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Old 22nd Feb 2021, 4:31 pm   #1
Al (astral highway)
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Default Running DC brushless PC cooling fans

Hi folks,

I have a few quite generic fan pulls from SMPS.

One here is rated 12V (quite typical) and 120mA. Out of curiosity I just ran it at different voltages, above and below the V rating. I know these things are normally run at variable speeds depending on load conditions and CPU temperature.

This only draws the rated 120mA at 19.4V.

Presumably this means it's perfectly fine to be run at this voltage?

In fact, could it be run even higher, for a few tens of mA above the rating? It won't be in continuous duty, just runs of 5 mins at a time maximum.

Thanks in advance,

Al
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Old 22nd Feb 2021, 4:47 pm   #2
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Default Re: Running DC brushless PC cooling fans

I wouldn't be so sure. I experienced an application where a 12v computer fan was run over voltage. In the initial design it was OK, but they upped the internal system voltage. The fan ran OK, but must have been running hot internally as it only took a few minutes before problems started, as in it started to smoke. It had got hot enough that the fan hub had softened so even after it all calmed down, the fan was physically stuck too because it was now eccentric.

The fan wasn't new and had been working fine as part of an original model that we were upgrading. The cost of the upgrade was priced on re-use of a number of components. The fan grille was virtually the only part re-used in the end.
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Old 22nd Feb 2021, 4:55 pm   #3
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Default Re: Running DC brushless PC cooling fans

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I wouldn't be so sure. I experienced an application where a 12v computer fan was run over voltage.

Hi Duncan, thanks for sharing that experience.

Only in this case, what do you make of the fact that the 120mA current rating is only achieved at 19.4V, not 12V?

It leads me to think that 12V is idling speed and the max rating is for when the CPU is running hotter.

For example, we're all familiar with the sound of a fan suddenly kicking into much higher revs. These sorts of revs are exactly what I'm hearing at 19.4V and 120mA - hence my curiosity.
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Old 22nd Feb 2021, 6:05 pm   #4
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Default Re: Running DC brushless PC cooling fans

I doubt a computer would run it at higher than 12V as I doubt they would fit a boost regulator just to power some fans, plus I believe modern fans are PWM controlled via a separate input so are not controlled by the voltage applied.
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Old 22nd Feb 2021, 7:15 pm   #5
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Default Re: Running DC brushless PC cooling fans

Maybe it needs an air 'load' to test it properly. In free air it just spins up.
Is there a CFM (Cubic feet per minute) number on them?
12 volts is the norm for PC fans. Any more is not going to give them a long life...
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Old 22nd Feb 2021, 8:11 pm   #6
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Default Re: Running DC brushless PC cooling fans

I've done quite a lot of experiments with speed controlling brushless fans, and it's quite easy to wreck them with fairly mild overvoltage. For a 12V fan, a supply of 19-20V is already in dangerous territory. The reason is that they have an internal controller chip (or, sometimes, just a couple of transistors) which has a pretty strict upper voltage limit. Exceed it and it's game over.

Chris
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Old 22nd Feb 2021, 8:20 pm   #7
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Default Re: Running DC brushless PC cooling fans

I agree with Chris, this is likely to end in tears unless you are just playing around with some surplus fans and don't care if they get wrecked. I don't know of any computer motherboard or PSU that runs fans overvoltage to improve cooling.
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Old 22nd Feb 2021, 8:24 pm   #8
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Default Re: Running DC brushless PC cooling fans

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Originally Posted by paulsherwin View Post
I agree with Chris, this is likely to end in tears unless you are just playing around with some surplus fans and don't care if they get wrecked. I don't know of any computer motherboard or PSU that runs fans overvoltage to improve cooling.
If Dell have anything to go on, they use a centrifugal fan for cooling, it's speed varies but will never run over 12v, it is very loud when it gets going properly and shifts a lot of air.
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Old 23rd Feb 2021, 2:06 pm   #9
Al (astral highway)
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Default Re: Running DC brushless PC cooling fans

Thanks for your replies, everyone.

The fans are surplus but armed with this input, I don't need to do pointless experiments or run with overvoltage.


Current drawn does increase by restriction of airflow as pointed out by snowman_al.

This draws a line under my Q.

Much appreciated, all
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Old 23rd Feb 2021, 2:21 pm   #10
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Default Re: Running DC brushless PC cooling fans

The 120mA will be a possible maximum, that is if you allow 120mA for the fan it will never take that much and always work, it's only a thing for the label after all.
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Old 23rd Feb 2021, 3:32 pm   #11
cmjones01
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Default Re: Running DC brushless PC cooling fans

If you want more airflow, and to be able to control it, I strongly recommend getting some ex-server fans secondhand. I bought some from a PC parts dealer via allegro.pl (our local equivalent of eBay) and have found them very useful. I think rack-mounted servers are often withdrawn and scrapped/dismantled and the parts can be had at very low prices. The fans can be very powerful and high-quality units (Papst, Panasonic or Sanyo Denki). They have excellent speed control via a PWM input.

For example, I have a couple of 120mm Sanyo Denki fans which, running at full chat, will draw nearly an amp from the 12V supply and produce something like a hurricane. But the PWM control can smoothly reduce the speed to little more than a whisper. For my application I rigged up a simple PWM controller using a 555 chip which works just fine.

Trying to control brushless fans by varying the supply voltage doesn't work well at all. They're not designed for it. Get a good quality fan, more powerful than you need, and control it via the PWM input, and you'll be pleasantly surprised at the range of control possible.

Chris
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Old 23rd Feb 2021, 4:29 pm   #12
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Default Re: Running DC brushless PC cooling fans

@MerlinM, good point. I hadn't thought of it as an overhead rather than current drawn.

@Chris - good to know. I'll rustle up a PWM controller and go for one of these more powerful fans that you recommend. I had a very powerful and beautifully engineered muffin fan from Papst - solid brand indeed.

Cheers.
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