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Old August 26th, 2009, 11:17   #1
DonP's Avatar
Join Date: Jul 2002
Location: Edmonton, AB
Exclamation How to Make a Basic MOSFET Switch Unit

Update Sept 2009: Parts kit now available! See this post.

(This is also posted on the web here.)

I sometimes get asked about basic (non-computerized) MOSFET switches. Here is how to make a basic one. If you have at least basic soldering and electronic skills, you should be able to put one together without too much trouble.

Terry Fritz of provided the design and kindly released it into the public domain. I have expanded it with some step-by-step and instructions. Enjoy!

If you’re reading this you probably already know what a MOSFET switch for Airsoft AEG guns is. (If not, then the short version is that it replaces the way a gun turns power on and off to the motor with solid-state electronics. A MOSFET switch unit controls power far more efficiently and reliably, especially for upgraded guns.)

It doesn’t have active braking, low battery warning, fire control, or any other of the fancy features that are part of computerized MOSFET switches such as my Trigger Master. It’s as basic as you can get while still being robust!

NOTE: Do not use a battery below 6.0V. You risk damage or destruction of the unit!

To work properly, the MOSFET needs to be switched on with a high enough voltage. Too low and the insides don’t work right. Under 6.0V is too low, and the unit might be damaged or destroyed. That sounds scary but it’s really not a big deal. Note that your average 8.4V airsoft battery is dead as a doornail at 7.0V so it shouldn't be an issue! What is really being said here is: “Don’t plug really small non-airsoft batteries into this. It won’t work anyway, and you might hurt it.”

A MOSFET is in many ways an efficient and tough device, but in other ways it’s quite fragile and needs to be protected.

Short answer: yes.
You may have seen some other MOSFET switch designs out there with fewer parts, but this design includes safety and protective parts which frankly others should not do without. There are simpler designs but not necessarily more reliable or tougher.

Some other final questions to nip in the bud before we get started:

Q: Can I substitute part “X” for “Y”? For example can I use part XXXXXX instead of the MOSFET you listed?
A: There is more than one way to skin a cat. But if you know what you’re doing, then you don’t need ME to okay your substitutions! If you don’t know what you’re doing, then stick to what is listed.

Q: “Bolt on, do not solder” to the tab of the MOSFET? Why can’t I solder to it?
A: The part (like many MOSFETs) is heat-sensitive and it’s easy to damage or destroy it with excessive soldering heat. Bolt the connections on with a connector like the one shown. Feel free to ignore that advice but don’t come to me when your unit blows or doesn’t work at all!

Q: Do I really need the huge diode?
A: Yes, you should use it. That part eats up the large voltage spikes that are normal in this kind of application. This is not a “theoretical” just-to-be-on-the-safe-side thing, there are some scary voltage spikes that can easily damage and destroy the MOSFET otherwise.

Update Sept 2009: The fat diode can be installed in a different orientation to make the whole assembly overall a little thinner. Just be careful to insulate all the connections so they don't touch where they're not supposed to. Here's an example that moved the huge diode further "up". The way it's drawn in the diagrams below is the easiest to hook up however.

With that all out of the way, here is my illustrated assembly guide.

The Basic MOSFET design, redrawn

Illustrated Parts List

Cut off the middle pin on the MOSFET (we won't be using it)
The middle pin and rear tab are electrically the same. We’ll be using the tab and not the pin.

Connect the 2.2k resistor and solder it.
This part ensures that the MOSFET turns off properly. When soldering anything to the MOSFET, try not to use excessive heat – solder long enough to connect the part and no longer.

Attach the 100 Ohm resistor to the GATE pin. Make sure you put it on the right pin! Check twice, solder once!

Connect the other components and MOTOR (-) wire to the MOSFET.
It is absolutely critical that no metal parts touch each other other than where they should. Assemble it so that there are no shorts, and we’ll insulate with heat shrink later.

The rest of the connections are wires are made and insulated. Compare with the drawing.

You’re done! Just twist up the wires as shown (it really does make life easier for the unit, trust me) and wire up into your gun! When installing the unit, leave as much of the wire twisted as possible (it won’t be able to be 100% twisted of course, just leave as much twisted as you reasonably can.)

A DIY parts kit is available if you can assemble it yourself but would like a package of the basic parts to work with. See this post.

And of course, if you'd prefer to buy a pre-made high-tech unit - you can buy one from me here.

Last edited by DonP; November 10th, 2009 at 11:15..
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