AM has a good tutorial on MOSFETS. If you need any help installing them or do not understand the tutorial then you can PM and I will help you out however I can.
MOSFETS are quite simple really, you have a Gate, Source, and a Drain. The gate is what operates the MOSFET, in N-Channel Mosfets, applying voltage to the Gate turns ON when there is a positive low voltage applied to it, the Source is where current flows into the MOSFET and the Drain is where current leaves the MOSFET. Most MOSFETS are voltage driven, however some are more current driven.
In either case the failure of the gun to operate is RARELY the MOSFET, an IRF1404PBF-ND N-Channel MOSFET (http://search.digike...e=IRF1404PBF-ND
) can supply 202Amps of current @40v, that is what I use in my KM16BR. An airsoft gun is never going to see even 25% of those loads. So let me say running dual fets is a waste of power (MOSFETS consume small amounts of power to operate) unless they are using very very cheap MOSFETs or trying to use low power MOSFETS.
By far those who do not get the MOSFET to operate properly or even those who experience issues with PTW's not operating correctly is because of an inadequate power source. When you pull the trigger the battery is going to experience its most severe load due to the simple nature of brushed DC motor operation, its a linear relationship where the higher the load is, the higher the torque will be and the lower the RPM until the motor stalls or reaches maximum load. Recall that torque = current and voltage = RPM in any DC motor. So as the motor is required to put out more torque it will require more current. Your battery does the same except with voltage, and current load instead of RPM and torque load. The higher the current load the battery the lower the voltage will drop, thus if the battery is inadequite for its application when you pull the trigger your battery will not be able to maintian enough voltage to fully open the gate or even open it at all. I experience this even with an 11.1v LiPo battery and my IRF1404, but only when the battery has expended all of its stored power, aka its dead. I have had 100% reliability with my MOSFET when it has adquite power for operation. All you need is some 22awg wire, a 30k ohm resistor and a 100 ohm resistor(1/8watt power rated is fine). Follow the above tutorial and you will not have to worry about the damaging effects of arching on the trigger contacts. Using a capacitor on the gate may also be a way to overcome the initial voltage sag if your battery is border line in regards to supplying ample voltage under load, something small like a 150mF cap may work depending on the the gate requirements of your particular MOSFET. Stay away from cheap NiMH and NiCD in general for power sources with MOSFETS. They often can barely supply enough current, a good NiMH cell can supply around 35 amps continuous, most cheap NiMH cell's i would estimate around 15~20, most AEG's pull around 15~25 amps depending on motor, spring, gearing. My KM16BR pulling over 20 on semi-auto as I was blowing 20amp fuses occasionally. You can expect current spikes, so its best to prepare for them!
Your ROF will be slightly higher as well depending the MOSFET you use because all MOSFETS have some capacitance.
Edited by WiN&LeGeNd, 19 March 2009 - 08:40 AM.