Convert Fan RPM to VDC?

Discussion in 'The Projects Forum' started by ifixtheinternet, Oct 29, 2015.

  1. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    I want to replace a fan in my television with a much better and quieter fan.
    The issue is the original fan uses a pass/fail monitoring circuit that puts out -10.88VDC at any RPM, then produces 0V when the fan stops.
    The fan I want to install is a standard PC fan with RPM monitoring.
    The fan runs at 2000 RPM.

    I want to build a small circuit that will produce the -10.88VDC for the Sensor at full fan speed, and cut the power if it drops below, say 1500RPM.
    I was hoping for the added security of it warning me if the fan is slow, not only if it is stopped.

    Could I use a frequency to voltage IC for this, with a potentiometer to adjust it?

    Thanks!
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You can use a LM2917 freq to volts chip, or a 555 timer, plenty circuits on
    tachometers,
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    part number of original fan?
    could just be a locked rotor alarm signal which is a very common option for most fans..
    Just source one with the same CFM rating and lower decibel rating.
     
  4. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    Yes, I think that will be the solution, I am just not sure how to adjust the circuit for my application.
    The Fan runs on 12v DC, So i just need to cut the voltage to 10.88, and then be able to send it when the fan is spinning, and cut it when it is not, or better yet when it is a specified RPM. I've seen several of these circuits, but Im a total noob about how to put it together, or what to change to get the voltage I need.

    Well I did look around, but I didnt find anything that I liked.

    The original fan is Nonoi 3110RL-04W-S19,
    Here are the specs:

    Product Type DC Axial Fans
    Rated Voltage 12 (V)
    Current 0.070 (A)
    Air Flow 24.01 (CFM)
    Air Flow 0.680 (m³/min)
    Mass 80 (g)
    Noise 22.0 (dB)
    Input Power 0.84 (w)
    Static Pressure 14.2 (Pa)
    Size 80 (mm)
    Thickness [L] 25.0 (mm)
    Speed 1900 (min¹)
    Static Pressure 0.050 (H20)

    Here is the Fan I want to replace it with:

    Model: Noctua NF-A8 FLX

    Rated Voltage: 12
    Current: 0.07 A
    Input Power: 0.84 W
    Speed: 2000
    Max. Airflow: 29.7 CFM
    Max. Static Pressure: .08 (in H20)
    Noise: 16.1 dB
    Mass: 80

    AS you can see the replacement fan moves considerably more air, while making much less noise.
    This is the perfect fan to go in a tv in my living room.

    Right now it is connected to the TV, but I have the monitoring defeated by connecting the lead from another fan to its header.

    I would appreciate a pointer towards the closest circuit that I am looking for, and perhaps how I may change or alter it to achieve what I need.
     
  5. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    Would it be a much simpler circuit if I just required 10.88 VDC to be applied when the fan is spinning at any RPM, and to just have no voltage when the fan stops spinning?

    That would be just like it is now anyway.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The fan will give out a pulsed dc signal when running, not a steady one, these pulses are fed into the freq to volts converter, as the fan slows down the pulses get less the voltage goes down. thats how a pc monitors its own fan, it will be the same in your TV, just feed the output voltage of the F to V to the sense pin on the tv.

    Does the new fan have a pulsed output pin, if so have you connected it to the tv sense pin?

    Figure 21 on page 10, they have set it to give 1v at 66hz, your fan runs at 2000 rpm thats 33hz, by increasing the values of R1, C1will give out more volts per rpm.

    Easy to Use; VOUT = fIN × VCC × R1 × C1


    http://www.ti.com/lit/ds/symlink/lm2907-n.pdf
     
    Last edited: Oct 30, 2015
  7. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    No, it really does not.
    I have researched this issue, and there are several different technologies for fan monitoring, only one of which is a pulsed DC signal based on RPM. That is done with a Hall-effect sensor, and is how 99.99% of all PC fans work.
    That is how the new (replacement) fan operates, as does pretty much every fan you have probably ever seen.

    This monitoring tech is not common, but it does exist, and it is what the TV fan uses.
    It did take me a couple days to find documentation on how this tech works.

    Here is the document I found describing how this technology works:
    http://www.comairrotron.com/methods-monitoring-fan-performance

    As I mentioned already I have measured the voltage coming from the monitoring pin of the fan, and I get a rock steady 10.88VDC at ANY RPM that it spins, the voltage never changes, it is either 12.88V or 0V.

    This is a PASS/FAIL monitoring system rather that an RPM based system.
    The Fan always puts out a constant 10.88VDC through it's third pin as long as it is spinning. And I am talking SLOW, like 20 RPM, and I still get a rock solid 10.88VDC.

    Only when the fan stops completely will the voltage drop to 0 and the TV will show an error message.
    This monitoring tech is very rare, as usually a PASS/FAIL system is done with High/Low voltage, rather than High/NO voltage.

    When I connect this fan to my PC. I do not get an RPM reading, so it does not use pulses.
    Even if it pulsed once per second, 4 times per second, I would get some sort of RPM reading, but I get nothing.
    I verified this with all 3 of the fans in the TV, they all put out 10.88VDC when spinning, and show no RPM when connected to a known, working speed sense capable fan header on my PC.

    I think maybe I should also re-phrase what I'm trying to accomplish.

