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I need to pot a 3kv capacitor, is silicone going to work ok?

Discussion in 'High Voltage' started by jpoopdog, Sep 14, 2016.

  1. jpoopdog

    jpoopdog Member

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    Im replacing the capacitor in this monitor inverter board http://imgur.com/eEEpPgd

    its neccesary however that i reapply insulation at least on the HV out.
    Can i use a regular 100% silicone caulking rubber for insulation? I cant for some reason find any high voltage insulation applied as liquid anywhere, leading me to believe that everyone uses some other common product that works so effectively that its not neccesary to produce a product that serves that specific purpose.

    Otherwise if not silicone, could i use liquid electrical tape? the brush on stuff made by starbrite? I dont know how well their stuff works, all i know is its solvent based
     
  2. gary350

    gary350 Well-Known Member

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    Use a hot melt glue gun.

    This section of the forum is dead this was posted years ago and no replies. I guess i wont post anything here I will die of old age before anyone sees it. LOL.
     
  3. jpoopdog

    jpoopdog Member

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    tried hot melt glue, my spark gap ignitors (50kv at least) arc easily through it, the silicone however i tried with completely insulated it so much that the charge was forced to build up and slowly and noisily leak through the wires over about 3 seconds from turning the power off, normally it discharges through the air in an instant. i think like 3mm of silicone blocked 50kv. Anyway, i now know it works.
     
  4. dave

    Dave New Member

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  5. vtech

    vtech Member

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    You are looking too much in to this;
    Your capacitor is only rated max at 3 Kilo volts.
    Why bother with a 50KV tester through the hot melt glue?
    If there is in fact a potential of more than 3KV across the cap, there is something else wrong and it is bound for failure.
    Any of the non corrosive RTV sealers will be more than enough for the application. such as:
    http://www.digikey.com/product-search/en?mpart=RTV167-85ML&vendor=473
    Too expensive for a single application though.:eek:
    What you see used (gray silicone) has a higher dielectric than 3kv meant for much wider application beside just a monitor. ie; (Shin-Etsu)

    CFL Hv cap failures in monitors are not that uncommon and most of the time caused by device quality issues or simply a design with no head room.
    In this case, if available, I would choose a same value with higher KV rating provided the space is not an issue.
     
  6. jpoopdog

    jpoopdog Member

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    damn, true i should have gotten higher values. I actually did buy some accidentally but then had my order changed from the 6kv ones to the 3kv ones.

    my 50kv spark gap ignitor was the only high voltage power supply i had available to test with.
    The silicone i wound up getting was neutral curing clear stuff. Not especially UV resistant nor especially resistant to mold. Basically just pure and clean.

    I did a test, all 6 capacitors on the board had different readings. although my multimeter barely reads 15pf, it did allow me to see some are wearing out.

    i also made a new developement in hunting the fault. It seems when the wires are leaking HV, and in the dark i can see a corona discharge, arcing into the grounded back board, the monitor works. When the wires arent allowed to do this, there is no noise, and no backlight starting up too, its like its triggering a over voltage sensor in the backlight circuit somewhere.

    the backlight inverter has 6 CCFL tubes i believe, since there are 6 outputs.

    When the backlight is up and running, the noisy arcing stops. i can however still see some violet glow comming off the other 3 of the 6 leads where they are all bundled together. These gave readings which suggested they werent failing.

    anyway though, point is, the silicone i have now appears to be working, but now the problem has spread or is at least being shared elsewhere. I might post a new thread somewhere else about this since ive done allot of studying on it and i need to explain everything before asking for help on it.
     
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Historically EHT triplers in TV's were potted in silicon rubber (so that was 25KV or so) - and because it was soft and easily removed, we did occasionally 'scoop it out', replace the faulty components inside, and then re-pot with silicon rubber.

    We didn't use caulking tubes, we used small tubes bought from RS Components (I don't even remember silicon rubber caulking been available back then?), but I would imagine caulking would be fine - and it smells identical :D
     
  9. Les Jones

    Les Jones Well-Known Member

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    Hi Nigel,
    I have often wondered if the acetic acid in the caulking tubes of silicone rubber (Which I think gives the silicone it's smell.) would cause corosion to the component wires over time.

    Les.
     
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's not a problem I ever found, and presumably the silicon used by the manufacturers also used acetic acid.
     
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  12. tronitech

    tronitech Member

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    Are you sure that's a capacitor and not a MOV?
     
  13. vtech

    vtech Member

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    Definitely not an MOV.
    The device is a low value ceramic capacitor usually in series with the CFL lamp.
     
  14. vtech

    vtech Member

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    Correct; while it may not be apparent, Acetic acid combined with oxygen can eventually corrode contacts and I have seen it.
    As a matter of fact, there is no acetic acid in Electrical silicon rubber which is more expensive. ie, it is a definite no-no in safety applications such as Avionics.
    Compare RTV162 vs RTV108
     
  15. jpoopdog

    jpoopdog Member

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    Ive since dicovered that while the capacitors were in bad shape the actual problem seems to be the CCFL tubes themselves. Looks like ill have to remove and replace them or switch them for LEDs
     
  16. RODALCO

    RODALCO Well-Known Member

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    In the electrical industry we use Guruflex. Which is a two component mix which needs to be mixed in 6 litre quantities. It is blue coloured and has a rubber texture.
    Will post some pics tomorrow as I am using it tonight.
    Rated at 10kV /mm.
     
  17. RODALCO

    RODALCO Well-Known Member

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    Couple of pics attached.
    Two component to be mixed, it ends up like a type of rubber, used in an 11000 Volts linkbox on outside High Voltage switchgear.
    2016-10-05_19-55-17.jpg 2016-10-05_19-55-53.jpg 2016-10-05_19-56-28.jpg 2016-10-05_19-56-57.jpg
     
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