(ebay) buck-boost converter not working

Discussion in 'General Electronics Chat' started by dk31, Feb 16, 2015.

  1. dk31

    Thread Starter Member

    Oct 30, 2014
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    Hi all,
    I recently bought this buck-boost converter from ebay:
    http://www.ebay.com/itm/400821372308?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT

    I checked with a multimeter the output voltage, before connecting any load, turned the pot to minimum voltage 1.25V then turned to max ~25v, then min and max once more, and then it stoped working. :(
    I checked the fuse for continuity and it's ok. Any suggestion how to fix it or what else I need to check? I have almost completed a project and it's the only component that holds the project back...
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,281
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    Maybe solder joints.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Advise the seller at the same time open a case with ebay before the alloted deadline, just in case.
    Max.
     
    Lundwall_Paul likes this.
  4. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Those Chinese modules can be a really great deal when they work. The key phrase is "when they work". Usually no datasheets. You are just guessing as to how everything should work.

    The description seems to mention a fuse "Short circuit protection: (enter the 10A fuse) double circuit protection, use more secure." Are you certain it is installed and not open?
     
  5. dk31

    Thread Starter Member

    Oct 30, 2014
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    I contacted the seller and opened a case with ebay to be sure. But I really wish I could fix it myself or make a new one - waiting another month will be a pain...

    @ronv : I will check for any broken solder joint. It should have been my first thought. Still learning...(but slowly)
    @spinnaker : the fuse has two solder joints and multimeter shows continuity between them.
     
  6. MikeA

    Member

    Jan 20, 2013
    125
    17
    Well you got 4-5 adjustments out of it. :D Isn't the life on those cermet potentiometers rated really low?

    Bourns spec sheet says "Rotational Life .......................200 cycles". And it's 25 turns stop to stop. Do they mean it's less than 10 full adjustments?o_O
     
  7. takao21203

    Distinguished Member

    Apr 28, 2012
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    Maybe OP wrecked it somehow, then "advising" the seller + opening a "case".

    Think, you wreck the item + ring Digikey "Get someone to fix it now" + "I've made a claim with authorities for $10 already just in case".

    What you maybe dont know, once you produce a Seller Defect with opening a case, there is no motivation to do anything for the buyers. The buyer could leave a negative comment, but since they cant produce more than one Defect actually scored, it doesnt matter.

    I had a customer recently who was thinking the product is defective. I checked with him, turned out he didnt install a freewheeling diode for the external relay.

    Had he opened a case I'd just have blocked him + done nothing. Think- can ebay fix the item or get a new one fast? They do nothing on their part, no one will actually read the case details.
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
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    Not good advice. ebay can do and will do nothing- all they do is to put funds on hold, and suggest a seller refund. there is no reward if the seller does anything beyond that. Plus they can prohibit you from buying.

    its invalid if the OP wrecked the item somehow. that's why OP came here- to know if it can be fixed.

    I have complained at least 20 times to ebay CS that cases for $2 are inappropiate without contacting the seller first, and same on the ebay community. I havent read their most recent reply because I'm anxious its from a cuesheet again.

    2) How about checking the adjustable resistor? Why did it fail suddenly? Describe that further. Can you measure a resistance? Replace with ordinary resistor? Is it totally dead now?

    Basically you need to measure input, output, the resistance, and what is used for supplying?

    it is remotely possible to wreck these just by adjusting. If the adjuster goes open circuit, the voltage will jump up. Its maybe hard to make it go open circuit, but certain people might be able to do so. 1/10 second and the MOSFET burns.

    Was there any noise?
     
  9. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    So your suggestion is that the OP shout just eat his purchase and do nothing.

    I have yet to get a refund from any seller when getting bad parts. It must be your attitude.

    Or stop buying from sellers with a 20 approval rating.
     
    Last edited: Feb 16, 2015
  10. takao21203

    Distinguished Member

    Apr 28, 2012
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    When you damage the parts they arent "bad parts".

    How about just contacting the seller? If they sell mostly fashion, they will not know + you can check that before buying. Sellers with good knowledge have higher prices.

    I havent yet come accross "bad parts", except from some 100,000 pcs, a few LED with malformed legs, one LED actually with only a partial top, and some foil capacitors moulded together.

    and one batch faulty NOS transistors, need to check each, and they vary very wildly. I sold some to a customer who wanted extra low hFE.

    2) If you rely on some parts, always order from two sources at one time, when it takes a long time to ship.
     
  11. takao21203

    Distinguished Member

    Apr 28, 2012
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    Besides, mostly the MOSFET/Diode on these blow, a major pain to remove from the PCB as well.

