Faking a fan's rpm sensor

Discussion in 'Overclocking & Cooling' started by Moosetracks, Feb 3, 2018.

  1. Moosetracks

    Moosetracks n00bie

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    So I am modifying a system to use a lower rpm fan than it came with.
    This is the original fan....
    http://partner.delta-corp.com/Products/FANUploads/Specification/QFR1212GHE-SP01(REV01).pdf

    The wiring says...
    Red = +
    Black = -
    Blue = STO
    Yellow = PWM

    The blue wire looks like a RPM reading off the fan.

    My thought was to use
    https://www.amazon.com/DROK-Frequen...17714814&sr=8-1&keywords=pwm+signal+generator

    And set it to the frequency I want the system to think its running? I thought I could get this to work, but it doesn't seem to recognize the PWM output from the DROK as a rpm.

    I have +12 going to the vin+, 0V going to the vin-, and the PWM out going to where the blue wire of the fan was.

    Do you guys have any advice on how to achieve what I need. I see a lot of references to using a 555 timer to create this, but I thought I could use the PWM siginal generator to have a visual representation of the frequency.

    Anyhow I am open to any suggestions / info..
    Thanks!
     
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  2. Pandur

    Pandur Limp Gawd

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    The PWM pin in the fan wiring is for sending a PWM signal to the fan motor, to control the speed of the fan. The fan rpm is read from the blue wire, like you thought, and this is where you need to fake your signal. The fan rpm signal is not a PWM signal either, afaik.
     
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  3. doyll

    doyll Kyle's Thermocouple is HOT

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    PWM signal is nothing like a rpm signal.
    My understanding is PWM control has a powered circuit from fan that PWM controller source opens/closes to signal fan PWM circuitry that controls 12v power pulses to fan motor.
    My understanding of RPM is the fan motor sends pulses based on it's rpm to fan header and these pulses are translated to rpm reading.
     
  4. nightanole

    nightanole [H]ard|Gawd

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    If i remember correctly, yellow is rpm, and it goes high twice a revolution. So if you want to make the system believe its running at 3000rpm, you have to set your frequency generator to 100hz.

    Edit never mind, that is to fake out the motherboard.

    Your problem is your fan is spinning at full speed, and you want to control it with your frequency generator.

    Why dont you just 5v or 7v mod it?
     
  5. VanGoghComplex

    VanGoghComplex [H]ard|Gawd

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    I'm having a hard time understanding what you're wanting to do, Moosetracks . Why do you need to "fake" an RPM signal back to your motherboard? Are you planning on installing a 2-wire replacement fan or something?

    Your mobo won't care if your new fan runs slower than the old one, unless it's got some kind of "low RPM warning," which is usually user-configurable. As long as it's getting an RPM signal at all, it'll be happy.
     
  6. Moosetracks

    Moosetracks n00bie

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    I believe your right about the blue wire being an RPM signal. My thoughts were the PWM signal generator I have could be used to create a rpm pulse train. It has two settings duty cycle and frequency.
    http://www.droking.com/index.php?route=product/product&product_id=819

    In this listing it says it can be used as a square wave generator. So I thought I could use it in that fashion.
     
  7. Moosetracks

    Moosetracks n00bie

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    Its not for a motherboard. Its for a hardware miner. The miner checks the fan rpm and shuts down if its not running at 4500 RPM. However I have heavily modified it so it cools with a different system and right now I just have to leave the other fans plugged in to trick the rpm sensor. My thoughts were I could use the DROK device to create a RPM signal. It has a frequency and duty cycle setting. And so far I have been unable to make this work.
     
  8. Absalom

    Absalom Limp Gawd

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    OP. I'm surprised you haven't blown the motherboard header by now, or at the very least your PWM board. Guess neither are pushing enough current for it to matter. Most motherboards add circuit protection (via some type of diode) to prevent voltage runaway. Anyway....

