Saturday, 1 November 2014

Get a MOSFET They Said

I have come to the conclusion that getting a MOSFET for the AUG is a good idea. That should spare the trigger contacts and make for a crisper trigger response. Sounds like a pretty good deal to me. I know there are ready made MOSFETs around, but I'd rather save some money, get the parts myself and try my hand at building one.

Now, I will readily admit that I'm not an electronics wizard by any measure. However, I can follow guides quite well, and have built other stuff in the past with success. After finding a promising looking how-to, I bought the parts and started soldering.

I could only get resistors in packs of 50, but at least they cost next to nothing. I got more than one of the MOSFETs as well in case something should go wrong

The AUG gearbox is handy in that you don't need to take it apart to be able to put a MOSFET on, since the wires are soldered onto the outside. This is apparently done to achieve the two-stage trigger mechanism. This thread on pretty much describes the process of installing the MOSFET.

As I started desoldering the wires, I found it was not quite as easy as I thought. I have an adjustable power soldering iron, but even at full blast it took ages to make the solder on the gearbox melt, and as soon as the heat was taken off, it would set again. What is in that stuff I don't want to know. In the end I was however able to suck off all the solder and wire in the MOSFET.

I have to admit it took me a bit of time to actually make sense of the wiring diagram, since the positive and negative marked wires didn't seem to make any sense. I have the default, small Tamiya connectors on all my batteries, and I went to Wikipedia to see and make sure I got them the right way around. Ok, positive is the square terminal, negative the other. Would not want to have my motor turning the wrong way, definitely.

Comparing the diagram and the connector didn't seem to make any sense though. The fuse seemed to be on the wrong wire on the diagram. I started to feel like I'm just dumb for not figuring this out, until about fifteen minutes had gone by and I went on Wikipedia again and read a few lines further, where it says "In some cases, Mini-Tamiya connectors are wired in reverse polarity. This is often the case with airsoft guns, where the square profile terminal is the negative terminal and the rounded terminal is the positive terminal." Right. Great. Why? I don't know the reasoning behind this non-standard wiring but at least I'm wiser now. Time well spent.

With the wiring issue sorted, I finished up my soldering. I hooked up the battery to see if everything works, and with building anticipation pulled the trigger. Everything worked! Brilliant. I'm not dumb after all!

My elation was rather short-lived though, as I apparently managed to touch the MOSFET on the soldered wires, thus shorting it, and with a bit of smoke rising towards the roof, the MOSFET burned up internally so that the circuit was now eternally closed, and the gearbox was running on full auto regardless of whether the trigger was pulled at all. I guess it's a good thing I bought spares. I took off the battery, boxed up the pieces and went to bed.

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