    Any MOSFET works as replacement. You only need special parts for high voltages (its possible to patch these converters).

    The IC isnt very likely to burn out.

    The diode is easy to check- if it reads 000 or "1" its blown.
    For the MOSFET you can make a small test circuit but its not that hard.

    Dissect it. You cant send back with USPS for free, to China. Learn from it. Maybe, you can fix it.
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    I suggest that you fix it. That will give you the most satisfaction. I doubt EBay will give you much satisfaction. OK, what are the numbers on the parts mounted to the heat sinks?
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Maybe that is your experience!
    I have usually succeeded in getting a satisfactory outcome.
    Max.
     
  14. takao21203

    Distinguished Member

    Apr 28, 2012
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    Like, the seller wouldnt do a thing UNLESS you pop open a case?

    Maybe. As a seller, most of the time the problem is not the $10. They want the item/need the item. There's nothing ebay can do for them.

    Isnt it just mean to put pressure on a seller upfront without saying a thing?

    So lets say the MOSFET is blown. How can ebay become involved?
    Do they give return labels for China? Wont that cost more than the item?

    2) Also "not as described" is incorrect case. OP needs "Return for refund" case- and that costs more than the item (I think so). Besides let the seller pay it, its no use! You wont get a new item faster or the item fixed.
     
  15. Denesius

    Member

    Feb 5, 2014
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    I agree with MaxHeadRoom. I have bought hundreds of electronic devices and PS's from Chinese sources thru ebay. As long as you stay with reputable companies (ebay stores with >1K+ feedbacks), I've found the following: 1-about 15-20% of items are defective or fail shortly, and 2-the sellers have always been good about following up and offering resolution, without getting ebay involved. Some have replaced without requiring return. As to the rest, I haven't sent anything back yet, since the shipping is usually a significant portion of the value of the item, and even with 20% failure rate, I'm still well ahead of the game compared to buying from US distributors.
    As to the problem experienced by the TS- I would go thru and reflow as many of the solder joints as you can comfortably do. While you're there, check the resistance of the suspected pot (it may be less than the published value on the device, but it should not be more). Ultimately, if this doesn't work, order another 2- one for a replacement, the second for the replacement's replacement.
     
  16. pilko

    Active Member

    Dec 8, 2008
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  17. dk31

    Thread Starter Member

    Oct 30, 2014
    36
    0
    First of all, thank you all for your interest in my issue :)
    My initial plan was to make a buck-boost circuit myself but I was discouraged - I have been reading and making progress in electronics but I am still fighting to get a grasp of the very basics.
    I would really like to repair it myself, first to learn something out of it and then to save me from waiting the delivery of the replacement (it takes me 1 month to receive packages from China, and it's mostly due to the local post service)

    @takao21203 : your posts are a mix of useful info (check the adjustable resistor, MOSFET and diode) and mistrust for the human race :) . I didn't wreck it and I don't really care for the <10$ spent on the item to start lying all over the net. Becides, I wouldn't need to lie HERE, as that wouldn't make my case stronger over ebay. Also, I have nothing but good experiance with ebay and all sellers there - I always pick up sellers with >=99% rating (unless for items that are dirt cheap).
    That being said, the potentiometer was 10kOhms when turned all the way clockwise providing 25volts output, and zero Ohms when all the way counter-clockwise. "All the way" is subjective, as it could take more turns at the min and max values but the voltage stops changing. I did turn the pot 4 or 5 times from min to max and back and also 2 times over the min-max range, if it makes any difference.
    Input voltage comes from a PC power supply (12V) and I thing it is not part of the issue.
    I am not at my lab right now but I am going to check all the components for continuity and check again the pot resistance and diode values. Please stay tuned as I am going to need help with the MOSFET and any other exotic component. I am willing to put the time to replicate the whole circuit if necessary with your assistance.
    Thanks,
    Dimitris
     
  18. dk31

    Thread Starter Member

    Oct 30, 2014
    36
    0
    Last update...
    The buck-boost converter finally works. It seems I turned by mistake the second potentiometer (controlling current) all the way right or left (can't recall) and that caused voltage to drop to zero. Today, I set the current poti to 6.9 KOhms (10 KOhms is the max value) and it works fine.
    Does anyone know where I need to set the current poti to get maximum output current ? Is it 10 KOhms or zero Ohms? I don't have a load big enough to check for current yet.
     
  19. takao21203

    Distinguished Member

    Apr 28, 2012
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    I hacked one of these some time ago (the smaller version).

    At 82 volts, the MOSFET made "pop". Then changed for higher voltage.
    I never really tried more than 100 volts. later used some time to power 60V LED circuit from a laptop brick.
     
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