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Everything You Need To Know About MOSFETS

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For all those who are looking into the world of MOSFETs, whether you want to learn how to build one yourself, how to install one, or you just need to know what they’re all about so you can buy one that suits your rig, this thread is for you.

 

INDEX OF LINKS: Quick links to the resources I use in this guide.

How to Make a Basic MOSFET Switch (Highly Recommended)

Video: Install MOSFET in V2 (Read Wired)

GoLgo 13's Wiring Guide

GoLgo 13 pt. 2

GoLgo 13's Video MOSFET Guide

AB MOSFET Guide

 

THE BASICS

The #1 reason to install a MOSFET in your AEG is to preserve your trigger contacts when using a powerful battery. A lot of AEG’s come stock with a cruddy 8.4V 1100mAh mini-type battery. This is usually OK to run a low-medium power stock gun at round 14 rps, but if you’re on this forum, it’s highly likely that you’re interested in upgrading your gun. There are plenty of threads on general tips for upgrading like this one: http://www.airsoftforum.com/board/Art-Upgrades-t137203.html so my object is not to repeat what has been said elsewhere; but if you want to see any sort of good performance out of your gun, the first thing it needs is a bigger, better battery. The problem with using more powerful batteries comes from the design of the switch (trigger) that closes the DC circuit (shoots the gun). When you pull the trigger of an AEG, the trigger contacts are physically moved toward each other until they touch, allowing the current from the battery to run through and power the motor. This mechanism, though simple, becomes a pretty big problem when you start introducing batteries that put out higher voltage and more current. That problem is electrical arcing

800px-Electric_arc.jpg

As the trigger contacts get near each other, but before they physically touch, an electric arc occurs in the gas-filled space between them and it results in a very high temperature, capable of melting and destroying a lot of materials. Here’s an example of what electric arcing can do

arcdamage.jpg

Do you want your trigger contacts looking like that? I sure don’t... not only because it slowly destroys them, but also because the buildup of carbon greatly increases the amount of resistance in your electrical system, and also encourages greater and more damaging arcing as it gets worse.

 

To avoid this problem, we have two options:

 

1) Use a battery that puts out a low enough voltage that the danger of arcing is minimized/neutralized

 

2) Install a MOSFET

 

The basic function of a MOSFET is to work around this flawed switch mechanism. It acts as a new, electronic switch that uses a signal from the old mechanical switch to divert the current down a new path where there is no possibility of arcing. As a bonus, installing a MOSFET often ends up improving the overall efficiency of your AEG’s electrical system in several ways. It can serve to absorb voltage spikes that can be potentially damaging to your motor and battery, and a new MOSFET normally comes with new, thicker wiring that is of much higher quality than the stock wiring; thus the overall resistance is reduced which often results in less heat and stress, and ultimately a higher RoF. There are also other reasons why one might want a MOSFET such as cool features like 3-round burst, LiPO protection, built-in poly fuses, RoF control, DMR mode, and others that come with some of the advanced computerized ‘FETs that are available these days, but more on that later…

 

So it’s important to use a battery that can deliver enough power to meet your performance goals, but you also need to avoid electric arcing. For those of us who want a simple, straightforward solution without having to mess with wiring and MOSFETs and all that, there is good news. The general consensus is that with the standard NiMH/NiCD/LiPO batteries we airsofters typically use, arcing doesn’t pose a real problem until you break the 9.6V ceiling, so you can with relative safety purchase just about any 8.4/9.6V NiMH, or a 7.4V LiPO battery as a drop-in upgrade.

 

Now, for some more quick info on Batteries and Motors before we go on to see if a MOSFET is right for you, and whether you should purchase one or just make your own:

 

 

 

 

BATTERIES

Basically, your battery determines whether your system will run to its fullest potential, or whether it will limp through or fail to cycle your gearbox. The most important question you need to ask about your battery is, “can it deliver enough current?” The amount of continuous current your battery can deliver is related to (a) capacity (in mAh) and (b) discharge rate (“C” rating) in the following relationship:

 

Amps of Continuous Current = C * [mAh/1000]

 

So let’s compare a stock mini to a nice Elite cell NiMH nunchuck pack and see what we see...

