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Need some help to identify blown up component

Discussion in 'Troubleshooting and Repair' started by SapeS, Feb 8, 2016.

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  1. SapeS


    Feb 8, 2016
    Dear all,

    I own a Chinese brand LED dive torch. Unfortunately I blew it up by using two individual 18650 cells while the original pack has two 18650 cells in series. As a result, the over-voltage blew up a SOT-89 component on the pcb. The failed component has a marking BH so my first guess is a BCX56 transistor. However, I have some doubts as this is an NPN transistor. Looking at the PCB, battery positive would be connected to the emitter while the collector would be connected to the anode of the CREE XML LED. This is completely opposite of what I know about NPN transistors where the emitter should be connected to battery negative. Is my identification wrong? Can an NPN transistor work like this? Can you please assist?

    Some additional information:
    LED kathode is connected to battery negative.
    The lamp has three settings: 25%, 100% and SOS.
    These are controlled be three hall sensors at the edge of the pcb.
    The hall sensors are activated by a magnet rotation around the barrel.
    There is an unidentified 8 pin SMD component that in my opinion provides a PWM signal to the "base".
    When using the lamp in the rain, I see the raindrops as short stripes which indicate a "high" frequency repetitive on-off signal.
    When the cells go flat, the intensity of the light decreases which IMHO means that the voltage drops and is not compensated with more % PWM.
    When I look closely at the SOT-89 component I think I can distinguish an L.
    There must have been another character in the middle, but there is a crater now.

    See pictures below and BCX 56 spec sheet.
    . led_driver_345.jpg sot-89_bh.jpg

    Attached Files:

  2. davenn

    davenn Moderator

    Sep 5, 2009
    If that large solder pad on the centre right is the positive, then it IS going to the collector ( the centre pin)

  3. SapeS


    Feb 8, 2016

    If it is indeed an BCX56, terminal 2 is the collector. it is attached to the big soldering pad on the pcb but it goes to L+ which is connected with a red wire to the anode of the LED. The lower terminal #1 (emitter) is connected to battery positive thru pcb bushings to the other side. Terminal 3 (base) is connected to an output of the 8 pin SMD chip.
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