Adding a Tachometer
to a 2-wire Fan
It's quite simple to add a tachometer
output to a 2-wire cooling fan so you can connect it to a fan header on
your motherboard and monitor rpms using Motherboard Monitor or similar
programs. I added a tachometer to a Pentium I CPU fan. First take your
fan apart (they are all different, you'll have to figure out this part
yourself)
enough to get access to the circuit board. It will look something like
this:
Look for the tiny copper wires from
the circuit board to the coils. There are usually 2 coils connected
together (labelled Common above), and a separate input to each one
(labelled Coil 1 and Coil 2). Solder a wire to either one of the coil
inputs - the blue wire above (which I later changed to a thinner red
wire because the blue one was too thick to allow the fan to be mounted
on
it's heatsink properly).
Next build the circuit below. You
need a 2.7K ohm 1/4W resistor, 4.7K ohm 1/4W
resistor, 2N2222 NPN
transistor, and a 5.1v
Zener diode - the Zener is optional, but will protect your
motherboard's sensor input if the fan ever melts down or the transistor
fails.
Install your tachometer circuit on
the fan leads however you like, mine looked like this before I covered
it in a blob of hot-melt glue to protect it and prevent it from
shorting out onto anything.
**IMPORTANT**
Test your circuit before you connect it to the
motherboard - if you've made a
mistake in the wiring, it will blow your motherboard sensor instantly
(or worse). You need a multimeter, the 4.7K ohm resistor left over from
the parts list above, and a test jig wired as shown below.
The
tachometer output should read around 1.5 - 2v on the DC scale, and 3 -
4v on the
AC scale - if it's more than 5v on either scale, something is seriously
wrong and needs to be
fixed before connecting the fan to the motherboard!
I also built a version out of
surface mount components I scrounged off the controller board of an old
hard drive. The parts are tiny and hard to solder, but the end result
is neater as you can see below.
Last Updated January 7th, 2004 by P2B
