[rescue] [off-topic] Vacuuming your servers is like cleaning them with a Van deGraff generator

Derrik Walker lorddoomicus at mac.com
Thu Jul 28 12:45:15 CDT 2011

Wouldn't a well designed DC fan circuit just have a diode on the positive
input to the fan?

That should protect from any back DC current if the fan is spun by compressed
air, a human hand or even the momentum from the fan blades after it's turned

If the negative is tied too;? ground ( which is typical ), a small capacitor
between the negative input and ground is also a good idea to stop transients
from variations in fan speed ( spinning up and spinning down, and maybe even
variations while its running ) from bleeding back into the ground bus.

- Derrik

On Jul 28, 2011, at 02:30 AM, Mark Benson <md.benson at gmail.com> wrote:

> On 28 Jul 2011, at 00:51, Phil Stracchino wrote:
> > On 07/27/11 19:43, Robert Darlington wrote:
> >> This generally works great, just remember to not let the fans spin. It's
> >> quite easy to get them going 10x faster than they were meant to go using
> >> compressed air. That means you're generating a good 120 volts back into
> the
> >> 12 volt regulator on the mobo or other parts in the PSU.
> >
> > Didn't we just cover (and debunk) this a couple of weeks ago?
> Yes, and like I said back then, there's a much plainer and simpler reason
> not spinning the fans with compressed air. On less well made ones (like you
> find in a most consumer PCs, for example) it kills the bearings meaning
> suddenly get mad cow disease just after you clean them out.
> The DC back-voltage thing would depend on how the fan is constructed. IIRC
> brushless motors don't generate current when spun, but straight up DC
> motors do. You can't always tell if a fan is brushless from the connections
> because the driver circuit is usually on a PCB inside the fan housing, but
> think moderns fans tend to be brushless as it's cheaper to manufacture and
> lasts longer because of the lack of contact parts. I could be way off the
> though, my knowledge is abased off radio control car motors :)
> --
> Mark Benson
> My Blog:
> <http://markbenson.org/blog>
> Follow me on Twitter:
> http://twitter.com/mdbenson
> "Never send a human to do a machine's job..."
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