12v to 4.5v on Childs Electric Car

Discussion in 'The Projects Forum' started by nickorossa, Jul 10, 2010.

  1. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
    54
    0
    My daughters electric car has a 12v sealed lead acid battery as its main unit. There is a separate music box that runs on 3xAA batteries (4.5v).

    I'd like to be able to connect the music box to the main 12v supply and save the hassle of changing batteries.

    I found some circuits on the web, but not using the same input/output voltages.

    To do this I think I need a Voltage Regulator such as this.
    http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=661-6226

    Then use a diode with a forward voltage of 0.5v to reduce the 5v output to 4.5v such as this.
    http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4689376

    The circuits that I came across then have capacitors on both input and output side of the regulator and this is where my very limited knowledge starts to fade as I don't really understand the purpose of capacitors.

    Can someone let me know what else is required to complete the circuit?

    Thank you for your help and patience.

    Nick.

    P.S. Is there a recommended free/low cost circuit design tool available?
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Yes.

    Use a LM317 regulator... it's the cheapest and simplest solution.

    Connect a 390 ohm resistor to the adjust pin from the output pin. Connect a 150 ohm resistor from the adjust pin to ground. This gives 4.5 volts ±10%.

    Connect your input voltage to input.
    Connect your output, to output.

    Connect the ground of the musicbox to the negative battery terminal.

    If the musicbox doesn't draw much current the regulator will stay cool, otherwise you may need a small heatsink.

    For improved output, a capacitor from input to ground and output to ground will take care of any momentary glitches in power caused by the motor accelerating. About 470µF, 25 volts will do. Remember to get the polarity right. These capacitors act as tank capacitors for when the regulator or battery is unable to supply current. The datasheet for the LM317 suggests small capacitors but for a battery powered device running a motor you will need bigger capacitors as I suggest.

    You can build this in a case and use point to point wiring.
     
    nickorossa likes this.
  3. Dx3

    Member

    Jun 19, 2010
    87
    7
    The small capacitors are used to suppress oscillation as the regulator tries desperately to regulate the output very closely. Big capacitors will probably work, but it is important to place them very close to the regulator chip. A few inches of wire can be enough to allow high frequencies to develop. If you can't get the large capacitors very close to the chip, you can add the small capacitors that are recommended by the manufacturer.

    I know, this doesn't make sense adding a tiny capacitor in parallel with a big one, unless you realize that a few inches of wire has some inductance at high frequencies and the regulator chip has a VERY fast error correction circuit.
     
  4. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Capacitors usually larger than 10µF prevent oscillation, as do values less than 100nF. I suggest these values because the 'voice box' most likely has a microcontroller in it and a large dip in voltage could cause a reset.
     
  5. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
    54
    0
    Hi

    Thanks for your quick responses; its much appreciated.

    The music box has a COB circuit so its probably impossible to tell what the chip actually is under the blob!

    The voltage circuit will be made up on some strip/Vero board and mounted in a suitable sized box or within the music box itself as there is lots of room.

    Thanks for your help.

    Nick.
     
  6. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    This should be very easy to accomplish with the components you had provided links to.....

    If you just stick with the 7805 5 volt regulator, just add a couple of .1uf caps to the input and output as close to the regulator as possible.... I personally do not think the diode is necessary since the sound module can handle 5 volts (I know since I did the same thing to my sons limited Edition Jeep Wrangler)... it does save on having to replace those AA's all the time.....

    7805 CIRCUIT.png

    7805 PCB.png

    B. Morse
     
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  7. SgtWookie

    Expert

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

    Small values of capacitance (<20nF) on the output can cause oscillations due to the LC time. Using 100nF or larger on the output swamps the effect. A metal poly or ceramic 100nF (0.1uF) bypass cap should be placed as close as possible to the OUT terminal.

    Minimum requirement for a 78xx type regulator is 0.33uF (330nF) on the input, 0.1uF (100nF) on the output.
     
  8. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
    54
    0
    Thanks to all for your help.

    I built the circuit and the output voltage measured around 4.55v with no fluctuation when the motors are started/running/stopped.

    Nick.
     
  9. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    awesome, glad to hear it worked out for you..... now just keep your feet out of the way when she's driving :)

    B. Morse
     
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