Case Fans in Series

[Jeff]

Has anyone ever run two 12V case fans in series in order to lower the speed
they both run? I was thinking about trying that as opposed to installing
resistors on each.

 

[Mike Walsh]

It will work if the fans are approximately the same resistance; i.e. have the same current rating.

 

[John McGaw]

Has anyone ever run two 12V case fans in series in order to lower the speed
they both run? I was thinking about trying that as opposed to installing
resistors on each.

Don't forget that, connected in series, if the circuitry in either fan
fails then it is possible that power to the other fan could be cut off.
Kind of a "worst of both worlds" scenario but entirely possible. It is
also possible that the startup voltage requirement of one fan or the
other or both might not be reached with a series connection even though
both might run fine once they get started.

 

[kony]

Has anyone ever run two 12V case fans in series in order to lower the speed
they both run? I was thinking about trying that as opposed to installing
resistors on each.

With many (DC brushless) fans it won't work. Use the
resistors.

 

[Sjouke Burry]

It will work if the fans are approximately the same resistance; i.e. have the same current rating.

But then would it not be easier to run them
from 5 volt??
That makes them independent from each other.
And there is not much difference between 5 and
6 volts.

 

[darklight]

With many (DC brushless) fans it won't work. Use the
resistors.

i would go with the above i done the same with variable resistors
so i could turn the fan speed up in summer

 

[GT]

How about just re-wiring them to run on the 5V rail?

 

[Mike Walsh]

But then would it not be easier to run them
from 5 volt??
That makes them independent from each other.
And there is not much difference between 5 and
6 volts.

There is not much difference between 5 volts and 6 volts, but 6 volts will make them slow enough. Some people have wired fans between 12 volts and 5 volts so the fan runs on 7 volts. It would be easiest to wire them on 12 volts in series because only one splice would have to be made.

 

[Jeff]

How about just re-wiring them to run on the 5V rail?

The fans I've got have 3-pin connectors, and I was planning to run them off
a single fan connector on the motherboard (the board I was looking at only
had a single connector). I was thinking about splicing up a 3-pin splitter
cable to put the fans in series and avoid adding resistors.

Since the OP, I've moved on to consider a board with 2 fan connectors so
I'll probably be going to Radio Shack and picking up some resistors.

 

[GT]

The fans I've got have 3-pin connectors, and I was planning to run them
off a single fan connector on the motherboard (the board I was looking at
only had a single connector). I was thinking about splicing up a 3-pin
splitter cable to put the fans in series and avoid adding resistors.

Since the OP, I've moved on to consider a board with 2 fan connectors so
I'll probably be going to Radio Shack and picking up some resistors.

Save yourself the trip and just wire them both, in parallel to the 5v
circuit from a floppy disk power connector that I'm sure you'll never need.
If you need rotation figures, then you can plug the yellow wire onto the
correct pin on one of your motherboard fan connectors.

 

[GT]

Save yourself the trip and just wire them both, in parallel to the 5v
circuit from a floppy disk power connector that I'm sure you'll never
need. If you need rotation figures, then you can plug the yellow wire onto
the correct pin on one of your motherboard fan connectors.

** That's the yellow wire from the fan, not the floppy disk power cable!

 

[kony]

How about just re-wiring them to run on the 5V rail?

One of the potential problems with this is that you don't
have any control, the fans might be faster than desired or
(more likely) too slow, possibly even too slow to spin
reliably in some fan models.

Usually the resistors work better, you have a choice of
which to use in values between roughly 47 and 120 Ohms (what
is typically needed), but it is a bit counter-intuitive to
pick the resistors in that the faster the default speed of
the fan, the lower the resistor value you can get away with
using before the fan doesn't spin reliably anymore.
Therefore the best fan to start with would be spec'd for
below 0.16A current @ 12V/stock speed, not 0.25A or more and
it can take some trial and error to find what will work best
with a fan having integral thermal sensor & control.

 

[Jeff]

One of the potential problems with this is that you don't
have any control, the fans might be faster than desired or
(more likely) too slow, possibly even too slow to spin
reliably in some fan models.

Usually the resistors work better, you have a choice of
which to use in values between roughly 47 and 120 Ohms (what
is typically needed), but it is a bit counter-intuitive to
pick the resistors in that the faster the default speed of
the fan, the lower the resistor value you can get away with
using before the fan doesn't spin reliably anymore.
Therefore the best fan to start with would be spec'd for
below 0.16A current @ 12V/stock speed, not 0.25A or more and
it can take some trial and error to find what will work best
with a fan having integral thermal sensor & control.

I've got a Vantec SF12025L fan rated for 0.08 amps at 12V. Which, if I've
done my math right, tells me the fan has a resistance of roughly 150 ohms.
If I put a 100 ohm resistor in series, I should end up with a 7.2V drop
across the fan. Is this too simplistic? It's been a long time since I've sat
in a EE class.

 

[UCLAN]

But then would it not be easier to run them
from 5 volt??
That makes them independent from each other.
And there is not much difference between 5 and
6 volts.

Oh, there is a WORLD of difference. Some fans will start up at
6v, but not at 5v. This becomes more of a problem as the fan
gets older.

 

[kony]

I've got a Vantec SF12025L fan rated for 0.08 amps at 12V. Which, if I've
done my math right, tells me the fan has a resistance of roughly 150 ohms.

Fan is not a steady resistance, it has discontinuous current
draw that averages less than what is stated on the label.
Sometimes it is a bit puzzling what the labeled current is
supposed to be, one could suppose it was a peak momentary
value but IIRC, the peak momentary is higher than the
typical labeled current even though the average is lower.

If I put a 100 ohm resistor in series, I should end up with a 7.2V drop
across the fan. Is this too simplistic? It's been a long time since I've sat
in a EE class.

It is too simplistic, BUT the margin of error is not so high
that you're wildly off by an order of magnitude or anything
like that. 100 Ohm is a reasonable value to try, though
only you can know if the fan still moves enough air.