    I actually don't need to "Convert" RPM into voltage, as during normal operation, the voltage from the fan never changes, it is either 10.88V or 0V. So what I really need is just to output 10.88VDC to the sensor wire to the TV all the time, and just CUT the power to that pin when the fan stops spinning, or better yet, when it slows to a specific RPM, such as 1500.

    So now that I've made it clear I really dont need to convert RPMs into voltage, do I still need a frequency to voltage converter?
    I still need to interpret the RPM and use a switch to cut power based on the RPM, but the voltage sent to the TV's monitoring pin should not change linearly with RPM, but just needs to be disabled at a certain RPM.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Then you could use a led and photo sensor through the fan blades, to detect movement, this could feed a 555 in a retriggered monostable mode, so when the fan stops the 555 goes to zero, and when running stays at 12v output.
     
  9. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    Ok, that makes sense.
    I knew there had to be a simpler way, as I was looking at circuits meant to convert RPM into linear voltage, and the circuits were a bit complex.
    (For me anyway) I realize now that isn't what I need.

    So to change 12v to 10.88v on the pin, would I just add a specific combination of resistors to lower it to 10.88V?
    Or possibly a potentiometer I can adjust manually?
    Also, could I setup the 555 sensor to kill power at a certain RPM, or would I be limited to ON/OFF functionality?

    Sorry for the dumb questions.
    I've repaired circuits before but never built one from scratch.

    Thanks.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Yes you can reduce the output voltage from 12v to 10v with a couple of resistors, you can alter the output triggered signal by setting the timing with the R C values on pins 6,7
     
  11. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    actually, I don't think that will work.
    This fan is sealed in a tunnel, and it is filled with hot heat sinks, and one is right behind the fan.
    I think it would be next to impossible to implement that.

    The 3 fans are also in different places, like a foot apart or more, so wiring would be a little nuts.
    I think I would really like to use the RPM data as a trigger.

    If I could accomplish that on a circuit, I could just duplicate it 3 times on the board for the other fans.
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Can you measure the current when running, and then force the fan to slow down and see if the current changes.
     
  13. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    The Fan always puts out a constant 10.88VDC through it's third pin as long as it is spinning. And I am talking SLOW, like 20 RPM, and I still get a rock solid 10.88VDC. only when it stops does the voltage go to 0V.

    SO really all I need is a switch that shuts off the voltage to that pin connected to the TV when the RPM from the fan drops past a certain point.
     
  14. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You've lost me here, you want to replace the Old fan for a new one, your New fan rotates at 2000 rpm, does it give out a pulsed signal or a voltage, ????
     
  15. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    Dave, I have to ask if you are reading my posts, because I have answered the last 3 questions you asked already in my very first post.

    The old (original) fan supplies the TV sense wire with a constant 10.88VDC when spinning at any RPM. Thats why it is so hard to find a suitable replacement.

    The new chosen (replacement) fan gives out a pulsed signal like any normal 3 pin PC fan.
    The constant voltage of 10.88V is what I need to replicate for the sensor pin on the TV.

    I want to open the circuit when the new fan falls below a certain threshold (such as 1500rpm), this will trigger the TV to throw a fan error code, as it does this when it detects no voltage in that pin.

    So logically I want the circuit like this:

    IF: RPM > 1500, supply voltage.
    IF: RPM < 1500, cut voltage

    Then I will lower the 12V supply voltage to 10.88.

    Worst case scenario I could just supply 10.88V if any rpm is detected, open the circuit if no RPM reading is detected.
    This is essentially the same pass/fail system the TV already has.
    in that case the logic would be:

    IF RPM present, supply voltage.
    IF RPM not present, do not supply voltage.

    Then I would lower the voltage to 10.88V.
     
    Last edited: Oct 30, 2015
  16. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Right so make the 555 timer monostable then, OR put a fixed 10.8V to the fan sensor detect input pin and run your new fan, why do you need to detect if the fan has stopped.
     
  17. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    I want to retain the warning feature, as the expensive LEDs in the TV will quickly burn up if not cooled by the fan.
    If I could create a circuit for a few bucks that can prevent several hundred $ worth of damage it seems like a good Idea.

    If I can create this circuit once, I can create it 2 more times and replace all the fans with better ones.

    So a 555 will switch a pin on and off based on RPM signal detected on the sense pin?
     
  18. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  19. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    Thanks Dave! I will get these parts and start putting it together.
     
    Last edited: Oct 31, 2015
  20. ifixtheinternet

    Thread Starter New Member

    Oct 29, 2015
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    Ok, some more newb questions here.

    My supply voltage is 12V for the fan and the circuit.
    Some of the parts are only rated up to 6.3V.
    Do I need to add a resistor to the + side to drop the voltage, or will this work as-is?

    Also, I read on the datasheet that the out pin of the 555 chip is limited to 2/3 of supply voltage.
    So even if the whole circuit was 12V I wont get the 10.88V output I need.
    I'm guessing I will need to add a relay to the TRI pin, connected to the 12V supply, and then limit it to 10.88V?

    Last one.
    Could I simply add a potentiometer to lower the supply voltage connected to the relay to 10.88v before it goes on to the TV?

    Thanks again.
     
    Last edited: Oct 31, 2015
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