Mosfet upgrade
 

Is there any of you guys out there that did a mosfet switch upgrade? If so, I'd like to know to do so, with pics if possible. Thx guys

Have fun reading...

http://www.airsoftmechanics.com/guid...?aid=14&page=0

you are my god
Thx bruce

You're better off to buy a pre-made switch from someone like Systema or Guarder. Although it looks easy, the fact they do not use a heatsink and only 1 FET tells me that this switch will simply not last and you will have wasted as much money in parts as it would have cost you to buy one from the start.

Frankly, if done correctly, FET switching is the way to go. Systema uses it in their PTWs to handle M150 systems.

This makes me realise that I have some 250A rated mosfets lying around in a box somewhere...... Should be great to make a switch.


By the way, you can lower internal resistance and rise current drain simply by placing 2 or more mosfets in parallel to each others... just "stac them and connect all the pins with the right ones for the other mosfets (Gate with gate, etc.)

You can still add a heatsink to this assembly, as long as you have sufficient space avalable. They sell some in electronic surplus stores for about 1$. Just add some heat compound and make sure nothing get's shorted. Usually we use the rear tab (connected to emitor) to screw a heatsink on.

Alsom make sure you but the right type of mosfet for your schematic. There is 2 types avalable, N type and P type. In this tutorial, they use N type mosfets. Basically, you will just need to revert all current flow in the circuit to use p type ones (Including gate and return flow. The gate is "switching" when grounded instead of "turned on"). I don't recommend the use of P type since they are less efficient compared to the equivalent N type. Just make sure you buy the right one.

I will make a few of these to sell here on ASC. Maybe 4-5 units.

pm johnnydo i think he has a design or knows a way to do it...

Just found my mosfets...

They are military grade high power mosfets. They can work in temperatures from -30°c to 175°c (ya it's crazy!!)

20V, 110A and rDS 0.0026 ohms. I just remembered that I got them to modify a speed controler.

I have 8 in stock. 1 will be mine, and the others will be sold here on the forums. Should be ready in 2-3 days. PM me if you want one, I won't make others...

I am building a PWM generator to drive these. It will allow anyone to use a 11.1v or 14.4v Li-Po pack in his gun with variable voltage control. This means that your gun will be able to fire like a 8.4v pack, or a 14.4v with the same battery, in about 10 sec, just enought to change the switches settings.

More to come.

Joo got mail. :D

Humm according to the web site bruce refered me too and according to the fact that wgc are using irf3037 mosfet, yours might not be sufficiant.
For airsoft purpose, looks like we need 30v, 210a and 2.8ohm

SEALS, go into the forum on the link Bruce provided, there is a dozen or more pages of info, questions, discussions on that topic alone.

It is sufficient. You don't need a dsV above 20, unless you plan on having a 15 volt pack wired to your gun.

110A Continuous is well enough. The MOSFETs I use can take a 250A burst for 30 sec. every 5 min. Do you know a lot of people that shoot their AEG at 12v for 30 seconds steady? I don't. They used 220A MOSFETs, but the on resistance was 2.6 ohms. The lower, the better. This is the power consumption of the actual MOSFET. Mine have on resistance of 0.0026 ohms. It will generate about 100 times less heat than the ones they used.

And as I said before, nothing prevent me from building 2 mosfets in parallel. This will give a 220A continuous with 500A burst capability. As a comparaison, these little MOSFET could drive a 1200 class motor. I think that the ones used in our guns are 400 or 500 class. They are garanty no to fail even when used at 175°c. Your gun will melt before these get smoked.

I got these MOSFETs back when they where in prototype release. They have been designed to be used as switching voltage regulator in extreme situations.

And on top of that, If they are used along with the other module I am building, they will work only 60-70% of the time. No worries about that.

But since I plan on mass producing these MOSFET modules, I will fry some on tape to show how much abuse they can take. I got a IR tempgun to show the actual temperature of the MOSFETs. due to the package, it will be extremely easy to add a heatink on top of the MOSFETs.

Anyways. I just got back from the electronic supply store. Bought all the things I need to make some prototypes of both the MOSFET and PWM modules.

alrigth thenbuilt a prototype and I might be interrested

There is one thing I found missing from that airsoft mech's article. DC motors are significant inductors. They are able to store some significant energy in establishing a magnetic field which means they can output a pretty high voltage when you disconnect them from a current supply. It may help to put a diode across the motor terminals opposite to the usual polarity applied by the external MOSFET cct. When you interrupt current flow through the MOSFET cct, the reverse biased diode provides an easy current path for the inductance current to dissapate throug so it won't fry the MOSFET. In normal operation, the reverse biased diode will not particapate and will appear to be an open cct so it will not affect the motors operation.

I thought that the motors already had some king of protection, be it diode or capacitors....

Well that is a good thing to add. There is one integrated in the FET, but if it fries, must change the whole FET.

Thank you for pointhing that! I think that a 1W 50V diode should do the trick.

There are usually caps across R/C car motor terminals, but I have not observed any on AEGs. Typically the caps are used to suppress EM noise from the motor so they are affixed to each motor terminal and to the motor can. In airsoft, we don't have remote controls so there isn't any EM noise issue. You can put the diode across the major current connections on the FET where you have more space to play with instead of jammed up in the motor space.