Bike lighting using 18V battery pack - Help!

Discussion in 'The Projects Forum' started by Zorch, Jul 14, 2008.

  1. Zorch

    Thread Starter New Member

    Jul 14, 2008
    4
    0
    I am putting together a lighting system for my bike using some 18V rechargeable batteries I have.

    I am running a pair of 12V LED lights and a small power inverter that runs on a pair of AA batteries.

    Sorry for the quality of the diagram (its been a few years since my last electronics class) Can someone tell me what the resistors at A & B should be?

    [​IMG]

    Any other tips are more than welcome too.

    Thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That isn't enough information, I'm afraid.

    What is the rating of the LEDs? Forward voltage @ current?

    Your inverter running on two AA batteries probably won't last more than a few minutes unless your 12v LEDs have very low current draw.
     
  3. Zorch

    Thread Starter New Member

    Jul 14, 2008
    4
    0
    Oops! Your right, they are 1 Watt bulbs (these are MR-16 replacements)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, so about 83mA @ 12v.

    Looks like you're trying to run the LEDs in series, but your inverter isn't hooked up in series with the battery.

    If you're running the LEDs in series, you will need > 24v.

    What is the output voltage and current of your mini-inverter?
     
  5. Zorch

    Thread Starter New Member

    Jul 14, 2008
    4
    0
    I figured that I would be able to run the LED lights directly off the DC current (bypassing any sort of inverter). The inverter that is there is made for my 15' of El wire - i don't know if i would trust it to run anything else!

    I'm not opposed the adding one if it is necessary/more efficient - but for time and equipment cost sake I would rather keep it as simple as possible
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, have a look at the attached.

    The LM317's are adjustable voltage regulators rigged as constant current regulators. You might get by using 1.5 Ohm resistors, but you'll be pushing the limits of the LEDs, shortening their lives.
    Note that the resistors are 2 Watts. That's a bit more than the amount of power to be wasted in them, a hazard of using linear regulators. But, this is the easiest way to power your LEDs, and ensure good brightness untill your battery is discharged to around 15v - by that time it's really dead anyways.

    For the 1.6 Ohm resistors, you could use several smaller-wattage higher-resistance resistors in parallel, such as four 6.2 Ohm 1/2 Watt resistors. That would give you 1.55 Ohms, which would be close enough.
     
  7. Zorch

    Thread Starter New Member

    Jul 14, 2008
    4
    0
    Awesome. That was exactly what I need. Thanks Sgt! I'll be sure to post about how it works out
     
Loading...