Free-Wheeling Diode
 

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.

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.

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;

http://upload.wikimedia.org/math/8/d...2d87fba6f9.png

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

You'll definitely also want to pose this on Airsoft Mechanics.

Its a shame my refrigeration license doesn't let me come up with stuff like this for my AEG's :(

Most modern MOSFETs already use a diode (built in the chip 99% of the time).
I also add a large one to prevent damage to the FET when I build some.

Now for trigger components, I guess it would work, but at the same time, people want some active braking because the collapsing EMF is not enough to slow the motor/gears in time. Adding a diode in a normal switch-based circuit would amplify the issue a little.

I have to redo the wires in my P90. I will see if it makes an appreciable difference with/without the diode.

The issue isnt solely the arcing. Another issue that requires the mosfet is over heating.
Especially common with 9.6v large nimh and 11.1v lipos, sustained or even rapid burst fire can cause the trigger block itself to melt or deform. This is solved by using the trigger contacts as a relay control only and having the high current go through the mosfet. This also inherently reduces resistance since our mechanical triggers offer so much resistance.

This might be a good solution for aegs with microswitches though since they are also prone to arcing but cant use mosfets due to bounce.

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


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.

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.

I should also add that the diode will effectively eliminate the cemf. Considering the motor type, cycle rate, and limited power supply, I am not so sure that is a good idea. Most AEGs take ridiculously small batteries, or use crappy lipos.

I think I am just going to forget the diode idea, and do a DIY mosfet.

Any thoughts on adding a braking mosfet, think it adds stress to the motor?

If a braking mosfet does add moor stress, is it worth the trade off of less gearbox stress?

Active Braking increases stress on the motor and mechbox.
The gears have to stop faster and the motor overheats way faster in semi.
However that being said, it doesnt create a LOT of extra wear, but of you dont need active braking then dont use it.
Typically if you have a neo magnet motor you dont need ab.