    This is pulled straight from the spec:
    Since you're trying to replace the fan PWM sense circuit with your custom (i.e. "fake") PWM, your custom PWM will need the same open-collector/open-drain output as the fan. Unless your PWM board already has an open-collector or open-drain setup, you'll need to add either a MOSFET or BJT in series to your PWM output signal. And depending on your choice of TTL, the logic may need to be flipped on your PWM side. For example, if your TTL's logic-high equates to open circuit, a duty cycle of 80% on your PWM will equate to 20% on the motherboard side.

    After you introduce that circuit isolation, the motherboard side (which we understand is your miner, but same concept applies) will be supplying 12V through a pull-up resistor - but since it's on the open-collector side of the TTL, it's isolated from the fan PWM circuit. In other words, even if your PWM only outputs +5V, the only thing you have to worry about is that +5V output will trigger the TTL to either open or close the open-collector side of the circuit. The pull-up resistor on the motherboard side will take care of the rest.

    As a side-note, if your TTL doesn't provide one, you'll also need a resistor in series with your PWM, assuming you're going to feed the TTL input your raw PWM output. Then again, it might be able to sink a lot of current. My advice is to consult your TTL's datasheet before attempting without one.

    One reason why the standards committee went with this design is:
    • There are two different voltage logics independent of each other. This abstracts either's internal design from the other, with only a common interface between the two.
    • If something goes awry on either side, such as a high current event, the transistor will, in theory, blow up saving the other side from an extinction event. Generally not going to be a problem on a real fan, as all it's really doing is counting revolutions here. But stranger things have happened.
     
    Last edited: Feb 4, 2018
  9. Absalom

    Absalom Limp Gawd

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    Now thinking about it, if the goal here is to trick the mining board into thinking the fan is spinning all the time, then the simple solution would be to short the Tach pin to ground. Most likely it will sense 100% duty cycle and think the fan is running at full tilt. If not, then it's smart enough to realize what you are doing.
     
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  10. Moosetracks

    Moosetracks n00bie

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    So would the 555 timer and RC circuit be a better solution to fake the rpm signal? https://www.techidiots.net/notes/fake-fan-sensor

    I can't find any thing on the specs about the output of the DROK signal generator.





     
  11. Absalom

    Absalom Limp Gawd

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    I took a look at that, and while that might work, it's a somewhat bizarre way of going about it. It looks like they are powering the 555 from the motherboard's 12V pin (I see all sorts of red flags thrown due to this). They are using the DIScharge pin on the 555, which according to the data sheet, is an open-collector type output, which serves that part of the requirement. The opening and closing of the collector output is a function of the charging/discharging of the capacitor in the configured interval.

    My understanding is that when the capacitor discharges, TRigger's voltage drops below the threshold (much lower than THR's threshold) which initiates Q (out) to go high. When Q goes high, the capacitor starts to charge, until THReshold reaches its threshold value, thus Q goes low again. The cyclic charging and discharging of the capacitor would be how you get the required opening and closing effect on the collector output. The choice of resistor and capacitor determines the frequency of this interval.

    It looks like it might work. There's a link there to an [H] thread where others claim it works, so I don't doubt it works. I mean, adding a resistor and capacitor to your 555 is going to be cheaper than coming up with your own open-collector type circuit. You already have the 555, so might as well.

    Edit: My bad, you have the DROK in hand, which is a whole different beast compared to a 555. It looks like the DROK (while vastly more expensive than a 555), is going to require adding an open collector to drive its output. I'd ditch the DROK and go the 555 route, if nothing for simplicity.
     
    Last edited: Feb 4, 2018
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  12. Moosetracks

    Moosetracks n00bie

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    Thanks for all the info so far!

    I built the circuit, but I wanted to test it before I plugged it into my motherboard. I have a handheld fluke scope how can I use it to see the signal. Would I need a 12V source and a resistor in line to see the response, or is there another way to test it with what I have?




     
  13. Absalom

    Absalom Limp Gawd

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    That will depend on how you built your circuit and what components you used. For the 555, you'll need to supply enough voltage that it allows the transistor to reach its saturation point. 12V should be plenty, I would guess. You can always refer to the 555 datasheet if unsure.
     
  14. harddime

    harddime n00bie

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    Hey Moose

    Any update on your attempts?