MOSFET Switches

For you gear head's out there here is an excellent article that explains what MOSFET is all about and how to install it.

http://www.airsoftmechanics.com/guides.php?aid=14

This is your normal trigger wiring:


This is your trigger wiring on MOSFET:



Any questions?

-Doug

I've seen this thread from Kos-mos and I think he's full of CRAP. He says his FETs are rated for 200+ amps peak and 110 amps continuous. In a TO-220 case size (as is the one above), I have never seen a FET (especially a MOSFET) rated any higher than 65 amps continuous and 130 amps peak, and that's with a heatsink (not present in his pics, is it). Without a heatsink, cut those numbers by 2/3, and you right on the money.

Having dealt with FETS in airsoft guns in the past (my Systema PTW has 2 50A in paralell), I can tell you that they can be problematic with low battery voltage and do tend to get destroyed. They are only for the seriously well-engineered guns and for the VERY experienced installer/user.

He has not put any pictures of his available MOSFET's up, just a comparison one from WGC (which does have a heat sink under the heat shrink.

http://www.airsoftmechanics.com/guides.php?aid=14
Is an excellent article that explains in great detail exactly what is happen when you install (ROF increase) and how to install.

Now, correct me if I am wrong but:
http://rocky.digikey.com/WebLib/InternationalRectifier/Web%20Data/IRF3703.pdf
is a MOSFET rated for 210amp continious drain @ 10V, 25ºC and 100amp @ 100ºC.

Now, SystemA PTW uses a 7.2v battery as a standard (higher is available). This is going on a 10.8v battery. The reports from FilAirsoft.com (they are the ones you think 500fps is slow) say that they have few issues with the MOSFET if the proper power dispersal is used (meaning use the IRF3703).

I am not sure exactly what MOSFET is being sold by KOS-MOS, but there are small sized MOSFET's available that do go as high, and higher, than what he is advertising.

-Doug

The temperature rating is the Junction temperature. As you use it, it's temperature will rise. They will always list a junction thermal limit in their specs. This is the temperature at which the juunction will be destroyed, rendering the FETs either open or closed.

Heatsinking delays this, or prevents it if thermal dissipation by the heatsink is greater than thermal build-up by the FETs.

You have to think of FETs like part switch, part diode and part resistor, all rolled up into one package. There are better FETs to use than MOSFETs, that will have higher current ratings, but their configurations, switching type (voltage vs. current gated), cost and availability.

MOSFETs for my PTW (50NO3) cost me $20 each (x2), plus with cost of labour to build, $40 seems to me a little cheap to be using rather exotic parts.