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OT Electrical (low voltage)

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  • OT Electrical (low voltage)

    I have a small lamp consisting of two LEDs powered by three AAA batteries connected in parallel. On the body of the unit is a standard socket for a power supply. Questions: 1. In connecting one or more external batteries to the power supply socket, is polarity an issue? 2. In either event should the AAA batteries be removed? Thanks in advance for answers, Shel

  • #2
    Polarity is almost certainly an issue and it is strongly recommended to keep
    + to + and - to -. Standard throwaway AA/AAA/C/D etc batteries do not
    take well to outside power being applied and may get hot and leak a little
    or expand and leak a lot. They should be removed. This is not absolutely
    necessary but advisable. Rechargeable batteries do better in circuits
    designed to recharge them under controlled conditions and can overheat
    as well. It is feasible to operate the LEDs with AC, by appropriate
    current limiting resistors, but with DC, wrong polarity means they won't
    light up. Batteries won't like AC at all. EDIT: above assumed an auxiliary
    wall wart type power supply plugged in. If you are just using a bigger
    external battery jumpered in, the internal batteries won't mind this at all
    and can be left in place.
    Last edited by sch; 05-10-2010, 02:25 PM.


    • #3
      If the lamp is intended for primary cell use and has an external power source, there is a remote possibility there is a forward biased diode off the primary cells, when you hook up an external supply which is usually slightly higher voltage than your AAA cells, especially wall-wart variety, then the diode will be reversed biased and prevent flow to the cells.
      With the supply removed, the AAA cells supply the current.
      Just an idea?


      • #4
        Polarity is an issue, but it should allready have isolation beween the two power supplys because it will SURELY rupture the batterys if it connects the power supply to the batterys.

        But, if in any doubt (espeicaly if its home made, or some really cheap chinese product) remove the batterys just to be sure.

        And from someone who as a kid did accidnetly recharge batterys and then accidently got in contact with the leaking alkaline battery goo.. Let me tell you.. it *BURNS*
        I washed it off within 15~20 seconds, and it felt like someone was puting a cigar out on my skin for the next few hours.
        Thermal burns are less painful.

        Also, Never use 'heavy duty' aka carbon zinc batterys, they will rupture just from being overdischarged (left in a flashlight thats 'on' for example)
        (Though I think there electrolyte is more benine, its still not good)
        and only have like 1/3rd the capacity of alkalines.
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


        • #5
          I would be surprised if the 3 cells are in Parallel. More likely they are in Series.
          Might want to check again to be sure.
          Is the External Power Jack marked with Voltage needed?

          Tom M.


          • #6
            I have an MP3 player that used two double As in parallel, and I added a D cell to the thing. Runs for about 5 months before I have to change the D cell, unless I leave it on too many times for the night.

            A diode in series with a single cell, or cells in parallel is going to waste at least a third and probably half of a cells voltage, which won't leave much for the led drive circuitry. Chances are there is no diode, and you will have to be careful to get the polarity right the first time if you use the external power supply. The dedicated power supply could have some limiting circuitry in it that supplies only enough voltage to run the leds, and therefore won't charge the internal batteries, but that's a guess. Chances are good that the light was designed for rechargeables, and if used on a power pack, those would be ok. If you had alkalines or carbon zincs in there, you would want to remove them before using the power pack. I have seen this recommendation on one of these led lights, so I know at least one manufacturer has considered this potential problem.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


            • #7
              My guess is that the batteries are in series, if a low cost shottky diode is used the drop is minimal, .15v max .45v at worst case.


              • #8
                Agreed, Shottky diodes would lessen the voltage loss from the battery. There's still going to be a voltage upconverter of some kind to run the leds, and that's going to be electronic, so a reversed polarity supply will probably kill that. If the batteries are in series, there will be enough from two batteries to run the leds, but not enough to overcome a series diode as well. With three cells in series, things get lots simpler.

                It sounds like you don't have the power pack, and would have to find one with a suitable voltage output and correct polarity on the plug. You may be able to measure voltage on the jack and determine the polarity if there is no series diode to isolate the power pack from the batteries. That's worth a shot at least, as all it would take is a properly fitting plug, then a voltmeter measurement. Whatever polarity you might read is the way you'd have to feed in from the power pack. If you read no voltage, then you don't know yet which way to wire it up. You may have to turn the light on in order to connect the battery voltage to the input jack- depends how it's wired.

                In any event, you could just test without the batteries in, but with the switch on. Momentarily touch a current limited voltage source to the input- if no light, reverse the wires and touch again. However momentary your contact is, when the leds even so much as flicker, you've got the polarity nailed down. If the cells are indeed wired in parallel, that's a guarantee that there's electronics involved, and this brute force test might just spell doom for the light. Maybe not, but maybe. More info is needed, particularly to clarify the battery connection- series or parallel. And check to see if there's any markings around the power input socket- often there will be some hieroglyphic telling of the polarity.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-