12vdc to 3vdc

Discussion in 'The Projects Forum' started by Lightfire, Jul 20, 2011.

  1. Lightfire

    Thread Starter Well-Known Member

    Oct 5, 2010
    690
    21
    Hello,

    Here we go again.:)

    I have a radio that uses 3 volts to powered.

    Um, now I want to use my 12 volt battery, so step down to 3 volts.:)

    Um, it has a battery holder as well as the one that it looks like a charger hole. I don't know what it is called. but it accept 3 volts.:)

    Now, I want to use my old charger cord. so that i could put the cord to to the charger hole and then power it by a 2 AA batteries.

    pls. help. i want to use 12 volt dc instead.

    Lightfire
     
  2. readym

    New Member

    Jul 19, 2011
    13
    1
    You could use a 9V zenner diode in series with the 12V line or use a 3v regulator.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    If you use a linear regulator, the radio will consume 4 X more power, as you have to "burn" the resting 9 volts away.
    A switching regulator will be more complicated, but will have a much more efficient power conversion.

    Bertus
     
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    Linear Regulators will waste tons of energy. Building a switching or Buck regulator will cost you time, more money, and quite possibly a migraine.

    The following device, Pololu Step-Down Voltage Regulator, will offer the 12V to 3.3V or 5V and up to 3.5A continuous current. It's fully assembled and takes up the same space as 1 AA battery. The 3.3V output should not be a problem as 2 AA batteries, New can often equal 3.2V. A single diode in series could drop the 3.3V down to near 3.23V.

    I am not associated with this company but have been very satisfied with their products in the past. I even have one of these exact devices.
     
    Last edited: Jul 20, 2011
  5. castley

    Member

    Jul 17, 2011
    31
    0
    If you can pulse the 12 v on off with a 25% on duty cycle you will get the equivalent energy as from 3V.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This is not correct.

    If the load were purely resistive such as a lamp or heater element, and the base PWM frequency were high enough for the items' wattage rating, then it could work. However, the radio has reactive components (L and C) which you cannot randomly power using PWM.

    Besides, plain PWM would generate a horrific amount of noise that would be heard on the radio speaker.

    A switching regulator would work, but our original poster is very new to electronics, and would have a very hard time understanding switching regulators at this point. We try to help educate people, but we don't want to bewilder them.
     
  7. pagoda

    New Member

    Jun 6, 2011
    6
    0
    After reading about zenner diodes, it seems as though you are right, but it almost seems too easy. Why do most people use the 7805 voltage regulator to get 5V from 12V? Why not use the cheaper and simpler zenner diode?

    Thanks
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Trying to regulate using a 9v Zener would not be good for the radio, as the radio voltage supply variations would be too large. You need to regulate the voltage to the radio, not the voltage drop from the battery to the radio.

    If a 12v lead-acid battery just came from being float-charged, it might have a terminal voltage of 13.7v.
    13.7v - 9v = 4.7v; too much for the radio.
    Even at 12.7v-12.8v, it would still be 3.7v-3.8v; too much.
    The voltage across the radio would not drop low enough until the battery was about 40% discharged; down to 12v. At that point, the battery really needs discharging.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,122
    3,046
    Of course Sarge meant REcharging the battery, not discharging it.
     
  10. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    I think it will be 3.3V - 0.7V = 2.6V for a silicon diode.
    Or if using a schottky diode it will give approx 0.25 to 0.4 (Vf) forward voltage drop ,thats makes 3.3 - 0.25 = 3.05 or 3.3 - 0.4 = 2.9.

    If using schottky diode check out its (If) forward current because they have lower forward current ratings.

    Anyway as per me the cheapest solution will be using a 317 regulator.

    Good Luck
     
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