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Discussion Starter · #1 ·
Hello all, I have been doing reading for a while on how to build an AB mosfet and attempted to build one the other night and got it working until there was some catastrophic failure (and foolishness on my part) that resulted in a non working gun, a smouldering mess and my missing the first chance at a game in 6 weeks. (aren't your curious now
)

Let me start with a parts list of what I was using:
1. N-channel mosfet http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&productId=618089&catalogId=10001&freeText=618089&app.products.maxperpage=15&storeId=10001&search_type=jamecoall&ddkey=http:StoreCatalogDrillDownView
2. P-channel mosfet http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&productId=1559851&catalogId=10001&freeText=1559851&app.products.maxperpage=15&storeId=10001&search_type=jamecoall&ddkey=http:StoreCatalogDrillDownView
3. 100 Ohm .25W resistor
4. 2.2k Ohm .25W resistor
5. Anderson Power Pole plugs http://www.andersonpower.com/products/singlepole-connectors.html
6. A whole bunch of 16gua wire

I started researching and ultimately just did it using this diagram as a reference:


Now, the problems...
1. Once I wired it up it would shoot... when it felt like it. I could get off a few shots on semi and short bursts on auto, but after about 20-30 shots it would just stop all together. Am I safe to assume that since it worked I wired the Mosfet correctly? and that the result of it not working is possibly from a bad connection or inadequate parts somewhere?

2. I foolishly left the battery connected when I ran out to work after realizing that I had spent way too long trying to get it to work, and when I got home, I looked at it again and realized that the mosfet unit itself had... frankly disintegrated from over heating. The terminals had desoldered themselves and one of the mosfets had turned to dust and the whole thing melted the heat shrink that it was in and left a sizable hole in my neoprene pad that was under it (thankfully it was under it or it could have been a lot worse). What could potentially have been the cause for this? or are mosfets supposed to run power when the gun is on safe "off" since the contacts are open (since mosfets essentially create a parallel circut). Also could this have been a result of not pairing the P and N channels properly? I looked at many many forums and people that have done this, and found no information on properly pairing the P and N to create an AB unit, so I just went with something as close as I could find from a guide that I found.

Any help would be greatly appreciated :-/
 

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Bob

There's a pretty extensive thread on the board regarding how to build a AB Mosfet, you can always check if you connected your FET correctly there.

1) It sounds like there's something that's shorting the circuit. Make sure there aren't metal parts touching each other when they're not suppose to.

2) Mosfets cannot be plugged to a almost dead battery or else it'll kill itself (it heats up and kills itself). I heard anything below 7.4(?) volts is really bad. I'm guessing that's what happened.

Hope this helps, and thank gosh you didn't burn your house down!
 

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Discussion Starter · #3 ·
Thanks,

I checked the thread here and saw his original post on ASF i think it was. My main tech. question is how to pair the P and N mosfet, but I'll go through the other thread again and see if that was answered here or there. I have never heard that about the low voltage battery before. I am running a 9.6 NiMh at the moment, but I want to be able to go to a 11.1 LiPo in the future, so that is the end plan. I am going to end up rebuilding everything and wiring everything again, so hopefully this time I will pay better attention and avoid any shorts or whatever happened on the last go around. Although I did heat shrink all the connections and insulate everything when I was done, so I am not sure where there could have been a short....
 

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Hmmmm.. then possibly wrong wiring like you said, thus shorting the circuit. That diagram seems correct though.

Are you sure you didn't plug the mosfet into a almost drained battery? a 9.6v that's almost completely drained is bad for the mosfet. Some ppl add a capacitor to make sure there's enough juice for the mosfet to close when there isn't enough power (correct me if I'm wrong on this one)
 

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its not a low voltage issue.. a low battery will not cause your FET to melt.
I use a 7.4 lipo as my power source never had any problems.

1st question... Did you remove the anti-reverse latch in the mach box?

If you removed it the second culpret could be a burned or bad fet to begin with..
you can also install re-setable fuses to help out overload.

Here is the link for the other post..
http://www.airsoftsniperforum.com/phpBB/phpBB3/viewtopic.php?t=810&start=0

I looked up the specs on your N & P FETs they are a little high but not out of typical range to have caused that much of a melt down..

I'm leaning most to a short in the system somewhere.
 

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Discussion Starter · #7 ·
remove the anti reversal latch?.... wow i feel retarded... i didnt know you were supposed to do that but it makes sense. i am pretty sure i didnt fry my fet in construction as it worked spuradically. im planning to put a fuse in my next one, what fuse should i be using? or how should i figure out what fuse to use? my stock one was removed from my gun before i got it so i dont even know what was in there from the factory.
 

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PolyFuses or PTC Fuses when tripped require a cooling/resetting time period that is longer than most impatient airsofters are willing to wait.

here is an example... you trip the fuse... in 3 minutes the fuse has cooled enough to reset contacts but it is not fully reset.. electricity will flow but it will trip very easily.. to fully cool and reset may take 10 min to 20 minutes.

I find this is a problem for shooters who use their weapon like a garden hose.
 

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Discussion Starter · #12 ·
A car fuse is basically what comes stock in your AEG. Either the tube type or the plug in type. They work fine, but when they blow you will have to replace them, so it would be a good idea to carry one or two in your ruck if you are worried about them blowing.