-Stock 1100 mAh mini batteries have absolutely horrible discharge rates, usually around 2-5C . Some of you may be familiar with this one which has a discharge rate of 2.2C :a-nonod:... so 2.2 * [1100/1000] = 2.4A

 

-Next the Elite cell 2000 mAh 15C nunchuck pack: 15 * [2000/1000] = 30A

 

The Elite pack delivers 12.5 times the current the stock mini does! That should be sufficient to demonstrate why a good battery should be the first thing you secure for your gun.

 

 

 

MOTORS

Now as we all know, it is the Motor that converts electrical energy into mechanical energy, which turns the gears to cycle the gearbox which makes the gun shoot. There are two factors from your battery that determine how your motor will perform: current, and voltage. Together these two factors determine how much POWER your motor will be able to use to do work. AEG motors are hungry little fellas, and the more you feed them (current), and the faster you cram it down their pie holes (voltage), the harder and faster they will work. On the flip-side, if you starve them with those puny stock batteries :a-gross:, good luck getting them to do anything for you. And here’s the thing you need to remember about motors: they will only take as much current as they want, and no more! Under normal working conditions, you can’t give them too much current because they decide how much they want!

 

As for voltage, the faster your feed them (higher the voltage), the faster they will work (higher RPM/torque). So although the important thing for motors is that they get all of the current that they want, it is the voltage that ultimately determines the actual level of performance. Put simply, Higher Voltage = Higher RoF.

 

So if you’ve decided that you want to get all the performance you can out of your AEG, and so you want to use an 11.1V LiPO, you’ll need a MOSFET to preserve your trigger contacts.

 

 

 

 

 

IS A MOSFET RIGHT FOR ME?

 

-If you are content with a battery that puts out 9.6V or less

-If you don’t want to mess with wiring and soldering and don’t want to pay to have a tech install it for you

-If you want to use a higher-voltage batter and don’t care that your electrical system is going to slowly deteriorate as your trigger contacts fry

A MOSFET is not right for you

 

-If you want to safely and efficiently use a higher-voltage battery to improve the performance of your gun

-If you want to improve your AEG’s electrical system

-If you want the cool features offered by computerized units

You should get a MOSFET installed in your AEG

 

 

 

 

 

TO BUILD OR TO BUY?

 

This of course depends largely on the features you are looking to get out of your MOSFET. For a guy like me who doesn’t particularly care about fancy features, this is a matter of cost… yes, because I’m a cheapskate :a-yesnod:. Considering that I’m not trying to set any performance records, getting my guns to shoot 700 FPS at 40 RPS ; ) and therefore don’t need the extreme power of a huge 18.5V LiPO running through my electrical system, I find that building my own ‘FET is the best option. Why? Because most of the high-quality MOSFET units out there are based on the same basic design (more or less), and are just implemented in various ways. Some use PCB and add nifty switches and sensors and programmable computers that do some really cool stuff, but it’s all the same concept.

 

If you have even basic soldering skills, or aren’t afraid to learn and try, then I’m all for building your own. I figure unless you’re having a tech install it for you, you’ll have to do some soldering and rewiring anyway, so why not just do the whole thing yourself?

 

With that said, some might want to avoid going through the trouble of having to locate and order all the different parts, measure out all the wiring, take the chance of screwing up, or what have you, and would prefer the ease and safety of a professionally-made MOSFET; or maybe you just want the awesome extra features. Unfortunately I can’t cover all of the MOSFET developers/sellers out there, and I’m sure there are plenty of good ones, but these are the current big competitors in the MOSFET arms race. These guys pretty much invented/developed MOSFETs to where they are today, and they’ve done incredible work. Special thanks goes out to Terry Fritz (A.K.A. Gandolf) of Extreme-Fire who supplies us with free access to all of his designs (which includes the one used in this guide) by ensuring that it all stays in the public domain :a-salute::

 

Extreme Fire: From super basic switch MOSFETs at $20 for light users, to heavy-duty, feature-packed computerized bundles at $85, these MOSFETS provide tons of extra protection and are guaranteed to please, and to WORK.

