MOSFET- Friend or Foe?

stephenshields2

Sorry lads, but who wants to explain MOSFET to me??



I did take the tame to try and explain it to myself via Wikipedia, but it dont make no sense!

drifterex

Far as I'm aware, mosfet is a very fancy fuse, which limits voltage going into your motor and in turn your gear box, and as far as I know they help with the issue of a spring being compressed in your gearbox if it hasn't fully cycled. Sposed to help with wear and tear.

Actually, have a look at this, much better explanation...
http://extreme-fire.com/faq

hitman__

stephenshields2 wrote: »

Sorry lads, but who wants to explain MOSFET to me??



I did take the tame to try and explain it to myself via Wikipedia, but it dont make no sense!

Well, I'm sure there are better explanations but here goes.
(1) Without a MOSFET, all the power from your battery passes through the trigger connections to the motor when you fire the gun.
This causes wear and early failure on the trigger contacts.
(2) The MOSFET is like an electronic high power switch, connected between your battery and motor. The trigger is now used to turn the MOSFET on, which only requires a tiny amount of power, the big motor power now travels through the MOSFET from the battery thus greatly reducing stress/wear on the trigger contacts.

Here's how to make a basic one if you have electronic skills.
http://www.airsoftcanada.com/showthread.php?t=89097

Hope that makes sense.
E.

extremetaz

Right - hitman_ has the function right, as to what it is:

A MOSFET is a Metal Oxide Semiconductor Field Effect Transistor.

In short - it's a transistor, and in the context of airsoft devices, it's a transistor that can pass a helluvalotta current. There's little point going into much more detail on the device itself beyond that.

The benefits are that it can pass more current, more cleanly than the types of switches used in airsoft kit, and that it won't wear out or get dicky.

In the most basic config you just use one in place to deliver charge to the motor without having it go through the switch.

In the more elaborate "active braking" setup, you use two of them with opposing chemistries (a P type and an Ntype) and in opposition in order to stop the motor dead as soon as the trigger cycle is complete.

stephenshields2

Very good answer there Extremetaz, that gives me a decent idea of what it is!

Wish they'd just call it a "big elaborate transisitor", that has a much better sound to it!

bullets

Have been looking at 3 Round burst ones and others with active breaking, the last day or so on the web they are selling for between 85-125 USD which I find crazy price for two mosfets worth 1.75 each a diode and a resistor or two.

I know nothing about electronics which is shameful considering I've a HND
in electronic engineering and work in the design dept of an IC company!
At some stage I may try and get my hands on the components and fumble my way through designing my own tiny PCB board with the software in work so everything fits into a fuse container sized package.

Ideally one with active breaking, 3 round bust and the ability to switch via a jumper or something from 3 round to auto with the trigger kept pulled would be nice. (So your not sacrificing one fire mode for another your keeping all)

Not sure how 3 round burst works. Heard depending on what you've got
on your gun battery wise or wiring etc and gearbox you may get 2 round burst, or 5 round as it operated on Time delay or something ?
Can anyone explain in nice simple terms ?

~B

Stercus Accidit

Burst fire was timed iirc from KD having the extreme fire model.

extremetaz

bullets wrote: »

Not sure how 3 round burst works. Heard depending on what you've got
on your gun battery wise or wiring etc and gearbox you may get 2 round burst, or 5 round as it operated on Time delay or something ?
Can anyone explain in nice simple terms ?

~B

Easiest way to do it would be by timing but naturally this could very easily lead to inconsistencies accross multiple platforms. You could overcome that by using a variable resistor in the timing circuit so that the user could tune the device though.

There's litterally hundreds of ways of generating a .25s pulse (and anything around that) but a 555 timer is probably the most obvious - with a bit of work though you could probably get the whole thing working with a resistor, a capacitor and a comparator (op-amp) though.


The more robust (and also more complicated) way of doing it would be to use a microcontroller with an ADC (Analog to Digital Converter) function - have it take a reference off the peak voltage and watch for deviation and return within a given threshold (you'll always get a voltage drop when you load the motor). Sounds harder than it is - something like a PicAXE 08 would do the job fine though, is easy to program, cheap to buy, and has all the necessary functions.

kdouglas

bullets wrote: »

Have been looking at 3 Round burst ones and others with active breaking, the last day or so on the web they are selling for between 85-125 USD which I find crazy price for two mosfets worth 1.75 each a diode and a resistor or two.

The extreme-fire mosfets, and their clones, are quite a bit more complicated than a couple of cheap mosfets, even the mosfets alone which are normally used for airsoft are more than 1.75 a piece, unless you buy in ridiculously high quantities (1000+). There are cheaper variants available of course, but they normally can't handle the same current and hence why the more expensive/higher current ones are used.

Anyway, the extreme-fire one is a small PIC based microcontroller, associated circuitry and dual mosfets (for active braking)

The programming in the microcontroller allows for some fancy things like automatic adjustment of 3 round burst timing to allow for battery size/voltage and charge state, along with motor speed etc... etc... it's probably the best way of building a 3 round burst module since straightforward timing is difficult to keep consistent across multiple different AEGs and normally requires a manual adjustment.

It also has a menu-drive interface, using the trigger to select options and giving feedback by vibrating the AEG's motor, also it warned you when your battery was running low and could be used to adjust your rate of fire.

The microcontroller can also be programmed to do plenty of other fancy things, not sure what other ones have been done but I know someone did make a lipo add-on board for it that made use of lipo batteries much safer.

They may seem expensive at first look, but they are definitely worth the money.

damagegt

kdouglas wrote: »

The extreme-fire mosfets, and their clones, are quite a bit more complicated than a couple of cheap mosfets, even the mosfets alone which are normally used for airsoft are more than 1.75 a piece, unless you buy in ridiculously high quantities (1000+). There are cheaper variants available of course, but they normally can't handle the same current and hence why the more expensive/higher current ones are used.

Anyway, the extreme-fire one is a small PIC based microcontroller, associated circuitry and dual mosfets (for active braking)

The programming in the microcontroller allows for some fancy things like automatic adjustment of 3 round burst timing to allow for battery size/voltage and charge state, along with motor speed etc... etc... it's probably the best way of building a 3 round burst module since straightforward timing is difficult to keep consistent across multiple different AEGs and normally requires a manual adjustment.

It also has a menu-drive interface, using the trigger to select options and giving feedback by vibrating the AEG's motor, also it warned you when your battery was running low and could be used to adjust your rate of fire.

The microcontroller can also be programmed to do plenty of other fancy things, not sure what other ones have been done but I know someone did make a lipo add-on board for it that made use of lipo batteries much safer.

They may seem expensive at first look, but they are definitely worth the money.

I hoping to pick 2 of these up but as much as i would like to think i could install them myself i dont want to make a balls of it.Is there anyone around that does installs of the SW-CHEETAH and connector changes to dean's and any idea of the cost of install?

kdouglas

I used to do it, but I don't really have the time at the moment, Richie in Eirsoft should be able to look after you no problem.

Beez

Theres a good thread on Mosfets over at arnies airsoft forum.

http://www.arniesairsoft.co.uk/forums/index.php?showtopic=161764

Big_G

damagegt wrote: »

I hoping to pick 2 of these up but as much as i would like to think i could install them myself i dont want to make a balls of it.Is there anyone around that does installs of the SW-CHEETAH and connector changes to dean's and any idea of the cost of install?

If its anything like a mosfet install its easy. I did one without any electronic knowledge whatsoever. I used infected airsoft's soldering primer and rewired my g36c with a low resistance setup including deans plugs and a mosfet. Wasn't hard. Shimming and gearbox work is more difficult, IMO.