NimbUx
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Running fan on 3.3 volts instead of the nominal 5 V

Tue Jan 14, 2020 4:10 pm

Running the RPi4 as a desktop box with a basic cooling fan (supplied in the "Labists" RPi 4 kit); fan did a great job of removing heat (ca. 15°C, or 27°F temp. reduction) at the slight price of noticeable sound. I therefore fancied to try feeding the fan from the Pi's +3.3 V pin, resulting in decreased rotation speed, appreciable noise reduction (it"s almost silent now, at least not noticeable/distracting). Of course cooling performance has suffered, but still lowers the temps by around 8°C / 14°F (vs. running w/ cover & fan removed).

I'm pleased with this arrangement, provided the hardware Guru will confirm under-volting is an approved mode of running small DC CPU fans, and importantly that the Pi's 3.3 V is adequate for this application. I"m just an old software kind of guy, then, what do I know ? ... TIA !
Last edited by NimbUx on Wed Jan 15, 2020 11:51 am, edited 1 time in total.

W. H. Heydt
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Tue Jan 14, 2020 4:53 pm

I got a couple of small nominal 5v fans that said they could run on 3.3v. I've had one running for a couple of months as a test. No issues so far. I was more concerned about the current load on the 3.3v line than I was about anything that might happen to the fan.

alphanumeric
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Tue Jan 14, 2020 6:48 pm

My SmartiPi Case came with a small 5v fan. It was a bit noisy to my liking so I moved it to the 3.3V GPIO pin. Its nice and quiet now and still cooling my Pi 4B. I hardly noticed any difference in temps maybe a degree or two higher.

NimbUx
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Tue Jan 14, 2020 8:02 pm

So, anecdotal evidence gathered by several posters seem to confirm it is OK. Still I'd appreciate to learn from the expert if the amps drawn from the 3.3 volts rail by the typical asiatic no-name mini-fan are well within the specified envelope... how (in)significant, or conversely near the limit ?
[edited, to add] ... Some search/browsing thru this forum found that note un an old post (2016) viewtopic.php?t=151871
The 3.3V regulator on early Pi was a linear regulator and severely limited, but the switch mode regulator on later Pi is rated at 1A, and testing indicates it can provide close to this to peripherals (PI power source permitting).

If that is confirmed I think it would imply running a case fan out of one of the Pi's 3.3 V pins is well within allowable limits. Righto ?
Last edited by NimbUx on Tue Jan 14, 2020 10:41 pm, edited 1 time in total.

hippy
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Tue Jan 14, 2020 8:20 pm

I don't think any "hardware Guru will confirm under-volting is an approved mode of running small DC CPU fans". I expect most would only go as far as "might work", "often does".

It's hard to say what current is being drawn, what effect that may have on other things connected to the 3V3, or the regulator providing that 3V3. That greatly depends on the fan and can probably only be determined by measuring and testing.

I would expect most advice for under-voltage fan use will be to use a separate regulator, fixed or variable, powered from the 5V supply. Given the risk of damaging or bricking PMIC chips on the latest Pi's that's what I would suggest.

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mahjongg
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Tue Jan 14, 2020 8:24 pm

NimbUx wrote:
Tue Jan 14, 2020 8:02 pm
So, anecdotal evidence gathered by several posters seem to confirm it is OK. Still I'd appreciate to learn from the expert if the amps drawn from the 3.3 volts rail by the typical asiatic no-name mini-fan are well within the specified envelope... how (in)significant, or conversely near the limit ?
It depends of the model RPI you have, some have 5V to 3V3 converters that are more robust than others.

In general terms running a 5V fan from the RPI's 3V3 supply is a bad idea, its much better to place one or more simple diodes in series with the 5V supply to drop 0.7V per diode, so with two diodes 1.4V, resulting in 5.0 - 1.4 = 3,6V.

NimbUx
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Tue Jan 14, 2020 10:51 pm

mahjongg wrote:
Tue Jan 14, 2020 8:24 pm
It depends of the model RPI you have, some have 5V to 3V3 converters that are more robust than others.
I have the newest RPI 4.
In general terms running a 5V fan from the RPI's 3V3 supply is a bad idea, its much better to place one or more simple diodes in series with the 5V supply to drop 0.7V per diode, so with two diodes 1.4V, resulting in 5.0 - 1.4 = 3,6V.
Ahem, would you kindly provide a suitable part number / link to locate it online, for a dummy such as myself ?

