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    isolating screws from taped heatsink for mounting TO-3 package

    It would be far quicker to tap holes and screw the TO-3 package but can screws be insulated from TO-3 case?

    The TO-3 case will go to ~800v - the screws must make contact with heatsink threads which require screws to be isolated from case.

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    Re: isolating screws from taped heatsink for mounting TO-3 package

    use a plastic (nylon?) shoulder washer

    examples:
    http://www.fastenercomponents.com/ny...hers-p-27.html

    put shoulder washer into mounting hole, put screw into shoulder washer


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    Re: isolating screws from taped heatsink for mounting TO-3 package

    Nylon parts can be torqued to TO-3 torque specs? https://www.digikey.com/products/en/...=1&pageSize=25



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    Re: isolating screws from taped heatsink for mounting TO-3 package

    i remember watching them being used in production

    the wiki article

    https://en.wikipedia.org/wiki/TO-3

    shows lots of plastic and fiber shoulder washers

    and references this article on mounting

    http://www.ti.com/lit/an/sboa020/sboa020.pdf

    which has this bolded
    The recommended tightening torque is 6 inch-lbs., with anacceptable range of 4 - 7 inch-lbs. ( 0.45 - 0.79 N-m ).

    and this:
    http://www1.futureelectronics.com/do...LOY/4804SG.pdf

    and this
    https://www.shopaavid.com/Portals/0/...talog-2018.pdf

    to answer your question:
    without specific torque specification:
    i expect the nylon shoulder washer to NOT handle the same torque as the metal TO3 case
    i also expect that they are acceptable to do the job


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    Re: isolating screws from taped heatsink for mounting TO-3 package

    Whats the approximate thermal resistance from TO-3 case into heatsink using slightly greased mica? https://www.digikey.com/product-deta...mSeq=293777268

    It would probably be better with more torque but nylon doesn't permit it.



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    Re: isolating screws from taped heatsink for mounting TO-3 package

    the issue usually isn't the torque
    its the contact between the heat sink versus the TO3 case

    one of the references in post 4 talked about:
    1) drill, don't punch the mounting holes and C E holes - punching leaves craters which reduce the contact area and the quality of the contact
    2) machine the area of the heatsink to a flatness of 0.001 inch per inch - the TO3 package is o.004 inch per inch


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    Re: isolating screws from taped heatsink for mounting TO-3 package

    Why won't the metal hole in the TO-3 arc over to the hole in the heatsink (via the path through the hole in the sil-pad or mica insulator hole)? That same arc path is there even wehn the shoulder washer & screw is completely removed or inserted


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    Re: isolating screws from taped heatsink for mounting TO-3 package

    the dielectric strength of air is about 3 x 10^6 V/m

    https://hypertextbook.com/facts/2000/AliceHong.shtml

    that's 3000 V/mm
    or 300 V/ 0.1 mm

    the space between the screw and the TO3 is on the order of 1 mm (0.03937 inch)
    at 1/64 th of an inch (0.015625 inch or 0.3969 mm) its about 1000 V
    much higher than the collector voltage of most TO3 components

    see page 100
    https://www.shopaavid.com/Portals/0/...talog-2018.pdf
    the pertinent part of shoulder wacher is about 1 mm
    so there is about 1 mm between screw and metal.
    Last edited by wwfeldman; 27th May 2019 at 04:22.



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    Re: isolating screws from taped heatsink for mounting TO-3 package

    If the case is at 800V, what keeps it from arcing from there to the heatsink metal which is extremely close by?
    You have a hole sandwich:
    A) TO3 --with metal hole at 800V
    B) Insulating pad...Very thin insulator with a hole
    C) Heatsink---metal with tapped hole

    There are 3 stacked holes.. The only thing separating metals A & C, is the thickness of hole B, regardless of whether a shoulder washer & screw is inserted into these stacked holes.

    Is this discussed some where??

    Certainly the washer keeps the screw isolated & provide "lots" of creepage. But what prevents the TO-3 from "arcing" to the heatsink? UL requires a much larger creepage distance at 800V.



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    Re: isolating screws from taped heatsink for mounting TO-3 package

    All too vague. The problem can be clearly discussed in terms of clearance, creepage and dielectric breakdown voltages. Would you mind to show a drawing or photo of the respective mounting? We'll try to interpret it in IEC1010 terms.



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    Re: isolating screws from taped heatsink for mounting TO-3 package

    Quote Originally Posted by FvM View Post
    All too vague. The problem can be clearly discussed in terms of clearance, creepage and dielectric breakdown voltages. Would you mind to show a drawing or photo of the respective mounting? We'll try to interpret it in IEC1010 terms.
    The voltage devices which heatsink will facilitate will not have sufficient voltage to break over and arc.

    Is there a small inexpensive tool or method which can determine whether surface is adequately flat?



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    Re: isolating screws from taped heatsink for mounting TO-3 package

    @hoyt
    is there an 800V rated TO3?

    see this thread:
    Transistor isolation question

    @FvM
    i have not found the information you want - at least not all in one place
    every article, ap note etc on mounting transistors, not just metal TO3, but plastic -220s -243s etc
    do essentially the same thing - screws, nuts, washers and shoulder washers.
    they do not talk about creepage distances or arc over.
    neither does Horowitz and Hill (at least, i didn't find it)

    @Zak
    I doubt it is small,
    and i doubt it is inexpensive
    especially since very few things seem to be in TO3 cases anymore



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