Simple durable DIY mosfet design?

[luke213]

Well guys I'm looking around for a simple durable mosfet, no fancy features needed just basic trigger contact protection. I can solder and I can assemble things but a electronics designer I'm not. I've never really spent the time researching to understand how to design circuits or really know what components to use. In the past I've looked at some howto's but generally things are out of date, either guys have found different components etc and since I don't know enough about what components are equal I really don't know what to substitute.

So I'm hoping one of you guys have gone into this and found a simple list of components and a wiring diagram that is still current in the sense that the components are easy to get and cheap.

My whole reason for this, some of you know I've got a fairly large collection of guns. Well I had a mosfet in my stubby M4, it failed. So I had to replace it with the only other mosfet I had laying around and I don't honestly trust it since I know nothing of how it was built and it looks like a cobble. So I'd like to be able to build around 10 of these for various guns and builds I've got. So I could buy something prefab for $15 or whatever they run these days, but I'd rather just be able to build them out as needed and repair them by swapping parts etc.

Any insight would be appreciated!

Luke

[ctres]

I've never soldered up a MOSFET myself since they are cheap enough for me to buy from others but I believe that this is the standard design based around the 3034 that most people use.
http://unconventional-airsoft.com/2009/08/26/how-to-make-a-basic-mosfet-switch/

[luke213]

I had read that I think before, for some reason I wasn't able to find parts or maybe it was a different diagram. Whatever the reason I didn't end up building from it. But I found all the bits I needed cheap so I'm going to give it a go as soon as the slow boat from China shows up with my parts. I ordered enough resistors to build 100 since they were cheap by the 100 pack, and I ordered the diodes of mosfet itself by a 10 pack. So I'll have enough parts to build 10 to start with and then I'll see how they work out. Best part was $15 or so shipped for the whole shebang, and 10 will get me through most of my guns that need mosfets;) I can always order some more diodes and mosfets for the rest.

Thanks sir just what I needed, lets hope they work out well;)

Luke

[ctres]

Definitely report back on how it goes. I bought a few 3034 MOSFETs from digikey a while back and never got around to doing anything with them so maybe I'll have to give it a try.

[luke213]

I certainly will, might even do a quick video on assembly if they work well just to help other guys out. I'm fairly new to mosfets in general since no one used them back in the day. So I'm still adapting to new knowledge;)

But since I've used them in a few builds I like the ability to lessen the trigger contact wear over time which is huge for me. I like to put things together once and not open it again for a long time if possible. Really because I've got so many guns if I don't do things that way it's a dang full time job just fixing things when the time comes;) Took me several months to get everything back online again after sitting so long;)

Luke

[kjones734]

A trigger mosfet is a pretty simple and strait forward circuit to build. You should be able to handle it. I didnt look at the circuit at all so im not sure if it is already incorporated in the design but make sure you use a diode to prevent the voltage spike from flowing back into the mosfet and killing it. When you let off the trigger the magnetic field from the motor will collapse and send a very large voltage spike back to the mosfet and if you dont have a diode there to stop it the mosfet will be toast. If you do find that you have issues with the design or building it let me know and ill take care of it for you.

[luke213]

I'm sure I can build it I just don't know enough about things like that voltage spike to be able to design it;) This one does include a diode so it should protect against that.

We'll see how it goes everything is on the slow boat from China at the moment

Luke

Sent from my XT1060 using Tapatalk

[luke213]

Alrighty bringing this back up since today I spent some time trying to build out my first DIY mosfet;)

So things learned, don't trust China when it comes to what you ordered. I got the parts in quite a while ago and hadn't had time to fiddle with it. So the resistors were the wrong ones, should have been 100 ohm and instead were 100k ohm, so that was the first problem. The 2.2k resistors I ordered were correct, so since I had a ton of those around I took 22 of them in parallel to get 100 ohms. Note not a good idea, too dang big but I wanted to make sure it would function and now I'm waiting on the proper resistors again.

