Low Current Circuits with Car (Automobile) Batteries

Discussion in 'General Electronics Chat' started by pandian, Sep 27, 2009.

  1. pandian

    Thread Starter Active Member

    Sep 27, 2009
    Hi frens and pros,
    I'm new here. I need some clarification. Hope you ppl will help.

    I'm trying to install a low current electronic circuit (eg.: LED indicators, LED based diagnostic panel, LED based room lamp etc.) in my car.
    Usual automobile batteries are rated at 40 to 70amps.
    Is it possible to use a normal zener/ resistor or IC based voltege regulator to power such low current circuits?
    Could the circuit or voltage regulator components stand for such a high current?
    Pls advice. :)

  2. Wendy


    Mar 24, 2008
    If you look at Ohm's Law (V=IR) you will see a device only pulls the current it needs, so connecting a low current device to a high current system is not a problem. The reverse is not true.

    Voltage is a different issue, but as long as the device is rated for the same voltage no problem.

    Cars tend to have dirty power electrically, which has to be adjusted for in the low power device.
  3. pandian

    Thread Starter Active Member

    Sep 27, 2009
    Thanks alot bro.
    that clears my doubt.
  4. Gustav180


    Aug 25, 2009
    Hello pandian

    As Bill says, cars have dirty power electrically, you must think for a higher voltage range than around 12 V.

    Normally when everything is OK in the car, the voltage is around 14 V, it is the charging voltage to the battery.

    If the charging regulator faild, the voltage can rise to arond 18 V.

    If the battery connection faild during charging, the volage from the generator under a short moment gan generate around 35 V! This is the max voltage you must calulate.

    If you start the car a cold day, the start engine will take a lot of energy. The battery voltage will drop to around 5 V.

  5. pandian

    Thread Starter Active Member

    Sep 27, 2009
    there's alot factors to consider?!
    thats a good explanation.
    Thanks alot Gustav
  6. Von

    Active Member

    Oct 29, 2008
    Mr gustav is technically correct.

    But if you bias LEDs for a worst case 35 volts, they probably won't be much use at 12-14v.

    Go ahead and design for max. voltage while engine running. That is how they will be used right? If you use these with the engine off mostly they would possibly be just a bit dimmer.

    The chance of failure of these other items (reg., etc.) are very rare in today's autos.
  7. BMorse

    Senior Member

    Sep 26, 2009
    You can always use a Voltage regulator to regulate your "auto" voltage to 5 volts, try MC78T05CT from Fairchild Semiconductor, can handle up to 35 volt input. Then power your indicators from that and your LED's should stay the same brightness even if the auto is off.....
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    No, they are rated at 40 - 70 AMP-HOURS which is the battery's capacity.
  9. zimbarak

    Active Member

    Feb 8, 2009
    normaly a led require a current of 30 ma and arround 1.5 v for its to glow so u can use the ohms's law to calculate the resistor that might be used in series with the led ! r=v/i (without forgetting the calculation of power dissipation ) p=r.i squar
  10. pandian

    Thread Starter Active Member

    Sep 27, 2009
    Any suggestion circuit diagram available?thanks
  11. sillyfrog

    New Member

    Sep 29, 2009
    THis is what I did on my car.