 

AWS Airsoft: AWS has been a source of extremely innovative products and features starting with a rock-solid basic ‘FET, pre-wired at $25. AWS computerized MOSFETS also offer fantastic protection, as well as the ability to automatically disable the Active Braking feature to prevent overheating! The new line of products they are about to release is even more impressive, and is definitely worth a look.

 

Hunterseeker Armory: Everyone who knows Hunterseeker5 knows his philosophy of no-nonsense, immaculate quality and ultra-high performance, PERIOD. : D He is the only tech approved by both AWS and Extreme-Fire to install their products, and he also sells his own MOSFETs made to order, which are excellent.

 

 

Personally, if I was going for a real high-performance setup with a battery bigger than 11.1V, I would probably opt for a professionally-made MOSFET from one of these developers; but the truth is, the great, vast, overwhelming majority of us would do just fine with the right design DIY ‘FET. You just have to take the initiative to do it!

 

 

 

 

 

BUILDING YOUR OWN MOSFET: This is the best guide I’ve seen so far, I highly recommend it: http://unconventional-airsoft.com/2009/08/...-mosfet-switch/

 

GoLgo 13’s video guide is also very helpful as a reference, but don’t follow it exactly as the newest design makes some changes to the way he does it here:

http://s7.photobucket.com/albums/y284/mean...rewire_0001.mp4

 

Finally, we’re getting to the point where we can get down to the nitty-gritty and talk about how you should proceed in making your very own MOSFET. But first we need to look at a couple of things.

 

ACTIVE BRAKING

AB is an interesting feature that is an option for DIY’ers, but what is it, and should you use it? Active Braking basically shorts the motor and uses the EMF generated by the spin down of the motor to produce a “braking” effect. It kind of “puts the motor in reverse” after you release the trigger in order to stop it faster. It is intended to prevent any overspin from a given cycle from starting a new, unwanted cycle (which is also what the semi-auto cutoff lever is intended to do BTW). This can be accomplished by wiring in a second MOSFET whose die is doped with the opposite polarity (don’t worry, we’ll get to that). Seems like a good idea, right? Well, it can be nice, and some people swear by it, but it also comes with some side-effects...

 

Without going into any more technical explanations just yet, AB does end up producing heat, and sometimes a lot of it. Heat in the motor because it essentially converts the inertia of the motor and drive-train to heat, which has been confirmed to lead to increased motor wear and carbon build-up, however minimal; and even more undesirable, heat build-up in the MOSFET unit itself. I’ve personally had problems with this in two different builds of ‘FET units that I bought from a reputable seller, caused by Active Braking. The units would overheat constantly because of the AB feature, even though they were both using the current industry standard IRF4905. However, once AB was disabled, they worked like a charm.

 

AB does have its advantages, of course. It is sometimes used in DMR setups both to help prevent overspin as well as to reduce the amount of time that the motor and gearbox is working in order to keep the gun as quiet as possible. Less time having gears spinning = less noise for our quasi-sniper friends. So although AB seems to be made for semi-auto shooting due to the fact that it only kicks in when you release the trigger, it is ironic that shooting in semi will then likely be the greatest source of wear and heat build-up. It should also be noted that once you get into higher-performance setups, the braking effect is not strong enough to actually work, and the increase in current running through the system means that it ends up becoming much more damaging, so AB is best reserved for less-demanding setups where the braking effect will actually be efficacious and the extra heat and wear will not be as significant. However, there are a lot of people who use AB, and I would imagine that most never have any problems with it; but some do and I think it’s proper to give fair warning of the issues at play. I would personally recommend against it; save yourself some cash and some hassle and just leave the braking ‘FET out. Of course, I ultimately leave it up to you.

