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

Trying to get back into the swing of circuits

  1. Jun 1, 2012 #1

    I realize this may appear as homework but I assure you that it is not for school. However i am new to the site so please let me know if i interpreted the rules wrong.

    I'm a mechanical EIT whose electrical engineering education ended in about second year when general studies were concluded. Its been years since I've used the information within and school never really focused on creating the circuits just solving them.

    For work I'm trying to come up with a (relatively) simple circuit to power two motors and a set of LED lights using DC batteries and a solar panel. My issue is that the lights require 12V while the motors require 24V each. Attached is a sketch of what i believe is a viable solution.

    The lights use 540mA. If i remember my formulas right that means that to "get rid" of 12V i would need a 22.2Ω resistor. (12/0.54 = 22.2).

    Basically all I'm wondering is if my logic is sound or if I'm totally off base.

    Thank you.

    Attached Files:

  2. jcsd
  3. Jun 1, 2012 #2


    User Avatar
    Science Advisor

    Yes, that looks OK.

    Would it be possible to get 12 volts from just the lower battery?

    Seems a pity to waste 6 watts on a resistor, assuming the LEDs already have a current limiting circuit in them.
  4. Jun 4, 2012 #3
    Thank you for your response!

    Turns out my parameters have changed and i no longer need to include the lights, which simplifies this whole thing.

    However for my own knowledge I would like to understand.

    i thought that you did not want to drain two batteries in a system like this at two different rates, wouldn't putting the lights on one single battery do this? I realize that lights are often insignificant in power draw, and maybe that is where the suggestion comes from, but in this particular case the motors run for only 10 minutes (about 3 amps each) but the lights may be on for up to 4 hours (0.5amps). This means that on the required power reserve for the lights(2Ah) is actually double the motor (1Ah).

    If this was still OK I'd be all for saving the energy lost to the resistor.
  5. Jun 4, 2012 #4


    User Avatar
    Science Advisor

    If the light use was trivial compared with the motor use, then I would go for use of just one battery, but in your case, this would not be a good idea.

    Some small imbalance would be acceptable, but not that much.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook