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Running 3 LED lights on one battery

Discussion in 'Electronic Design' started by [email protected], Jun 18, 2013.

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  1. Guest

    I have a six LED lanterns that I bought really cheap. They are similar
    to those old looking kerosene lanterns they advertise on tv which use
    LEDs, but in a less fancy plastic case. The lanterns are designed to
    use three AA batteries, thus 4.5 volts. I want to connect them to a 12V
    deep cycle battery which I have connected to a solar cell to charge the

    What is the easiest way to connect them to the battery and supply the
    correct voltage to the light? I was thinking of wiring three lights in
    series across the battery. That's actually needing 13.5 volts, but most
    auto type batteries actually charge to around 14V anyhow. Is it
    possible to do this, or wont the series method work on LED lighting? I
    do realize that I'd have to keep the switches rurned on on each light or
    none would work, so I'd have to have a single switch in the source

    What are my other options?

  2. gregz

    gregz Guest

    Measure the voltage of one led using original circuit. If they are pulsed,
    might need scope. How many LEDs does I actually use ?

  3. Phil Allison

    Phil Allison Guest

    ** There must be a resistor of about 39ohms in series with each LED.
    ** What sort of battery, SLA ( gell cell ) or other ?

    ** Three in series is fine, then a resistor of about 82 to 100 ohms as well.

    The white LEDs take about 3.4V each to run at 30mA.

    .... Phil
  4. Phil Allison

    Phil Allison Guest

    "David Eather"
    ** No they don't.

    The resistance of a good AA alkaline is only 0.1 ohms.

    Even a carbon zinc AA cell has only 0.5 ohms.

    .... Phil
  5. Robert Baer

    Robert Baer Guest

    Connect the lanterns in series, and drive them with the Joule Thief
    circuit powered by whatever source you want (one 1.25V NiCd cell to ...).
  6. Martin Brown

    Martin Brown Guest

    Some of them do after a fashion, but they use an incredibly weird
    discrete step up circuit from two AA cells in parallel to charge them
    from the cheapest possible solar cell configuration and then a flyback
    transformer. The circuit fails to oscillate with only one battery in!

    The solar cell doubles as both charger and sensor. I traced the circuit
    of one once to adjust them so that they coped with the UKs long summer
    twilight. Otherwise the battery would run out just after it got dark!

    The OPs best bet would be something like a simple one chip and an LC
    buck converter to convert 12v down to something appropriate for an LED
    or one of the devices intended to do constant current LED driving but I
    can't think of any of those in a DIY amateur friendly packaging.

    Amateur astronomers here had a habit of replacing the lamp in those
    cheap ticktack box torches with a red LED and using spent batteries in
    them and under those brutal conditions the LED does seem to survive. If
    they ever forget and put fresh cells in the LED will surely die though!

    These days you can buy cycle lamps which are LED based (and *much* more
    cheaply than the ones they still try to sell to amateur astronomers).
  7. Phil Allison

    Phil Allison Guest

    "Martin Brown"

    ** Get your hand off it - wanker.
  8. Guest

    On Jun 18, 12:04 am, wrote:
     I want to connect them to a 12V
    Look on Ebay for LED replacements for lamps in cars. They work on 12
    volts and are cheap.

  9. ehsjr

    ehsjr Guest

    You asked for the -> easiest <- way.

    If putting three lanterns in series disappoints you (which
    it probably will, as nominal voltage for your battery is
    about 12.6 V), then probably the easiest way is to use a 330
    ohm resistor in series with the lantern, and wire that
    combination to the battery for as many lanterns as you want.

    For the six lanterns you have, it would look like this:

    | +|---+---[330R]---[Lantern]---+
    | | | |
    | Battery | +---[330R]---[Lantern]---+
    | | | |
    | | +---[330R]---[Lantern]---+
    | | | |
    | 12V | +---[330R]---[Lantern]---+
    | | | |
    | | +---[330R]---[Lantern]---+
    | | | |
    | | +---[330R]---[Lantern]---+
    | | |
    | -|----------------------------+

    With a 330 ohm resistor, current through each lantern
    will be about 24 mA when the battery is at nominal
    voltage. The lanterns will dim as the battery discharges.

    A little more complex (but better) is what David Eather
    mentioned: using an LM317. To do so, replace each
    resistor above with this LM317 and 51 ohm resistor circuit:

    + -----|Vin Vout|-----+
    | LM317 | |
    | Adj | [51R]
    ------------ |
    | |
    - ------------+

    That will provide a constant ~24 mA to the lantern, so
    the lantern won't dim as the battery discharges.

  10. josephkk

    josephkk Guest

    Assuming that you are using a proper solar charge controller, just add a
    11 to 15 V input to 5 V output SMPS module. Then you can make the
    existing lantern LED drivers happy as well.

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