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Capacitor Substitution

Discussion in 'Power Electronics' started by H2814D, Aug 24, 2019.

  1. davenn

    davenn Moderator

    13,294
    1,758
    Sep 5, 2009

    this pic ?
    [​IMG]


    The case for those MOSFETs is a TO-247AC
     
  2. WHONOES

    WHONOES

    723
    147
    May 20, 2017
    Can you provide close up photo's (as close as you can whilst still in focus) on both sides of the board around the burnt area?
     
  3. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    Sure. Here you go. Note that although there was substantial heat there, the resistors are still measuring out correctly. The top portion of the board does not have any burn marks. The shroud on the mains does though. There are cotton fibers from a q-tip on some of the pointy solder bits in the bottom view.

    The arcing that I mentioned above was visible directly in front of the middle post (Drain) of the mosfets (IRFP450) in the picture above through the board. There were no burn marks there either before or after seeing the arcing.
     

    Attached Files:

    Last edited: Oct 8, 2019
  4. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    I replied above.
     
  5. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    Are you able to tell me if these are the "sense resistor(s)" you are talking about? They are low value (.1 Ohm) and are in parallel with each other. One end is in circuit contact with the number three leg (source) of both of the mosfets (IRFP450). You can see the connections in the photos attached to this reply.

    Now if I was to replace these resistors, do I look for wire wound or current sense resistors? Are they the same thing as long as I get the correct values? They look like wire wound. They are 24mm long and 8mm diameter. I am guessing 5 watt?
     

    Attached Files:

  6. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    OK all. I have to report a success here. Thanks for your help.

    Today, since there weren't really that many of each, I decided to pull a leg off of all of the resistors, diodes, transistors, etc., and other easily testable components and do just that...test them. Everything tested ok, so I soldered each of the pulled legs/components back to the board. As I was re-soldering on the bottom of the board, I happened to glance at the top/inner leg of where "RT1" comes through the board from the other side. It is a thick coiled wire (Someone tell me what that (RT) is please? I tried but couldn't find "RT"). It was then that I noticed a cold solder joint that had not been there before. You can actually see that there is a difference between an earlier message pic of the bottom side of the PCB and the pic I have posted in this message.

    So I am confident that this is what happened: After removing the converter from the RV to begin my initial inspection I checked for visual signs of failure. This included cold solder joints and damaged/burned electronic components. Finding none, I decided to start testing individual components. In order to remove the heat sink from the PCB to get to the mosfets and the larger diodes attached to it, I had to remove the single vertical white resistor. It was in the way of a screw that holds a metal plate that holds the two mosfets and one of the thermal components to the heatsink. I had to take a long screwdriver and snake it between the RT1 and the two larger caps to get to the screw. The screw was tight and difficult to remove. I am sure I put pressure against the RT1 wire and that was what caused the solder joint to break. After testing the components, replacing everything, and powering back up- because I did not have any idea that I had caused the break- I am certain that that was what caused the flickering/arcing below the mosfets drain legs. As stated above, at that point I stopped and came back here for ideas. Because I had already inspected for broken solder joints, and found none, I assumed that wasn't the issue. Hindsight tells me I should have checked again. Amateur. :)

    Anyway, today, after finding that break, reflowing the solder around it, and putting everything back together, I can happily say that I am getting 13.8vdc at the outputs, I can power a 12vdc LED fishing light without a substantial voltage loss, and am confident I have a working converter again. The original issue with the caps needing to being replaced and later finding the solder break, with your help, has saved me about $250USD!!! So thanks for that. And as a side note, hopefully someone else will come across this thread and get as lucky as I did.

    I have attached pics of the break, the RT1 coiled wire, and a working converter powering a very bright LED fishing light (covered so it didn't wash out the photo).

    On a side note, those two vertical resistors standing right next to each other, were about 250F when measured with an infrared thermometer. They are 10W1K5J resistors. Can anyone advise if there would be a more suitable/cooler replacement for these? The heat they have produced, that actually marked the board, is my concern.

    Thanks again, guys. I appreciate it.
     

    Attached Files:

  7. WHONOES

    WHONOES

    723
    147
    May 20, 2017
    RT1 may be the current sense resistor which would make sense if you found that it had a dry joint and all is now working OK since you rectified it. Well done, glad you have got it sorted.
     
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