On 15 Mar 2005 20:18:20 -0800, wrote:

I would like to know the way to reduce the speed of a fan inside a
small device in order to cut down the noise from the fan. If I
understand this correctly, I am supposed to do this by undervolting the
fan from 12-volt it is now to something like 6-volt using some kind of
"inline resistor" or someting. I can soldering wires together. But I
really don't know much about resistor and such. I am hoping someone can
give me the "exact" information about the way to do this.

Take original fan and cut it's power + lead in half, maybe
in the middle so you have enough slack to work with.

Next look at the length of the resistor you'd use,
accounting for a few (maybe 3) millimeters on each of the
leads, and cut off that much more from one of the ends of
the wire you'd just cut. Essentially you're shortening the
lead such that both leads will be same length after the
resistor is soldered on, which is theoretically unnecessary
but looks better.

Get a 2W resistor, the value depends on the fan but a
ballpark range would be 47-200 Ohm. A value of about 100
Ohm is a good first guess with no further info. Since you
don't yet know what the resultant RPM & noise will be at
"6.0V", it could be that you don't actually want exactly 6V?

Another alternative could be to start out with a 200 Ohm (or
make-do with whatever is available) rheostat, dialing in the
resistance value you find to be optimal then measuring the
resistance, to "size" the resistor value you need. Then
choose the closest commonly available value (or whatever you
have on hand).

So you've found the resistor value you want, simply slip a
length of heat-shrink tubing on each of the cut wire ends,
back as far as possible away from the heat of soldering.
Then solder on the wires- could help to tin the wires first
and/or clamp them to the resistor while soldering. In theor
a mechanical joint is best before soldering, but a good
solder joint alone with heatshrink over it is sufficient for
a non-stressed connection (no later mechanical force will be
applied to it).

After resistor is soldered on, slip the heatshrink over the
soldered, bare metal area and heat it up to shrink it.



The small device is a LinkSys gigabit switch that has a very noisy 40mm
fan in it. I have replaced it with a slightly quieter Papst 60mm fan.
My intention is to oversize the fan and then reduce the fan speed; then
I will be able to maintain the same air flow as the 40mm fan but with
less noise. Now, I have the oversized fan mounted inside the gigabit
switch. And I can feel that its air flow is definitely stronger than
the 40mm fan. This means I can go ahead to reduce its speed without
worrying about the possibility of not getting enough air flow. I need
to figure out how to reduce its speed.

My questions a

- I am under the impression that I should use something
called an "inline resistor". Is it the right product?


Inline simply means it's a serial connection, that you cut
the power lead and put each cut end on opposite ends of the
resistor. It would be called a "2W" resistor. There are
different resistor compositions like metal or carbon and
ceramic, etc. They don't matter, just use smallest 2W
resistor you can find, OR the cheapest, or whatever's
available where you normally purchase parts (to combine with
other orders since resistor is tiny fraction of the cost to
ship it) or whatever you have handy.


- Because the gigabit switch has very limit space
inside, the "inline resistor" must be small enough
to fit inside. Will it be small enough?

Yes, so long as you don't get a giant cermet (cement filled)
wire-wound type. They are usually rectangular and white
colored, and quite overkill. The typical resistor you might
seek is about the diameter of a thin pencil and just under 2
cm long.


- The fan is rated as 12V and 0.6 watt with two wires.

It's a relatively slow fan already, again I'd try around 100
Ohm resistor first, maybe slightly higher.


What type of "inline resistor" should I use? I am
under the impression that inline resistor is rated
by "ohms". Can someone give me a RadioShack
product-number or something like that?

Well Radio Shack might have them, I'll check that in a
moment but these are common parts, any fully stocked
electronics outfit should have something suitable. Seems
like Radio Shack brick-n-mortar stores carry fewer misc.
parts than they used to, you might not be able to assume
what's available online is actually stocked in any
particular store- I'd call ahead first to ask as one store
might have it but another not.

I didn't see any on Radio Shack's website, maybe I
overlooked them but I dug up another source, might as well
get a good deal on 'em?

http://www.goldmine-elec-products.co...m=2&mite m=11

You might call and ask them if they'd just throw 5 (pieces
instead of 200) of them in an envelope, if you're trying to
get ultra-cheap shipping.

Since the fan has such a low RPM already, it's possible that
it's not a good candidate for (around) 6V operation, but it
might be suitably quiet at higher voltage. You might try a
68 Ohm resistor, or solder two 120 Ohm resistors in parallel
(for 60 Ohms), which is just connecting both ends of each
resistor together and one end of the cut wire (mentioned
previously) to each junction.



- Which wire should I solder the inline resistor to the
fan? Red-wire or the blue-wire of the fan?

Positive, usually red-wire is the typical way, but if you
wanted to solder it to the blue wire instead that would work
too with no drawbacks since this is only a two (power) lead
type fan connection.

- Does this matter which way I orient the inline resistor?

no