 

 

 

 

COMPONENT/DESIGN UPDATE

 

There is another great DIY MOSFET guide (and pt. 2 here) here on ASF, which has been a great reference for me in the past, especially with the great photos and the advice on what wiring and connectors to use, etc., but it needs a little bit of updating.

 

First, that guide recommends a 20-30KOhm resistor between the gate and source pins, but it has been determined that a much lower 2.2KOhm resistor is more appropriate. It turns out that using such a high-impedance resistor makes the MOSFET’s shutoff time significantly longer as compared to the 2.2KOhm :a-crazy: and tends to be subject to increased noise and contamination. Terry recommends 2.2KOhm at most!

 

Second, GoLgo 13’s guide also recommends the IRL1404z for the main (N-channel) MOSFET, which has been firmly beaten out by the now-standard IRLB3034PBF. It can handle much higher current and voltage, requires less voltage to turn it on (more sensitive to the signal from the gate), and keep it on. This is the schematic you should follow:

LARS-2009-06-22.jpeg

 

And my front-wired adaptation

V2MOSFETforwardwiring.png

 

If you use the IRLB3034PBF, which you should, YOU CANNOT USE THE DRAIN (center) PIN (poor internal connection), AND YOU CANNOT SOLDER ANYTHING TO THE DRAIN TAB! If you do you will almost certainly kill it. The die in this MOSFET are very large and very vulnerable to thermal expansion during soldering, and even if it survives the soldering it will be set up to fail later. As the schematics above recommend, it’s best to bolt a ring connector onto the drain tab instead and just cut off the middle pin. If you’re using a P-ch ‘FET for AB, you’ll find it difficult to orient the two ‘FETS together any way other than back-to-back, bolting them together through the hold in the drain tab. I only mention this because some like to sort of piggyback them over top of each other in order to create a thinner, longer profile like so:

5.JPG

 

Third, Terry’s design also integrates a new component, a TVS diode (5KP18A). Technically the MOSFET will work without this, but it is highly recommended that you include it. The TVS is the component that filters off and absorbs dangerous voltage spikes that could damage the main ‘FET, causing premature failure. So if you want your home made unit to live a safe, long life, I wouldn’t ignore it. Just be sure to hook it up the right way:

DiodePolarity.png

 

I’ve found that the best way to do this is to crimp your Motor (-) wire into a Ring Tongue along with the (-) terminal of your TVS diode, and then solder them in. Sort of like this, but this awful picture (sorry, iPhone...) shows a wire already crimped down and soldered without the TVS inserted as well:

ringtongueconnection.jpg

 

NOTE: Be sure to trim the TVS terminal so that when you bend it down next to the MOSFET it lines up without too much slack. You can then solder the (+) end of the TVS to the Source pin, and bolt the Ring connector onto the Drain tab as demonstrated in the guide.

 

And Lastly, it’s best to twist the battery, motor and trigger wires together if possible... that’s the best way to reduce noise and voltage spikes due to inductance.

 

Some of these components can be hard to come by, but you can usually find them somewhere. Good places to look are http://www.newark.com/ and http://www.digikey.com/

If you’re in the UK, try http://uk.farnell.com/

 

 

 

 

OTHER CONSIDERATIONS/WHICH MOSFET QUESTIONS

 

As noted in the Active Braking section, the standard braking MOSFET is the IRF4905PBF, and if you’ve decided to go ahead with AB, here is another great guide:

http://www.cerberusairsoft.co.uk/Mosfet/

 

Again, just so everyone is aware of the situation, a lot of developers dislike the 4905 ‘FET and have been trying to find a better solution for quite some time, some using alternatives like the SUM110P, the SPP80P06P as in the above guide, and some even attempting to drive other N-channel MOSFETs using a charged pump IC that makes them act as a P-channel. The issue with effectively implementing AB is applying the right strength of the braking effect. In the driver IC scenario, the PWM (pulse width modulation) that is applied controls how much braking there will be, but the more braking (higher PWM) applied, the more power is dissipated in the MOSFET due to switching losses, creating lots of heat in the ‘FET. Lower the PWM and you get less heat, but also a weak braking effect, and much faster motor overheating instead! If AB is introduced in an off-balance manner, that heat has to go somewhere, and the right balance is hard to find.

 

Here’s a recent quote from Terry on AB MOSFETS, “Sometimes, the best AB MOSFET is like finding the best way to cure a bullet hole in your chest. It's just NOT that simple...

 

But now you have many options from do it yourself to picking your poison from us sellers :a-jester:

 

So Gandolf does not have any advice for you this night. But I wish you luck! ...many designs, even in the basic AB stuff, are in flux now.”

 

Recently there have been several new developments in the P-channel MOSFET world (and N-channel for that matter) that are exciting, but unfortunately none that suit a DIY scenario because they all come in tiny packages like the PQFN that would not be viable to implement outside of a machine-installed, PCB application. You just can’t hand-solder those things.

 

Here’s a quick size comparison:

MOSFETsizecomparison.jpg

As you can see, at 3x3mm, and with no pin leads, it would be pretty difficult to get those puppies soldered on without cooking their die. For now we’ll just have to stick to the conventional packages, and believe me the 3034 we’re using is more than enough to cover you. All of that to say, if you’re dead set on AB, you’re kind of stuck with the IRF4905 or the SPP80P06P ‘FETs for now, even though they’re not the ideal components.

 

As for the main ‘FET, it’s probably best to stick with the 3034, but there are alternatives. Terry uses the IRF1324S-7PPbF (beast) in his higher-end FETs, and AWS has switched over to using two of the tiny BSC042NE7NS3. As mentioned, there are several great components that have just recently come out on the market, but their tiny, delicate packaging makes them impractical for our purposes. Don’t worry, for now the standard IRLB3034 will treat ya right.

 

 

 

 

 

INSTALLING MOSFETS:

 

I can’t possible cover all the different ways to wire MOSFETs into all the different AEG’s out there, so I’m just going to cover some basic concepts here regarding how to install a MOSFET.

 

To start, here’s a very helpful video showing how to install one into a rear-wired V2.

http://infectedairsoft.wordpress.com/tech-...active-braking/

 

Sorry for using the same picture twice, but I’m going to reference my forward-wired schematic again:

V2MOSFETforwardwiring.png

 

Notice how there is only 1 signal wire running to the gearbox trigger contacts. Now compare this to Terry’s drawing above, and note how the second trigger wire runs straight from the battery. Can you see why I did this? When a V2 gearbox is wired to the front, the wires have to pass through the inside of the GB, where the trigger leads are housed. So instead of running a separate signal wire, I just used the Positive (+) wire by stripping a small section in the middle, soldering it to one of the trigger leads, and letting it continue on its way through the GB, eliminating the need for a separate wire.

 

Important Note: when I actually installed mine after drawing this diagram, I actually soldered the Red (+) wire to the lower trigger lead and the thin signal wire to the upper lead; it turned out to be much easier that way.

 

Another issue that comes up with forward-wired V2 guns, is that when you want to take the upper and lower receivers apart, you have to have a way to disconnect the wires from the battery/MOSFET so they can pass through the little wiring hole. I solved that problem with some 2mm Bullet Connectors and they work like a charm.

bulletwiring2.jpg

 

 

On the other hand, if you wanted to rear-wire your MOSFET, you would need that thin signal wire coming from the battery, as well as the one from the MOSFET Gate pin to run through your gearbox, down and around, all the way to the front and solder to the trigger leads, while the heavier current-carrying wires just run straight to the motor. Yes, V2’s are a pain in the :a-censored:

 

Another reference, the forward wired diagram above would correspond to 3b/2b in this chart, while the rear-wired configuration just described would correspond to 4a

MOSFET-WiringDiagram.jpg

 

 

So, if you’re forward-wiring a V2 gearbox with your MOSFET, then you can leave out the signal wire from the battery, but otherwise you’ll probably want it:

done1.jpg

This shot, borrowed from the linked guide, shows how this should look.

