Jetson nano fan running when unit powered off

Hi peoples, I have two Jetson Nanos 4gb B01 units that i have connected up with Core Electronics supplied 5v Fans. However the fans run at full speed when the jetson nano is turned off. The j15 header on the Nano is supposed to be PWM type however it has 5v on pin 2 and pin 4 even when powered off. I noticed that both fans were supplied from Core with the wrong wiring on pins 3 and 4 of the female connector but even after swapping these wires over the full speed when turned off problem persists.
Note that i am able to control fan speed when the unit is on using the automagic fan control. It is just a problem that when the nano is shutdown the fan runs at full speed.

I suspect there’s still power on the 5v rail (else the nano couldn’t boot) and the fans - being powered and received no valid PWM signal - are behaving as designed. It’s probably a (quite sensible) fail safe design feature.

As for how to fix it - you’ll either need to cut power to the fans (maybe use a mosfet connected to a spare pin so they only have power when the OS is running) or send them a valid 0 speed PWM signal while the power is off. Could be as simple as a pulldown resistor on the PWM wire - you’ll have to check what pwm signal they’re expecting.

Thanks Oliver. I am cracking on with a CD4066BE quad cmos switch and using pin 18 from the HDMI connection to obtain a switched 5V signal to disconnect 5V from the FAN input. It doesn’t matter what state or setting the Fan setting it set to prior to powering down the Jetson Nano, as soon as the main system is off line the fan shoots back up to full speed. Cheers for the input, nice to know that I am not going crazy! :slight_smile:

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Hmm, actually I just had a quick google. Seems like there should be built in fan control on the Jetson - there’s a dedicated 4 pin header for it. Maybe it’s a config thing?

I also found a script someone’s written to get variable speed control:

You are correct that there is a PWM compatible header on the B01 Nano board with 4 pin header. I was already using the Pyrestone automagic fan control, which works perfectly when the Nano is up and running but when I go to Power off (shutdown -P) the Jetson it would cause the fan to go to 100% speed - and it didn’t matter if prior to shutdown the fan was turned off completely in the automagic json file or which 4 wire fan I was using (tried a few including a noctua $$)

But ! your post made me consider what if there may be a pull down resistor missing in the B01 developer kit reference design.

So with a 1/8 watt 3.3k resistor between ground (pin1) and pwm signal (pin4) and the result is Problem Solved!! - thanks for the help. Much simpler than using a quad cmos switch and a bunch of other stuff.

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Nice find! Gotta be pretty chuffed catching and fixing a mistake of the engineers at Nvidia!