Voltage Drop

Discussion in 'General Electronics Chat' started by sasek, Nov 28, 2011.

  1. sasek

    Thread Starter Member

    Jan 3, 2010
    64
    0
    Hi,

    Voltage drop happen during long way travel of power from supplier to load. The standard allowable voltage drop is between 3-5%. Normally variable that affect voltage drop is cable length(internal resistance), load resistance & impedance. Basiclly we can calculate this voltage drop from online voltage drop calculator.

    The question is....

    Is it any voltage drop occur during transferring signal that take 1km long (i.e from transmitter to PLC). From my understanding, I believe voltage drop will occur due to length of cable(internal resistance). But since transmitter and PLC is provided with external power supply and since transmitter can be consider as power supply while PLC can be consider as load, is it PLC will affect the voltage drop.

    How the calculation and parameter affect the voltage drop?

    Any reference i.e books, notes can be used as reference?
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    The type of voltage drop is the simplest equation you can imagine, Ohm's Law.

    The total resistance of the cable, along with the current, is where the drop is coming from.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    If the signal includes anything other than DC or very low frequencies, then a 1km long cable may cause more attenuation than its DC resistance would imply.

    This is due to an interplay of distributed resistance, inductance, capacitance and dielectric losses, and at higher frequencies skin effect may be relevant. (But perhaps the frequencies involved with a PLC are too low for much of this to matter?)
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,523
    Some thing that many don't consider is that the voltage drop of your wire is twice the distance. One is the distance/drop from supply to device. The second is the distance/drop from device back to supply, completing the circuit.
     
  5. sasek

    Thread Starter Member

    Jan 3, 2010
    64
    0
    All 3 replies state that the voltage drop during transmitting signal is only apply on power loss due to cable length. Means PLC not contributing on voltage drop.

    I just compare voltage drop different between transferring power to load with transferring signal. As my understanding during my study, voltage drop during transferring power to load also effected by it own load beside cable length it self.

    Thanks 4 the answer
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The load will have a significant effect on the cable voltage loss if its value has an important effect on the cable current, which is usually the case in power systems.

    Signal transmission systems often use a defined load impedance matched to the cable, in which case an incorrect impedance will have harmful effects.

    Finally, some systems use a defined current signal, which may be the case with your PLC.

    If this is so, the cable voltage drop is defined by the sender current and the cable resistance, and the load only affects the voltage required. The value of the load voltage may however be significant in defining the maximum tolerable cable voltage drop, beyond which the driver voltage capability will be exceeded.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    What gauge is your 1km wire run, what is the signal voltage, and the impedance of the load at the far end?

    For a 26 gauge conductor (cat 5, phone wire, etc), 1km with a 10mA load will drop 1.32V. 20mA current will drop 2.64.

    This is assuming temp is 72 degrees, if colder, you don't lose as much, 2.38V in the 20mA case over the same line.
     
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