O
o-chan
If I hook a case fan to a 5V power source can I expect it to run at 5/12
the max rated rpm? Or might it not run at all?
the max rated rpm? Or might it not run at all?
J
John Doe
o-chan said:
I guess this might sound like a typical put off, but if I were you I
would try hooking it to a mainboard fan speed controller or a power
supply fan speed controller (new Antec power supply).
P
philo
o-chan said:
it will run a bit less than 1/4th the speed
R
ric
o-chan said:
Depends on the fan, but it's usually not that linear.
Again, depends on the fan. And its condition. Some new fans will run
on 5v, but not start up on 5v. Others may do both fine when new, but
not as they get older/dirtier.
Play it safe and use a potentiometer to adjust your fan voltage.
R
ric
philo said:
Really? All manufacturers? All models? I think not.
P
philo
ric said:
check out the "mean squared" law in your physiscs book
D
DaveW
Yes, it will run at 5/12's speed.
S
Spajky
could happen if is dirty or weared out to not to start by itself &
would need some "kick on" circuitry to start. But usually 8x8cm fans
start running with 4V or even less, but not all of them ... you can
try to see ... (there on my site are some fan circuitry under
electronics for that "kick on" ...
T
Trinity
Zalman thinks otherwise. Ever heard of a Zalman Fanmate? They cut the
voltage to the fan to 5v. I'm using one right now on my other PC with
no problems.
R
ric
philo said:
Irrelevant to the situation. Look at the published specs for 3 or 4
12v fans. They're all different. A case of quoting a law of physics
where it doesn't apply.
R
ric
Trinity said:
Fine. It works with your fan - now. Fans have a spec called minimum
startup voltage. Most are around the 5v mark. CPU fans are especially
prone to not start at 5v when they get dirty. Good luck to you.
J
John Doe
ric said:
I was in a hardware store looking at room fans. Was chatting with
someone and out of curiosity asked him why the first setting is
always high. He said it might be to make sure the fan starts.
Glad I asked. Probably one of the brightest answers in a passing
conversation.
J
John Doe
Spajky said:
Horse hockey.
And what happens when you're not looking and it doesn't start?
Depending on the application, things might burn up.
Z
Zotin Khuma
John Doe said:
The guy was spot-on. After all the disparaging remarks I've read
about ignorant sales people, it's good to know that at least some
of them can provide an accurate, helpful answer.
Most electric motors starting up under load while being fed a
lower than normal voltage, try to compensate by consuming
more current. In the worst case, they can burn out simply
because they're receiving too low a voltage. Such a
catastrophic failure may not happen in the case of a low-
power brushless fan, but that's the principle.
D
David Maynard
Zotin said:
Well, if the voltage droops it's because it's drawing so much current.
When a motor is running the spin creates a back electric field (EMF) that
counters the voltage being applied so, effectively, only the remaining
difference is applied across the winding resistance, which determines the
current draw. (if load increases spin decreases, which decreases back EMF,
which increases current draw since less of the applied voltage is
countered. As one would expect: more load, more current)
When starting, however, there *is* no back EMF being generated, no spin, so
the full applied voltage goes straight across the winding resistance. It
drops off rapidly as the motor spins up and, fortunately, winding
inductance also limits the initial surge to some degree.
But hold the motor still, as in a stalled rotor, and it'll likely burn up,
well, if it doesn't have a thermal trip, because there's no back EMF to
counter the applied voltage.
As for starting, you've got to break the static friction, which is higher
than rolling/sliding friction and, with brushless motors, you have the
added problem that the winding phase isn't optimum for creating torque when
stalled (starting). PLus, you're not just 'running the load', you're trying
to accelerate it, which takes more power than a constant speed.
D
David Maynard
ric said:
Most 12 volts axial fans I've seen spec'd 7 volt minimum guaranteed start.
S
Spajky
most generic PC fans run at approx.
half speed (-/+ 10%) when connected from 12 to 5V; larger ones usually
run faster than that, smaller ones slower than that ...
you have not been experimenting enough with generic PC fans obviously
you haven´t browsed thru my site´s , didn´t ya ? ..
S
Spajky
yep, 99% of them are rated so, but if not too much weared out or
really dirty would start @5V around of almost 90% of them.
But it can depend also on PSU how its engineered:
if on power on their protection does "slow rise" or sequential
switching voltages on, there could be some problems even with fans
connected to "7V trick way" starting them @ bootup of PC.
This happened to my "MosFets HS" fan & chipset_HS one with my newest
PSU, while with older one PSU had no problems starting @ 5V ! So I had
to make additional "kick On" circuitry to resolve the problem ...
Its always good to check stuff to be sure before closing the PC case
for good ... !
D
David Maynard
Spajky said:
Yes, but it's dicey and they don't have to be 'dirty' to have problems.
I'm sort of 'stuck' with a 12 volt fan on an iopener, mainly because I'm
too lazy to find a 5 volt one that'll fit, and it's operation is similar to
your number: It starts about 90% of the time. The other 10% I give it a
manual kick start.
But that's a play toy and if I were going to do anything serious with it
I'd get something proper.
I haven't seen this. Why would power sequencing cause a 7 volt fan to not
start?
S
Spajky
http://freeweb.siol.net/jerman55/HP/Pics/elSchem/rpmSensFan5v.gif
(leave out 2,2k & lower other circuitry for tacho wire & dashed switch
if you do not need that)
me either before last year with my newer PSU; seen again with some
other PSU ..
if 5V rail is switched on before 12V rail with lasts its "slow rise"
voltage (to prevent peaks to electronics & motors of optical drives &
HDs - usually with better quality PSUs),; that few tenth of ms-s may
miss a kick-on peak voltage to some fans to start properly IMHO ...