12V fan not working

[Andrew-H]

Hi everyone - first post - I hope you're all well?

I've been using a prebuilt HeaterMeter 4.3.4 with a BBQ Viper fan (12V <1A) on my MiniMax BigGreenEgg with no issues for quite some time - I love the thing so big thanks to Brian et al.

Yesterday, the fan stopped working. The rest of the board seems fine - the conversation with the Pi works fine and you can see the device online. The fan itself tested fine, as did the cable but when anything up to 100% is commanded, I don't see more than 0.005-0.011 VDC at the blower pin on the RJ45 connector. 5V is there, servo pin is 0.63V but nothing useful at the blower pin.

I've seen the post earlier this month regarding a fan not working but I believe this is a different fault. I don't seem to have any voltage at any of the pins to the mosfet at the top of the board. Power source is and has always been my 12V motorcycle battery (for portability)

I have 12V at the bottom of the diode D3(?) and 12V and 5V at IC1 so all good there.

However, as the blue line (in PCB layout diagram) connects the bottom of D3 up to the Mosfet (left leg in PCB layout diagram) where I only have 0-0.07VDC then surely it's a board fault - I've tested that connection and this seems to be a point of failure but I'm worried why it failed - has the MOSFET failed, shorted something and popped the board but also done damage elsewhere...? None of the mosfet pins are shorted to each other and the line from 12V blower to the mosfet centre pin is fine.

Any guidance on tracing the fault please?

Thanks in advance, Andrew

 

[Bryan Mayland]

I would check the board without being powered and see if you have continuity from the bottom of D3 to the left leg of the MOSFET (the blue line). If this reads as open then that's pretty crazy because it would mean the trace burned (or the solder joint is bad, might want to just hit both of those with the soldering iron to check). If it is, then you can just jumper it with a wire to test if everything works again.

If the MOSFET failed open then it just won't turn on at all, but if it is fused closed, then it will just act as a straight passthrough to the fan and just always give it the full 12V. Just test it without a blower connected and see if it gets hot, be prepared to power down immediately if that happens. If it feels cold still, plug the fan in and see if the fan runs fully at all times. At that point the MOSFET shouldn't overheat because the fan doesn't pull that much current.

 

[Andrew-H]

Thanks for that Bryan. No bad solder joints so the craftsman that made it can rest easy. What I was getting at in my original post was that I was unsure about soldering a jumper wire from the bottom of D3 to the left leg of the MOSFET without understanding the cause - fixing the symptom not the cause and all that...

Either way, the board has failed as discussed so I've soldered the jumper wire and it seems fine - I guess we'll never know what happened.

Back to making pastrami....

Thanks again, Andrew