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led heats up,while using mobile battery(3.7V,1000mA)

Discussion in 'Power Electronics' started by skilkhan, Jun 16, 2013.

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  1. skilkhan


    Jun 14, 2013
    I made a small pocket size torch using mobile battery 3.7V,1000mA,
    i use 4 gboss led(named by electronics shop who gave me this shown in pic)
    i connect 4 leds in series as shown in diagram atched
    6 diodes are connected in serirs to drop 0.7v each(since i m using 6v dc nokia mobile charger for charging) and final voltage reaching battery is about 3.7V
    now problem is that leds became very hot after few min.
    can anyone please tell me how to reduce current with out reducing brightness of leds
    any thing small(like diode is used to reduce voltage)
    tnx in advance

    Attached Files:

  2. BobK


    Jan 5, 2010
    Hi and welcome to the forum.

    There are several problems with your circuit.

    Your LEDs are not in series as you state, they are in parallel. This is not a good way to connect multiple LEDs. No two are identical and the current will not be equally distributed. Worse yet, the one getting more current will heat up more, causing it to take even more current.

    You have nothing to limit the current in the LEDs. Even if the forward voltage of the LEDs is 3.7V, a LiIon battery will be 4.2V when fully charged, which will put too much current through them, unless you run them a quite a bit lower than the max current they are designed for.

    Those LEDs are designed to be attached to a heat sink. They will get very hot and not last very long if are not.

    And finally, you cannot charge a LiIon battery by putting 3.7 V across it. You need a current limited supply of 4.2V to charge LiIon batteries.

    Please read this tutorial for help in running LEDs.

  3. skilkhan


    Jun 14, 2013
    tnx for ur reply
    i will try ur sugestion later (after my exam)
    i tried these led in series but none turned on,,,,
    i also will try a 4watt resistor in circuit before leds,and will post my result

    i m uploading led sink picture,,a small sink that come with these led
    Tnx again

    Attached Files:

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    That's correct, they wouldn't. (I think it was an observation that you said you placed them in series -- but I expect you were meaning the other diodes.

    Not just any 4W resistor, an appropriate one. Preferably one for each LED. (and given the variance in voltage -- especially considering the Vf of the LEDs -- it's not going to be a good solution.

    A better solution id a switchmode boost current source driving all the LEDs in series. However that's not exactly a jelly-bean component.

    The star will act as a heatsink, but its real purpose is to allow you to connect the LED to a larger heatsink.
  5. somanshumehta


    Jun 22, 2013
    Put your leds in series so as to distribute the suply voltage and also put current limiting resistors if practically possible.
    At 3.7V, u may blow ur LEDs
  6. skilkhan


    Jun 14, 2013

    I have tried using 460ohm and 2watt resistor in series and is know working fine for me.
    thx to all for response.
    I am inserting images
  7. skilkhan


    Jun 14, 2013

    thanks again for ur interest

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