Blown Mosfets

Controllers Smoothie
1

Smoothieboard v1.1 (from Openbuilds)
I have already flashed fresh firmware and used fresh config files multiple times.

My plan is to attach a DC/DC 24V to 12V converter to power some fans and I’d like to be able to turn them on/off. When I hooked it up to large mosfet Q5 (p1.23), I checked the converter’s output voltage and it was 24V. When I sent the fan_on command, output voltage was 12V. Turned off, it would go back to 24V again…
Apparently I blew the mosfet completely because now I am getting 0V no matter what.
Q4 and Q5 both did the same thing. I wired separate 24V in for the small mosfets (Q8 and Q9) and had the same result here.
I hadn’t soldered an extra diode on since v1.1 comes with surface mounted diodes pre-soldered.
So, when I replace the mosfet, should I add another diode, or can I not use the converter through a mosfet at all? Did I receive a faulty board to start with? Not that it matters now that I can’t return it, I’m just curious…

Imported from wikidot

2

I couldn’t explain the electronics reason why, but I remember a similar case a year ago, and the conclusion was : don’t do that ( converters after mosfets ), it’s bad. You want the converters before the mosfets.

3

Thanks for the reply, Arthur.

I accidentally found a way around it in an unrelated post you had replied to years ago.

In case anyone else has this issue, you want to connect the converter’s input + to the power supply and the input - to the mosfet output. I am getting consistent 12V output from the converter now and I’m able to switch it on and off with gcode without destroying the mosfet. I’ll update this post of I have problems with it later on, but so far, so good.

Arthur, I can’t tell you how much I appreciate your dedication to helping the community. My build wouldn’t have been possible without the incredible contributions from you and others on this forum.