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    Constant current source for led lamp

    HI,
    Please help me to understand following ckt diagram.Current source.bmp

    •   Alt14th February 2013, 12:55

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    Re: Constant current source for led lamp

    |The 1n4148s act as a low voltage zener so the base is held at a constant voltage, so the emitter voltage is constant, so the emitter current is constant ( Ve/33). This current is also the collector current, so nits the LED current. If the designer has done a good job, the small temperature variations of voltage will cancel out, giving the constant current over a range of temperatures.
    Frank


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    •   Alt14th February 2013, 13:13

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    Re: Constant current source for led lamp

    If I replace BC547 in mentioned diagram by 2N3055, Can it be possible to operate over wide range(from 12VDC to 220VDC) for constant current source....



    •   Alt15th February 2013, 06:25

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    Re: Constant current source for led lamp

    No!

    The transistor has to meet two critreria, it must be able to witstand at least the maximum voltage across it and it must be able to dissipate the heat produced in it. 2N3055 ratings vary by manufacturer but are typically around 60 V and you should use a transistor that could witstand surges so one rated at 300V+ would be more suitable. It must also be able to dissipate the heat (voltage across it multiplied by current through it). For example if you set it to 20mA current and it had 220V across it, 4.4W of heat would be produced which would almost certainly require a heat sink or forced cooling of some kind. Don't confuse the power rating with the power dissipated. The rating is the maximum posible power before damage occurs but only if the temperature is kept under control.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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    Re: Constant current source for led lamp

    Okay.....Thank you....



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