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Fluctuating voltage drop in radio

  1. Nov 16, 2009 #1
    Hi Experts :!!),

    I tried to use a 5v dc voltage source to power a normal handheld 3v radio using a resistor to regulate the voltage.

    The voltage drop across the resistor seems to fluctuate constantly by about +/- 0.8 volts.
    So, the voltage drop across the resistor fluctuates between 1.5v to 2.3v. The reception sounded bad like with intermittent noise. It seems that I could not get a constant 2v drop across the resistor to provide the 3v(2 x AAA) needed by the radio. When I use the batteries, the radio performs normally.

    I could not understand why the voltage drop was not constant when the volume is kept constant and the why using the batteries the radio works normally.

    Thanks in advance.

    Last edited: Nov 16, 2009
  2. jcsd
  3. Nov 16, 2009 #2


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    This is because in normal speech, people talk then stop talking. Music is not constant, it varies with the tune.

    So, the radio draws current then it stops drawing so much current. This variation gives you a different current through the resistor and so you get a different voltage on the radio.
    So, it may produce distortion.

    You need a voltage regulator that gives 3 volts. The LM317 is one that can be adjusted to 3 volts.
  4. Nov 16, 2009 #3


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    A cheaper way might be to put 3 silicon diodes in series with each other and the radio so they are conducting and each dropping 0.6 volts.

    This would give you a fairly constant 3.2 volts
  5. Nov 16, 2009 #4
    Thanks for enlightening vk6kro !

    Will try your suggestion.

    Have a great day.

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