But I ran into an issue I haven't figured out, I've got a couple possibles but my understanding of circuits is super limited so we'll see. The mosfet is getting very hot on 11.1v lipo at the end where the negative wire connects. Very hot. One possibility is that the 3034 mosfets I got are counterfeit, and not switching properly which is causing the heat and possibly restricting current. Another option is the resistors are 1/4watt each, and in parallel maybe that is bringing the voltage on the trigger wires and gate up to 5.5watts which I don't know if it works that way or not, or if that effects output since I just flat don't understand entirely how a mosfet works. So possibly it's causing the issues because of my 100ohm cobble. If so that will be fixed when I get the right parts in. The other issue is possibly the wiring is too small on the main lines, but it's fair sized(didn't check gauge), about the same size as normal airsoft wire but also silver from a computer power supply which is supposed to be better.

So minor issues, far too much time invested today identifying the resistor since I didn't notice the diagram saying 100 ohm rather then 100k, once I solved that I ran into the heat issue. So we'll see how I solve that, suggestions from guys with working electronic knowledge would be appreciated.

Overall though I would say it's not fast, rewiring a gearbox isn't a fast process for me typically. But not too bad, If I were building several at a time I'm sure I could make it much faster to build out. But overall not a terrible DIY project so long as you get the right parts etc.

Luke

[Snarf]

I don't know much about circuits aside from highschool physics, but I don't think wiring 22 2.2kO resistors in parallel will give you a final resistance of 100 O...isn't it 1 / R = 1 / R1 + 1 / R2 + 1 / R3 +... so wouldn't you get ~0.01 Ohms (and then there's wire resistance though)? Again, I'm not super knowledgeable on this, but since you might have much lower resistance, the current going through the circuit might be higher than you want. Also, I'm curious as to where exactly you bought your parts.

[luke213]

On the resistance it does work that way I metered it to check my work and make sure I was thinking properly, I also soldered the tips so they were all connected so to speak. But since then I've heard the wattage I suspected is likely the issue but it will be a few days till I get the proper resistors in.

The parts I got via ebay from China random suppliers, so playing the part lottery mostly because of price. I got a hundred resistors for $1 shipped, in both cases. So it's not cost just spec that has me irritated. The 3034's were slightly more but they came with the diodes at right around $10 for 10 of each, so $1 per mosfet plus a couple pennies worth of resistors.

I do think though once I get the 100ohm 1/4watt resistors it will likely work properly. But I'll test it to see once they come in. Also figured out the wire I used was 18ga silver for the main leads to the motor, which from what I've read should be sufficient. So we'll see what I find as I dig a little further into it.

Luke

[kjones734]

is this the mosfet you have? http://www.mouser.com/ProductDetail/International-Rectifier/IRLB3034PBF/?qs=9%252bKlkBgLFf0jZeWnRhawEQ%3D%3D

If so you should have absolutely no problem with current or voltage, this mosfet can handle a very large amount of energy.

I am assuming that you followed some sort of diagram from the internet, do you mind linking it here so I can see if it is appropriate.

If you have a ammeter can you check and see how many amps you are drawing from the battery through the mosfet into the motor.

I would assume you are drawing around 10 amps, that would be 10a X 11.1v = 111 watts. That is a pretty decent amount of energy flowing through a rather small space, it is expected that it will get hot. looking at the data sheet this mosfet can handle a rather large amount of heat (Maximum Operating Temperature:   + 175 C) that equals 347F. If the mosfet is not getting so hot that you are risking fire or damage to the gun then you will be fine. You could also attach it to a smallish heatsink to pull heat away if it is getting too hot.

[luke213]

This link from above is what I'm using as a guide: http://unconventional-airsoft.com/2009/08/26/how-to-make-a-basic-mosfet-switch/

And that's pretty much the mosfet I'm using other than it's likely a clone of it based on my reading, since I bought it on ebay. Knowing likely it would be a clone but it was cheap and I figured I'd give it a try on this application.

