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

[Robert Darlington]

Why would I, as a designer of boards for mass production, include an extra
$0.02 part when the fan will never run without me powering it?  I can save
$20,000 on a one million board run just buy cutting out that part.  If you
break it by doing something dumb, my warranty won't cover it and you'll buy
another one from me.  It makes sense to not include protection there.

-Bob

On Thu, Jul 28, 2011 at 11:45 AM, Derrik Walker <lorddoomicus at mac.com>wrote:

> 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
> off.
>
> 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
> for
> > 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
> they
> > 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
> brushed
> > 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
> I
> > 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
> mark
> > 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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