Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Voltage Regulator, Battery resistance, current

  1. Jun 15, 2012 #1
    Dear Experts

    Lets say I have a power source of 9 volts.
    A 5v output voltage regulator is connected to this power supply.
    So, input 9 volts, output is regulated to 5volts constant.
    the output pin is connected to a battery with has a constant internal resistance.

    Does this mean, because the resistance of battery and the charging voltage is constant, the charging current going into the battery will not increase even when the power source is increased say from 9 volts to 12 volts?

    So, my question is can a voltage regulator giving constant 5 volt output charge with higher current when the source voltage is increased?

    Thanks for reading.

    Best regards
  2. jcsd
  3. Jun 15, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The point of the voltage regulator is to provide a constant output (i.e. regulate the output to within certain tolerances) over a range of input voltages. So, even with a 12 volt input, the output of the regulator is still 5 V, (assuming that both 9 V and 12 V are within the range of acceptable inputs for your regulator). So, regardless of input voltage, it's still 5 V at the output, which means that the same thing will happen when you connect the thing to a battery in both cases.

    A voltage regulator steps down voltage by basically wasting power. So, with a higher voltage input, more energy will be wasted as heat in order to step down to 5 V. That's about it.
  4. Jun 16, 2012 #3
    Hello Cepheid

    Thank you for your response.
    To finalise, the output current from voltage regulator cannot be increased as the battery resistance is constant.

  5. Jun 16, 2012 #4
    That's correct for your example, but it's potentially misleading.

    If the load needs 100mA, the regulator gives it 5 volts at 100mA. If the load changes, the regulator current follows it (within design limits.) You can double the voltage going into the regulator, but if the load doesn't change, the current doesn't change.

    As cepheid said, the higher voltage is simply dissipated as heat by the voltage regulator.

    (Of course, like all electrical circuits, if you supply higher than its rated voltage, bad things tend to happen.)

    ETA: Look at the information panel on some power supplies -- such as the charger for your phone. Mine says:
    Input: 100-240v, 50/60Hz, 0.1A
    Output: 5.9v DC, 375mA​
    You can see that a wide range of input voltage has no effect on the output.
    Last edited: Jun 16, 2012
  6. Jun 22, 2012 #5
    Thank you very much Mr Pantaz :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook