Running a Raspberry Pi 5V fan off a 3.3V GPIO pin

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johno1234

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#309299 6-Oct-2023 13:28

Hi all, the fan on the pi (3b) is a bit loud and I have read that putting it on a 3.3V pin runs it slower and more quietly for a little less cooling. However this fan's connector is a two pin block not two single pin blocks, so can't span the diagonal between the 3.3V power pin and a GND pin. I've also read that a GPIO pin can power the fan and there are a couple of those that are opposite a GND pin. So plugged in the fan on one of those (Pin 3) and used a little python script to switch the pin on.

However the fan doesn't spin. It seems to only work with 5V.

Anyone else tried this?

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#3143605 6-Oct-2023 13:45

Test the fan on two AA batteries (3V)?

That would tell you if the voltage is the issue or the script. If it won't work on 3V it probably won't work on 3.3V

Mike

johno1234

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#3143606 6-Oct-2023 13:51

There's something off about the whole thing. If I measure the voltage across the two pins (GPIO3 and GND) it is always 3.3V regardless of whether my script thinks it is and has been set to!

[correction] I was using the BOARD pin numbers not the BCM numbers.

Either the fan won't run on 3.3V or the pin doesn't put out enough current to get it started.

frankv

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#3143641 6-Oct-2023 15:33

I'd be surprised if a GPIO pin could provide enough current to run the fan. And it may fry the GPIO if you pull too much current through it.

https://forums.raspberrypi.com

I'd try first from a 3.3V supply rail, and if that works, measure the current going through it. But the above suggests something between 40 and 50mA for a fan.

A maximum of 16mA per pin with the total current from all pins not exceeding 50mA[sic] (Source 4)

I think the above applies to your model 3b, but haven't checked.

If you really really wanted to do this, try hooking up (say) 8 GPIOs in parallel to drive the fan, and (obviously) switch them all at once. [Edit] But if you do this, you can't drive any other GPIO outputs.

But really, get a transistor, or a fan with a built-in transistor (3 pins).

Tinkerisk

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#3143686 6-Oct-2023 18:33

johno1234:

Anyone else tried this?

Yes, I used a 15x15mm fan whose characteristic curve at 3.3V still allowed it to start up. However, not at all directly at the GPIO pin, but a transistor that switched the Vcc. A GPIO pin does not supply the necessary current.

edit: or follow this small project https://github.com/JeremySCook/RaspberryPi-Fan-Control

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johno1234

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#3143698 6-Oct-2023 19:33

Thanks all. It did seem a poor idea to use a gpio. Just goes to show you must treat anything found on the Internet with a little care.

The fan would probably run fine off the 3.3v pin if the connector would fit.

SCUBADOO

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#3143835 7-Oct-2023 00:03

Slightly related.

I have a couple of small 300 PSW inverters cooled by 5V fans that were a little noisy in the dead of night.

Adding a 2c 1N4007 diode in series dropped the voltages by c0.7V making it a lot more bearable.. 2 diodes in series was even quieter but I preferred at bit more airflow through the lightly loaded inverter.

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#3143837 7-Oct-2023 00:19

Yeah, a diode or suitable resistor (depends on the fan current) in series with the fan is a better way to reduce power. It also removes the load from whatever is supplying the 3.3V - probably an LDO, not a proper buck converter.

Wombat1

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#3143838 7-Oct-2023 01:19

Most of the fans are noisy, does yours have 3 wires or 2? If it has 3 you can use the blue wire to set it up to only come on when required in the Raspberry Pi Configuration tool. I did this and now it only seems to startup when I am using it, otherwise its turned off most of the time.

https://www.raspberrypi.com/products/raspberry-pi-4-case-fan/

toejam316

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#3143844 7-Oct-2023 07:15

Just a thought regarding fan noise, if space isn't an issue, bigger fans at lower RPMs tend to be quieter with equivalent airflow. Unsure if it would be practical for your application,

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#3143854 7-Oct-2023 08:03

The best way to deal with fan noise is to move the fan away from you. Alternately, I have this Argon One m.2 case, which has a built in fan. It comes with a script that turns the fan on over a specific temperature. I also have the Pi up in a wardrobe away from where anyone will hear it. It sits on about 40 - 45 degrees, but there's quite a few things in that cupboard making it warm - router, fiber thingy, switch, DC UPS, etc.

johno1234

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#3143865 7-Oct-2023 08:48

Wombat1:
Most of the fans are noisy, does yours have 3 wires or 2? If it has 3 you can use the blue wire to set it up to only come on when required in the Raspberry Pi Configuration tool. I did this and now it only seems to startup when I am using it, otherwise its turned off most of the time.

https://www.raspberrypi.com/products/raspberry-pi-4-case-fan/

Unfortunately just the simple 2 wire one.

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johno1234

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#3143867 7-Oct-2023 08:51

timmmay:
The best way to deal with fan noise is to move the fan away from you. Alternately, I have this Argon One m.2 case, which has a built in fan. It comes with a script that turns the fan on over a specific temperature. I also have the Pi up in a wardrobe away from where anyone will hear it. It sits on about 40 - 45 degrees, but there's quite a few things in that cupboard making it warm - router, fiber thingy, switch, DC UPS, etc.

Yeah once I’m done fiddling with it it can go sit in a cupboard with the router then I won’t care about the noisy fan.

fearandloathing

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#3143876 7-Oct-2023 09:26

You can adjust the temperature at which the fan starts and stops. I adjusted mine by a couple of a few degrees to stop it coming on as often.

Wombat1

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#3143878 7-Oct-2023 09:41

johno1234:
Wombat1:

Most of the fans are noisy, does yours have 3 wires or 2? If it has 3 you can use the blue wire to set it up to only come on when required in the Raspberry Pi Configuration tool. I did this and now it only seems to startup when I am using it, otherwise its turned off most of the time.

https://www.raspberrypi.com/products/raspberry-pi-4-case-fan/

Unfortunately just the simple 2 wire one.

You can buy the 3 wire fans off aliexpress for a couple of bucks, but saying that I have one of those passive cooling fanless cases for one of my Pi4's and it cools better than a fan to be honest.

SomeoneSomewhere

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#3143927 7-Oct-2023 12:41

Three wire fans are usually pos/neg/tach - the tachometer wire allows the fan to tell the system how fast the fan is spinning so you can monitor for fan failure, but doesn't usually let the system control the fan.

You would need a four wire fan which adds a PWM signal.

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