Mosfets, Fets, AB, AEGs electronics, overspin, and other confusing concepts.

[nukeduster]

An AEG at its core is an electrical circuit. A battery is connected to one end, and a motor at the other, with a trigger in between. The trigger engages a switch which completes the circuit.

In a conventional gearbox, the circuit is completed by a moving part called the trigger trolley. It rides along with the physical motion of the trigger, and at the engagement point, completes the electrical circuit sending power to the motor.  In full auto mode, this circuit stays completed until you remove your finger from the trigger which breaks the circuit.

This is where a mosfet comes into play. This mechanical switching circuit has several inherent weaknesses. One, the switch carries the entire load of the electrical circuit through it every time, meaning you need heavy gauge wires going to and from the switch. Two, because it is carrying such a high amperage load, the current is such that it arcs, and actually causes bolts of electricity to jump between the switch contacts before they physically make contact.

Why is this arcing bad?

This is a new trigger trolley:



This is what happens to a trigger trolley once  it has been run with a lipo for any real amount of game time:



In fact, its probably the #1 thing that I fix in broken AEG’s, and its why I decided to start making mosfets. Here's just the handful I still have on my bench from the last couple of jobs:





Arcing literally is used in the industrial world as a method of welding. It will melt almost any metal given enough amperage. The kind of amperage that a lipo is all too capable of producing.



But why a mosfet? What does it improve?

The mosfet acts as a relay of sorts, except it has no moving parts. It carries all of the heavy current that normally would arc through the trigger contacts (over 30 amps in modified AEG’s) and instead only relies on a very small (it is so close to zero that some EE books assume it is zero to simplify some equations) signal to be sent through the trigger contacts instead, thusly preventing arcing and preserving your trigger for years to come.

AEG’s that say that they are “lipo ready” and that do not have a mosfet are just time bombs waiting to happen. It is NOT a matter of if, but a matter of when. If you’re lucky, it’ll simply stop conducting and when you pull the trigger the AEG will not fire. If you are unlucky, the current will arc weld the two pieces together and the gun will shoot at full auto until you unplug the battery.. this is not a fun occurrence, and can actually ruin a perfectly good lipo.





So if a mosfet is good, are active braking mosfets better?

The short answer is: no.

The long answer is a bit more complex, and basically people use them as a bandaid to fix a poorly tuned gearbox. In essence, the gearbox experiences overspin due to using too strong of a battery or motor with an inappropriately weak spring, without compensating for it in some way.

Active braking (AB) literally switches voltage sent to the motor from positive to negative the moment you release the trigger causing the motor to momentarily try to spin backwards. The effect of this is that the motor stops spinning (almost) immediately, preventing overspin.




Sounds great, right? Not so fast.



AB’ing comes at a cost. The cost is primarily on your AEG’s motor. Causing the motor to switch polarities suddenly puts an intense amount of strain on the brushes of your motor, and will shorten its life measurably. To some, this is a tradeoff worth having since it makes a gun work within the constraints they have set out for themselves, but in almost every circumstance, you are better off having your gearbox professionally tuned so that you don’t need AB in the first place.