Tiny MOSFET

[Knappster]

I've been asked to make a basic MOSFET switch as small as possible but with polyfuses and some circuit protection. I came across Gandolf's design here: http://extreme-fire.com/Background/LARS-2009-06-22.jpeg and have copied it into Eagle PCB like so: http://openairsoft.net/stuff/TinyMOSFET.png

Question is, am I going to get away with a smaller 600W TVS or do I 'NEED' to have something close to the 5000W range? I have supplies of 600W TVS's but I've yet to find 5000W devices in the UK. Although I'm going for as small as possible it'll still be built onto a PCB as I don't like free wiring something like that, maybe if it was just the 2 resistors but it wouldn't be that reliable I don't think.

[Assault Team Luke]

I'm almost certain you've already looked here, but I thought it might be worth posting. Their minimum order is one and have prices in pounds, although it seems they still ship the items from America.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=5KP18ALFCT-ND

[Knappster]

Yeah it's the shipping and import costs I'm trying to avoid, it's really not worth it unless I order 100's of them. If I can use what I already have the better .

[Knappster]

Finished a preliminary design anyway:





I could make it smaller by placing the resistors and TVS on the back but I want the polyfuses to rest flat against the board to minimise the risk of them breaking.

[Assault Team Luke]

So, how small are we talking about roughly? It'd be great to have an alternative to the Infected MOSFETs. They're really small but I've already had one try to flambe my M16, and I'm nervous about using it in my UMP except it's the only thing I can fit in there.

[Knappster]

The board size is 34.5mm by 16mm, can't give you the height till I've built one.

[hunterseeker5]

So far your dimensions sound comparable to one of my fets.......  I need to go get some calipers and come up with exact numbers but somehow I think that 3.5cm long is a bit on the large side for a "micro" fet.

[Assault Team Luke]

I'm glad I'm not the only one thinking that. An SW-AB-LONG is ~50 mm long and the same width, so really your Tiny MOSFET isn't all that tiny.

[Knappster]

It would be a lot smaller if I used through-hole connections for the cables, removed the polyfuses and placed the resistors and TVS on the other side of the board. From recent experience I prefer to solder the cables flat on pads and having all the components on one side makes it dead easy to solder the board with a hotplate. A soldering iron would only be needed to attach the cables. Tiny was probably a bad choice of word . It's basically a standard mosfet with a bit more than just a fet and 2 resistors and built properly unlike some freewire monstrosities I've seen in the past. The extra copper on the back of the board gives a little heatsinking as well, not that the IRLS3034 needs it .

[Gandolf]

The power the TVS has to absorb is proportional to the voltage squared.  However, the high current LiPOs tend to kick up higher voltages too.  I think 8.4 and 9.6 volt NiMH guns would be OK with 600W TVSs and they should work on 7.4V LiPO too.  But anything higher should probably go to the omni-directional 1500W TVSs like the Panther uses.  They seem to also work on the 14.8V LiPOs.  Beyond that the 5000W TVSs would rule.  But the 1500W and the 5000W  SM TVSs are the same size so cost is the only factor there.

Gandolf

[kiwi_mat]

So, how small are we talking about roughly? It'd be great to have an alternative to the Infected MOSFETs. They're really small but I've already had one try to flambe my M16, and I'm nervous about using it in my UMP except it's the only thing I can fit in there.

I have installed a SW-AB-LONG inside a UMP for someone, he is using a 11.1V 1250 mah lipo in it, works really nice and fits good too..

[Assault Team Luke]

I just about had an AB-LONG fit but it was fiddly to say the least. Plus, the battery I'm using in it is an 11.1V 20C 1600mAh LiPo battery, which may be a little larger, and a Wolf Dragon LiPo alarm. It was a bit on the cosy side, but now I've substituted it for an Infected AB MOSFET it's much easier to install batteries. I'm just concerned about the lack of fuse and Infected's distaste of voltage spikes. Finger's crossed. Sorry for derailling your thread, Knappster. 

[hunterseeker5]

Part of the reason the infected fets are so small is because they contain NO voltage protection. They are just an N and P fet back to back with two resistors, no heatsink, TVS or anything. I have pics of a few burned out ones if anyone is just dying to know whats inside, but really it isn't particularly surprising or interesting to look at.

[Knappster]

Breakage is something I want to avoid, as we all know replacing a broken part in any AEG is a right PITA. 1500W TVS's I think I can get hold of. Found one: http://uk.farnell.com/fairchild-semiconductor/smcj33a/diode-tvs-33v-1500w-smc/dp/1017857

[Assault Team Luke]

When I ordered the second one I stripped off the heatshrink so I could add a second trigger wire (just because I prefer it that way), and I was amazed at how simple it was. I really feel, even with my childish grasp of electronics, that I could make one myself. I did notice it was literally the bare minimum AB setup; I suppose you can't have everything. It seems it's a choice between small or clever.