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mahjongg
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Tue Jan 14, 2020 11:03 pm

Any small signal silicon diode would do, the canonical example would be a 1N4148, from farnell or any other electronics supplier:
https://nl.farnell.com/search?st=1n4148

another popular diode would be a 1N400x (for example an 1N4001), or an 1N914.

X-Gen
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 2:56 am

I've been running the fans on my 3B+ on the 3.3V for a while now.
The only issue at the time was, that the 3.3V line could cause a little ground lift; and the moderators on the forum at the time, cautioned in case you're using more than 1 fan.
1 fan works fine.
2 fan designs seem to work fine as well.

A minor overclocking adjustment on the 3B+ was necessary (5Mhz lower overclock) for stability reasons.

If the fans are DC motors, they should handle lower voltages just fine (PC case fans and CPU heatsink fans, as well as GPU fans operate the same). Lower voltage just lowers output.

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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 10:55 am

I like mahjongg's idea....seems doable (oh goodie some experimenting later)

but still prefer the 5v powered full throttle quiet fan that is the noctua.

and I've good successes silencing those noisy cheapo 5v fans running on 5v (have several still running now)
reading 20.8db to 22.4db at 10cm distance

am still to find a less than 10db 5v powered fan for the RPi (does that even exist?) ;)

ok going back to using 3.3v for the fan
if it were me I'll get 5v from the microUSB side, get a 100mA 3.3v LDO regulator ( or those itsy bitsy tiny buck converters) and power the fan from there.
as I really don't/never like using the RPi's 3.3v pin
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Come to me with 'problems' and I'll help you find solutions"

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PeterO
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 11:13 am

Another option would be to run a 12V fan from the (more robust) 5V supply.
I did that on my Pi4 until I got a Pimoroni fan-shim.
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NimbUx
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 11:41 am

Thanks to all and every adviser ! Although I appreciate the wisdom and warnings against using the 3.3V directly for this purpose, I also noted that those doing it are not reporting failures, much less catastophic issues. Meanwhile I'm taking the risk (if any) and leaving the cheapo fan on its ration of 3 volts, I've already powered the Pi on and off several times and so far so good (am I stubborn !)
What I am going to do next time I shutdown the system is to fetch my cheapo multimeter & measure currents drawn by the fan in operation, and when starting and shutting down : I hope the figures - that I'll be sure to report here for discussion - will be low enough to assure my and everyone's peace of mind ;=)

hippy
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 3:04 pm

NimbUx wrote:
Wed Jan 15, 2020 11:41 am
What I am going to do next time I shutdown the system is to fetch my cheapo multimeter & measure currents drawn by the fan in operation, and when starting and shutting down : I hope the figures - that I'll be sure to report here for discussion - will be low enough to assure my and everyone's peace of mind ;=)
I would take any such multimeter readings with a pinch of salt. Multimeters tend to average readings and won't reveal the in-rush current, transient peaks or ripple. It is those transients which have the potential to cause issues even if a regulator is capable of providing the current needed to run a running fan. There's also the potential for back-EMF when the 3V3 rail is shutdown or clamped to 0V.

I am not sure how the professionals measure such things but I would imagine they put a resistor in series and then measure the voltage across that, capture with a high-speed digital scope and calculate current draw from that. With correct amplification and adjustment what is shown will be instantaneous current.

Unless soak-testing for a long period of time, it's not always clear how the characteristics will change as the fan is used, as bearings wear, friction increases, what happens if the fan jams.

I don't doubt it does work, but that doesn't mean it will keep working forever, or what the consequences will be when it's less than perfect.

I am not saying don't do it if prepared to take the risk, but the risk of using 3V3 seems quite high compared to the cost of avoiding that risk by using 5V.

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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 3:29 pm

Personally I like the Pimoroni Fan Shim, and have plans to replace the noisy fan in my SmartiPi case with one. I currently have a Pi 3B+ in that case. When my Pi 4B gets swapped in there the Fan Shim will go with it.

