Ohm resistor

Discussion in 'General Electronics Chat' started by sol, Nov 29, 2010.

  1. sol

    Thread Starter New Member

    Nov 1, 2010
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    I’m working on a project where I want to charge a mobile phone and I want to approximate the resistance in the circuit.
    Any ideas what resistor in ohms I need?
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you mean measure the current going into the phone while charging?

    If so, something like a 0.01Ω precision resistor inline with the source used with a multimeter can give you the current using ohms law. You may get inaccurate results with cheaper meters as the value will be in the millivolt range.


    To give you a rough idea, many US mobile phones need a regulated 5V charger, between 350mA and 1A capacity. This fact means there are a lot of spare power supplies around for logic circuits. :)
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    I think he wants to simulate the load.

    Like using a 50ohm resistor to simulate a 50ohm antenna.

    It this correct?
     
  4. sol

    Thread Starter New Member

    Nov 1, 2010
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    Yes that’s right I wanted to simulate the load of a phone battery with a resistor.
     
  5. thatoneguy

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    A battery can't be modeled with a resistor, the best way to go about it would be logging the current from an existing charger. Most chargers simply supply a voltage with a max current, the phone itself has the charge controller. You'll need to verify that, though.
     
  6. tyblu

    Member

    Nov 29, 2010
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    Modern battery chargers work by following a specific current vs. charge plan. These are available as graphs for many battery datasheets -- you may be able to get one for your phone battery. They are regulated by a controller (sometimes a micro, sometimes an ASIC, maybe but not likely analog) which controls a constant current source and monitors battery charge.

    What type of chemistry are you going to be charging? If it's just lead-acid, then you can build an analog circuit to do this, but it still doesn't follow Ohm's law (a resistor-type load).
     
  7. sol

    Thread Starter New Member

    Nov 1, 2010
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    I guess it’s a normal LI-ION Battery.
    I was doing a wind power experiment and wanted to see if there was enough power under load to charge the battery.
    So I wanted to simulate the load and do some tests before I hooked my phone and battery up to it.
    Some resistor would approximate that load i thought?
     
  8. tyblu

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    Nov 29, 2010
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    A resistor can approximate the load at a certain charge point, yes. Should be able to look up a certain point on the datasheet and match the specified IV.
     
  9. retched

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    Dec 5, 2009
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    You would have to use a variable resistor that changes it value throughout the charging cycle to continually match the "current*" status of the battery.

    *(not amperage but the resistance at a particular point in time during the charge cycle)

    A batteries internal resistance changes with different variables including total charge percentage and temperature.
     
  10. sol

    Thread Starter New Member

    Nov 1, 2010
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    How about the peak load that expected to see if there is enough power in the wind. That is when the electrical load is highest also will the mechanical load on the prop be as well.
     
  11. thatoneguy

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    Both my camera battery charger and my phone charger are rated for 2.5 Watts output, 500mA @ 5V
     
  12. sol

    Thread Starter New Member

    Nov 1, 2010
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    What would be the internal resistanc ethen at its peak?

    No datsheets seem to have this info?

    Also my wall charger is 5V at 350mA but this has nothing to do with load?
     
  13. thatoneguy

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    The internal resistance is typically as low as possible in a power supply. Otherwise it will be dissipating too much heat.
     
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