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Silent_Assassin

How to build an AEG MOSFET

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Thanks for the AB mosfet link, I'll order the parts at the weekend to build one of those (and yes I'll make a guide).

 

I'm also going to post guides with pictures on how to fit a MOSFET into both Version 2 & 3 gearboxes.

 

Josh

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Hey,

 

To all those that contribute to expanding the general knowledge within the sport we all love and obsess over, I really appreciate the work you guys have done. Please keep it up. I have spent the last year learning about gearboxes, shimming, gears, switches etc, and the next logical step is to move onto lipo's, mosfets so this thread is invaluable.

 

It's time for me to move on from fixing breakdowns onto asking that intriguing question 'what if i tried', and i guess this summer i do just that.

 

Cheers

 

J.

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Thanks for the AB mosfet link, I'll order the parts at the weekend to build one of those (and yes I'll make a guide).

 

I'm also going to post guides with pictures on how to fit a MOSFET into both Version 2 & 3 gearboxes.

 

Josh

 

if you could make a full AB Mosfet guide that would be awesome, i really need some more but i simply just cant afford to pay other people to make them...

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Thanks for the AB mosfet link, I'll order the parts at the weekend to build one of those (and yes I'll make a guide).

 

I'm also going to post guides with pictures on how to fit a MOSFET into both Version 2 & 3 gearboxes.

 

Josh

Please don't forget to include specific suppliers & part no's, to save us plebs any confusion. ;)

 

Many thanks,

 

 

Greg.

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i dont know if double tap wants to post his instructions in here, but i found them usefull, and with further modification im happy with the units in using.

 

Your guide is good for the beginner, and would only need the inclusion of a good circuit diagram and one nr two extra steps for the p fet.

 

So before just reposting some existing information, the overall design could be optimised for use here.

 

I want to see some testing also, of the current flowing in the motor and switch sections of the circuit.

 

From the current flow a suitable fuse can be installed. Where lipo are being used over current protection is a must have

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This is a fantastic article for DIYers, but I have a few questions. Where would one read up on MOSFETs to understand them better? For example: if I wanted to build a MOSFET unit for my gun, what kind of stats should the FET have and what kind of resistors should I use with it. I like DoubleTap's guide and will probably build a couple using his specifications, but I would like to know what alternatives are available as those particular FETs are out of stock where I looked.

 

 

EDIT: Airsoft mechanics has a guide that is really great, pretty much explained what I needed to know. Those interested should check it out.

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WARNING!


This mosfet should not be used in conjuction with high powered motors.

I have just wired it up to my high ROF gear box with a Systema Magnum motor. When I pulled the trigger the mofet heated up to such an extent that it melted the solder holding it together and then began smoking.

I can confirm that the mosfet itself is not to blame as I have used it in conjuction with a standard GB without any problems. I am therefore drawing the conclusion that due to the power requirements of the Systema Magnum motor this type of mosfet was pushed beyond its designed limits which caused it to fail.

Josh

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Well, the MOSFET is not to blame. This ties in neatly with crackisbad's question actually. The Magnum motor (and Turbo) draws a hell of a lot of current. You're talking up to 100A at a burst. Using a FET that's only rated to 70amps burst will cause the FET to overheat, and possibly burn out/catch fire/melt. Standard motor's will only really need a FET capable of handling around 40 amps continuous, 70 amp burst. Even that's being generous. If I had a Magnum motor, or big spring set up, I'd make sure I was running with 100 amp continuous draw capacity.

In a way, you're to blame for useing a motor that draws a lot of current, with a FET that's not up to the job.

 

Similar thing with the Li-pols actually. Li-Pols are easier to estimate power drain for. They come with information about how much current they will give out, both continuously and at peak. Li-pols can also give out over 100amps, if it's a big one. So just check the power of your battery, and match the capacity of the FET. I'd be generous in all circumstances with the FET though. Doesn't hurt to have 30A spare. Might mean a bigger unit, but I think that's a fair trade to stop your gun from melting.

 

Brilliant guide. I look forward to your version of the AB FET guide.

Also, can anyone explain to me what P channel and N channel FET's are?

 

Edit: Just read wikipedia FET page... I think I understand the difference. P channel allow current when the voltage drops, and N channel when the voltage increases.

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...what kind of stats should the FET have and what kind of resistors should I use with it...

 

For the FET, there are 3 specs that you want to look for;

1. ID = Continuous Drain Current, in Amps. You want this to be a minimum of around 180Amps (at 25°C). There will be two figures quoted, one at 25°C and one at 100°C, which will be approx 30% lower.

2. PD = Power Dissipation, in Watts. You want a rating upwards of 200W. The higher it is, the more heat it can dissipate.

3. VDSS = Breakdown Voltage, in Volts. Wants to be around 35V or higher.

There are many other specs that will be quoted, but these are the three most important ones, get those right and the others will be good.

The other thing you're looking for is the 'package type', go for TO-220.

For a simple switching MOSFET build you want an N-Channel type, as opposed to a P-Channel, unless you want to build an AB MOSFET, in which case you'll need both types.

 

The ones I use mostly are the IRF1404zpbf and IRF1404pbf.

You'll find quite a few suitable ones with similar specs.

You should pay no more than around £2.50 each.

Try RS Components RS MOSFETs or Farnell Farnell

 

The resistors you need are 1 x 100 ohm and 1 x 30k ohm (30,000 ohm).

You can go a little either side of those figures but I don't think there's any advantage to it, they work and are readily available everywhere, and there's nothing in the prices, I pay less than £1 for 50 resistors, the more you buy, the cheaper they are.

I use 1/4 (0.25W) Watt resistors, as they're nice and small, but you can use higher Wattage ones.

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Thanks for the additional info guys, that was definitely what I wanted to know. Looking forward to building my own soon! :)

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I've ordered more MOSFET's to build both AB and non-AB circuits so I should be able to post up some guides for you all soon (I'm taking the day off work to do it ;) ).

 

Josh

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Just to add a wee note, as silent has stated, the mosfet he posted indeed wont handle a systema magnum or similar.

 

BUT

 

IRL1404's can handle the current of a high speed systema magnum, and a 11.1v lipo.

 

I have built a few, and they are still holding up strong ( holy cow its fast )

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Thanks for that Blood-Clot I'll add them to my order. Any particular model or will any IRL1404 do?

 

If I wanted to build an AB Mosfet that can handle a LiPo/Systema Magnum setup, which would you recommend (model No pref).

 

I've got an airsoft day booked this Sunday and I'm not going to be the parts in time so I'm going to revert back to the fet that I built in the guide and stick with the stock motor for now. Then once I've got enough time I'll build an AB MOSFET that will able to handle the power requirements of my Systema motor.

 

Josh

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i dont know if double tap wants to post his instructions in here, but i found them usefull, and with further modification im happy with the units in using.

 

Your guide is good for the beginner, and would only need the inclusion of a good circuit diagram and one nr two extra steps for the p fet.

 

So before just reposting some existing information, the overall design could be optimised for use here.

 

I want to see some testing also, of the current flowing in the motor and switch sections of the circuit.

 

From the current flow a suitable fuse can be installed. Where lipo are being used over current protection is a must have

 

I've only just noticed this thread. I have no problem with people using my images or reposting my guide on here or elsewhere, I'm chuffed if it helps some people and all I ask is that if it's reposted you acknowledge the original source.

 

I posted the guide on our forums so as not to increase bandwidth use for Arnies, but if you want to put it on here, feel free.

 

Gzus11: I like the idea of the resettable fuse, very neat. I tried a few positions for adding one but it was still too bulky but I don't think I tried it like that, now, where's my soldering iron...........

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What was the increase in ROF when you MOSFET them ( any other parts you change ie voltage of battery )

Can't say I've ever measured it but it is noticable, the main benefit for me is the improved trigger response and less wear on the switch contacts. Nothing else needs changing, you don't even need to open the gearbox on the Star one.

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quick note i have decided to situate the fuse closer to the battery,

aids with installation slightly, also i think i noticed when fets warm up

this can limit the currnet through the polyswitch as its temp cut out.

 

im planning to remove the small resistor from the switch and keep 30K ish

pull down in the next unit i build as i cannot find any reason to keep it in

the circuit

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Yes, of course the heat from the fets will affect the fuse in that position. Back to the drawing board then :)

 

If you've got a good enough FET, surely it shouldn't heat up enough to make any difference? I know fets get hot, but normally that's when they exceed their continual drain threshold is it not? I was useing a pair of FET's built to take around 150amps in my high speed set up with no noticeable heat, even when holding the unit. That was a Systema turbo, 16awg wire 8.4v min battery giving out around 350fps. And low voltage batteries cause more heat through the unit. I don't think a resetable fuse in that position would cause any problems

I'm not exactly a giften electrician though.

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