 

Starting with the wire pair in the bottom-left “To Motor,” we know that the black wire is attached to the TVS and bolted onto the MOSFET with a ring connector, but what about the red wire? If you look closely, the red wire just passes through the heatshrink without connecting to anything, and comes out the other side where the black wire picks up from the Source pin. This is also true of the orange signal wire. This is done, as previously noted, to reduce noise and inductance. Also notice that the orange and red wires are soldered together on the battery side… It’s all coming together now isn’t it. . . This is the sort of arrangement that you’ll use most often, and would be suitable to use in a rear-wired V2 or a gun with a V3/6/7 GB.

 

 

 

 

Well I hope I’ve covered all the important stuff. Any questions, comments, critiques, mistakes… just let me know and I’ll do my best to help out. Hope you all find this useful!

 

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Excellent guide Buppus. Much more than I was expecting. Thanks for also including all the links, particularly the ones to Golgo's guide. I will pin this after a couple of days that way people see it before it is pinned. People tend to skip over the pinned stuff, even though it is usually the most helpful. Once people know its here, ill pin it.

Edited by airborne101

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Very nice guide. Seems all the sites you listed are out of the IRLB3034PBF fet =/ got an 82 day lead time from newark and digikey estimated the earliest time they could ship these out was november.. Anywhere else to buy this from? or maybe a substitute that is comparable to it?

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Hi! Just asking about AB fets, how much amps must the braking (P channel) FET handle? My gun setup is a JG G36K with stock gears, 120 spring, stock motor and a 8.4v 1100mah battery. For example, is a IRF9540N P-FET suitable as it can handle 23 amps? It's a lot easier to get here in Finland instead of a SPP80P06P which is used in golgo's AB mosfet guide.

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It should be able to handle more continuous amperage than the motor requires. I've not seen many setups that require more than 30-35A continuous, but I would go for a 'FET that gives you a little breathing room. Additionally, the IRF9540N has a very high rds on... try to find one with >25 mOhm if possible.

 

I should warn you about the implementation of a P-'Fet without PWM control though - the consensus now is that it is not really safe to include AB in a simple circuit like this because it is uncontrollable and unstable. The P-channel 'FET makes the whole platform unstable and is the source of many failures, which generally look like the gun shooting uncontrollably until you disconnect the battery. Unless you're dead set on having AB, I would recommend you stick with the simple N-ch switch circuit. If you want AB, purchase a unit from AWS, Bravo, or Extreme-Fire.

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Good stuff Buppus. I haven't been keeping up with the latest and greatest improvements on these. I still treat every one of my AEG's to a mosfet rewire.

 

Since I am in japan I have met some crazy airsofters who have DIY precocking Fets among all kinds of other stuff. I would like to post more stuff, but these days I seem to have less and less time.

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Sorry about the ridiculous necropost, just wanted to share my experience with the the diy mosfet module I made following the guide at unconventional airsoft.

 

It's cheap, and it's the best mosfet module I've used so far, all my previous modules have failed one way or another, some even caught fire. :S This module runs cool and easily handles the powerdrain of a heavily upgraded AEG, I definitely recommend it. The guide was very easy to follow. :)

Edited by Lefse

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If I'm not mistaken that type of mosfet is more for fire control than trigger contact protection and is much more complicated to make on your own since you need to take into account timing. If you are actually talking about making a mosfet that is removeable so you can swap it from gun to gun, you can easily do it with the designs shown here, just need to put connectors in the right places. You still have to rewire your gun though, no way to get past that.

