Voltage Drop ?

Discussion in 'General Electronics Chat' started by trader007, Jun 13, 2013.

  1. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
    19
    Dc circuit, resistive load, 18awg cable, 12v, 5 amps

    In that combination, if i want to run longer then 20ft, it would require heavier wire to minimize voltage drop.

    However, if I have a power supply that I can bump the voltage on, it effectively compensates for the voltage drop of the cable.

    In this example, if I wanted to get a full 12v at the end of 30ft, I would need to bump up the voltage at the source about .4v.

    I can't see any reason this would be frowned upon or dangerous, but I want to make sure I'm not missing something. I know at a certain point, the cable will run warm and efficiency will be horrible, but if I'm just going .4v over I don't see what bad could happen, as long as my power source is up for it. Am I right?

    Probably a dumb question but I just want to make sure. Thanks in advance.
     
  2. ramancini8

    Member

    Jul 18, 2012
    442
    118
    No problem except that your source has a higher output resistance that leads to the downsides you mentioned. The no load voltage will be 0.4V higher, but that shouldn't pose a problem on a 12V circuit.
     
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  3. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
    19
    Thanks. The only reason I questioned it, was because everywhere I read seems to highly stress using the proper sized cable in low voltage applications or you won't get what you expect. Because I have been through electronics schooling, I know it's safe to bump the voltage on many power supplies, so I wasn't sure why it's never mentioned.

    I think it really has to do with most power supplies having a fixed 12v output, and most people don't want to modify the supply for their custom application.
     
  4. Andreas

    Active Member

    Jan 26, 2009
    68
    4
    Presumably you are using multi stranded wire...? Solid copper 18AWG has a x-sectional area of about 0.8mm and about 21Ω per 1000m. From my reckoning that equates to about 0.21Ω for every 30ft (~10m). Your load resistance will determine what current will flow.
     
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  5. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    If load current does not vary and your load does not depend on having a low source impedance power source it is likely OK.
     
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  6. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
    19
    LED ropelights, so its not purely resistive, but its close. And the current of course never varies that much.

    It's important to compensate for voltage drop though, because even .1v make a noticeable difference in brightness. So I do the math to get it close, but then I tweak the power supply until the end of the ropelight sees 12v under load. From my experience so far, this has been working great.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    In critical applications, remote load sensing is used to regulate the voltage at the point of use. Separate wires (they can be small gauge) go to the load from the power supply to sense the load voltage (called Kelvin sensing) and adjust the output to compensate for any voltage drops in the line. Of course the power supply must have this capability (which most commercial lab supplies have).
     
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  8. trader007

    Thread Starter Active Member

    Feb 27, 2010
    222
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    That sounds neat. I suppose what I'm doing then is just a manual form of that... in a way...
     
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