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Boosting a very low voltage

  1. Jun 21, 2012 #1
    Hi all,

    I'm currently trying to build a sort of anemometer using an old computer fan. I have the hardware side of things set and was able to get my voltmeter on it whilst it was spinning, unfortunately it puts out somewhere between 1 and 8mV, which was a little less than I was hoping for as the other models i tested put out about 500-1200mV. However they are not suitable for this task.

    I have tried boosting this signal using an old op amp I had (an ne5532p) but have had no luck (possibly due to no switching power supply on it). I've also tried an npn transistor amp, but unsurprisingly I have no transistors that will flip at such low voltage.

    If anybody has any ideas about what component to use to boost this signal, to about 500-3000mV I would be very grateful

    Thanks in advance
  2. jcsd
  3. Jun 21, 2012 #2
    You should be able to achieve such amplification with any reasonable opamp.

    You could also take a look at some ECG amplifier circuits as most of them operate within the mV range.
  4. Jun 21, 2012 #3
    Ok, thanks for the rapid response, I'll get my power sorted out properly now that I know that the op amp should work in principle.
  5. Jun 21, 2012 #4


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    What sort of motor does this fan have? Many of them are not just simple DC motors. Yours may well have a control circuit in it that won't allow it to act as a generator. Is that 8mV that you measured, AC or DC? Before doing anything else, have a look at it in detail and see what's actually inside it. You may find another motor will do the job for you.
    Last edited: Jun 21, 2012
  6. Jun 22, 2012 #5

    jim hardy

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    indeed, is that millivoltage even proportional to motor speed?
    if it has brushes there'll be friction torque
    if it has permanent magnets there'll likely be reluctance torque

    you may well be ahead to adapt the optical sensors from a computer mouse to the fan shaft.
  7. Jun 22, 2012 #6


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    If you want to get down and dirty, like that, you could just hang a magnet on the shaft and look at the current in a hand wound coil.
    I think, for an anemometer, you want really free movement so, as Jim implies, a motor really may not be the way to do it. Optical or weak magnetic coupling would probably be better. In any case, you will need to amplify any signal you get. But that's not a problem.
  8. Jun 22, 2012 #7
    Computer fans are brushless. You'll get near zero signal out if them.

    This is because there's a little chip that switches coils in and off to impell a set of magnets. But chips like these aren't bidirectional for power flow.

    You need to take the chip out and connect to the coil(s). You should get very nice impulses when the magnets go past.
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