Add MOSFET wiring to an M4 or M16

[Killbucket]

MOSFET's, 'FET's...
Questions about them?
Just what is a 'FET?
Why use them?

When any switch is closed or opened, there's an instant spark at the contacts. This is because power is trying to flow before the surfaces are completely together, and when the switch is opened, the current want to resist being interrupted. That spark is material burning. After a number of cycles, the condition worsens as the contact points erode and result in increased resistance. At some point, the resistance is too much, and the switch fails to function.
Now in a stock AEG, this usually won't become a problem during the service life of the assembly. The contacts in most of them are pretty robust. If your gun is staying stock, you're done reading here, skip this whole process...But add a stronger spring, a beefed motor, etc, and the contacts have to work harder. Plan on eventual switch failure, especially if more battery cells were added to the setup.
Modern electrical/electronic systems only use mechanical contacts for VERY heavy loads, that are not frequently switched. For anything less, the transistor is the solution.
A transistor (in our case, the Metal Oxide Silicon Field Effect Transistor) is a neat little device that acts as an electronic "switch". Usually, they are used to interrupt the NEGATIVE power lead in a system. This is the opposite of nearly every circuit that uses switch contacts. Including AEG's. This is important, because it means the wiring the gun came with needs to be changed in operation. Instead of the trigger giving POSITIVE to the motor to activate it, we are going to feed NEGATIVE to the motor to turn it on.
I've constructed a simple circuit to illustrate MOSFET wiring:

This is the AmmoDrive for the HK21 I'm cobbling together.
"a" is the JR servo that spins the drive wheel (preparing a post on how to do this) on the hicap.
"b" is the itty-bitty switch I got to run it. (It would last about a week if I tried to put it in between a battery and an inductive load like a DC motor.)
"c" is the familiar Tamiya power connector.
Note that the negative lead goes into the 'FET and back out, then going to the orange negative lead on the servo. The red, positive lead from the servo goes right to the positive lead from the battery. Note the black wire that comes out of this connection, going to the switch "b". Closing switch "b" feeds positive to the 'FET, allowing the negative leads to be connected.
Going to the inset pic, the leads on (most) 'FET's are laid out:
GAIN-DRAIN-SOURCE
"GAIN" is the lead that waits for you to pull the trigger or close a switch, giving it POSITIVE voltage. "DRAIN" is the item needing power, and suitably, "SOURCE" is the power available from the negative side of the power source.

Now you can decide if you even NEED this for any particular gun. As far as actually converting a gun, more well-thought-out information is available here:

http://www.airsoftmechanics.com/home...nt/view/23/30/
You'll find comprehensive information on choosing components, and the actual wiring changes you need to do.


Modify a ver 2 gearbox (M4 style!) for MOSFET or 3-TAP installation:
The stock wiring:

See the small grey part at the bottom? If your M4 trigger feels "notchy", it's in upside-down. The stock wire is 18gauge, silicone-jacketed. This is suitable for any stock gun. I'm unsure of how far this will go, so I pulled all the stock wire in favor of brand-new 16gauge. My new wire is vinyl-jacketed, so the diameter difference is minimal, which is important for getting everything back together. There is limited space in the gun body for bigger wire. Silicone is better insulation, but I'll make the sacrifice to use larger conductors.

First, the stock neg lead shown in the top of this next photo:

I'm going to eliminate the part circled, because it does a poor job of keeping the negative lead against the gearbox wall as it goes around the motor shaft. I learned this little trick from a cheap clone, bend the wire as shown, and it will stay there until you want it back out, if ever. Note that the negative lead doesn't connect to anything inside the gearbox. It just passes through to get to the motor. Limited room again.

Now to put in the new positive lead. You'll be happier replacing the whole lead with a setup like I show here. Trying to solder TWO wires to ONE contact will end in frustration, melted parts, or both, unless you're a seasoned solderer.

So I bend a red wire as shown, THEN cut the insulation away from the bend. Now we have ONE wire to solder to the delicate contact, and we know every conductor is whole from the battery to the motor. Technically, a whole solder joint and it's resistance are eliminated as well.

Then another red wire is soldered to the other trigger contact. This wire can be really, really, tiny, and the circuit will still operate, as only a microscopic current is present while firing. In the interest of space/reliability, I went with 22gauge.


Then we button it all up. Next, building a basic MOSFET circuit.

[CDN_Stalker]

Very well written.

[mcguyver]

It should be Source-Gate-Drain, and they are in the wrong positions on your diagram of the FET. Gate is always in the middle

[Killbucket]

The basic MOSFET install.
NOTE: If you've read the tutorial at the airsoftmechs pages, you'll know the resistor values, and why they're arranged the way they are. You may also know that the 'FET I have here won't survive long pushing even a mild M4. I'd parallel another one to it if I was actually going to run this, and add heat sinking as well. But this is just a layout pic, I have another step to add to this.
When the trigger is pulled, there's negative voltage output at the center pin of the "FET. If we hook this to a relay coil instead of an AEG motor, this little 'FET will be happy, even without a heat sink. And then we can add the fun part: A 3-tap system.

Pinout depends on the particular FET. Always refer to the data sheet.


Rated at 250vac/8amps, I'll be using both contacts paralleled at 9volts or so DC. Should survive a long time. The bad news? It's a surplus part. Here's the info on it, good luck finding more...

[Naglfar]

Just so you know, a DC current in the range that airsoft guns use may blow that relay...
AC current is actually much less strainful to components than DC because AC does have a 'cooldown' time, where DC is just the solid voltage/current. 8AC Amps may not be enough

[mcguyver]

Originally Posted by Naglfar

Just so you know, a DC current in the range that airsoft guns use may blow that relay...
AC current is actually much less strainful to components than DC because AC does have a 'cooldown' time, where DC is just the solid voltage/current. 8AC Amps may not be enough

Agreed. Relays rated for larger AC voltage will only pass the same current in DC as it's rated for in AC, unless it specifies a lower current for DC use. This is quite common. An 8 amp rating will reduce those contacts to nothing if used in even the most basic of gun systems. It's just a matter of time.

Even though AC has current reversal at 50 times/second (Europe) and 60 times/second in North America, it doesn't render much difference to actual heat build up on the contacts, as current is passing through them still, regardless of direction. A better way to see if a relay is suitable, look to see if it has different rating for resistive or inductive loads. This is representative of use for motors/discharge lighting which is inductive and incandescant lighting which is resistive. A relay rated for 25 amps resistive might only be rated for 18-20 amps inductive. This is due to inrush currents present during motor/ballast start-up.

[Killbucket]

Good call. However, several of these are still working fine after several months. Remember I'm using both poles, So I'm doubling the 8A rating. In my production Brownings, we use a 30-am 12volt Auto relay. There's just not enough room for them in an M4. The relay spec'd above isn't here yet to play with yet. This more about concept than component selection.

A closer look at the relay with the 'FET on top. The Relay comes on if we hook positive to the other coil contact and pull the trigger. Adding a small capacitor to the 'FET drain will feed the relay a few seconds longer after the trigger is released. Next, how the wiring looks for this.

EDIT:
After comparison to similarly-rated relays, I've determined that this one will support about 30A inductive per contact at 9volts. That's why they're surviving.