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Step Down DC Buck Capacitor Blow up

Discussion in 'General Electronics Discussion' started by NVZN, Dec 17, 2016.

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  1. NVZN


    Dec 17, 2016

    First, sorry for my bad english.

    I Have a Step Down converter DC from 8-60V to 1-36V continously. It named :
    200W 15A DC Buck Step Down Converter Wide In 8-60V to 1-36V Adjustable

    So here's the problem :

    I have a LiFePo4 Battery from my Electric Bike, it's battery specs is 48V 10Ah.
    I wanted to step down it to around 24Volts, so i bought stepdown.

    After received the product, i'm going to test it out. from the battery i connect + to "IN+" to the product and - to "IN-".
    (The battery voltage now is 50Volts)
    then, i connect the output to my multitester.

    I turned it on, the red led indicator from the product is turned on. the multitester jumped to 50Volts. Strange. the specifications shows only 36V max. Yeah there's a trimmer, but all i know if i turn the trimmer to increase it's volt output, when it reach 36v it should be still 36V. (when i test it i dont change the trimmer, yet)

    after 10 seconds, suddenly the one of capacitors explode. I panicked and turned that thing off.

    I asked the seller, seller says "the polarity is wrong" but im VERY SURE the polarity is correct.

    What if i put wrong polarity?

    There is 4 capacitors on it,
    2 capacitor near IN socket, 220uF 63V, and
    2 capacitor near OUT socket, 470uF 35V.

    the strange is : assume you put wrong polarity on it, the result should be 2 or 1 capacitor near IN socket blew off, but in my case the capacitor near OUT socket explode.

    So is the controller or IC the product defective or something? is it my fault or vendor's fault? My friend says it's Chip fails/defect so it doesnt step down, then made the Out caps explode.

    i need your answers, to prove it to the seller to get replacement.
    Here is the pics.

    (at the left capacitor which it blew off)

    Last edited: Dec 17, 2016
  2. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    Th electrolytic is 35v The battery is 48v . . . .BANG !
  3. NVZN


    Dec 17, 2016
    Its step down. From range 8-60V. So a 48V battery should be accepted.
  4. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    How can IN=8v OUT=36v be a step-down converter???

    You obviously put 48v on a 35v electrolytic.
  5. davenn

    davenn Moderator

    Sep 5, 2009
    you obviously didn't read his post correctly

    the input can be up to 60V, he only put 48V in
    and that doesn't explain why the output cap was the one that went bang

    that could really only be explained by either
    1) accidently connecting his 48V to the output or
    2) the converter was faulty

  6. duke37


    Jan 9, 2011
    Congratulations on your english.

    A led lighting up indicates the correct polarity.
    You may have had the polarity right but did you stuff 50V into the output?
  7. NVZN


    Dec 17, 2016
    No, i put the battery source to IN socket, and i connect the multitester to OUT socket.
  8. AnalogKid


    Jun 10, 2015
    Not automatically. LEDs have a reverse avalanche just like zener diodes. For a standard little red LED, put 8 V across it backwards (with current limiting) and you get red light. His onboard indicator shows correct polarity *only* if there is another diode, a standard rectifier, in series with it.

  9. AnalogKid


    Jun 10, 2015
    You obviously are wrong.
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    My guess is that the battery voltage is higher than the nominal 48V. My guess is that fully charged it would be a little higher than 50V (assuming 12 cells in series). So there's the source of the 50V that was read at the output -- the board didn't need to act like a boost converter.

    if the voltage adjustment pot is open, or in some fashion not sampling the output voltage, the buck converter would likely leave its pass device turned hard on and the input would flow to the output via the switch device, an inductor, and a diode. In this case, given that the load is not substantial, the output voltage would rise to that of the input.

    The visible failure is just a simple consequence of this.

    I would get my multimeter on the ohms range and measure the resistance from the +ve output terminal to the three pins of the output voltage trimmer. If my guess is correct, at least one of these will read open.

    Another possibility is that the designer saved themselves the cost of a resistor and connected the trimpot directly across the output (rather than using another resistor to create a potential divider. The consequence of turning the trimpot all the way in one direction would be to produce a completely unregulated output leading to the consequences you observed.

    If this latter possibility is correct then at least one of the three pins on the voltage trimpot will be connected directly to the negative output rail (i.e. you see less than a few ohms).

    As a general note, whenever you're testing out one of these boards it is wise to start with an input voltage that is within the range of valid input *and* output voltages, and set any trimpots roughly mid-way. If possible, you also start with a current limited power supply.
    hevans1944 likes this.
  11. Bluejets


    Oct 5, 2014
    With LiFePo4 Battery, nominal is around 3.2v so maybe 15 cells, makes the top end even worse at 3.8v going to 57V.
    (*steve*) likes this.
  12. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    That says it all. The product is defective because the maximum output should have been only 36 V. You measured 50 V. Clearly, the input polarity was correct, because the product did produce a DC output, albeit too much output.

    Demand a refund! Then buy a different replacement from a different manufacturer, one that does not specify a 36 V maximum output and then installs capacitors rated at only 35 V.

    The product is supposed to be a buck converter, not a boost converter. Since it is rated for up to 80 V input, even a fully-charged LiFePO4 battery that is nominally 48 V should have been acceptable as a power source. The fact that the output jumped to what appears to be the battery supply voltage is an indication that the product was defective.

    The biggest "mistake" the OP made was leaving the product powered on when the output was measured at 50 V.
    davenn likes this.
  13. Alec_t


    Jul 7, 2015
    As I replied on another forum, it may be that the converter requires some minimum output current to regulate properly. A meter alone draws little current.
    davenn likes this.
  14. BobK


    Jan 5, 2010
    Although I agree that this might be the case, that is a piss poor design if it blows the output capacitor when you fail to have a large enough load.

    davenn likes this.
  15. cjdelphi


    Oct 26, 2011
    I've had a couple of defective step down converters

    The cap that exploded would be in the low side, but instead of switching like it supposed to, it dumped the load to the output, thus the caps exploding..

    Faulty unit.
  16. cjdelphi


    Oct 26, 2011
    Yes, i once had an lm2596 self destruct over a 16v input lol, the reg exploded, nothing else though as it wad just 16v input/16v out

    And several regs that just fed out what came in, but these issues seem more common with a step up dc dc converters
  17. NVZN


    Dec 17, 2016
    the maximum voltage of my battery is 54.6V.
    i see. the seller finally give some choices : the seller will repair it free, and i only pay for the shipping costs.

    Yes, i agreed. and when i retrieve back, and tested it, it work again. (the caps also fixed too)
    i've tried from 1 volt to 24 volt, still working fine.

    when i asked what's cause of this, the seller won't give the answer.

    well i think if it's working now, so no problem.
  18. NVZN


    Dec 17, 2016
    is it same case like me, blown capacitors?

    do you have pics? i would like to see one
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