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In order to get trigger protection, you HAVE to rewire. You HAVE to take the trigger out of the main power loop and make it just a gate. Plug n' Play MOSFETs like the one you linked, or the Burst Wizard simply CAN'T protect your trigger because of how they and the stock wiring are designed.

 

So in the end, you would still have to open the gun up and rewire it. However you could install 3 disconnects to the wires so you could swap one MOSFET between multiple guns.

 

The Plug n' Play stuff like what you linked is only for fire control settings and such. No actual protection until they are hardwired.

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I'm considering building Terry's style of MOSFET from scratch. It will need to be durable when wired to a 11.1V lipo battery with 2600mAh <AT> 20C. From my understanding, continuous amperage would be 20C x 2.6Ah = 52A. What I don't see in this guide is anything about a heat sink for the MOSFET. Any recommendations on how to approach component selection to tailor towards my high current setup?

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I'm considering building Terry's style of MOSFET from scratch. It will need to be durable when wired to a 11.1V lipo battery with 2600mAh <AT> 20C. From my understanding, continuous amperage would be 20C x 2.6Ah = 52A. What I don't see in this guide is anything about a heat sink for the MOSFET. Any recommendations on how to approach component selection to tailor towards my high current setup?

While your battery can supply 52 amps, that doesn't mean your gun will draw that. Electricity is pulled from the battery, never pushed. In reality, most guns draw under 30 amps. Most guns have between a 15 and 25 amp fuse, which means if they were trying to draw more than that, the fuse would trip. Terry's fet should be fine for most any setup.

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airborne101 is right. Find the datasheet for your motor and look up the stall current, internal resistance, etc. The motor isn't a linear device but it won't draw more than if it were as per ohm's law. For instance, if the internal resistance is 500m-ohm, it won't draw more than roughly 12V / 0.5ohms = 24A. Note that this is the worst case current, in reality, the current may never reach that, and even if it does, it won't average to that over time. But once you have found the upper limit, then you can design your circuit from there.

 

Heatsink generally isn't a concern unless you use active braking, a really high end motor, or do a great deal of tapping the trigger. Also if you are willing to spend a little more money (ie. $11. or so) you can use a BTS555 high-side switching module which is simpler to wire up and doesn't require any external components to protect the transistor ( TVS diodes won't last forever (if stressed).

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Great guide, but I think some info on specs can be clarified.

 

I recently realized that each of the "dead" li-ion drill battery packs I have use two IRF1404Z's I can salvage. When I compared the specs & applied what we'll be using them for, the differences between the IRF1404Z and IRLB3034PBF are almost negligible.

 

True, the 3034 can turn on with less voltage, but I'm certain only a few mosfet's will ever see less than 10 volts (3x LiFe cells), and the 1404 can pass 75 amps at 10V and still be at it's minimum resistance. Even at 6V it can move it's package-limited 75A and stay cool. When you look at the difference in MAX Rds(on), resistance at a given voltage & amperage, it's 2 mOhms, or 0.002 - at 12.6V (full 3S LiPo) that equates to a 0.06V voltage drop at 30 amps. I'm sure some comp guys would give their left nut for an extra 0.06V, but not someone building their own on a budget.

 

Another thing, I think the rating on that TVS is a bit crazy. Technically the smallest recommended is 400W (12.6v x 30A), so compare that to the 5000W rating. Since there's already some internal protection, even 1500W would be plenty. Also you can technically use a TVS with a working voltage a bit higher than worst-case (peak) battery voltage and less than the max breakdown voltage (40v for both mosfets). I was able to find some 5KP22A's for half the price when I wasn't being picky. If my motor ever produces a spike over the 5KP22A's 141A limit for any tiny period, the wires will melt & vaporize the insulation in the process.

 

 

TLDR; Great guide, however either MOSFET will work fine and any TVS rated at least 1500W and between 15-35V will work fine.

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