Tuesday, 1 December 2015

Quick Mod: Bullpup RS MOSFET Killswitch

After having my Bullpup RS pusher go full runaway at an MHvZ event (I believe due to a stuck switch), I decided to install a master power switch.

Firstly however, was the issue of the switch itself. To not impede the performance of the RS (and to survive properly), the switch has to be able to handle a current of at least 30A to be safe (calculated using stall currents of 2xFalcons + 1xFK180PH-3050). A switch capable of handling this much current would be pretty big, if it was even sourcable. So I was recommended by Foam Data to use a MOSFET.
For those uneducated in such, MOSFETs are basically switches that switch on an electrical signal instead of a physical button or switch. MOSFETs have a number of advantages over standard microswitches, such as the following:
  • MOSFETs can handle ridiculously high currents compared to a similarly sized switch. For instance, the one I've used is capable of handling 32A and is absolutely tiny (see pictures below).
  • Since MOSFETs are electrically controlled, there's less worry about user error causing issues.
  • Since MOSFETs handle the large current switching, there's less concern about the current rating for the physical switches. For instance the stock switches could be reasonably used. This is especially useful for the Rapidstrike, for which it is a pain to replace all 3 switches with properly rated microswitches.Being able to use the stock switches would be a big plus for convenience. Though MOSFETing a Rapidstrike has it's own issues that I won't go into.
  • Since MOSFETs are electrically controlled, going from MOSFET'd to microcontroller'd is not too big a step. MOSFETs greatly simplify burst/single fire and ammo counters (for Rapidstrikes at least).
The main disadvantage with using MOSFETs is wiring complexity, it requires an extra set few wires and a resistor on top of the rest of the circuitry.
Anyway here's my set up, sort of. Things of note, the new MOSFET with heatsink (the heatsink is mostly precautionary). I used a 30N06L as my friend had one spare. The resistor (which I believe is a ~2000 Ohm one if I read it correctly) leads to the new killswitch, which switches between power and ground. The red wire leads to the negative end of the rest of the circuit, while the black leads straight to ground. Since this picture everything has been wrapped in e-tape, no worries about shorting.
Here's the wiring packed into the rear end. It was bad before the killswitch, it's even worse now. I barely managed to fit everything in.

I'm liking the new killswitch. It works perfectly and doesn't appear to affect performance at all. The switch is low profile and out of the way, but still very easy to reach and use if necessary.
I don't think MOSFETs are necessary for most uses, regular microswitches can handle the currents necessary well enough for the most part and the extra wiring gets annoying quickly. For instance I would never MOSFET a Stryfe, it's nearly doubling the wiring required and is pretty unnecessary. But if you're looking at installing a master killswitch for a Rapidstrike a MOSFET is probably the best way to go unless you want to try fitting in a big microswitch.

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