Thanks for the advice Mojo, I think that I would be able to deal with that amount of time, plus I hardly ever ride the trigger that much... its just not tasteful


Still waiting for another question to be answered before I build another one. How do you pair the fets correctly? When I got the last ones I got 3 of each of the ones in the OP, so I still have 1 or 2 left of each if those would work well together, if not ill have to go and get some more before I do this which isnt a big deal since Jameco is basically right down the street from me.
 

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I use these from DIGIKEY.com ( http://www.digikey.com )

IRL 1404 ZPBF N-ch 40v(Vdss) 75A([email protected]) $2.25pcs
SPP 80P06P P-ch 60v(Vdss) 80A([email protected]) $3.30pcs

and 2 good alternatives for N channel FETs
IRL 1404 PBF N-ch 40v(Vdss) 160A([email protected]) $3.35pcs
IRL 1404 Z N-ch 40v(Vdss) 75A([email protected]) $2.10pcs


I use the Lime Green set only out of habit and that they were suggested to me.
Since I have had good luck using them I have never looked for alternatives.

In comparing what you chose.. I don't see enough of a change in specs to worry about it.

In terms of paring them I just try to keep the Volt/Amp rating comparable to each other.. which you have with yours.

here is the original post I used for source information..
http://www.airsoftmechanics.com/home/content/view/23/30/1/0/

and here is his new site with his latest equip.
http://www.awsairsoft.com/
 

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Discussion Starter · #14 ·
Well my friend took some of the extra fets that I bought and hooked them up to a bread board with a 9v and a motor and tested the circut and let it sit for 12 hours and it didnt even get warm, so it looks like it was user error/design flaw in the way that I laid it out... Im planning to make another one with the same components when i get a chance, with a better lay out this time on a project board. Ill post some details and build pics when I get a chance... around my 75 hour a week work schedule.... :-/
 

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Discussion Starter · #15 ·
Ok guys heres an update and some problems...
decided to try again, but this time I wanted to use a project board to try and prevent shorts and smaller wire (still 16g) to hopefully fit in the grip better. So first pics, then problems:


This picture is of the side of how I decided to lay it out using the wiring diagram in the OP. The N Fet is on the left with the tab down, the P Fet is on the right with the tab up. this allowed me to keep the pins in line (showed in another pic) while being able to place that plate over the top to tie the drain pins and also let it double as a heat sink.


Picture of the top of the assembly. The wire on the right is the (-) wire to the motor from the drain pins.


Pic of the mech box wired up.


this is a pic of the bottom of the assembly where the pins are. The Middle drain pins were soldered and cut and the 2.2k Ohm resistor was soldered across the N Fet on the left of the board (resistor is the two long leads coming out.) Because of the way that I arranged the fets, the Gate pins are in line on the top, so I wired in the 100 Ohm, folded the pins together and soldered the whole thing. The down side to this is that the (+) and (-) leads to the battery are on top of each other. The (-) goes to the source on the N Fet on the left and the (+) goes to the source pin on the right both on the bottom of the board in the pic. since it was kind of cramped it turned out messy but there werent any visible shorts.

The problems:
My friend (an EE student) came over and helped me trouble shoot it since it was not cycling at all after I had it wired up. He found a short between the (+) source and the drain. I visually inspected the whole thing and thought that it could possibly be from the motor. After removing the motor there was no short.

The next thing was to test the motor. So I soldered temporary leads to it and touched it to my battery and nothing happened. There was continuity across the motor and my battery read 7v. (I may have blown a cell a while ago), touched it to a 9v. battery and it turned over.

Hooked it back up to the fet and battery and still nothing, the N fet got warm after a few trys pulling the trigger, but nothing to the motor.

So I am thinking 4 possibilities.
1. bad motor? (doubtful, but im not sure if there is supposed to be continuity across it that is possibly causing the short)

2. Bad battery? More likely since it wont charge over 7v right now, but if the battery is bad, there is still a short.

3. There is a short somewhere else that I didnt notice and didnt/couldnt test? Possible, but there should have been continuity across the short with the battery and the motor removed if it was not in either of those.

4. I have no idea what I am doing since my friend and I are not very familiar with Mosfets, so I could be testing it completely wrong.

help? I have been working on this for 3 weeks and missed the last game because of it. And since I'm the one the coordinates the games I won't be able to play until I get this fixed :'(
 

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1) As I remember, 7v isn't enough for the Fet.

2) Is the wiring for the trigger to the fet correct? Would there be any affects if you switch the wiring? (i.e. trigger wire to the back, + from motor and fet to the front)? <-- I'm not really sure about this one

3) If that's a JG gun, you have to watch out for shorting with the selector plate (?) I'm not 100%. I took a look at mine and saw that the solder from the trigger contact doesn't touch the plate at all.
 

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Discussion Starter · #17 ·
just an update for those following this...

my friend and i ran some tests on the fets and found that they they were working, but were burning out because of too much amperage. We ran them on the motor, with the motor installed and no spring and then again with the spring. They ran in every set up except with the spring installed (thus the reason for the new thread I posted). They ran, however they also burned out in the first two set ups, because without the resistance against the motor from having to pull the spring, the motor kept drawing as much power as it wanted and eventually burned the units out.

I am waiting to do a final build on it until I get the mechanical issues worked out. thanks to everyones input on this! without your comments and input I would probably still be burning fets out trying to figure out what was going wrong
 
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