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crackisbad

The Mosfet Thread

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Since we have a Lipo thread that is getting filled up with mosfet questions, it seems it would be a better idea to seperate the two topics to make it easier to search for information.

i would start this thread off with a bang but my first mosfet is still in the mail wink.gif


Edit:
Interactive Guide on MOSFETs
http://img401.imageshack.us/img401/1889/mosfetpr3.swf


Guide on how to install Mosfets:
Clicky Here!


For those of you just looking for where to buy Mosfets from:
Our very own, Infected:
Infected Airsoft

Black Talon Concepts
Extreme Fire
HH Airsoft


list will be updated periodically

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Excerpt from my website (http://infectedairsoft.wordpress.com):

Okay, you guys have all heard of this thing called the MOSFET. Many of you just think it’s too technical to bother with. Other’s can’t justify buying one, and some of you just don’t know what they do. Well, I’m here to help you out.

 

What does a MOSFET do for me?

Glad you asked! A MOSFET will remove the current that normally has to travel through your trigger switch to your motor to power your AEG. Trigger switches have a higher resistance that you might think. That means reducing the amount of power going to your motor. Think of it this way, can you get all the performance out of your car by removing 2 of the cylinders? Nope. The power is there to be had, but you’re just not using it all. The MOSFET redirects the power so when you pull the trigger, the power goes from the battery through the MOSFET and to your motor. There is almost 0 resistance, therefore almost every drop of power reaches your motor!

But what’s this “Active Braking” thing?

A lot of people have been selling MOSFETS, but not ones including Active Braking. Active braking is a method of stopping your motor from over spinning. When you release the trigger normally it takes a moment for the inertia of the motor to subside and the motor to come to a stop. This means added wear and tear on your motor, brushes, pinion gear, anti-reverse latch, and the rest of the gear assembly. Active Braking halts the motor almost instantly after you release the trigger. This reduces wear, and will make sure that after you fire a complete shot in semi, that you’ve uncompressed the spring as much as possible. Saving the life of the spring and gearbox. In full auto it will prevent overspin wearing down your components.

But I don’t have an upgraded gun. Why do I need this?

Because it will prolong the life of your AEG. You’ll have less wear and tear on your trigger switch (I’ve seen some fail without this system), and your gearbox will thank you. In addition you’ll get better trigger response. Have you ever seen a target and fired a shot at it, but they had just a slightly faster trigger response and hit you first? Put an end to that. The trigger response is shortened as well as an increase in the ROF (rate of fire).

 

You all know where to find me if you have questions, or want to order.

 

 

CrackisBad... please post pictures of the guts of the mosefts you ordered from Echo1. I'd like to see them. :)

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i've been building my own, wasnt too difficult the guide at airsoftmechanics sorts you out pretty well.

 

i used IRF1104, rated for 100A. total cost was NZD5 for the FET and bugger all for the resistors, first one took me 3 hours to put into the AKS74U and then i did the AUG and P90 those only took an hour since you dont have to open the mech to fit them. next lot i do i am going to use IRF3703 since they are rated for 210A and only costs a dollar more

 

what it looks like installed

 

949929cfaa524b.jpg

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Honestly... and it's just my opinion... mosfets without braking are not really worth it. Sure you save wear and tear on your trigger switch and increase your ROF, but you loose out on the reduced wear and tear for other mechbox components without the braking. Once you put the fet together, the install is the same.

 

Carbacca... you really should heat shrink that moseft. You don't want to short it against that metal body.

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its kinda different here in NZ....since are are only allowed Semi the voltage spikes just keeps killing contacts, active braking would be nice but i am too lazy to implement it. i only started going lipols 2 months ago and already have a dead set of contacts (SRC RPK with M145 spring and 11.1v 2200mah 25c Lipol O_O)

 

yeah i will heat shrink it once i get some heat shrink from

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Honestly... and it's just my opinion... mosfets without braking are not really worth it. Sure you save wear and tear on your trigger switch and increase your ROF, but you loose out on the reduced wear and tear for other mechbox components without the braking. Once you put the fet together, the install is the same.

 

Carbacca... you really should heat shrink that moseft. You don't want to short it against that metal body.

 

Well the statement needs to be qualified, a FET is astronomically better than a regular trigger switch. Its just you saying that you might as well go all the way.

 

It needs to be noted that true active breaking requires 2 Fets( p and n type combination) while there is such a thing as passive breaking which "slows" the motor instead of giving it a little juice in the opposite direction.

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With active breaking I assume that you are putting a short reverse pulse to the motor to stop it? Wouldn't that though cause undue stress on the pinon and gear? I would have thought allowing it to spin down under its own steam (or with slight assistance) would be better. Wouldn't sudden halting of the motor cause backlash on the pinion and thought the gear train, wearing the back face of the gears/mount shafts (and possibly the anti reversal latch)?

 

I'm just interested in your opinions on this rather than saying you are wrong, as I'd be interested in how you are applying the reverse power and to what degree :)

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With active breaking I assume that you are putting a short reverse pulse to the motor to stop it?

