Fan stuff Part III
Monitoring RPMs without 3rd wire? Yup, possible ...
Somebody maybe would say that is not possible with ordinary computer fans, but actually can be done few ways.

- One is that some fans already have inside all circuitry necessary & only lack of external 3rd wire; in that (lucky) case you have to open (under sticker) the fan & solder it there a wire & connect it to proper pin on MoBo header. You can check that if there is on internal small PCB additional contact already set there; using an ohm-meter just see if there is with slow manual rotation any shorting circuit involved between that contact & the black wire, according to rotations the ohm value should drop to zero in pulses.

- The second way can be to use some smaller fan with dead driving electronics (but with Ok Rpm circuitry) & glue rotors thru rubber band together & so use the bad fan for sensing ... or resolve the problem like a usenet poster P2B did ... or getting an idea also here ...

- The third one (just to once check its rotation once) could be using s small tube to blow the air to its wings rotating & comparing the tone you get with some musical instrument like a guitar with some tone frequency table & use of some math formula to find out the actual Rpm speed of the fan (will be explained at the end & later added ).

Maybe there could be some another easy way to do it, but right now it does not pop in my mind ...

- or ... using some kind of electronic circuitry like I did! ... (someone also tried something similar) ...

The idea popped in my mind time ago was to check the current consuming the fan while spinning with my audio card & using some proper software to check, if any other signal other » ...

  ... than a straight current thru a fan is showing there on some inserted resistor in series with a fan even if there are not any brush contacts inside these fans.

I did it with some different fans, but instead of using a resistor, I used a 1N4007 diode (ok till 1A current), to prevent higher voltage drops on fans with bigger consumption. Well, the diode gives quite a constant voltage drop on it, but also compacts & diminishes any AC signals, so I knew if there would be any useful pulses, have to be amplified further for any use. There on that diode really showed some pulses in accordance with fan RPMs & with almost new TT Volcano-II fan looked like on the first slide, while the signal from its tacho wire looked like on the second one.

on diode signal

tach wire signal

Rpm gadget schematics   terminals
Explanation of this circuit:

it is composed from first stage which rapresents an DC amplifier with its input connected to the negative wire of a fan (& parallel to serial added diode to a fan negative supply) thru a diode & resistor getting there also the same time a biasing current for proper operational (working) point of it. The electrolytic cap there serves to bypass the input resistor for AC part of a signal. The other caps serve only for RF decoupling of circuitry. The input diode serves for additional DC drop before entering transistor & at the same time expands a bit a dynamic of a AC signal entering it ("passive amplifying"). Since there on the "fan diode" is a voltage drop around 0,75V when the fan is running, the input transistor gets because of that serially resistor & diode about 0,2V less voltage on its B-terminal for proper biasing. The second stage gets amplified signal & direct biasing (since direct coupled) from the second resistor (working load of first stage), a bit reduced by serially added diode to its E-terminal & the last transistor output (open collector!) is tight to the Rpm input on the MoBo header. This second transistor acts the same as does the actual one in the inside circuitry of that tach capable fans with 3rd wire.

How the signal is produced on that serial "fan diode"?

Since the fan gets power a bit reduced for that diode voltage drop, it starts spinning powered by its inside electronics producing with its (one, two or more) inside coils a rotational magnetic field driving its brush-less rotor part. Depends of the number of coils inside it how many times per revolution the inside electronics sucks current more (current pulses!) feeding the inside coils. Those increased current pulses produces also increased current pulses thru that serial diode, so parallel to it you have a DC voltage with spikes (DC+AC signal together) which is further amplified & filtered with this mine circuitry feeding the MoBo on_Board HWmon chip Rpm sensing. Thats how you can reveal RPMs also without a that famous 3rd wire! The forth slide (diagram) shows the signal on the output of this mine circuitry;

See, it is similar to original fan tacho signal & when I saw it, I knew that this dongle will work even before I actually tested it practically with my MoBo!

circuitry out signal

But here can also be a drawback! Since mostly these days powerful & bigger fans mostly have inside 2 coils, you could get a doubled frequency signal & so revealing double value of Rpm. This could be a problem with older boards & their HWm chips, but IMHO not with newer ones, since they can be programmed thru MBM or similar software in Windoze their internal divider (under settings, Fan/Divisor or so), but in Bios could be still shown a double Rpm rate. For my experience with a stock Intel Celly Mendocino HSF that was not a case since it has only one coil inside like mostly all very small fans like those for chipset or graphic cards ones. If you can by yourself deal with that in your mind, there won´t be a problem knowing in these cases that for example 6000 Rpm in Bios shown means in reality 3000 Rpm. But if this is bothering you anyway, you can insert some additional discrete elements or IC circuitry at the end of this mine circuitry (additional divider by two - an idea can be found here ) or somehow modify so this circuitry to show exact RPMs also in Bios. Since this can be made on a proto_board for few fans w/o tach wire just a single dongle, than with a some few positions switch you can manually switch between them the same way (for 2-fan) like there is shown on that mine circuitry slide.

If you do not believe that this stuff actually works, make it & try! . . . ... continues more further ... ...



 © Zdenko Jerman-Spajky