of anything else on that path between the battery's positive and negative terminals. We say the mosfet's resistance 'dominates' the behavior of that path. When the mosfet's resistance is low, it's much smaller than the resistances of anything else on that path, so the other compoents dominate the path behavior.
To control the speed of the pump, you use a technique called 'pulse width modulation' or PWM.
Current measures the amount of charge that goes from here to there in a certain amount of time. If you let current flow freely half the time and shut it off completely half the time, the average current over time will only be half what it was if you didn't shut it off.
That principle extends all the way down. If your unimpeded current rate is 1 Amp, but you only open the switch for 1 microsecond per second, the average current per second will be a microamp. Over the long term, the on/off switch acts like a resistor (1 million ohms in this case) but doesn't waste energy as heat like a normal resistor does.
A mosfet makes a good on/off switch, and you can set its effective resistance by changing the relative amounts of on-time and off-time. The part where the switch is open and current can flow is called a 'pulse', and you control ('modulate') the current by changing the width of the pulse.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.