mosfet problems
 

Ok so here is the situation

I have a Echo 1 AK700. The stock motor is a POS so I wanted to upgrade it. I purchased a systema turbo high speed motor. Upon installation I noticed my DIY mosfet getting really really hot. So hot that it promptly burnt it out. So I built a new one and tried it and noticed it would still get hot on semi but was ok on full auto. I checked my shimming, replaced gears with a fresh SHS 18:1, lubed etc. It still had the problem. So now I'm thinking that maybe the motor is bad or the spring is too heavy so I picked up a lonex A1.Popped it in and It is still having the problem. My last step has been to change out to 13:1 gears just to see what happens. I would have expected this to make it worse but no real change (maybe even slightly better).

The gun runs very smooth besides the overheating. It is also worth noting that the motor stays cool and so does that battery and wires. Im honestly at a loss here. The only other thing I can think of is that I got a bad batch of mosfet materials but I haven't had problems in my other guns so I don't think so.

Any ideas?

any good gun docs in the guelph area?

What FET are you using, and how are you wiring it in?

Show us your schematic.

this design

http://unconventional-airsoft.com/wp...ple-mosfet.jpg

That's the basic FET layout, how are you wiring it in to the gun?

not sure what you mean by wire it into the gun? It was done just like the diagram. Its a version 3 gearbox so all wiring is external. I use 16AWG for the motor wires and 18 for the trigger leads.

There should be 1 trigger lead, and you can use as small as 22ga wire for it. The other is a Y going battery +/motor +/trigger.

And there should be a resistor in-line with that 1 trigger lead, otherwise your MOSFET gets in triode mode and that sucks.

My mosfet design is sound. I have used it in all my other guns and they work 100%.

If your MOSFET design and wiring were sound, you wouldn't be experiencing this issue. Can you show us a picture of the wiring?

do you want a close up of the chip or just the wiring as a whole (need to know if I need to cut off my cover)

More interested in how you wired your gun as a whole, I'm going to assume the FET itself was assembled correctly.

http://i824.photobucket.com/albums/z...psoxvnouze.jpg

http://i824.photobucket.com/albums/z...psmphnxu2p.jpg

Well I guess the heat shrink is covering up the part I wanted to see.

So the FET is on the negative wire, which goes battery --> FET --> motor, then your positive wire goes straight from your battery to your motor. Your signal wires go from the FET --> trigger contacts --> battery positive?

sound clip of the gun operating? Don't record in a tin can room either lol... if all you have is a shitty mic try and dampen the sound a bit by pulling a tshirt over the mic.

thats correct

Does the problem still persist if the gearbox is not in the receiver?
I know it sounds weird but I witnessed a v3 with this problem the other day, the pistol grip was causing tension on one of the motor leads.

The wiring sounds correct. Could you post a picture of the FET unit itself? Can you also check with a wattmeter and see what kind of current draw you're dealing with? I've run 3034 based FETs on 14.8V batteries in DSG setups with no heating of the FET itself.

Ill try and get a watt meter reading tomorrow. Whats the correct sequence to hook up my multimeter in to get the reading?

yes the problem is still there

I'm not trying to be condescending with this picture lol. Its just what I found on Google.

http://www.cdn.sciencebuddies.org/Fi...re-current.jpg

ok so here is a shooting video for sound. I have tried to get a reading on my multimeter but the spikes were too fast for it to read. Also theres no way to get watts directly on it only amps or volts. Seemed to be spiking at 19 amps on a 7.4 battery but Im really not confident in that value

http://i824.photobucket.com/albums/z...pskjbbmwfe.mp4

That's fine, Watts are units of power, you get that from multiplying current (amps) times potential difference (volts). 19A on a 13:1 setup with Lonex motor sounds normal. Your wiring sounds like you've done it correctly. You may have to open up your FET and make sure you didn't accidentally put the 100ohm where the 2.2kohm is supposed to be or something along those lines. Also, did you use a TVS? It doesn't look like you can fit the 5kW TVS specified in that diagram in your contraption.

gun sounds okay as well. There is a bit of a screech but it could just be the recording, sometimes well shimmed guns will sound a bit like that on small mics. 19A for 13:1s on bearings and an A1 is pretty damn good, probably as good as it's going to get.

Probably look at the fet more closely in that case as pesto says.

ok so here is a pic of the fet itself.

http://i824.photobucket.com/albums/z...ps4px6pofa.jpg

Your wiring looks correct, so does your FET unit.