 

Mosfet active braking is usually done with P-channel mosfet which sort circuits motor when user releases the trigger. This basically turns motor to a generator which transfers kinetic energy of the gear train into electricity, which in turn turns into heat since motor is sorted. Active braking circuits don't generally prevent mechbox component wear as some people state, since non-braking circuits just let the kinetic energy of gears gradually decrease, resulting quite smooth stop. Braking on other hand basically stomps on the brake and thus there are a bit more impulse energies going around in the mechbox. But for records sake I must state that either way you do stopping real impulses are going to happen when you start the motor, since then motor is just turned on with 100% of battery power to drive it.

 

On other hand braking is nice feature since gearbox stops to same position every time and there isn't overspin which in some cases can result gun shooting one or two rounds after user has released the trigger.

 

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It also has a positive effect because guns that shoot high rof(30+) will overspin to catch the piston midway, mine will catch intermittently and i have to keep shooting it till i get the piston to rest(about 3-5 shots) in semi. Alot of the old highspeed gears used to be lightweight(i remember systema used to have a badass looking sector gear that was skeletonized) so the momentum is maintained longer. Overall the energy has to go somewhere so wear is still going to happen to the gun, the energy doesn't magik into nothing.

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can anyone confirm or debunk the rumors that active breaking mosfets wear down the brushes of the motors? I heard that systema and some others had this problem.

 

 

Depends on how you look at it. What wears down brushes is use. If you install a mosfet you're going to send more current to the motor, and the motor will do more work. With that line of thinking you'll be causing more wear on the brushes.

 

The breaking feature itself does send current to both sides of the motor to stop it. Some motors seem to make a "spark" at the brushes when that happens. Honestly, I haven't seen any extreeme wear from using braking mosfets in my setups. I have them on every one of my guns, and the only motor to die was a Guarder high speed... but they have a short lifespan as it is.

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I'm planning to build my own AB mosfet and here are the parts that I ordered from digikey:

 

SPP80P06PIN-ND (p-channel)

IRL3713PBF-ND (n-channel)

20KW-1-ND (20k ohm resistor)

P100W-3BK-ND (100 ohm resistor)

 

Are these the right parts? I'm not sure I chose the right resistors because the 20k one was supposed to be a "pull down resistor" according to the guide I'm using, so I just decided to buy a normal 20k one instead since I don't know what that is

 

btw, I used this guide: http://www.airsoftretreat.com/forums/index.php?topic=58079.0

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You should be fine, but that pulldown resistor value is much higher than it needs to be. I don't know why people are using such high values.

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When looking for MOSFETs to build my own one what should i be looking for in the ratings?

 

I assume that the drain current, voltage and power dissipation are important. Would a MOSFET such as the one linked below be able to handle a 1 joule AEG on a 11.1v 1400mah 20c battery?

http://www.maplin.co.uk/Module.aspx?ModuleNo=22997

It's rating of 115w might be a little low.

 

Cheers

 

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I'm confused as to why people are building mosfets - aren't these things available on the electronics market, prepacked into tiny IC chips? Surely the space saving from using such a device would outweight the cost of buying one?

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er.... not sure what you mean hedge. You do indeed sound confused.

 

I don't think many people are making mosfets from scratch, rather wiring up an existing mosfet for use in an AEG rather than buying one that's already been wired up with the correct resistors and connecting wires.

 

Or have i misunderstood you?

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I'm confused as to why people are building mosfets - aren't these things available on the electronics market, prepacked into tiny IC chips? Surely the space saving from using such a device would outweight the cost of buying one?

 

They aren't making mosfets, but rather creating a circuit that uses mosfets to redirect the current from the trigger switch to the motor. This make the trigger switch just an on off switch and the mosfet bears the current.

 

People (such as myself) are also using the mosfets to create circuits that incorporate braking to stop overspin in their gearboxes. This requires multiple mosfet's.

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Hedge: from my understanding of it, when someone says 'building a MOSFET' in airsoft, they actually mean 'wiring up a MOSFET with the other components it needs to work in an airsoft gun'.

 

You're right - MOSFETs are readily available and are indeed tiny IC chips which would be impossibly difficult to make without all of the computer-guided machinery, laser etchers etc. that the electronics industry uses. But just a MOSFET won't work on its own - it needs some extra components, and putting these together is what most people mean by 'building a MOSFET'.

 

If you look at carbacca's picture or click tome's link, the actual MOSFET is the small black square thing with the metal tab (heatsink).

 

Hope this clears things up for you!

 

Stu.

 

EDIT: Dammit guys, beat me to it! :P

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The circuit for the Active breaking does actually warrant wiring up on a proto board because its more complex with the two FETS

 

There is a Japanese group who have these amazing skills with bending the wires and resistors to make the package smaller than some of the crumbled receipts in my pocket, but i dont have the skinny asian fingers for that. It does take a bit of soldering work tho to put one together. The process has to go just right with the soldering due to the size of some of the leads.

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I'm sold on the benefits of getting one, but I think I'll leave it to a professional to fit one for me...

 

Well, just let me know if you're planning on ordering one from me ;) They might be going on sale soon too :).

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