So heat wise, with an 11.1v lipo it was getting hot enough to melt solder connections so too hot. Fairly surprisingly it didn't seem to harm the mosfet itself. But I'm still waiting on 100ohm 1/4w resistors to test and see if that is somehow causing the heat issue. With 7.4v it seems to get hot but not hot enough to worry about, I didn't take time to hit it with the IR temp to see what it was actually but felt acceptable.

I admittedly don't understand how that mosfet works, IE I don't know the reason for the various values of resistors etc on which leg and why. I do know running a higher resistor on the leg in place of the 100ohm caused it to cycle very slow like it was running on around 5v or something around that range. So I'm not sure how the magic in the mosfet converts voltage etc, but since I don't have the correct part I'm sort of assuming that's the issue. Once I get the resistors(hopefully today), and see if that solves it or not. If not then my next guess would be the 3034's I got aren't any good. Though I might take and try each of them since from what I understand if they don't switch entirely then they can cause excess heat to be generated.

Luke

EDIT: Alright did a bunch more testing tonight and I think I've drawn some conclusions which may or maynot be correct.

So the 100ohm resistor from what I can tell just drops the voltage/amperage down for the gate on the mosfet to trigger. I found some guys who swear up and down that 3034 mosfet's don't fully open the gate until they hit 10v or so. So I pulled my resistor off the trigger line and redid my test. Same results mosfet still getting far too hot. And by too hot if I didn't mention above we're talking the solder started to melt at the joint. Without the IR temp gun handy I'm not sure what it was, but freaking way too hot.

Now testing wise, what I did was shoot around 20 times on semi auto. With the 11.1v that's enough to get the mosfet that hot which isn't right. With the 7.4v it seems happy, warm/hot but not dangerous levels about what I had expected. So my theory is that these are just garbage mosfet's that I got, the more I read the more I realized how crappy the knock off parts really were.

So I went ahead and ordered from digikey 4 IRLB3034's that I know are genuine and by then I'll have the 100ohm resistors since I'll start with the build as the spec lists on the site. Only if I find I'm getting hot then will I drop that from the circuit and try it the way it's wired now. I expect it's just bad mosfet's at this point, but it could also be wiring, but until I can test the new ones I'm just going to leave the wiring as is and try a new mosfet in the existing circuit.

So more info coming as I get more components etc. But right now despite the set backs I'm cautiously optimistic. And I know at this point I could have bought at least one decent mosfet for the money invested, but I'm still winning if I can make all this work. Since I've got a bunch of guns to install these in, and I've learned a boat load about how they work in the process which will make it much easier to diagnose and repair going forward.

[luke213]

Ok guys bringing this back to the top;) Since this morning while doing some other stuff in the shop I got a chance to build out another mosfet and throw it into my CA M4 test gun. So I ordered proper 3034 mosfets from DigiKey and assembled this morning along with the right resistors.

Much better performance and temps are right where I figure they should be. I'm running around 100F under load after shooting full auto or semi. Where with the other 3034's from Ebay it was hot enough to melt the solder joints. So key to this project which I should have known is getting 3034's from DigiKey so they are legit.

Overall though I'm very happy with it, the ROF is close to what an 11.1v would have done on a 7.4v so it's a big improvement with these 3034's.

I'll plan on taking some pictures later and running through it a bit more.

Luke

[kjones734]

good to hear that you got it all working good. I hope any information I provided was able to help you.

[luke213]

I appreciated the help for sure, since I posted that last message I've built another three of these and all work flawlessly. So if other guys are interested in taking this path, it's not difficult if you know how to solder and apparently the wire isn't a problem. So if you do go down this path make sure to buy legit 3034 mosfets, rather than ebay cheapies and you should have more luck. From what I've read it's a roll of a dice whether or not you'll get good ones via ebay and not worth throwing the money away. On the diodes and other common components it's hard to beat the prices though direct from HK.

Overall I'm very happy with the results though. I've currently got one in a build I've been planning in a Snow Wolf M82, that is pulling an M150 with a Matrix 3000 motor. And I'm still tuning, but right now with a 7.4v lipo it doesn't turn over much less than a normal spring would and I'm pretty dang impressed.

Luke