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Is It SOP to Use TO-220 ..

Discussion in 'Electronic Components' started by Watson A.Name - \Watt Sun, the Dark Remover\, Jul 12, 2005.

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  1. Is it SOP to use TO-220 mounting hardware for TO-126 packages?

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  2. Tim Shoppa

    Tim Shoppa Guest

    Is it SOP to use TO-220 mounting hardware
    I thought that TO-126 packages have a hole sized for 3mm metric screws,
    while TO-220's have holes sized for 6-32 (non-metric) screws.

    You will not be able to put a 6-32 screw in a TO-126!

    All of the associated insulator hardware (mica sheet, fiber or nylon
    shoulder washers,etc. ) for TO-220's is completely unnecessary for a
    TO-126, right?


  3. Maybe I shouldn'r have said mounting hardware, I really meant mica
    insulator and washer and hardware (of the appropriate size), like what
    you get when you buy a TO-220 mounting kit from Rat Shack.

    I've seen TO-126 devices (mostly Japanese) mounted directly to a steel
    heatsink which is soldered to the PC board. I've seen 'em mounted to
    the PC board for low power apps, with a clip-on HS, or even freestanding
    w/o a heatsink. But I've never seen 'em mounted to a HS with a mica or
    other insulator.

    The On Semi AN-1040/D shows 'em mounted to a HS with screw and nut and a
    conical washer, but no mica insulator. That's fine if you use an
    electrically isolated heatsink. But if you want to mount them to a
    chassis or bigger heatsink, say as driver transistors with the output
    trs on a common HS, then they have to be insulated.

    I've seen several TO-220s mounted to a common HS with the gray rubbery
    thermopad insulators and an aluminum bar across them all to hold them to
    the HS. That way, there's no hassle with insulated washers, bushings,
    or screws. But I've never seen the TO-126 with mica insulator and white
    silicon goop. So I'm just trying to come up with some kind of solution.
  4. Fred Bartoli

    Fred Bartoli Guest

    Why wouldn't you do it? Just because you've never seen it before?

    Just check the mica hole size against the TO126 hole isolation and go for it
    if it's OK.
  5. The TO-126 transistors I use (BD135, BD136) are completely insulated. What
    TO-126 transistors are you using, that have exposed metal like a TO-220?
  6. TO-220?

    No, the BD135 has the flat metal area on the back, which contacts the
    heatsink, electrically tied to the center pin which is the collector.
    The BD135s I have are made by Philips. The same is true for the 2SD975.
    However the 2SC4934 is completely encapsulated in plastic and is
    electrically isolated from the heatsink.
    See datasheet here
  7. "Fred Bartoli"
    My original Q was "Is it SOP (standard operating procedure) to use
    TO-220 mounting hardware for TO-126 devices?"
  8. ehsjr

    ehsjr Guest

    TO-220: Use a sil-pad (the gray rubbery thermopad you mentioned),
    incur the wrath of John Larkin (rightly so, as experiments
    demonstrated), endure more heat in the device - but if you need
    insulation... mica would be better, sil-pad easier to modify (if needed).
    I haven't experimented yet to see the Rt difference between
    a sil-pad and mica. I've never seen a TO-3 sil-pad, but
    TO-3 micas abound. Makes you wonder. Mica is supposed to
    be a better heat conductor than sil-pad.

  9. [snip]
    I've seen the spacer, looks like it may be anodized aluminum, for the
    TO-3. It's thicker than mica, and dark, almost black. I don't know
    what it's made of, because I've never had to remove one. Seems like all
    the PSes I've come across are cheapo, with TO-220 or similar devices.
    It used to be that heatsinks were massive with convection cooling and
    TO-3s. Nowadays in order to reduce size and weight and cost, they are
    all smaller and cooled with fans and use epoxy cased devices.

    The On Semi AN-1040/D app note shows some good info on why the TO-220
    and similar are not the best way to mount a power device to a heatsink.

  10. Hmm, interesting. The BD135s I've been using are by Fairchild, and they are
    completely insulated, no exposed metal.

    What a pain! I don't think I've seen mica insulators with the smaller
    TO126-sized hole myself, although Google does turn some up ("TO126
  11. The Fairchild datasheet I have is dated Feb 2000 and doesn't show
    whether or not the 'backside' is insulated; it doesn't show the
    backside. Philips' datasheet does so and states so. SGS-Thomson's
    datasheet calls the package SOT-32, and does show a graphic of the
    backside, which looks like it has a metallic area that contacts the
    heatsink. But it doesn't state that it is connected to the collector.
    The general standard is that most transistor packages have the metallic
    area electrically connected to the collector.

    ON Semi has the BD135 and says it's in the Case 77-09, what it calls
    TO-225. Here it shows the metallic
    heatsink contact area, but doesn't state that it is electrically
    connected to the collector.

    Fairchild's BD135 series should conform to the rest of the industry,
    which is probably like Philips, since Philips or another Euro
    manufacturer is probably the original maker of the series. Also it
    seems to me that in order to dissipate that much power on a heatsink, it
    would have to have a metallic area on the backside.
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