Basic Nerf Stryfe Electrical Modifications, Part 2

Or, "Fuses and MOSFETs, oh my!"

In Part 1, we showed how a simple pair of 14500 sized rechargeable IMR batteries, without any rewiring or shell cutting, can take your dart speed from 60 fps to nearly 100 fps.  The one practical consideration I skimmed over was how to run two cells in a four cell blaster.  "Dummy cells" are the answer.

You can usually pick up a set from the same place you buy your batteries, or make your own:
On a graph of performance verses effort/cost, IMRs represent the biggest bang for the buck and the "lowest-hanging fruit".  It is all uphill from here, so if your thirst for performance is already satisfied, quit now while you still have some cash left in your pocket.

The graph, while meant to be humorous, also provides a bit of a roadmap of where we are going.  Everything from here on out will require you to violate the "do not modify blaster" molded into the shell.  You will need to open it, and learn how to solder. Time to become a rebel hardware hacker!

If you are still tempted to bypass your fuse at this point, let me show how little it will gain you, and why upgrading the wiring and adding a MOSFET is a better choice.

Comparison of average dart speed in feet per second of fused and bypassed fuses in stock blaster:

Comparison of average dart speed in feet per second of stock wiring and upgraded wiring with MOSFET control:

The increase in current that comes with MOSFET control means we need to change out the stock fuse if we are going to use anything other than AA or NiMH batteries.  The higher the current we subject a fuse of a given rating to, the quicker it will activate, and at some point that will start to occur during the normal starting surge.

A fuse rated at "1 amp" doesn't actually tell us everything we need.  Since starting current and stall current are the same, it is time we are really interested in.  We need the fuse to activate sometime after the starting current normally subsides (< 1 second) and before the motor is damaged (> several seconds).  The datasheet for a fuse series will give us what we need to select the correct fuse.  If we want the fuse to activate at 5A, we just need to find which fuse line crosses in the "seconds" time-frame.  In this case, it looks like the AHEF100 (C), which is confusingly identified as a "1 amp" fuse will activate in about 3 seconds at 5A.  Just remember that the graph, like our earlier starting current graph, is log scale.

As far as the actual operation of a MOSFET goes, and trying to keep this more practical than theoretical,  see the circuit diagram below in conjunction with the following points:
  • The "Gate" of the MOSFET is the signal from the rev switch that allows current to flow through the motor and from "Drain" to the "Source" to ground.
  • A small resistor from the "Gate" to ground keeps the MOSFET turned off until it gets an actual signal from the rev switch.  This prevents "floating" signals from accidentally activating the motor.  It can happen, and it is spooky.
  • An inductive load like a motor or solenoid will generate a voltage spike when de-energized as the magnetic field collapses.  This can damage the MOSFET, so a "off-ramp" is provide in the form of a "kickback" diode.
  • And finally, the MOSFET must be on the low side, or down-stream side of the load to work.  Much frustration will result if you try it the other way around.  Ask me how I know.
A MOSFET with a kickback diode and pulldown resistor can be just "wired into" a circuit, as demonstrated masterly by Tungsten EXE in this video, but I find it much easier and less error prone to design and test a circuit board.  That also lets me add a fuse!  In the Stryfe, I'm using the existing wiring tray cover screw locations and all components are on the back to avoid interfering with the darts.

My designs are freely shared as Open Source, so you can buy the PCBs and parts yourself, or modify the design to meet your own needs.  You can also save yourself the trouble and help me pay for this type of experimentation and design by buying a kit from me:
Tune in to Part 3 where we will finally upgrade the motors and hit the big league of starting currents.  And more fuses. 

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