Fuse test mode

A project log for REMB - Raz0r Electric Mountain Board

An electric mountain board, built from scooter parts.

Daren SchwenkeDaren Schwenke 09/02/2019 at 06:003 Comments

It was all going so well.  I tested forward and brake for both channels and it was working perfectly.


That is the magic smoke being released from the left braking mosfet. It was quite the event and I can tell you that mosfet never gave up.. it was positively glowing before I cut power.  Perhaps I should have actually included the fuses in the test.  :)

Somehow it seems to have taken the two drive mosfets with it during its demise.  So it seems I will need to rebuild half of my board here to fix it.  Ugh.

I have since discovered that the 2N3904 transistors I picked to drive my braking P channel mosfets have two different ratings online.  One group is rated for 40v, the other for 60v. My battery voltage maxes out at 42v, so can only assume I have the cheaper ones.  I am driving the P channel via a voltage divider to keep the gate voltage within the max Vgs of my mosfet here, so I'm basically left with that.

Currently rebuilding it with spade connectors, and seeing if I have some higher voltage rated NPN transistors here.  Cross your fingers.


Xasin wrote 09/02/2019 at 17:13 point

Ouch, that really sucks ... 
I know and hate the feeling of vital or hard to desolder components giving up. Had to deal with a few (like a 40 pin SSOP), but luckily nothing as bad as a BGA or so.

In any case, I really hope fixing things will go well, and that you can figure out the problem with the FET!

Broken BJTs due to the voltage are definitely a possible culprit, especially if they drive the FET gate improperly.

  Are you sure? yes | no

Daren Schwenke wrote 09/02/2019 at 18:27 point

Pretty sure it was either the gate of the BJT floated up (somewhat unlikely) or it failed shorted due to overvoltage (pretty likely).  I didn't have a pulldown resistor on that BJT gate but I won't need it when actually hooked up to the MCU.

In either case it turned on my braking FET while I was testing forward... so.. pop.  I got spares of everything I used and a handful of BC337 NPN transistors rated at 50v, so.. just more work.

  Are you sure? yes | no

Xasin wrote 09/02/2019 at 18:58 point

As usual.

I'm working on a small series of Lasertag sets, 4 at once, so... That'll be fun to solder <.<

Good luck!

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