The trigger switch of an AEG work just like one of those old throw switches that you used in science class or saw on cartoons (where a bar is essentially pushed between two live contacts to complete the circuit).
Both of those fins are live...one connects to the motor, one to the battery. When the switch block (the plastic block with the stub of metal in it) gets pushed between, it completes the circuit and the motor receives power.
Now every time the block is inserted...power arcs between the fins and the block...until a solid connection is made (then it flows through just like a solid wire). Same when the block is kicked out of position by the cut-off lever...it arcs just as it breaks connection.
So...semi-auto, where the block goes in and out for every shot, leads to more arcing than full auto.
And...big batteries (i.e. 9.6v, 11.1v, large cells, high discharge rates, etc...) will also cause more arcing.
All that arcing leave scorching/burning/corrosion/oxidization on both the fins and the block. It also "vaporises" the surface. Both of these make the situation worse and worse...and less power gets to the motor, more arcing because the contact surfaces are less and less pristine.
So...eventually you burn out your switch. You can simply pull out the old one and install a new one (typically with the whole wiring harness intact). Contact Jugglez (Airsoftparts.ca) and ask him for an AK one.
A mosfet is a circuit that "replaces" the switch. The switch is still there, but basically the mosfet acts as a gate to dump power to the motor. The MOSFET senses a very small electrical current through the switch (when you pull the trigger and push the block inbetween the fins). When it senses it, it dumps power directly from the battery to the motor. The "gate" wires don't have to carry much current...so they are typically smaller than the motor/battery wires. When the switch is disconnected, the MOSFET stops/cuts the flow of electricty to the motor. So in effect, your switch lasts forever....there's so little current/voltage going through it that doesn't burn out.
The MOSFET obviously has to be able to handle the power throughput...and turn on/off fast enough over and over so your AEG shoots properly. Just because it says MOSFET...doesn't mean that all MOSFETs are created equally. Take a look at extreme-fire.com...it's the basis of what DonP cooks up for us Canucks. Don's a good guy too...knows tons more about this stuff than I ever will.
Wordy explanation...but hey, it's lunch time.
Hope that helps,
Last edited by m102404; June 11th, 2009 at 12:37..