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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 4:13 pm

PeterO wrote:
Wed Jan 15, 2020 11:13 am
Another option would be to run a 12V fan from the (more robust) 5V supply.
I did that on my Pi4 until I got a Pimoroni fan-shim.
PeterO
That's actually a great idea!
I have 80mm 12V case fans connected to a USB output, and they offer great low flow cooling, and super quiet, can't hear them at all.

Only issue with these fans is that they're pretty large for a pi.

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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 4:19 pm

hippy wrote:
Wed Jan 15, 2020 3:04 pm

Unless soak-testing for a long period of time, it's not always clear how the characteristics will change as the fan is used, as bearings wear, friction increases, what happens if the fan jams.
Nothing happens on one fan of it jams.
They don't short circuit. The fans have magnetic coils that still offer some form of resistance.
If the fan slows down, also nothing out of the ordinary happens.
Single fan =safe.
Dual fan seems pretty safe under normal working conditions.

What would happen if you spray compressed air over the fans? Or abruptly stop the fans? They'll generate a voltage or back current that could damage the pi.
Not sure if it would though, haven't tried it yet.

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PeterO
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 5:15 pm

X-Gen wrote:
Wed Jan 15, 2020 4:13 pm
Only issue with these fans is that they're pretty large for a pi.
The latest PIs (ones with metal covers over the SoC) are designed to spread the heat from the SoC out into the board, so a larger fan that blows air over more then just the SoC will be beneficial.
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Imperf3kt
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 8:04 pm

X-Gen wrote:
Wed Jan 15, 2020 4:19 pm
hippy wrote:
Wed Jan 15, 2020 3:04 pm

Unless soak-testing for a long period of time, it's not always clear how the characteristics will change as the fan is used, as bearings wear, friction increases, what happens if the fan jams.
Nothing happens on one fan of it jams.
They don't short circuit. The fans have magnetic coils that still offer some form of resistance.
If the fan stalls its the same as any other motor that stalls, the current drawn goes through the roof.
55:55:44:44:4C
52:4C:52:42:41

FlirtingWithDisaster
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Re: Running fan on 3.3 volts instead of the nominal 5 V

Wed Jan 15, 2020 11:33 pm

Just to add, the CanaKit Pi4 4GB kits I have state in their instructions regarding the included fan:
"You may also choose to operate the fan at a slower speed for a quiet operation. In this case, connect the red wire to pin 1 instead."

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Re: Running fan on 3.3 volts instead of the nominal 5 V

Thu Jan 16, 2020 12:08 am

Imperf3kt wrote:
Wed Jan 15, 2020 8:04 pm
If the fan stalls its the same as any other motor that stalls, the current drawn goes through the roof.
Maybe not. Nearly all computer type muffin fans are BLDC with integrated controller hidden inside and do not behave at all like the toy brushed type hinted at in this thread. Most use a ground shunt or the low side driver FET itself inside the controller IC to sense overcurrent causing immediate shutdown.

Many are fooled by the red and black power wire but in reality they are 3 phase brushless with quite sophisticated operation involving multiple coils and h bridge drivers with MCU using fancy feedback algorithms. Usually there is a slight* bump in current at startup and maybe jamming the motor to a stop. Roof remains intact.

Initially I was poking a piece of paper into the blades of the fan in one of my scopes that was specially loud. Eventually came to my senses and just cut the wires like we are supposed to in this situation. lol

*by 'slight' I mean only a few percent.

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Re: Running fan on 3.3 volts instead of the nominal 5 V

Thu Jan 16, 2020 12:28 am

Imperf3kt wrote:
Wed Jan 15, 2020 8:04 pm
X-Gen wrote:
Wed Jan 15, 2020 4:19 pm
hippy wrote:
Wed Jan 15, 2020 3:04 pm

Unless soak-testing for a long period of time, it's not always clear how the characteristics will change as the fan is used, as bearings wear, friction increases, what happens if the fan jams.
Nothing happens on one fan of it jams.
They don't short circuit. The fans have magnetic coils that still offer some form of resistance.
If the fan stalls its the same as any other motor that stalls, the current drawn goes through the roof.
I withdraw my comment.
Stalling fans (fans in the process of stalling, or jammed fans), at worst is nearly equal to a cold start power draw.
And for dual case fans like below, my Pi3B+ easily handles both fans stalled through me pressing on them:
Image

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