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Insulating hardware, esp.washers

Discussion in 'Electronic Design' started by [email protected], Dec 21, 2013.

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  1. Guest

    I've got an electrically and thermally hot TO-220,
    that needs mechanically securing to a grounded
    heatsink.

    Anodizing isolates the TO-220 from the heatsink, but
    what do you guys use to isolate the mounting hardware
    to prevent it from shorting the device to the sink?
    Something that'll maintain the mechanical mating
    force, despite long-term operation at >80c?
    .----.
    |////|
    |////|
    ~ ~
    TO-220 ~ ~
    split | |////|
    washer V |////|
    | .-. |////| nut
    V | | |////| /
    .-. | | |////| .---./
    _ | | | | |////| |___| bolt
    / || | | | /
    _ '-^^^^^^^^^^^^^^^^^^^^^^^^^./
    _| |
    .-vvvvvvvvvvvvvvvvvvvvvvvvv'
    \_|| | |___|
    | | | | |////| | |
    '-' / | |////| '---'
    / | |////|
    / | |////|<--.
    | | |////| \
    | | |////| Heatsink


    The split washer maintains compression.

    Is there something better than fiber washers / bushings?
    Somehow I don't trust nylon heated in compression, esp. over
    time. ISTM it will cook, then crumble.

    TIA,

    James
     
  2. Den lørdag den 21. december 2013 19.34.12 UTC+1 skrev :
    use a spring clip instead?

    either a heatsink that is meant for it or something like this:
    http://www.tarapath.com.au/productRange_image/240/3120_3121.jpg


    -Lasse
     
  3. Guest

    Nylon 6/6 GF30 (30% glass-fiber reinforced) is really durable.
    http://www.plastic-products.com/nylon66.htm

    PPS is a good/better alternative, this is more available:
    http://www.aavid.com/product-group/accessories/washers
     
  4. I like Mica with a full body clamp or the package sandwiched between
    heat sink.
    Jamie
     
  5. Use mica insulators and a bar clamp. You can also get spring clamps
    from mouser.

    CHeers
     
  6. Yep, constant spring pressure is the best.
    As long as the spring can keep it's K.

    Cheers
     
  7. Which they usually don't!

    Jamie
     
  8. Beryllium Copper contacts have good spring tension. Run high current
    thru them and they loose their K. In my Tahoe the dash lights used to
    blink when it was hot, it turned out to be a bulkhead connector
    contact, the main B+ for the instrument cluster. Re bent the one
    terminal an all was good. There is some thing about those auto
    designers that they never seem to get electrical stuff right.

    Cheers
     
  9. Guest

  10. Guest

    That's what I'm leaning towards. I'm working on it today, so I'd
    hoped for something I could get locally.

    Just for this prototype I'm considering using a clamp--I can make that
    here, today:


    .----.
    |////|
    clamp |////|
    | ~ ~
    V ~ ~
    .----------.|////|
    split | .--------'|////| lock
    washer| | |////| washer nut
    | | | |////| / /
    V | | |////| .-. /
    .-. | | |////| | | .---./
    _ | | | | |////| | | |___| bolt
    / || | | | | | | | /
    _ '-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^./
    _| |
    .-vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv'
    \_|| | | | .-. | | |___|
    | | | | | | |////| | | | |
    '-' | | | | |////| | | '---'
    | | | | |////| '-'
    | | | | |////|
    | | __| | |////|
    | | / | |////|<--.
    | \ | | |////| \
    \ \ | | |////| Heatsink
    \_| | |


    Nice. Thanks.

    Cheers,

    James Arthur
     
  11. Guest

    This unit was designed to run cool, loaded, as operated by a
    sophisticated user (it drives part of a process experiment).
    But, the user is proving electronically inexperienced.

    Normally it switches 50A (screamingly audible--hearing protection
    required) and dissipates a few watts. Unloaded--i.e., under fault conditions--the FETs dissipate the entire input power. So instead
    of 3W conduction loss I'll get 150W of avalanche, and probably all
    in one device. That's the rub.

    I included thermal shutdown, but I didn't expect to actually
    need it--the design wasn't meant for this kind of abuse.
    That looks like the way to go. Easy, and all sorts of advantages...

    Cheers,

    James Arthur
     
  12. Guest

    Sounds good. A colleague told me woodworking outlets sell 24x24 inch
    mica panels, cheap. They use it for decoration.

    Cheers,

    James Arthur
     
  13. Guest

    I've seen that, but forgot. Sounds like a good combo.

    Thanks,

    James
     
  14. Guest

    Yes, hard anodized.

    Teflon squeezes out in slow-mo. I like PTFE, but not compressed.

    Thanks for the thoughts.

    Cheers,

    James Arthur
     
  15. Guest

  16. Phil Hobbs

    Phil Hobbs Guest

    Roight, I was looking at the nylon ones. Silly me.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  17. There are application notes by many of the Semiconductor manufacturers where
    they describe mounting arrangements that can be used with their devices.
    These include the Mica and Silicone based Insulations washers and insulated
    sleeves and collars for the mechanical fixings.

    <http://www.onsemi.com/pub_link/Collateral/AN1040-D.PDF>

    --
    ********************************************************************
    Paul E. Bennett IEng MIET.....<email://>
    Forth based HIDECS Consultancy.............<http://www.hidecs.co.uk>
    Mob: +44 (0)7811-639972
    Tel: +44 (0)1235-510979
    Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
    ********************************************************************
     
  18. Hi James, Nice art work! So are we talking about the washer between the back side nut and the heatsink? (not shown in drawing) How about an anodized aluminum washer? There's a washer company up in Idaho that has all sortsof options. (Do you do any prep to the to-220 surface so it doesn't scratch the anodization*? What about where the screw goes through the hole in the heat sink?)

    Re: nylon and crumble at ~80C.. have you seen this? (Does your device run 24/7?) I've used teflon shoulder washers for ~120C heater operation.. and nylon at lower temps.

    George H.
    *I've had issues with burrs left on a threaded hole punching through sil pads and shorting the to-220. I don't know if the same thing can happen withhard anodization.
     
  19. Guest

    Thanks Michael.

    Cheers,

    James
     
  20. Guest

    It's been decades since I read that, and I needed the reminders.
    (I hardly do anything that needs heatsinking any more; I'm always
    trying to use all the energy and waste none. Heat = waste.)

    It's the first published reference I've seen that recommends anodize
    on Al (as I'm doing here) as electrical isolation for low-voltage
    circuits.

    I also had mistaken "wavy" washers for Bellville--that was good
    to learn too.

    Thanks.

    James Arthur
     
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