[mcguyver]

Originally Posted by Pointy

I understand using a mosfet to protect the switch contacts when pulling a higher current, or to allow fancy firing control (3-round burst etc). In effect the mosfet is being used as a relay.

Yes, it is essentially a solid state relay, no moving parts, no arc formation during closure.

My understanding is the cause of arcing at the switch is not related to the closing of the switch, but it’s opening. There is insufficient voltage to cause a breakdown of the air and cause arcing during a switch closing. When opening a switch on a inductive load you can see very high voltages, which causes the arcing.

Your understanding is incorrect. Arc formation occurs at both openning and closing, and is larger at closing. The motor inrush current of an AEG motor is something like 25 amps (just a number used to express a relationship, and it may in fact be something else), and that inrush is always 25 amps, no matter what. The motor is always drawing that from the battery, whenever is turning, and even when it is not but still receiving voltage and current. Wen it is turning at it's full speed, it now becomes a generator, and creates counter EMF (cemf for short) that produces upwards of 70% of the inrush current, so in this case let's say 18 amps. That cemf is 180 degrees out of phase with the inrush current, and it cancels out 18 amps of it. So the net current draw at motor running would be 7 amps. If the gearbox jammed up, and the armature of the motor was locked and could not turn, that motor would draw its full inrush current, until younblew a fuse or another weak point due to resistance presented itself and you end up with a burnt motor, selector, switch, wiring, battery, etc.

So, arc formation is larger at closure that at openning.

A motor is an inductive load (ie has a magnetic field) and when opening the switch contacts the energy within the magnetic field needs to be dissipated somewhere, hence arching at the switch. See the below formula;



v=volts
L=inductance
di=change in current
dt=change in time

Based on a everything being equal, a small change in time (ie opening the switch) will result in a high voltage spike and this high voltage leads to arching across the switch contacts.

I have been involved in industrial electrical engineering for ~13 years, and we always install a free-wheeling diode across relay coils (another inductive load) on DC circuits. A free-wheeling diode should work the same on a DC motor, and I believe this would be a simpler fix then installing a mosfet circuit.

I have searched google and have found no mention of installing just a free-wheeling diodes on a AEG. Has anyone consider or installed just a free-wheeling diode?

Reference links;

http://en.wikipedia.org/wiki/Flyback_diode
http://en.wikipedia.org/wiki/Inductance

The inductive spike that the armature could generate is expressed above as a function of time. When you compare the openning speed of an AEG switch to a motor starter or other contactor, they are not even close. Then, compare the materials used for contacts and an AEG uses at best a plated copper contact block and plated copper fingers that are closed by the contact block. Arc formation does occur, always will occur in this scenario. The best bet is to have materials that will handle it, or use a FET. The FET will act as a switch, a diode and a resistor, with some control systems using the resistive properties to effect dynamic braking on the motor.