Too much voltage, Not enough current

Discussion in 'The Projects Forum' started by ruddager, Jun 5, 2011.

  1. ruddager

    Thread Starter New Member

    Jun 5, 2011
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    The concept of the project is taking a power source measuring 51.8VDC and 23.8mA, and using this to operate multiple small fans. The fan I have chosen is battery operated by 2 D-cell batteries. I have a limited physics based understanding of circuits, but unfortunately my math isn’t translating well to real-world applications.

    I have attached a photo of some concept circuits, and associated thoughts about each. The first hurdle I encounter is: What current is really needed to run this fan? There’s a port for an AC adapter that says 3VDC, 500mA, however I’m obviously getting no where near 500mA on 2 batteries. With my trusty multi-meter I measure a total of 2.4-Ohms of resistance through the fan, and 3.4V / 8.6mA on a pair of new batteries. This doesn’t make sense to my limited V=IR education.

    The second hurdle is using resistors. I thought in a series circuit that current remain constant and voltage dropped across resistors, however upon adding a resistor between my meter and the power source I noticed a drop in voltage, but also a significant drop in current. Hence by the time I apply enough resistors to get down to 3V, I have no current left.

    I know there are several components out there (voltage regulators, diodes, etc..) that I probably need, but these are beyond my basic understanding. I would greatly appreciate any help, or anyone who could diagram the correct circuit to ideally power as many fans as possible given this fan and power source.
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
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    51.8V X 0.0238A = 1.23W. That's very little power to work with. The first goal would be to choose fans that can operate with this small amount of power. Any system that converts the voltage will waste most of the power in losses before it gets to the fans therefore the second design goal should be to find fans that will operate on the supplied voltage. There are fans designed for 48VDC so it may be possible.

    These are the smallest fans I know of. They're designed for low voltages but the wattage requirements would be similar if they were made for your voltage.
    http://www.sunon.com/pro2.php?c1=10&c2=2
     
    Last edited: Jun 5, 2011
  3. wayneh

    Expert

    Sep 9, 2010
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    Measured how? I mean, was there a load attached, and what was that load?

    For simplicity, I'd think about putting as many fans in series as you can, without resistors. Any resistors will just burn off power, which you have little of. Small CPU cooling fans (typically 12v and less than 100mA) would work, and you could put 4 in series with no other components. The 24mA current might be iffy but I'm sure they'll spin.

    You could use many more of the lower voltage fans, but let's be sure about the actual power supply capacity before we consider that.
     
  4. ruddager

    Thread Starter New Member

    Jun 5, 2011
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    Thanks for the input. The source was measured with a multi-meter.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    OK, but how? If you measured that current with the meter as the load (ie. in parallel to the output), then you measured the current into a near short (the meter's shunt resistor). You were lucky to not destroy your meter, or at least blow its fuse.

    Any idea what the voltage was when the current was measured? You'd need a second meter to determine this. If the voltage was pulled very low, which I suspect it was, then your supply probably does not have enough power to run many fans. It has a high internal resistance.
     
  6. ruddager

    Thread Starter New Member

    Jun 5, 2011
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    Yes waynah, you’re correct, the current was measured in the same way the voltage was, using the meter connecting the leads. The meter has a 200mA setting, so no destruction, still works fine after multiple testings. I don’t really have a reliable second meter to test voltage and current simotaniously.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    We don't know for sure what the ohm rating of the meter's internal resistor is, but probably less than 1Ω, more likely closer to 0.1Ω. You can do the Ohm's law math and see that your voltage when "powering" the meter had to be very low. Probably well less than 1V. It's unlikely this supply could spin ONE fan, let alone multiples. Do you have any other information about this thing? I'd hardly call it a power supply, since there's almost no power.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It sounds to me as if you want to operate low-voltage fans using power from your telephone line. Is that correct?

    Be aware that your telephone service provider is not a power provider; they provide telephone, fax, and perhaps digital connectivity but not power.

    If you attempt to use your telephone lines as a power source, the phone company will see the load as a fault condition, and will disconnect your telephone service until the fault is corrected.
     
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