FanPico using Aux power alone, without USB

[lowjoel]

Hello, I recently bought a FanPico 0804D and I've been powering it using aux power alone - there's no USB connected. When it's already booted, the steady state will keep running (fans, metrics etc all work.)

However, when there's a power loss or restart, the device will turn on (all the lights turn on) then when the LED on the RPi lights up, the entire board power cycles. It then gets stuck in this loop until the USB cable is plugged into a computer.

Is there a way to make it completely headless and run only on aux power?

Thanks!

[tjko]

Are you using latest firmware (currently v1.7.4)?

I just did test by powering unit only using AUX connector and couldn't reproduce the issue you describe.

Do you have "MAIN", "AUX", "5V", and "3.3V" LEDs all lit? (are LEDs uniformly bright and no flicker when powering on / rebooting?)

[lowjoel]

Yep, 1.7.4. FWIW it's a 2A power brick.

[tjko]

It could perhaps be that the power supply can't handle initial load spike when fans start. Does the problem go away if you disconnect the fans?

If it works withouth fans connected. Then try adding fans one by one to see how many fans it can handle... (btw, what is the current rating on those fans you are using?)

[lowjoel]

https://noctua.at/en/nf-f12-pwm/specification x4; 0.05A max rated per fan. For a 2A power adapter, go figure. I can start fine with 3 fans, the 4th makes it power cycle. I guess Noctuas have high initial load spikes.

Is there a way to make the fan spool up staggered? Or maybe we could have some filtering for the RPi using a capacitor? The fans do start spinning before the RPi switches on, so I'm not sure if we can PWM signal it down to maybe 20% load first before spooling up.

[tjko]

2A Power supply should be plenty. It would seem power supply has problem, or could be that there is poor connection somewhere (or too thin power cable could cause significant voltage drop as well).

Are you able to try another power supply or power cable?

[lowjoel]

I'll try to look around. I've checked the connections, seems to be OK. I'll have to look for an alternative power supply.

I see that the board only uses the 12V supply so maybe I could replace the floppy connector with a barrel jack, but that will take time.

[tjko]

Newer FanPico PCBs support installing either PC Floppy Power connector or standard DC barrel jack for AUX power.

I've been using SATA or 4-Pin MOLEX to FDD power connector adapters to power my units. Something like these:

Good quality adapters typically have thick (18 AWG) wires that should work fine. But there exists cheap adapters with very thin wires that might not work withouth causing significant voltage drop...

[lowjoel]

Thanks! I've tried multiple molex-Berg adapters from different sources (including those from my Corsair PSU) and the same thing happens. They're officially 18AWG but you're right I can't actually measure them.

[tjko]

I cut off SATA connector from a SATA to Floppy power adapter cable and connected DC connector to it.
This (cheap) adapter had nice thick looking wire labeled "18 AWG" but these cables clearly were "fake" , actual copper thickness is at most 22 AWG:

Adapter still "works" but at at 0.5A load voltage drop was about 0.5V so Fans were only seeing about 11.5V (so would not quite reach their rated top speed).

[lowjoel]

Yeah - I noticed that my fans only report ~1250rpm max, vs 1500rpm on the spec sheet. There's probably some significant voltage drop somewhere. I'm checking with my PSU manufacturer, probably will get another one -- because the PSU itself seems to be cutting off altogether when it cycles.

[lowjoel]

I've fashioned my own barrel-to-Berg connector with a new PSU, and it starts and boots fine. My RPMs are also back to the rated fan speeds at 100% -- so I'll conclude that it's my faulty Molex PSU. Many thanks!